Patent Application
20240218489
Oxide-dispersion strengthened (ODS) alloys, exhibiting extraordinary properties at high temperature, are in high demand in fields such as aerospace, nuclear power, and other applications with demanding conditions. They may also be used in additive manufacturing and welding. Current fabrication methods for ODS alloys such as ball milling have been limited by the high cost (hundreds of dollars per kilogram) and low production volume. Nanoparticles such as Al2O3 and Y2O3 cannot be incorporated into molten alloys without severe sintering due poor wettability. Other in-situ methods are also limited by high cost due to the requirement of fast cooling. Similar problems can face other ODS alloys.
Provided in one aspect is a method of producing an oxide-dispersion strengthened (ODS) alloy, the method including:
In some embodiments, allowing the ODS to solidify includes casting the ODS alloy.
In some embodiments, the metal or metal alloy of the master alloy powder and the molten diluent alloy are individually at least one of Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, or Zn. In some embodiments, the metal or metal alloy of the master alloy powder and the molten diluent alloy include Fe, Mg, Mn, Co, or a mixture of any two or more thereof. In some embodiments, the metal or metal alloy of the master alloy powder and the molten diluent alloy are the same. In some embodiments, the metal or metal alloy of the master alloy powder and the molten diluent alloy both include a steel.
In some embodiments, the particles of metal oxide particles include Al2O3, Y2O3, ZrO2, MgAl2O4, yttria stabilized zirconia, MgO, or a mixture of any two or more thereof. In some embodiments, the particles of the metal oxide particles are microparticles or nanoparticles. In some embodiments, the particles of the metal oxide particles are nanoparticles.
In some embodiments, the wetting-enhancing metal comprises an alloying element. In some embodiments, the wetting-enhancing metal comprises Nb, Cr, or a mixture thereof.
Provided in another aspect is a bulk metal matrix formed by any one of the methods disclosed herein.
Provided in another aspect is a bulk metal matrix including a metal matrix and a plurality of uniformly dispersed metal oxide particles, wherein the oxide particles include at their surface a wetting-enhancing metal that reduces the interfacial energy and vdW attraction between the oxide particles and the bulk metal in molten metal. In some embodiments, the wetting-enhancing metal comprises an alloying element.
In some embodiments, the metal matrix includes at least one of Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, or Zn. In some embodiments, the metal matrix includes a steel.
In some embodiments, the metal oxide particles include Al2O3, Y2O3, ZrO2, MgAl2O4, yttria stabilized zirconia, MgO, or a mixture of any two or more thereof. In some embodiments, the wetting-enhancing metal comprises Nb, Cr, or a mixture thereof.
Provided in another aspect is a method of manufacturing an ODS master alloy with high oxide nanoparticle loading for dilution to final ODA alloys including rapidly heating one or more metal of the alloy with the oxide nanoparticles for a period of time that less than the amount of time to sinter a majority of the oxide nanoparticles.
In some embodiments, the metal of the alloy with the oxide nanoparticles are rapidly heated using arc melting or laser melting.
In some aspects of the present disclosure includes the following embodiments. These are illustrative embodiments using steels, but it should be understood that the disclosure related to other embodiments as well.
Various embodiments are described hereinafter. It should be noted that the specific embodiments are not intended as an exhaustive description or as a limitation to the broader aspects discussed herein. One aspect described in conjunction with a particular embodiment is not necessarily limited to that embodiment and may be practiced with any other embodiment(s).
As used herein, the singular terms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to an object can include multiple objects unless the context clearly dictates otherwise.
As used herein, the terms “substantially,” “substantial,” “approximately,” and “about” are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. When used in conjunction with a numerical value, the terms can refer to a range of variation of less than or equal to ±10% of that numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%. For example, a first numerical value may be deemed to be “substantially” the same or equal to a second numerical value if the first numerical value is within a range of variation of less than or equal to ±10% of the second numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to #1%, less than or equal to ±0.5%, less than or equal to ±0.1%, or less than or equal to ±0.05%.
Additionally, amounts, ratios, and other numerical values are sometimes presented herein in a range format. It is to be understood that such range format is used for convenience and brevity and should be understood flexibly to include numerical values explicitly specified as limits of a range, but also to include all individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly specified. For example, a ratio in the range of about 1 to about 200 should be understood to include the explicitly recited limits of about 1 and about 200, but also to include individual ratios such as about 2, about 3, and about 4, and sub-ranges such as about 10 to about 50, about 20 to about 100, and so forth.
Disclosed herein are methods relating to a manufacturing method for incorporating and dispersing oxide nanoparticles in molten alloys (such as steels), and thus provide a novel and cost-effective way to manufacture oxide-dispersion strengthened (ODS) alloys (such as steels). ODS alloys (such as steels) exhibit higher strength, better thermal stability, better creep and irradiation resistance over alloys (such as steels) counterparts that are not reinforced by oxide nanoparticles (unreinforced alloys). Conventionally, ODS steels are produced primarily through solid-state processes, in which the oxide nanoparticles or oxygen carriers are ball milled (for dispersion) with steel powders at the desirable concentration, and the ball milled powders are consolidated to form bulk ODS steel. This method is time consuming and costly due to the long duration needed for ball milling and the high-cost metal powders, as well as the thermomechanical processing procedures to make plates, rods, or tubes. In contrast to the conventional method of manufacturing ODS steels, this disclosure described methods to enable the production of ODS alloys (such as steels) by liquid metallurgy more quickly and at lower cost.
The present disclosure addresses at least one of two key challenges of manufacturing ODS alloys (such as steels) by liquid metallurgy: 1) Molten alloys (such as steels) have naturally poor wettability with oxide nanoparticles, which will force the nanoparticles to sinter instead of dispersing in molten steels. 2) During the incorporation process, oxide nanoparticles can easily sinter together before having the chance to disperse in molten steel due to the very high temperature they experience. The present disclosure improves the interfacial adhesion, and thus wettability, between the molten steel and the oxide nanoparticles, and reduces the van der Waals attraction significantly between oxide nanoparticles by adding suitable element(s) into the melt. In some aspects, the methods prevent the sintering of oxide nanoparticles during incorporation by protecting them within a master alloy.
The manufacturing methods described herein may be applied to various types of alloys (such as steels) with various oxide nanoparticles. Specifically, the manufacturing methods described herein are scalable due to the conventional liquid metallurgy process used to achieve bulk ODS steels, provide bulk ODS steels with uniformly dispersed small nanoparticles (e.g., 10-50 nm), are relatively low cost, compatible with various steel/oxide systems, and allow for tunable volume percentage of oxide nanoparticles. For example, some embodiments include Y2O3 nanoparticle strengthened stainless steel, Al2O3 nanoparticle strengthened stainless steel, Y2O3 nanoparticle strengthened low carbon steel, and Al2O3 nanoparticle strengthened low carbon steel. The manufacturing methods described herein are advantageous over current methods as nanoparticles such as Al2O3 and Y2O3 cannot be incorporated into molten steels without severe sintering due poor wettability.
Provided in one aspect is a method of producing an oxide-dispersion strengthened (ODS) alloy. The method includes providing a master alloy powder including a metal or metal alloy and particles of a metal oxide; adding the master alloy powder to a molten diluent alloy to form an oxide-dispersion strengthened (ODS) alloy; and allowing the ODS to solidify. In the method, the the molten diluent alloy includes a molten metal or metal alloy. Additionally, a wetting-enhancing metal may be added to the molten diluent alloy either prior to, during, or after adding of the master alloy to the molten diluent alloy, where the the wetting-enhancing alloy reduces an interfacial energy and vdW attraction between the particles of the metal oxide and the molten alloy diluent to achieve a stable nanoparticulate ODS solid. As used herein, the master alloy is typically a powder of intimately mixed metal or metal alloy with the metal oxide. This can be prepared by a variety of methods including ball milling. The master alloy is typically more concentrated in the metal oxide prior to dilution with the diluent metal or metal alloy. Master alloys are typically used in the industry and their production is not particularly limited here.
In some embodiments, the allowing the ODS to solidify includes casting the ODS alloy. In some embodiments, the ODS is allowed to solidify slowly (e.g., cooling rate <5 K/s).
In some embodiments, the metal or metal alloy of the master alloy powder and the molten diluent alloy are individually at least one of Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, or Zn. In some embodiments, the metal or metal alloy of the master alloy powder and the molten diluent alloy include Fe, Mg, Mn, Co, or a mixture of any two or more thereof.
Examples of suitable sources for the master alloy powder include but are not limited to commercial stainless steel 316L, medium manganese steel (Fe-7Mn-1Nb-1C-1Si in wt. %), Fe—Nb binary alloy (Fe-1Nb), and pure iron. In some embodiments, the metal or metal alloy of the master alloy powder is at least one of Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, or Zn. In some embodiments, the metal or metal alloy of the master alloy powder includes Fe, Mg, Mn, Co, or a mixture of any two or more thereof. In some embodiments, the metal or metal alloy of the master alloy powder includes Al. In some embodiments, the metal or metal alloy of the master alloy powder includes Mg. In some embodiments, the metal or metal alloy of the master alloy powder includes Fe. In some embodiments, the metal or metal alloy of the master alloy powder includes Ag. In some embodiments, the metal or metal alloy of the master alloy powder includes Cu. In some embodiments, the metal or metal alloy of the master alloy powder includes Mn. In some embodiments, the metal or metal alloy of the master alloy powder includes Ni. In some embodiments, the metal or metal alloy of the master alloy powder includes Cr. In some embodiments, the metal or metal alloy of the master alloy powder includes Co. In some embodiments, the metal or metal alloy of the master alloy powder includes Au. In some embodiments, the metal or metal alloy of the master alloy powder includes Zn.
In some embodiments, the molten diluent alloy is at least one of Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, or Zn. In some embodiments, the molten diluent alloy includes Fe, Mg, Mn, Co, or a mixture of any two or more thereof. In some embodiments, the molten diluent alloy includes Al. In some embodiments, the molten diluent alloy includes Mg. In some embodiments, molten diluent alloy includes Fe. In some embodiments, the molten diluent alloy includes Ag. In some embodiments, the molten diluent alloy includes Cu. In some embodiments, the molten diluent alloy includes Mn. In some embodiments, the molten diluent alloy includes Ni. In some embodiments, the molten diluent alloy includes Cr. In some embodiments, the molten diluent alloy includes Co. In some embodiments, the molten diluent alloy includes Au. In some embodiments, the molten diluent alloy includes Zn.
In some embodiments, the metal or metal alloy of the master alloy powder and the molten diluent alloy are the same. In some embodiments, the metal or metal alloy of the master alloy powder and the molten diluent alloy are different. In some embodiments, the metal or metal alloy of the master allow powder and the molten diluent alloy both include a steel (e.g. stainless steel and low carbon steel).
Examples of suitable oxide nanoparticles include but not limited to Al2O3, Y2O3, and yttria stabilized zirconia (YSZ). In some embodiments, the particles of metal oxide particles include Al2O3, Y2O3, ZrO2, MgAl2O4, yttria stabilized zirconia, MgO, or a mixture of any two or more thereof. In some embodiments, the particles of metal oxide particles include Al2O3. In some embodiments, the particles of metal oxide particles include Y2O3. In some embodiments, the particles of metal oxide particles include ZrO2. In some embodiments, the particles of metal oxide particles include MgAl2O4. In some embodiments, the particles of metal oxide particles include MgAl2O4. In some embodiments, the particles of metal oxide particles include yttria stabilized zirconia. In some embodiments, the particles of metal oxide particles include MgO.
In some embodiments, the particles of the metal oxide particles are microparticles or nanoparticles. In some embodiments, the particles of the metal oxide particles are nanoparticles. In some embodiments, the particles of the metal oxide particles have a particle size distribution of from about 1 to about 100 nm, from about 1 to about 75 nm, and from about 1 to about 50 nm, including about 1 nm, about 10 nm, about 20 nm, about 30 nm, about 40 nm, about 50 nm, about 60 nm, about 70 nm, about 75 nm, about 80 nm, about 90 nm, and about 100 nm. In some embodiments, the particles of the metal oxide particles have a particle size distribution of from about 1 to about 50 nm. In some embodiments, the particles of the metal oxide particles have a particle size distribution of from about 10 to about 50 nm.
Suitable wetting-enhancing agents may be alloying elements, such as Mn, Si, Cr, and Nb. Such wetting agents may be effective in reducing the van der Waals (vdW) attraction between oxide nanoparticles, because they are likely to form a thin interfacial liquid layer around the oxide nanoparticles. In some embodiments, the wetting-enhancing metal may be any alloying element. In some embodiments, the wetting-enhancing metal includes Nb, Cr, or a mixture thereof. In some embodiments, the wetting-enhancing metal includes Nb. In some embodiments, the wetting-enhancing metal includes Cr.
Provided in another aspect is a bulk metal matrix formed by any one of the methods disclosed herein.
Provided in another aspect is a bulk metal matrix including a metal matrix and a plurality of uniformly dispersed metal oxide particles, wherein the oxide particles include at their surface a wetting-enhancing metal that reduces the interfacial energy and vdW attraction between the oxide particles and the bulk metal in molten metal.
In some embodiments, the metal matrix includes at least one of Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, or Zn. In some embodiments, the metal matrix includes Al. In some embodiments, the metal matrix includes Mg. In some embodiments, the metal matrix includes Fe. In some embodiments, the metal matrix includes Ag. In some embodiments, the metal matrix includes Cu. In some embodiments, the metal matrix includes Mn. In some embodiments, the metal matrix includes Ni. In some embodiments, the metal matrix includes Ti. In some embodiments, the metal matrix includes Cr. In some embodiments, the metal matrix includes Co. In some embodiments, the metal matrix includes Au. In some embodiments, the metal matrix includes Zn. In some embodiments, the metal matrix includes a steel.
In some embodiments, the metal oxide particles include Al2O3, Y2O3, ZrO2, MgAl2O4, yttria stabilized zirconia, MgO, or a mixture of any two or more thereof. In some embodiments, the metal oxide particles include Al2O3. In some embodiments, the metal oxide particles include Y2O3. In some embodiments, the metal oxide particles include ZrO2. In some embodiments, the metal oxide particles include MgAl2O4. In some embodiments, the metal oxide particles include yttria stabilized zirconia. In some embodiments, the metal oxide particles include MgO.
In some embodiments, the wetting-enhancing metal may be any alloying element. In some embodiments, the wetting-enhancing metal comprises Nb, Cr, or a mixture thereof. In some embodiments, the wetting-enhancing metal includes Nb. In some embodiments, the wetting-enhancing metal includes Cr.
Provided in another aspect is a method of manufacturing an ODS master alloy with high oxide nanoparticle loading for dilution to final ODA alloys including rapidly heating one or more metal of the alloy with the oxide nanoparticles for a period of time that less than the amount of time to sinter a majority of the oxide nanoparticles.
For the manufacturing methods described herein, rapid melting with an extreme heat source prevents sintering of the oxide nanoparticles. In particular, this disclosure recognizes that the combination of rapid heating (>1000 K/s), extreme processing temperature (>2200 K), and short processing duration (<5 s) for important for minimizing sintering. In some embodiments, the metal of the alloy with the oxide nanoparticles are rapidly heated using arc melting or laser melting. In some embodiments, the metal of the alloy with the oxide nanoparticles are rapidly heated using arc melting. In some embodiments, the metal of the alloy with the oxide nanoparticles are rapidly heated using laser melting.
Some embodiments include a method of producing an oxide-dispersion strengthened (ODS) metal/alloy. The method includes providing a bulk molten metal/alloy comprising one or more metals; adding a plurality of oxide particles that are surface enriched with an element that reduces interfacial energy and vdW attraction between the oxide particles and the bulk molten metal/alloy; and casting the ODS metal/alloy.
The ODS metal/alloy may be melted or welded without substantially affecting the oxide-dispersion. In some embodiments, the metal/alloy comprises one or more selected from Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, and Zn. In some embodiments, the bulk molten metal/alloy is steel, such as stainless steel or low carbon steel. In some embodiments, the oxide particles are surface enriched at nanoscale with Nb, Cr, which should be carefully selected for each nanoparticle and metal/alloy system. In some embodiments, the oxide particles comprise a higher concentration of the element that reduces interfacial energy and vdW attraction within a few atomic layers (e.g., 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 layers) or about 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8 or 2.0 nm from surface than the concentration of the alloying element that reduces interfacial energy between nanoparticle and metal/alloy and vdW attraction between nanoparticles in the high temperature melt. In some embodiments, the oxide particles comprise at least one of Al2O3, Y2O3, ZrO2, MgAl2O4, yttria stabilized zirconia and MgO.
In some embodiments, the oxide particles are microparticles or nanoparticles. In some embodiments, the oxide particles are nanoparticles. In some embodiments, the nanoparticles can have at least one dimension (e.g., an effective diameter which is twice an effective radius) or at least one average or median dimension (e.g., an average effective diameter which is twice an average effective radius) in a range of about 1 nm to about 1000 nm, such as about 1 nm to about 100 nm, about 1 nm to about 80 nm, about 1 nm to about 60 nm, about 1 nm to about 40 nm, about 1 nm to about 20 nm, or about 1 nm to about 10 nm, although other ranges within about 1 nm to about 1000 nm are contemplated, such as about 1 nm to about 500 nm or about 1 nm to about 200 nm).
Reducing vdW attraction between the oxide particles in the bulk molten metal/alloy may be demonstrated in
Additional embodiments include a bulk metal/alloy matrix comprising a metal/alloy matrix and a plurality of uniformly dispersed oxide particles that are surface enriched with an element that reduces interfacial energy and vdW attraction between the oxide particles and the bulk metal/alloy when molten metal/alloy. In some embodiments, the bulk metal/alloy matrix is formed by the method of any one of the embodiments herein.
Additional embodiments include a method of producing an oxide-dispersion strengthened (ODS) metal/alloy, comprising providing a bulk molten metal/alloy comprising one or more metal/alloy; adding an alloy comprising a metal/alloy and a plurality of oxide particles uniformly dispersed therein; and casting the ODS metal/alloy.
In some embodiments, the alloy comprises one or metal/alloy found in the bulk molten metal/alloy. In some embodiments, the metal/alloy comprises one or more alloys selected from Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, and Zn. In some embodiments, the bulk molten metal/alloy is steel. In some embodiments, the alloy comprises Nb or Cr.
In some embodiments, the oxide particles are microparticles or nanoparticles. In some embodiments, the oxide particles are nanoparticle. In some embodiments, oxide particles are nanoparticles that have at least one dimension (e.g., an effective diameter which is twice an effective radius) or have at least one average or median dimension (e.g., an average effective diameter which is twice an average effective radius) in a range of about 1 nm to about 1000 nm, such as about 1 nm to about 100 nm, about 1 nm to about 80 nm, about 1 nm to about 60 nm, about 1 nm to about 40 nm, about 1 nm to about 20 nm, or about 1 nm to about 10 nm, although other ranges within about 1 nm to about 1000 nm are contemplated, such as about 1 nm to about 500 nm or about 1 nm to about 200 nm).
Additional embodiments include a method of manufacturing ODS master alloy with high oxide nanoparticle loading, comprising: rapidly heating one or more metal/alloy of the alloy with the oxide nanoparticles for a period of time that less than the amount of time to sinter a majority of the oxide nanoparticles. In some embodiments, the metal/alloy of the alloy with the oxide nanoparticles are rapidly heated using arc melting or laser melting. In some embodiments, the oxide particles are microparticles or nanoparticles. In some embodiments, the oxide particles are nanoparticles. In some embodiments, the oxide particles are nanoparticles that have at least one dimension (e.g., an effective diameter which is twice an effective radius) or have at least one average or median dimension (e.g., an average effective diameter which is twice an average effective radius) in a range of about 1 nm to about 1000 nm, such as about 1 nm to about 100 nm, about 1 nm to about 80 nm, about 1 nm to about 60 nm, about 1 nm to about 40 nm, about 1 nm to about 20 nm, or about 1 nm to about 10 nm, although other ranges within about 1 nm to about 1000 nm are contemplated, such as about 1 nm to about 500 nm or about 1 nm to about 200 nm).
Some aspects of the present disclosure relate to manufacturing ODS alloys (such as steels) by liquid metallurgy is a two-step process. First, the ODS master alloy may be fabricated. The ODS master alloy is an alloy that contains higher volume percentage of oxide nanoparticles than what is needed in ODS steel. The ODS master alloy may be diluted in a steel melt that contains no oxide nanoparticles to create the desired volume percentage of oxide nanoparticles in the final steel product. Depending on the designed volume percentage of nanoparticles in the ODS master alloy, it may be manufactured differently.
Illustrative matrix steels for the ODS master alloy may include, but are not limited to, commercial stainless steel 316L, medium manganese steel (Fe-7Mn-1Nb-1C-1Si in wt. %), Fe—Nb binary alloy (Fe-1Nb), and pure iron. Illustrative oxide nanoparticles include, but are not limited to, Al2O3, Y2O3, and yttria stabilized zirconia (YSZ).
At low volume percentage of nanoparticles (<2 vol. %), the ODS master alloy may be referred to as a “powder master.” To manufacture the powder master, steel powders are mixed with 2 vol. % of oxide nanoparticles and organic solvent (pure ethanol or acetone) to create a slurry mixture. The particle size distribution of the steel powders is between 15 to 45 μm, and of oxide nanoparticles is between 1-50 nm. The slurry mixture is placed in a glass container and treated in an ultrasonic bath for 10 min. The ultrasonic bath breaks up the oxide nanoparticle clusters and improves the uniformity of mixing, while the organic solvent absorbs the moisture. The slurry mixture is then dried at 60° C. to evaporate the organic solvent. The dried powder mixture is rigorously ground in a mortar and pestle set for 0.5 to 1 hr for uniform mixing. The ground powders are transferred to a stainless steel mold for cold compaction. The cold compaction pressure is between 200 to 400 MPa in uniaxial compression.
Liquid state processing routes that require a temperature above the melting point of steel are not able to prevent sintering, due to the temperature requirement being significantly higher than the sintering temperature of the oxides. Rather, the present disclosure provides a combination of rapid heating (>1000 K/s), extreme processing temperature (>2200 K), and short processing duration (<5 seconds) to bypass the sintering issue in the master alloys containing a high volume percentage of oxide nanoparticles. Without being bound by theory, it is believed that rapid heating allows the process to reach desired temperature in an extremely short time window (within 2 seconds). This means that the sintering that could take place during a prolonged heating stage may be mitigated. Also, extreme processing temperature significantly enhances the wettability between the oxide and the molten steel. It also significantly increases the thermal energy (kT) available for the oxide nanoparticles to move around in the liquid. Finally, the short processing duration allows the system to solidify before any sintering can happen. Nanoparticle sintering in high temperature melt is a statistical event, in which long duration results in more sintering. Therefore short processing duration is necessary to eliminate the sintering.
The combination of the three features can only be offered by extreme heat source, such as an electrical arc, plasma, or laser.
Therefore, at high volume percentage of nanoparticles (2 vol. % or above), the ODS master alloy is manufactured by a rapid melting method, including, but not limited to, by arc/plasma melting or by laser melting, in which they are referred to as “arc master” and “laser master”, respectively.
To manufacture the arc master or the laser master, the starting powders may be prepared as described above. However, as the volume percentage of nanoparticle rises, the uniform mixing becomes more difficult to achieve. To this end, powder-mixing methods, such as ball milling, may be used to achieve better mixing. In a ball-milling step, steel powders are mixed with a high volume percent of oxide nanoparticles and placed in a steel jar. Hardened steel balls are added. The weight ratio between the hardened steel balls and the powder is 10 to 1. The steel jar is sealed and then shaken by a mechanical shaker for 8 up to 24 hr. The uniformly mixed powder is referred to as the “feedstock powder”.
For an arc master, the feedstock powder is cold compacted to form a pellet. The compacted pellet is placed in an electrically grounded pure copper melting bowl or a conductive ceramic crucible. Then, the compacted pellet is melted with a tungsten inert gas (TIG) welding or plasma torch placed 1-2 mm above pellet. The electrical arc or plasma torch can heat up and melt the pellet extremely quickly, and as it is turned off, the molten droplet quickly solidifies. In this process, almost no particle sintering will happen. For laser master, the feedstock powder is directly laid flat in an alumina crucible. The feedstock powder is then melted by a laser melting equipment. The lase melting process takes place with laser power of 100 to 200 W, laser scanning speed of 50 to 200 mm/s, and a hatch space of 0.1 mm. Similar to the arc melting process, the laser melting process heats up, melts, and cools the powder at very high rate, which results in almost no particle sintering.
The ODS master alloy contains high percentage of oxide nanoparticles, beyond the practical percentage that is needed for ODS alloys (such as steels). Therefore, the ODS master alloy may be diluted in a molten steel base that does not contain any oxide nanoparticles to create the ODS steel with desired percentage of oxide nanoparticles by adjusting the ratio between the master and the base.
During this process, the oxide nanoparticles in the master alloy can still sinter in the melt. Some aspects of the present disclosure address this issue by modifying the melt chemistry through the addition of Nb element. In a high temperature melt, the dispersion or sintering of nanoparticles is determined by the potential energy balance between the vdW attraction potential of the nanoparticles, the interfacial energy barrier due to wetting, and the thermal energy due to temperature. To prevent sintering, the interfacial energy barrier due to wetting needs to be high. But, while the interfacial energy barrier prevents sintering, the nanoparticles can still aggregate in pseudo-clusters. To achieve dispersion is to allow the nanoparticles to break free from the vdW energy well in a pseudo-cluster. The addition of Nb to the melt achieves both goals. The Nb improves the wettability between the nanoparticles and the melt, therefore, the interfacial energy barrier is increased, reducing the chance of sintering. Additionally, the Nb reduces the vdW attraction between the nanoparticles by forming a thin Nb-enriched layer around each nanoparticle, which allows the nanoparticles to break free from pseudo-clusters.
The parameters involved in the dilution process are summarized in Table 1. A steel with desired composition is melted at 1560° C. under protective atmosphere. To adjust the wettability of the molten steel with the oxide nanoparticles, 0.8 to 1.2 wt. % Nb is added to the melt. Stirring is applied to promote the uniform mixing of the Nb in the melt. The ODS master alloy, either in the form of small powder pellet (powder master), small ingot (arc master), or thin sheet (laser master), is added to the molten steel.
Stirring is applied to promote the dissolution of the ODS master, and the uniform mixing of the oxide nanoparticles. Due to the high wettability between the molten steel and the oxide nanoparticles, the nanoparticles will disperse in the molten steel rather than sinter. The furnace is then turned off, allowing the molten steel to solidify slowly (cooling rate <5 K/s).
wtoxide + wtmetal = wtmaster
Voxide + Vmetal = Vmaster
ODS stainless alloys (such as steels) may be used in high temperature and chemically aggressive service environment such as nuclear reactor fuel cladding. To manufacture ODS 316L stainless steel, the ODS master alloy needs to be fabricated first. The ODS master alloy is consisted of Fe-1Nb-6.76Y2O3 (in wt. %), equivalent to 10 vol. % Y2O3. Pure Fe powders (APS 50 μm), pure Nb powders (APS 50 μm) and Y2O3 nanoparticles (APS 10 nm) are uniformly mixed using the method introduced in 1. The mixed powders are compacted into pellets and then melted with electrical arc using a TIG welding torch at 2.5 kW of power for 5 s, producing the arc master. Instead of producing arc master, producing the laser master is also a viable alternative. Commercial 316L ingot (Fe-17Cr-13Ni-2.2Mo-0.8Si-0.2Mn-0.03C in wt. %) is melted at 1560° C. under protective atmosphere. The molten 316L is added with 1 wt. % of pure Nb pieces. The melt is stirred and held at the same temperature for 10 minutes to ensure homogeneity. To create 316L-1Nb-0.13 wt. %Y2O3, the ODS master alloy is added at DR=50, based on Table 1. To create 316L-1Nb-0.26 wt. %Y2O3, the ODS master alloy is added at DR=25. Upon addition of the master at designed DR, the melt is stirred for 3 minutes to ensure the full dissolution of the master. The furnace is then turned off, allowing the melt to solidify slowly.
Low carbon steel is the most versatile and commonly used type of steel in the world. The manufacturing of ODS low carbon steel by liquid metallurgy is an indication of the robustness and the wide-ranging applicability of some aspects of the present disclosure. In this case, the ODS master alloy is consisted of Fe-7Mn-1Nb-1C-1Si-6.76Y2O3 (in wt. %). Medium manganese steel (MMS) powders (Fe7Mn1Nb1C1Si, APS 20 μm) is commercially made by gas atomization. The MMS powders and Y2O3 nanoparticles (APS 10 nm) are uniformly mixed using the method introduced in 1. The mixed powders are compacted into pellets and then melted with electrical arc using a TIG welding torch at 2.5 kW of power for 5 seconds, producing the arc master. Instead of producing arc master, producing the laser master is also a viable alternative.
A steel with chemical composition of Fe1.2Nb0.2Mn0.1C0.1Si is melted at 1560° C. under protective atmosphere. The melt is stirred and held at the same temperature for 10 minutes to ensure homogeneity. The ODS master alloy is added at DR=25. Upon addition of the master at designed DR, the melt is stirred for 3 minutes to ensure the full dissolution of the master. The furnace is then turned off, allowing the melt to solidify slowly. The final composition of the steel is Fe1.19Nb0.5Mn0.14C0.14Si-0.31Y2O3, in which the base composition of the steel resembles that of a common low carbon steel. The final composition may be easily tuned by changing the starting composition of the melt, the composition of the ODS master, and controlling DR.
The present invention, thus generally described, will be understood more readily by reference to the following examples, which are provided by way of illustration and are not intended to be limiting of the present invention.
Ionocovalent oxides have higher thermodynamic stability than the metal-like ceramics. Molten steels have very limited reactivity with ionocovalent oxides. If oxide nanoparticles may be incorporated into liquid steel, the reaction issue encountered in previous chapters will be largely bypassed. However, ionocovalent oxides, due to having partial ionic and partial covalent bond, do not tend to form strong bonds with molten steel, resulting in poor interfacial adhesion. The poor interfacial adhesion manifests in the measurable engineering parameters such as work of adhesion and contact angle. In addition, oxides and alloys have large difference in their Hamaker constants, which are in the range of 100-150 zJ for oxides and 300-400 zJ for alloys. Such difference in Hamaker constants resulted in high vdW attraction potential between the oxide nanoparticles in molten steel, which can cause the nanoparticles to be trapped in pseudo-clusters or be sintered if the interfacial energy barrier is overcome.
To this end, the oxide nanoparticle incorporation and dispersion in molten steel can only be achieved when two conditions are satisfied. First, the oxide nanoparticles have a good wettability, due to a strong interfacial adhesion, with molten steel. This gives rise to large interfacial energy barrier against particle sintering. Second, the vdW attraction between the oxide nanoparticles is low. The thermal energy would allow the nanoparticles to break free from pseudo-clusters caused by vdW potential well, and disperse uniformly in the melt.
The wetting behavior between oxides and molten steels is well studied in conventional steel metallurgy. Oxides, such as MgO, Al2O3, CaO, and SiO2, are the main components of slag, which is considered detrimental to the steel product. Therefore, in a conventional steelmaking process, to separate the slag from the molten steel, a poor wetting (high contact angle) between the two is often favored. In this work, the reverse is favored. This chapter examines the effect of several alloying elements on the interfacial adhesion and wettability between the oxide and the molten steel using theoretical modeling approach, then validate their effect using experiments. The vdW attraction is also studied under the same theoretical framework.
Pure liquid iron has poor wettability and low work of adhesion with most of metal oxides. Oxides such as Cr2O3, TiO2, and NiO exhibit better wettability with pure liquid iron (θ˜60-90°), but these oxides have low thermodynamic stability and are generally not selected as reinforcing phase in ODS steels.
Some literature works suggest that the contact angle between liquid may be tuned when certain alloying elements are added. Verhiest et al. demonstrated the effect of Cr and Si addition on the contact angle between liquid iron and oxides (Y2O3 and Al2O3), and found that 20 wt. % Cr reduces the contact angle by 20° in pure iron while 0.5 wt. % Si reduces the contact angle by 10° in Fe-9Cr melt. Yu et al. studied the effect of Nb (0-1.48%) addition on the contact angle between liquid medium Mn steel (8% Mn, 1% C) and Al2O3. It was found that 1.15% Nb reduces the contact angle to below 50° [Journal of Materials Science and Technology, vol. 19, no. 6. pp. 625-627, 2003]. Due to the different chemical composition in the liquid steels and the experimental setup including temperature, holding duration, atmosphere, etc., it is difficult to determine the relative potency of each alloying element in improving the wettability. Therefore, a theoretical modeling of the contact angle and work of adhesion between an ideal binary Fe-x (x=Cr, Si, Mn, or Nb) and Al2O3 (or Y2O3) is performed. Element Cr is selected because it is the main alloying element for stainless steel, while Mn and Si are the most common alloying element in plain carbon steels. Element Nb is selected due to its reported potency for improving wettability at as low as about 1 wt. %.
Common engineering alloys such as steels, Al alloys, and Mg alloys typically contain one or more alloying elements. The interaction between multiple alloying elements and nanoparticles becomes highly complicated. Li et al. laid groundwork for understanding and predicting the influence of alloying element on the wetting behavior of oxide particles in a binary alloy system (A-B system). To understand the effect of adding B solute into A on the oxide wettability, the surface tension between the melt and the oxide (σSL) needs to be calculated. When the molar fraction of B (xB) is small, the change of the surface tension (σSL) is also small and may be approximated with a linear relation. The slope of this linear relation is given by Eqn. 1:
The term
is the slope of surface tension change when solute B is infinitely diluted in A. R is gas constant and T is temperature. ΩM is the molar interfacial area given by Eqn. 2, in which N is Avogadro's number and ω is the average area of a metallic atom at the interface, VM is the molar volume of the metal. The term ESL(B)A is the energy of adsorption of atom B at the metal A-oxide interface in AB alloy with infinitely diluted B, and is given by Eqn. 3:
where σLVA and σLVB are the liquid surface tension of pure A and pure B, WA and WB are the work of adhesion of pure A and pure B on the oxide, m is a structure parameter equal to 0.25 for liquid alloys, λ is the molar exchange energy of the liquid AB alloy, given by Eqn. 4, in which εAB, εAA, εBB are the interaction energy of the AB, AA, BB atom pair, and z is the coordination number in the liquid (number of nearest neighbors). The term ESL(B)A describes the tendency of atom B to adsorb onto the surface of the oxide in liquid AB, creating an interfacial layer with enriched B composition (yB). The ratio between interfacial liquid layer composition (yB) and bulk liquid composition (xB) is given by Eqn. 6. Eqn. 5 is the energy of adsorption of atom B at the surface of the metal A from AB alloy infinitely diluted in B, which is used to calculate the variation in surface tension oσLV due to the addition of metal B, as given by Eqn. 7.
Finally, the variation of work of adhesion between the oxide and the metal A with infinitely diluted metal B is given by Eqn. 8, and the variation of contact angle (θ) in Eqn. 9.
Where θA is the contact angle between the oxide and pure metal A. Based on this set of equation, the change of the work of adhesion (W(xB))and wetting angle (θ(xB)) between AB alloy and oxide is summarized in
The theoretical framework to predict the change in work of adhesion and contact angle between a liquid binary alloy A-B and an oxide was established by Li et al., J. Mater. Sci., vol. 24, no. 3, pp. 1109-1116, 1989. This framework is based on three basic assumptions: 1) the binary alloy A-B has a very low concentration of metal B, that is, element B is infinitely diluted in metal A; 2) the binary alloy A-B is considered as a regular solution, that is, the enthalpy of mixing is non-zero; 3) the liquid A-B alloy does not react with the oxide. The parameters and their definitions used in this model are shown in Table 3. The molar exchange energy of the liquid AB, λ, as given by Eqn. 4, is difficult to calculate due to the lack of literature data on the bond energy, εAB, for some of the systems, such as Fe—Mn, Fe—Nb. Instead, this parameter is evaluated from the tabulated values of partial mixing enthalpies of binary alloys AB at infinite dilution.
The same theoretical framework is also used to calculate the potential enrichment of metal B on the surface of oxide in liquid AB by using Eqn. 6. Under this framework, it is proposed that a monoatomic interfacial layer with composition yB will exist on the surface of the oxide. The presence of the interfacial layer can serve as a “dynamic coating” for the oxides that effectively reduces its difference in Hamaker constant with the melt. The schematic of the vdW attraction between two nanoparticles without interfacial layers in a melt, and may be calculated using Eqn. 10.
where D is the distance between two nanoparticles in nanometres, ASiC and AMg are the Hamaker constants for the van der Waals interaction and are 248 zJ and 206 zJ for SiC and molten magnesium, respectively. R1 and R2 are the radii of two nanoparticles.
The vdW attractions between two nanoparticles with interfacial layers is illustrated in
In this scenario, the interfacial liquid layer has a variable thickness T. The Hamaker constants for the solid phase (oxide nanoparticle), interfacial liquid layer, and bulk liquid are A11, A22, and A33, respectively. The vdW attraction potential between the two oxide nanoparticles is given by Eqn. 11, and the associated Hamaker constants are calculated by Eqn. 12, Eqn. 13, and Eqn. 14.
The AB alloy/oxide system in this work includes Fe—Si/Al2O3, Fe—Cr/Al2O3, Fe—Mn/Al2O3, Fe—Nb/Al2O3 and Fe—Nb/Y2O3. The numerical values of the parameters required for the theoretical modeling are shown in Table 4. Using Eqn. 3 and Eqn. 5, the ESL(B)A and ELV(B)A values are calculated for each system, as shown in Table 5. Based on the inequalities between ESL and ELV, ΣLVB and σLVA, WB and WA, the type of change in work of adhesion and contact angle may be assigned to each system as shown in
Using the ESL(B)A and ELV(B)A adsorption energy values in Table 5, the slopes for the change of the work of adhesion and the contact angle in alloy A-B/oxide systems are calculated using Eqn. 8 and Eqn. 9. The values for the slope for each alloy AB system are summarized in Table 6. The slope values are used to generate W vs. xB plot and 0 vs. xB plot for each alloy A-B/oxide system, as shown in
The Fe—Si/Al2O3 and Fe—Mn/Al2O3 systems share the most similarity, in which Si and Mn have slightly higher W value than Fe and the slightly lower θ value than Fe, resulting in the type 1 energy inequalities of ESL<ELV<0, σLVB<σLVA, and WB>WA. In either case, the work of adhesion and contact angle transitioned slowly between xB=0 to 1, indicating that Si and Mn will slightly increase the wettability between the liquid and the oxide, but not significant at low concentration. For Fe—Cr/Al2O3 system, although the energy inequalities are still type 1, Cr has much higher W value and lower θ value than Fe, resulting in a steeper drop in θ. At 20 at. % Cr, which is the normal Cr concentration in stainless steel, the model predicts the contact angle to be around 90°.
The Fe—Nb/Al2O3 and Fe—Nb/Y2O3 systems are the only systems with the energy inequalities of type 3b due to Nb having higher liquid surface energy than Fe, resulting in adsorption energy ELV>0. The type 3b system is unique because the model predicts a specific composition in which a minimum contact angle exist that is lower than the contact angle with either pure A or pure B. Yu et al.'s experimental work seems to confirm this prediction, in which 1.15 wt. % of Nb achieved a lower contact angle (50°@1823K) than 0.83 wt. % of Nb (60°@1823K) or 1.48 wt. % Nb (70°@1823K), in the Fe—Mn—C-xNb(x=0-1.48%)/Al2O3 system. The model here deviates from Yu's work in predicting the exact concentration of Nb needed to reach minimum contact angle and the value of the minimum contact angle. Yu's work suggests that an interfacial reaction between Nb and Al2O3 takes place and further reduces the contact angle. This theory cannot be accounted for in a model for non-reactive system.
In summary, among the 5 binary alloy systems examined, element Si and Mn showed limited effect in reducing the contact angle. Element Cr is more potent than Si and Mn, but requires high concentration (˜20 at. %) to reach a favourable contact angle (<90°). The Fe—Nb/Al2O3 and Fe—Nb/Y2O3 are the only type 3b systems indicating that element Nb potentially has the strongest effect in improving the wettability and requires experiment to confirm its effect.
Based on the results from previous section, three A-B/oxide systems, Fe—Nb/Al2O3, Fe—Cr/Al2O3, and Fe—Nb/Y2O3, are evaluated for the effect on vdW attraction. The vdW attraction potential between two oxide nanoparticles are calculated using Eqn. 11-Eqn. 14. The Hamaker constants, A11, of Al2O3 and Y2O3 are 150 zJ and 130 zJ. The Hamaker constant, A33, of the bulk liquid is assumed to equal to that of Fe, 392 zJ , due to the high dilution of Nb or Cr. The Hamaker constant, A22, of the interfacial liquid layer is approximated using a simple rule of mixture based on the interfacial composition, yB: A22=yB*AB+(1−yB)*A33, in which AB is the Hamaker constant of the metal B. The nanoparticle distance, D, of 0.4 nm (two atomic layers) is used. The vdW attraction potentials between two oxide nanoparticles are shown in
The interfacial layers are highly effective in reducing the vdW attraction between oxide nanoparticles in steel melt. When the interfacial layer is one atomic layer (0.2 nm), the vdW attraction potential is almost reduced by half in all three systems. When the interfacial layer is three atomic layers (0.6 nm), the vdW attraction potential is reduced by about 75%. Despite having very different yB/xB values in the three systems (Table 7), the Hamaker constant of the interfacial layer, A22, is relatively unaffected, due to the similar Hamaker constants for Cr (371 zJ), Nb (464 zJ) and Fe (392 zJ). For a nanoparticle with a larger radius (R=20 nm), the thermal energy, kT, is lower than the vdW attraction potential. However, the reduction of vdW attraction potential allows for a significantly higher probability for the nanoparticles to break free from the pseudo-clusters. The probability for a particle to overcome an energy well is proportional to e−ΔE/kT. Therefore, at a fixed kT of 25 zJ, a particle is 400 times more likely to overcome a 150 zJ well than a 300 zJ well (e−150/25/e−300/25=400), or 8100 times more likely to overcome a 75 zJ well (e−75/25/e−300/25=8100).
In summary, among the 3 systems examined, Cr and Nb are highly effective in reducing the vdW attraction potential between two Al2O3 or Y2O3 nanoparticles. The presence of the metallic interfacial liquid layer (yB) has a strong screening effect to the large Hamaker constant difference between A11 (oxide) and A33 (bulk liquid). The vdW attraction potential is reduced for up to 50% even with only one atomic layer thick of the interfacial liquid layer.
To verify the effect of Nb on the wettability between different steel compositions and oxide nanoparticles, liquid metallurgy experiments were conducted. In molten metal/nanoparticle system, high wettability between the melt and nanoparticles provides a strong interfacial energy barrier that makes nanoparticle contacting and sintering with each other heavily energetically unfavorable. However, for two nanoparticles that are already sintered together, the high wettability offers little help, because the thin liquid film separating them no longer exists. To this end, it is crucial to prevent the sintering of nanoparticles by physically separate them during the incorporation stage, because they have yet to be wetted by the molten metal. For nanoparticles like carbides and borides, molten salt assisted incorporation works well because carbides and borides can form stable colloids in molten salt through the formation of a layer of surface-bound solvent ions which prevents aggregation and sintering, but the minimization of interfacial energy allows carbides and borides to migrate to molten alloys. However, for oxide nanoparticles, molten salt assisted incorporation does not work due to the strong chemical reaction between the two.
In this study, metal micro-powders were used as “spacers” to provide separation between oxide nanoparticles. First, steel micro-powders of different composition were mixed with 2 vol. % oxide nanoparticles. Organic solvents (ethanol or acetone) were added to the mixture creating a slurry. The slurry was treated in an ultrasonic bath for 10 min. The ultrasonic bath helps breaking up the oxide nanoparticle clusters and improves the uniformity of mixing, while the organic solvent absorbs the moisture. The slurry mixture was then dried at 60° C. to evaporate the solvent. The dried powder mixture was rigorously ground in a mortar and pestle set for 0.5-1 hr for uniform mixing. The ground powders were transferred to a stainless steel mold for cold compaction. The cold compaction pressure is ˜400 MPa in uniaxial compression. The compacted powder pellet is referred to as the “powder master” herein.
A steel with desired composition was melted at 1560° C. under argon protective atmosphere. To verify the effect of Nb on the wettability of the molten steel, 1 to 1.2 wt. % Nb was added to the melt. Stirring is applied to promote the dissolution and uniform mixing of the Nb in the melt. The melt is referred to as the “base” herein.
The powder master pellets were added to the molten steel base at a weight ratio of 1:10. Stirring was applied to promote the dissolution of the master, and the uniform mixing of the oxide nanoparticles. The furnace was then turned off, allowing the molten steel to solidify slowly (cooling rate <5 K/s). A schematic of the overall process is shown in
The different compositions of steel micro-powder, steel base, and oxide nanoparticles tested in this study are summarized in Table 8. The experiments may be divided into 3 categories. In category 1, the micro-powder was pure Fe (APS 100 μm) and the steel base was a binary Fe-1.2Nb alloy. The oxide selected for this category was Al2O3 (APS 300 nm). Category 1 was designed to be a simple system that verifies whether Nb improves the wettability and dispersion capability. In category 2, the micro-powder was stainless steel 316L (APS 40 μm) and the steel base was also 316L but with 1% Nb added. Category 2 represents the scenario in which both Cr and Nb are present in the melt. In category 3, the micro-powder was medium Mn steel (MMS) and the steel base was a lean low carbon steel with 1.2% Nb, resulting in a final composition of Fe1.2Nb0.62Mn0.15C0.06Si, closely resembling that of a commercial low carbon steel. Category 3 was designed to test the effect of Nb in the most commonly used and versatile type of steel—low carbon steel. For each category, a steel base that contained no Nb was used as a blank reference. After the melt was solidified, the ingot was cut, polished, and examined under SEM.
The microstructures of Fe-1.2Nb-0.12Al2O3, equivalent to 0.2 vol. %, are shown in
316L is one of the most frequently used stainless steel. Its ODS variant is of great scientific interest. In the literature reviewed, mainly Y2O3 are used as the strengthening phase in ODS stainless, due to its excellent high temperature properties. Liquid metallurgy experiments were nonetheless conducted for Al2O3 as well as Y2O3 to examine the effect of Nb. The as cast microstructures of the samples are shown in
In comparison, the addition of Nb provides clear improvement to the wettability between the 316L matrix and the oxide nanoparticles (Y2O3 and Al2O3) and nanoparticle dispersion The oxide nanoparticles retained their small size (approximately 50 nm) and equiaxed morphology. Without Nb, the 316L matrix still has limited wettability with the oxides due to the presence of 17% Cr, resulting in a small portion of the oxides being dispersed while the rest being sintered.
The microstructures of the ODS low carbon steel share similarity with that of ODS stainless steel from previous section, in which, the samples containing Nb have good dispersion of oxide nanoparticles (Y2O3 and Al2O3), as shown in
Same sample from
In the theoretical modeling portion of this study, the effect of Nb, Cr, Mn, and Si on the work of adhesion and contact angle between Fe and Al2O3 is predicted. It is found that Mn and Si have a limited effect, Cr has a moderate effect, and Nb has the strongest effect. In the experimental portion, this prediction is verified. The Nb concentration is 1.2% for the Fe—Nb and the low carbon steel systems, and 1% for the stainless steel. The Nb concentration is maintained at this level since Yu's results indicate that the optimal Nb concentration is in the vicinity of 1-1.2%. In the Fe—Nb and the low carbon steel systems, the addition of Nb fundamentally changes the wettability from non-wetting to wetting. Without Nb, the oxides sintered in the melt, leaving no individually dispersed nanoparticle. With 1.2% Nb, the dispersion took place, and no sintered phase is observed. In the stainless steel system, the addition of Nb improves the wettability. Without Nb, the stainless steel melt still offers limited wettability and likely little reduction in vdW potential, which result in a portion being dispersed but a portion of the oxide being sintered. With 1% Nb, the sintering no longer takes place, resulting in individually dispersed nanoparticles. Furthermore, we found that Nb not only improves the wettability for Al2O3, but also for Y2O3, a more favorable oxide choice for application. In summary, we found that Nb addition improves the dispersion of both Al2O3 and Y2O3 nanoparticles in various steel compositions, including the simple binary Fe—Nb, the low-alloyed low carbon steel system, and the high-alloyed stainless steel system.
The modeling on vdW attraction potential (
As described above, non-reactive ionocovalent oxide nanoparticles such as Al2O3 and Y2O3 are incorporated into various molten alloys (such as steels) containing 1-1.2% Nb, since the addition of Nb greatly improved the wettability between the oxide nanoparticles and the molten alloys (such as steels) while reducing the vdW attraction between the nanoparticles. This chapter proposes methods to fabricate ODS master alloy to enable possible scalable manufacturing of ODS alloys (such as steels) by liquid metallurgy.
While a melt with a good wettability to oxide prevents the oxide nanoparticles sintering by a high interfacial energy barrier, it is unable to pull apart nanoparticles that are already sintered. For metal-like nanoparticles such as WC and TiB2, molten salts are effective vehicles to carry nanoparticles into the melt, since nanoparticles can form stable colloids without sintering in molten salts. For ionocovalent oxides, similar method is ineffective due to the reaction between oxides and molten salts. Therefore, the oxide nanoparticles are first mixed with metal micro-powders to create physical separation in order to prevent sintering before they are wetted by the melt. However, the metal micro-powder has a very low limit as to how many volume percentages of oxide nanoparticles it can carry. It was found during experiment that metal micro-powder becomes less effective when the volume percentage of oxide rises beyond 2%, in which the oxide nanoparticles start to form thick shells around the metal micro-powders that can no longer be infiltrated by the molten metal, which eventually leads to severe sintering issue during incorporation. To this end, a more effective method to separate the oxide nanoparticles prior to incorporation are necessary.
Xu et al. proposed the concept of metal-nanoparticle superstructure, in which the microstructure changes with the volume percentage of nanoparticle. At low volume percentage, the superstructure takes the form of nanoparticle-coated metal powder. At high volume percentage, the superstructure becomes networks of nanoparticles separated by alloys in-between. These metal-nanoparticle superstructures may be used as masters and diluted into molten alloys to manufacture MMNCs at desired nanoparticle volume percentage. This method of MMNC manufacturing transforms the nanoparticle incorporation, which is normally a high-cost process, into a process that is akin to alloy-making in conventional liquid metallurgy. In conventional liquid metallurgy, necessary elements that are difficult to dissolve, dangerous to handle, or have other reasons making them not suitable for direct addition are often made into master alloy at high concentration and then diluted into the alloy melt to achieve desired composition. Elements such as B, Nb, Li, Ti, V, W, etc. all fit into this category.
For ODS steels, the ideal master alloy should contain high volume percentage of non-sintered oxide nanoparticles, in a form, such as small ingot or thin sheet, that may be easily diluted into molten steel. Currently, there is no mature technology that allows the manufacturing of bulk steel containing high volume percentage of nanoparticles. Kimura et al. demonstrated the production of Fe-24Cr-15Y2O3 (in wt. %), equivalent to 22 vol. % Y2O3, composite powders by ball milling. The prolonged ball milling process induced super-heavy deformation that caused Y2O3 to mechanically decompose in the ferritic matrix. However, the size of Y2O3 nanoparticles after re-precipitation hinges upon the heat treatment temperature. An average Y2O3 size of 25 nm was reported if the powder is heat treated at 1000 K, but around 150 nm if the heat treated at 1600 K. As such, steel composite powders containing mechanically decomposed oxide phase is unlikely a suitable candidate to be used as master alloy for ODS steel, despite having very high concentration of oxide, because the molten steels will have temperature around 1823 K and significantly higher diffusion kinetics than solid state heat treatment. The decomposed oxide will not be able to retain a small size distribution in this condition. In addition, solid state processing route involving ball milling and hot consolidation have very high cost and requires long processing time, hence not conducive for scalable manufacturing. Majority of the ODS steel produced via this method is limited to below 0.5 vol. % of oxide nanoparticles. Conventional liquid state processing route which needs temperature above the melting point of steel is also unable to manufacture master alloy with high oxide nanoparticle loading, due to the lack of proper incorporation method.
In this work, a novel method of manufacturing ODS master alloy with high oxide nanoparticle loading is described. The sintering issue may be bypassed when nanoparticle loading is high (6-10 vol. %) by the combination of three processing parameters: 1) rapid heating rate (>2000 K/s), 2) extreme melt pool temperature (>2273 K), and 3) short melting duration, for the following reasons. Without being bound by theory it is believed that rapid heating allows the process to reach desired temperature in an extremely short time window (within 1 s). This means that the sintering that could have occurred during a prolonged heating stage in which the nanoparticles are not wetted may be mitigated. Additionally, the extreme melt pool temperature significantly enhances the wettability between the oxide and the molten steel, due to the significant reduction in solid-liquid surface tension (σSL) and liquid surface tension (σLV). It also significantly increases the thermal energy (kT) available for the oxide nanoparticles to move around in the liquid. Finally, the short melting duration allows the system to solidify before any sintering can happen. Nanoparticle sintering in high temperature melt is a statistical event, in which longer duration increases likelihood of sintering to occur. Therefore, a shorter melt duration is favored when manufacturing ODS master alloy with high nanoparticle loading.
The combination of these processing conditions can only be provided by extreme heat sources, such as an electrical arc or a laser beam, which are known to provide rapid heating rate and melt pool temperature as high as 4000 K. Therefore, ODS master alloys with high nanoparticle load are manufactured using arc melting or laser melting, in which they are referred to “arc master” or “laser master” herein.
To manufacture the arc master and laser master, metal micro-powders were first mixed with oxide nanoparticles. The mixing method is consistent with that previously shown. For arc master, the mixed powders were cold compacted into pellets about 1 g each. The arc melting was conducted using a tungsten inert gas (TIG) welding equipment. The compacted pellet was placed in an electrically grounded pure Cu bowl for quick heat dissipation after melting. The experimental setup is shown in
For laser masters, the mixed powders were cold compacted into thin pellets with thickness around 1 mm. The pellet was placed in an alumina crucible. The surface of the pellet was melted with a selective laser melting (SLM) equipment, as shown in
To verify the effect of the ODS master alloys, they were diluted into various molten steels. A steel with desired composition was melted at 1560° C. under Ar atmosphere. The melt contained 1 wt. % of Nb to improve its wettability with the oxide nanoparticles. Stirring was applied to promote the uniform mixing of the Nb in the melt. The ODS master alloy, either in the form small ingot (arc master) or thin sheet (laser master) was added to the molten steel at calculated ratio; this ratio is referred to as “dilution ratio” (DR) herein. Stirring was applied to promote the dissolution of the ODS master, and the uniform mixing of the oxide nanoparticles. The furnace was then turned off, allowing the molten steel to solidify slowly (cooling rate <5 K/s). The parameters used the control the dilution experiment are summarized in Table 10. The percentage of oxide nanoparticle in the master alloy (WM or VM) is related to the final oxide percentage in the ODS steel after dilution by a simple factor of DR+1. The as cast ODS steel samples were prepared into metallographic samples for SEM examination. The Vickers hardness of each sample was measured. Series of control experiments were conducted using the same procedure, however the master alloy used in the control experiments were blanks, which contained no oxide nanoparticles.
Two different matrix compositions were used to manufacture the arc master samples, Fe7Mn1Nb1C1Si and Fe1Nb (in wt. %). Fe7Mn1Nb1C1Si is a medium Mn steel (MMS) that is suitable for dilution to achieve the desired composition of other carbon steels due to its relative high concentration of C (1%) and Mn (7%). Fe1Nb is a simple binary alloy that may be diluted in other low-alloyed steels and stainless steel, since it does not impact the overall composition significantly if a high DR value is used. Y2O3 nanoparticle (APS 10 nm) was selected due to its prevalence in literature work. For each matrix composition, arc master samples with two different Y2O3 nanoparticle loading were made, 4 and 6.8 wt. %, equivalent to 6 and 10 vol. %, respectively. The microstructures of as-solidified Fe7Mn1Nb1C1Si-4Y2O3 ODS and −6.8Y2O3 ODS master alloy are shown in
The microstructures of as-solidified Fe1Nb-4Y2O3 and -6.8Y2O3 ODS master alloy are shown in
For laser masters, the matrix composition is Fe7Mn1Nb1C1Si. Y2O3 nanoparticle (APS 40-50 nm) was used. Samples with two different Y2O3 nanoparticle loadings were prepared, 4 and 6.8 wt. %, equivalent to 6 and 10 vol. %. The microstructures of the as-printed Fe7Mn1Nb1C1Si-4Y2O3 and -6.8Y2O3 ODS master alloy are shown in
ODS low carbon steels were produced by diluting the ODS master alloy in a melt with fixed chemical composition. Different DR values were used to control the Y2O3 nanoparticle percentage as well as the chemical composition in the final steel. The combination of these parameters is summarized in Table 11. The microstructures of the as-cast ODS low carbon steels are shown in
The primary goal of conducting the dilution experiment is to validate that the oxide nanoparticles can retain a stable self-dispersion after being diluted from high concentration, when in ODS master, to low concentration, when in a steel melt with no nanoparticles. This goal is achieved both using arc master and laser master. The arc master contains very fine Y2O3 nanoparticles (APS 9.7 nm), after diluting at DR=21, the resulting ODS low carbon steel is shown in
ODS stainless steels were produced by diluting the ODS master alloy in a commercial 316L melt modified with 1 wt. % Nb. The ODS master alloy used in this experiment were Fe1Nb-6.8Y2O3 with smaller Y2O3 nanoparticles for arc master (APS 12.3 nm) and larger Y2O3 nanoparticles for laser master (APS 50.0 nm). The combination of these parameters is summarized in Table 12. The microstructures of the as-cast ODS stainless steels are shown in
Stainless steel 316L is a highly alloyed steel that contains over 33 wt. % of alloying elements. Therefore, when diluting the Fe1Nb-6.8Y2O3 master alloy, the DR was deliberately set to a high value of 47, to preventing the excess dilution of the remaining alloying elements such as Cr and Ni. Similar to the nanoparticle dispersion observed in the ODS low carbon steels (
The Vickers hardness of as-cast ODS low carbon steel (LCS) and ODS stainless steel 316L are shown in
The LCS 1 reference sample has a hardness of 152 HV. The two ODS variants of LCS 1, one produced by diluting the arc master and the other by laser master, have hardness of 149 and 172 HV, respectively. Despite being designed at the same 0.32% of Y2O3 nanoparticles, the ODS LCS 1 produced by laser master has noticeably higher hardness. This is primarily due to the carbon loss during the manufacturing of the master alloys. The matrix of the master alloys contains 1% carbon. The arc master was produced by the electrical arc from a TIG welding torch, as shown in
For the stainless steel samples (
An electrical arc and a laser beam are two comparable heat sources. Both of them are capable of delivering rapid heating rate and extreme temperature. However, despite the similarity, the microstructure of the ODS master alloy produced using these two methods have very different microstructures. On one hand, the arc masters (
Due to the difference in experimental setup between the arc melting and SLM process, the master alloys were affected by oxygen contamination differently. The arc masters, despite showing the better nanoparticle dispersion, suffered from carbon burning, which resulted in the lower hardness value after dilution, comparing to the sample with the same designed composition but produced using laser master. If combustible alloying elements are not present in the master, such as the case of Fe1Nb-6.8Y2O3, then oxygen does not affect the quality of the arc master. To this end, if the gas protection may be further improved in the arc melting setup, then the arc master would be able to retain its combustible alloying elements while providing better nanoparticle dispersion.
This work presents a potential scalable manufacturing process for ODS steels by liquid metallurgy. High concentration of oxide nanoparticles is first incorporated and dispersed into a steel matrix by arc melting or SLM, producing the ODS master alloys. The ODS master alloys are then diluted into a steel melt at a high dilution ratio, releasing the oxide nanoparticles. Due to the presence of Nb in the steel melt, the oxide nanoparticles are able to wet with the melt and disperse uniformly. This work validates the theoretical and experimental study: 1. the wettability between the oxide nanoparticle and the melt may be improved to prevent sintering, and 2. the vdW attraction between the nanoparticles may be reduced to promote uniform dispersion. Despite having an initial high cost of producing the ODS master alloys using arc melting or SLM, the final cost is offset by the dilution ratio. When DR is 10, the master alloy only contributes to less than 10% of the final ODS cost. When DR is 50, the master alloy is only less than 2% of the final ODS cost.
Para. 1. A method of producing an oxide-dispersion strengthened (ODS) alloy, wherein the method includes: providing a bulk molten alloy comprising one or more metal elements; adding a plurality of oxide particles that are surface enriched at nanoscale with one or more element that reduces interfacial energy and van der Waal (“vdW’) attractions between the oxide particles and the bulk molten alloy in order to achieve a stable nanoparticle dispersion in the liquid alloy; and casting the ODS alloys.
Para. 2. The method of Para. 1, wherein the metal includes one or more metal elements selected from Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, and Zn.
Para. 3. The method of Para. 1, wherein the bulk molten alloy is steel.
Para. 4. The method of any one of Paras. 1-3, wherein the oxide particles are surface enriched at nanoscale with various elements, for example Nb in liquid steel, which are capable of reducing interfacial energy and vdW attraction between the oxide particles and the bulk molten alloy.
Para. 5. The method of any one of Paras. 1-4, wherein the oxide particle surface include a higher concentration of the element that reduces interfacial energy and vdW attraction within the first a few (e.g. 5) atomic layers in the molten alloy.
Para. 6. The method of any one of Paras. 1-5, wherein the oxide particles include at least one selected from Al2O3, Y2O3, ZrO2, MgAl2O4, yttria stabilized zirconia and MgO, and any suitable oxides for different metal/alloy systems.
Para. 7. The method of any one of Paras. 1-6, wherein the oxide particles are microparticles or nanoparticles.
Para. 8. A bulk metal matrix comprising a metal matrix and a plurality of uniformly dispersed oxide particles that are surface enriched at nanoscale with an element that reduces interfacial energy and vdW attraction between the oxide particles and the bulk metal in molten metal.
Para. 9. A bulk metal matrix formed by any one of the methods of Paras. 1-7.
Para. 10. A method of producing an oxide-dispersion strengthened (ODS) metal, including providing a bulk molten metal including one or more metal; adding an alloy including a metal and a plurality of oxide particles uniformly dispersed therein; and casting the ODS metal.
Para. 11. The method of Para. 10, wherein the alloy includes one or metal found in the bulk molten metal.
Para. 12. The method of Para. 10 or 11, wherein the metal includes one or more alloys selected from Al, Mg, Fe, Ag, Cu, Mn, Ni, Ti, Cr, Co, Au, and Zn.
Para. 13. The method of any one of Paras. 10-12, wherein the bulk molten metal is steel.
Para. 14. The method of any one of Paras. 10-13, wherein the alloy includes various elements such as Nb, Cr to reduce interfacial energy and vdW attraction of oxide nanoparticles in molten metals.
Para. 15. A method of manufacturing ODS master alloy with high oxide nanoparticle loading for dilution to final ODA alloys including: rapidly heating one or more metal of the alloy with the oxide nanoparticles for a period of time that less than the amount of time to sinter a majority of the oxide nanoparticles.
Para. 16. The method of Para. 15, wherein the metal of the alloy with the oxide nanoparticles are rapidly heated using arc melting or laser melting.
While certain embodiments have been illustrated and described, it should be understood that changes and modifications may be made therein in accordance with ordinary skill in the art without departing from the technology in its broader aspects as defined in the following claims.
The embodiments, illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms “comprising,” “including,” “containing,” etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the claimed technology. Additionally, the phrase “consisting essentially of” will be understood to include those elements specifically recited and those additional elements that do not materially affect the basic and novel characteristics of the claimed technology. The phrase “consisting of” excludes any element not specified.
While the disclosure has been described with reference to the specific embodiments thereof, it should be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the true spirit and scope of the disclosure as defined by the appended claim(s). In addition, many modifications may be made to adapt a particular situation, material, composition of matter, method, operation or operations, to the objective, spirit and scope of the disclosure. All such modifications are intended to be within the scope of the claim(s) appended hereto. In particular, while certain methods may have been described with reference to particular operations performed in a particular order, it will be understood that these operations may be combined, sub-divided, or re-ordered to form an equivalent method without departing from the teachings of the disclosure. Accordingly, unless specifically indicated herein, the order and grouping of the operations are not a limitation of the disclosure.
In addition, where features or aspects of the disclosure are described in terms of Markush groups, those skilled in the art will recognize that the disclosure is also thereby described in terms of any individual member or subgroup of members of the Markush group.
As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof. Any listed range may be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein may be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as “up to,” “at least,” “greater than,” “less than,” and the like, include the number recited and refer to ranges which may be subsequently broken down into subranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member.
Other embodiments are set forth in the following claims.
This application claims the benefit of priority to U.S. Provisional Patent Application No. 63/160,433 filed Mar. 12, 2021, which is hereby incorporated by reference, in its entirety for any and all purposes.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2022/020062 | 3/11/2022 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63160433 | Mar 2021 | US |
