Patent Application
20150275337
Embodiments of the present disclosure relate generally to iron-based metal alloy compositions having dispersed oxides of rare-earth metals, and to methods of making and using such compositions.
Iron-based steel is commonly used in a wide variety of applications. Nano-ferritic alloys and oxide-dispersion strengthened (ODS) steels are alloys having particles of an oxide material (e.g., yttria (Y2O3)) dispersed within the alloy. ODS steels have enhanced mechanical properties at high temperatures, yet maintain high thermal conductivity and low thermal expansion. The oxide material appears to slow degradation processes, such as recrystallization.
Historical fabrication methods generally employ powder metallurgy to mechanically disperse oxide particles within alloys. Thus, ODS steels are conventionally formed by mechanically alloying powders of the oxide material with the alloy in solid form. Such processes are time-consuming and labor-intensive, making ODS steels relatively expensive and difficult to make in large batches, which affects their limited application in industry. Furthermore, the directionality in microstructures formed during processing, such as rolling to form plates and foils, and extruding to faun tubes, generally produces undesirable anisotropic mechanical properties.
The density difference between the oxide particles and the alloy typically prevents welding of ODS steels by conventional techniques such as fusion welding. When ODS steel is melted to form a weld joint, the particles of the oxide material separate from the alloy by density. For example, Y2O3 has a density of about 5.0 g/cm3, so Y2O3 particles tend to float to the top of molten steel (which has a density from about 7.75 g/cm3 to about 8.05 g/cm3). ODS steel may be welded by more difficult and expensive techniques, such as magnetic-pulse welding, friction-stir welding, or pressure-resistance welding. Such techniques, which are expensive and experimental, must generally be carefully controlled to limit or prevent melting of the ODS steel because melting of the ODS steel may lead to separation of the oxide material from the bulk alloy.
In some embodiments, a composition includes a metal alloy matrix comprising iron and a plurality of nanoparticles dispersed within the metal alloy matrix. Each nanoparticle of the plurality comprises an oxide of a rare-earth metal and at least one metal selected from the group consisting of tantalum, niobium, vanadium, and titanium.
In other embodiments, a composition includes a metal alloy matrix comprising iron and a plurality of nanoparticles dispersed within the metal alloy matrix. Each nanoparticle of the plurality comprises at least two different oxides of rare-earth metals.
A method includes mixing an oxide of a rare-earth metal with a first metal and a second metal. The first metal comprises iron, and the second metal comprises at least one metal selected from the group consisting of tantalum, niobium, vanadium, and titanium.
In some embodiments, a method includes mixing a plurality of particles comprising at least one oxide of a rare-earth metal with a molten metal comprising iron. Each particle of the plurality may exhibit a density between about 6.9 g/cm3 and about 9.0 g/cm3.
In some embodiments, a composition includes a metal alloy matrix comprising iron and a plurality of nanoparticles dispersed within the metal alloy matrix The composition exhibits a stress rupture life of at least 50,000 hours at an applied stress of 100 MPa (megapascals) and a temperature of at least 650° C.
While the specification concludes with claims particularly pointing out and distinctly claiming what are regarded as embodiments of the present disclosure, various features and advantages of embodiments of the disclosure may be more readily ascertained from the following description of example embodiments of the disclosure when read in conjunction with the accompanying drawings, in which:
The illustrations presented herein are not actual views of any particular composition or microstructure, but are merely idealized representations that are employed to describe example embodiments of the present disclosure. Additionally, components common between figures may retain the same numerical designation.
Compositions disclosed herein include a metal alloy matrix having dispersed particles including an oxide of a rare-earth metal. The metal alloy matrix may include iron or an iron-containing alloy, such as any of various grades of steel. The particles may optionally include one or more other materials, such a metal having a low solubility in the metal alloy matrix. The particles may be formulated to have a density approximately the same as the density of the metal alloy matrix, such that the particles may remain dispersed in the metal alloy matrix even when the metal alloy matrix is in a liquid state.
The particles 104 may include nano-sized particles (i.e., particles having a characteristic dimension, such as a mean diameter, of less than 1 nm). For example, in some embodiments, the particles 104 may have a characteristic dimension of less than about 100 nm, less than about 50 nm, less than about 20 nm, or even less than about 10 nm.
The particles 104 may include a rare-earth oxide 106 (i.e., an oxide of a rare-earth metal), such as an oxide of scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, or any combination or mixture of such oxides. For example, the rare-earth oxide 106 may include Y2O3, La2O3, Pr2O3, and/or Nd2O3. The particles 104 may include one or more distinct phases, and may define a phase boundary between the particles 104 and the metal alloy 102.
The particles 104 may optionally include one or more metal 108, such as tantalum, niobium, vanadium, and/or titanium. The metal 108 may be formulated to promote dispersion of the particles 104 throughout the metal alloy 102. For example, the metal 108 may have a greater affinity for the rare-earth oxide 106 than for a solid phase of the metal alloy 102. The metal 108 may be relatively insoluble in the metal alloy 102 when the metal alloy 102 is in solid form. For example, the metal 108 may have a solubility in the metal alloy 108 of less than about 15 at % (atomic percent), less than about 5 at %, or even less than about 1 at % when the metal alloy 102 is in solid form (e.g., at a temperature of about 100° C. or lower). In some embodiments, the metal 108 may be slightly soluble in the metal alloy 102, and the metal 108 may be present in the composition 100 beyond the solubility limit of the metal 108 in the metal alloy 102. Thus, at least a portion of the metal 108 may be in a distinct phase from the metal alloy 102. For example, the metal 108 may form a coating over the rare-earth oxide 106.
Alternatively, in some embodiments, particles 104 may include oxides of the rare earth metal with the metal 108 and small amounts of oxygen impurities. The oxygen may form complex oxides, such as NbO—Y2O3, NbY2O4, NbY2O5, or NbYO4, even though these complex oxides were not added directly in that form. That is, such complex oxides may form during processing of the composition 100. Furthermore a mixture of such oxides with metal 108 may be present in particles 104 some embodiments. Some embodiments may include particles 104 having the metal 108 in addition to small amounts of an oxide of the metal 108 along with the rare-earth oxide 106. For example, particles 104 may include Nb metal with NbO2 and Y2O3, and the metal alloy 102 may be a distinct phase from the particles 104.
The particles 104 may have approximately the same density (defined for each particle 104 as the mass of that particle divided by its volume) as as the metal alloy 102. The rare-earth oxide 106 and the metal 108 may each have different densities, and the particle 104 may have an overall density different from either the rare-earth oxide 106 or the metal 108 alone (e.g., the density of the particle 104 may be between the density of the rare-earth oxide 106 and the density of the metal 108). The particles 104 may have a density within about 10%, within about 5%, within about 2%, or even within about 1% of the density of the metal alloy 102. If the metal alloy has a density of about 7.9 g/cm3, the particles 104 may have a density from about 7.1 g/cm3 to about 8.7 g/cm3, from about 7.5 g/cm3 to about 8.3 g/cm3, from about 7.7 g/cm3 to about 8.1 g/cm3, or from about 7.8 g/cm3 to about 8.0 g/cm3. If the density of the particles 104 is relatively near the density of the metal alloy 102, the particles 104 may tend to remain evenly dispersed even when the metal alloy 102 is in liquid form. Thus, liquid-phase processing operations may be performed on the composition (e.g., centrifugal casting, induction melting, conventional welding, etc.) without introducing composition gradients.
The particles 104 may include a material having a melting temperature above the melting temperature of the metal alloy 102. For example, the rare-earth oxide 106 and/or the metal 108 may remain in solid form even at temperatures at which the metal alloy 102 is a liquid. In some embodiments, the rare-earth oxide 106 may have a melting point above the melting point of the metal alloy 102, and the metal 108 may have melting point below the melting point of the metal alloy 102. In such embodiments, when the composition is heated to a temperature above the melting point of the metal alloy 102, the metal 108 may melt, and may at least partially mix with the metal alloy 102. However, if the metal 108 has a greater affinity for the rare-earth oxide 106 than for a solid phase of the metal alloy 102, the metal 108 may tend to solidify (e.g., precipitate) near the rare-earth oxide 106 as the composition 100 cools. If the metal 108 has a greater affinity for the rare-earth oxide 106 than for the metal alloy 102, the dissolution of the metal 108 in the molten alloy 102 may tend to induce a finer dispersion of the rare-earth oxide 106. Upon cooling the alloy 102, the metal 108 may solidify near the rare-earth oxide 106. The particles having associated rare-earth oxide 106 and metal 108 may influence the retention of the microstructure and mechanical stability of the ODS steel alloy composition 100 at high service temperatures, such as from about 800° C. to about 1,200° C.
As non-limiting examples, the composition 100 may include at least about 0.10% rare-earth oxide 106 by weight, such as from about 0.10% to about 3.00% by weight, or from about 0.25% to about 1.50% by weight. The composition 100 may include from about 50% to about 95% iron by weight, such as from about 60% to about 91% by weight. The composition 100 may also optionally include from about 5% to about 20% chromium by weight, such as from about 9% to about 15% by weight. The composition 100 may include up to about 0.5% carbon by weight, such as from about 0.1% to about 0.25% by weight. The composition 100 may include up to about 20% nickel by weight, such as from about 0.5% to about 15% by weight or from about 5% to about 15% by weight. The composition 100 may include up to about 2.0% molybdenum by weight, such as from about 0.3% to about 1.2% by weight. The composition 100 may include up to about 2.0% manganese by weight, such as from about 0.4% to about 1.6% by weight. The composition 100 may include up to about 1.0% silicon by weight, such as from about 0.4% to about 0.6% by weight.
The composition 100 may include up to about 3.0% of the metal 108 by weight, such as from about 0.1% to about 0.25% by weight. As discussed above, the metal 108, if present, may include one or more of tantalum, niobium, vanadium, and/or titanium. The metal 108 may include any combination of these elements, in any selected concentration. The amount and type of elements of the metal 108 may affect the density and other properties of the metal 108, as well as of the properties of the composition 100.
In some embodiments, compositions may be formed by mixing an oxide of a rare-earth metal with a first metal and a second metal. For example, to form the composition 100 illustrated in
In some embodiments, mechanical alloying may be used to form pre-alloys or master alloy precursors. For example, chromium may be milled with the rare-earth oxide 106 to form particles having diameters in the range of 100 to 5,000 microns. The chromium can be combined with the metal 108 and/or the metal alloy 102 while the metal alloy 102 is in a molten state. The material may then be solidified, and the solidified composition may represent the composition 100 or a pre-cursor alloy composition to be added to iron and other components to produce a composition 100. Alternatively, metal 108, such as niobium, may be milled with the rare-earth oxide 106 to form particles in the range of 100 to 5,000 microns or larger.
In certain embodiments, a sub-oxide of niobium, such as NbO or NbO2, may be added to supplement the oxygen content of the alloy in a controlled way. That is, finer control may be exercised over the amount of oxygen added when added as a component of a solid in comparison to the introduction of oxygen as a gas (i.e., as O2 with bubbling, sparging, or other methods). Addition of a sub-oxides of niobium may increase the affinity of the metal 108 toward the rare-earth oxide 106, due to increased affinity of the metal 108 for the sub-oxides of niobium as compared to that for the rare-earth oxide 106, and the expected increased affinity of the rare-earth oxide 106 for the sub-oxides of niobium as compared to that for the metal 108. In general, the oxophilic nature of niobium and the rare-earth metals can be used to adjust to some extent the degree of association of the rare-earth oxide 106 for the metal 108, such that the particles 104 comprise metals 108, rare-earth oxides 106, and complex chemical composites of these constituents with excess oxygen arising from either contaminants or purposeful additions of oxygen-containing compounds such as NbO.
As the metal alloy 102, metal 108, and rare-earth oxide 106 cool from the melt down to room temperature, the metal alloy 102 and the metal 108 may separate based on the solubility of the metal 108 in the metal alloy 102, which may be a function of the temperature of the metal alloy 102, and may change significantly at the freezing point (i.e., melting point) of the metal alloy 102. That is, if the metal 108 is present at a concentration above its solubility limit in a solid form of the metal alloy 102, the metal 108 may form a distinct phase as the materials cool. The metal 108 may coalesce adjacent to the rare-earth oxide 106 to form the particles 104 shown in
As a non-limiting example, molten iron may be mixed with molten tantalum and niobium. Particles of Y2O3 or another rare-earth oxide may be added to this mixture. As another example, molten iron may be mixed with particles of tantalum and niobium and with particles of Y2O3. In another example, molten iron may be mixed with particles of Y2O3 coated with tantalum and niobium. The tantalum and niobium may at least partially dissolve during processing.
The particles 104, 104′ formed by the rare-earth oxide 106 and the metal 108 may have a density closer to the density of the metal alloy 102 than either the rare-earth oxide 106 or metal 108 alone. That is, as the metal 108 coalesces with the rare-earth oxide 106, the density of the resulting particles 104, 104′ may become closer to the density of the metal alloy 102. Thus, the particles 104, 104′ may become or remain suspended and/or dispersed in a continuous solid matrix of the metal alloy 102 as the composition 100 cools and solidifies. The composition 100 may be normalized and tempered, such as by heating to dissolve carbon in the metal alloy 102, cooling, and then reheating to a to temper, forming fine carbide precipitates.
In some embodiments, compositions may be formed by mixing particles with the metal alloy 102. For example, particles 104 (
The composition 100 is hereby derived from unique and novel design and control of three forces: solubility, chemical affinity, and density. The solubility of tantalum and niobium in solid iron alloys 102 is low enough to cause tantalum and niobium to precipitate from alloy 102 during and/or after solidification. In the molten state prior to solidification, the chemical affinity of the metal 108 toward oxygen may cause oxygen (whether from incidental impurities or purposeful additions) to concentrate more strongly toward the metal 108 as compared to iron in the alloy 102. The chemical affinity of the metal 108 for oxygen will also induce association of the metal 108 with the rare-earth oxide 106. The oxygen that concentrates with metals 108 may augment the tendency for the metal 108 to associate with the rare-earth oxide 106, because oxides tend to separate from metals. The association of particles 104 (i.e., the agglomeration of the rare-earth oxide 106 with the metal 108) therefore presents a range of possible interactions between metals 108 and rare-earth oxides 106 such that additional oxygen impurities may be present, sub-oxides of metals may be present, or more complicated ternary and quaternary oxide phases may be present, any of which may form while the alloy 102 is molten or upon solidification of the alloy 102. The individual components that make up the compositions are selected so that the association of the chemical components of the particles 104 will tend to maintain their isolable association during fusion welding with a density that tends toward the density of the molten alloy 102. Fine control of the density through judicious choices of chemical components of the particles 104 will tend toward fine control of conventional fusion welding behavior and performance.
In some embodiments, particles may be formed having a mean particle diameter of less than about 100 nm, less than about 50 nm, or even less than about 10 nm. The particles may form during cooling (e.g., by the coalescing of metal 108 adjacent rare-earth oxide 106), or may be formed prior to introducing particles to the metal alloy 102. For example, to form the composition 130 shown in
In some embodiments, particles may be formed having a ratio of metal 108 to rare-earth oxides 106 from 0 (zero, i.e., no metal 108) to about 5 by weight (i.e., about five times as much rare-earth oxide 106 as metal 108 by weight).
The compositions formed may optionally be reheated to a melting temperature of the metal alloy 102, such that the compositions 100, 120, 130, 140 (
In some embodiments, compositions may be formed by mixing a plurality of particles comprising at least one oxide of a rare-earth metal with a molten metal comprising iron. The particles may exhibit a density between about 6.9 g/cm3 and about 9.0 g/cm3, and may exhibit a density close to the density of the molten metal. The particles may include two or more rare-earth oxides, such as Pr2O3 and Nd2O3, Y2O3 and Nd2O3, or Y2O3 and La2O3. For example, the composition 140 shown in
Particles 104 (
In some embodiments, particles including a rare-earth oxide 106 over a metal 108, such as the particles 104″ shown in
Compositions described herein may exhibit enhanced performance with respect to conventional materials. For example, composition as described may exhibit a stress rupture life of at least about 50,000 hours at an applied stress of 100 MPa (megapascals) at temperatures of at least 650° C. or at temperatures of at least 750° C. Iron-based ODS alloys generally provide increased properties in a temperature range from about 650° C. to about 850° C. At lower temperatures, the benefits of the dispersed oxide are not generally needed. At higher temperatures, accelerated oxidation and microstructural changes may warrant the use of high-nickel alloys instead of iron-based ODS alloys.
Yttria (Y2O3) is milled with niobium (Nb) to form particles having a mean diameter between 1 and 750 microns. The yttria is impregnated in the niobium metal, forming particles of varying concentrations of the components. Some unassociated particles of yttria may remain. The outside diameter of the particles may be less than about 7 nm. The particles are added to a liquid steel alloy, such that the overall composition includes about 1.0% yttria, about 2.0% niobium, and 9-15% chromium (Cr), with the balance iron (Fe). The niobium coating melts and mixes with the steel alloy. The composition is then cooled. The solubility of the niobium in solid steel is relatively low, so a relatively small portion of the niobium remains dissolved in the steel alloy after cooling. During cooling of the composition, the niobium remains associated with the yttria particles
Alternatively, a precursor alloy of iron that is high in niobium and chromium can be milled with yttria prior to addition to a liquid steel alloy such that their addition reduces the amount of chromium and niobium needed to reach a desired final alloy composition.
Alternatively, uncoated yttria particles may be combined with a liquid mixture of the niobium, chromium, and iron. In the molten state, the mixture may be centrifugally cast or inductively stirred to promote a uniform distribution. The mixture may then be cooled, during which the niobium tends to remain associated with the yttria to form isolable particles that may be observable with a transmission electron microscope, for example. The niobium may partially or completely encase the yttria, with yttria presenting itself as the core of the particle.
The result of any of these alternative processes is a continuous matrix of steel with particles of niobium-coated yttria dispersed throughout. The density of the particles may vary based on the size of the yttria cores and the concentrations of yttria and niobium.
Yttria (Y2O3) is milled with NbO2, niobium (Nb), and tantalum (Ta) to form particles having a mean diameter between 500 and 750 microns. The NbO2 and yttria are impregnated in the metal niobium and tantalum. The presence of the NbO2 augments the oxygen level in a controlled way and decreases the time required for milling as compared to its absence. The particles are added to a liquid steel alloy, such that the overall composition includes about 1.0% yttria, about 1.4% niobium, 0.2% NbO2, 0.4% tantalum, and 9-15% chromium, with the balance iron. The niobium and tantalum coating dissolves and mixes with the steel alloy. The composition is then cooled. The solubilities of the niobium and tantalum in solid steel are relatively low, so relatively small portions of the niobium and tantalum remain dissolved in the steel alloy after cooling. During cooling of the composition, the niobium and tantalum coat the yttria particles again.
Alternatively, uncoated yttria particles may be combined with a liquid mixture of the niobium, NbO2 and/or NbO, tantalum, chromium, and iron. The mixture may then be cooled from the molten state, during which the niobium and tantalum associate with the yttria particles.
The result of either process is a continuous matrix of steel with particles of niobium- and tantalum-coated yttria dispersed throughout. The density of the particles may vary based on the size of the yttria cores and the concentrations of yttria, niobium, and tantalum. For example, because tantalum is approximately twice as dense as niobium, particles having a higher density may be formed by increasing the ratio of tantalum to niobium. Likewise, particles having a lower density may be formed by decreasing the ratio of tantalum to niobium.
Niobium and tantalum are milled with yttria powder to form particles having a mean diameter of less than 500 microns The particles are added to a liquid steel alloy, such that the overall composition includes about 1.0% yttria, about 1.6% niobium, 0.4% tantalum, and 9-15% chromium, with the balance iron. The niobium and tantalum cores dissolve and tend to weaken the yttria coating such that the yttria coating may break into smaller particles. The niobium and tantalum mix with the steel alloy. The composition is then cooled. The solubilities of the niobium and tantalum in solid steel are relatively low, so relatively small portions of the niobium and tantalum remain dissolved in the steel alloy after cooling. During cooling of the composition, the niobium and tantalum coalesce adjacent the smaller yttria particles formed from the yttria coating. By breaking the yttria into smaller particles, the number of yttria-niobium-tantalum particles may be increased, thus decreasing the average separation between adjacent particles.
Compositions are formed as described in any of Examples 1 through 3, with the rare-earth oxides and metal phase shown in Table 1 forming the particles. The concentrations shown in Table 1 are in percent based on the total mass of the composition, with the balance being a steel alloy comprising chromium and iron (i.e., composition #4 has an iron concentration of 82.1%-88.1%).
The particles formed of each of the compositions 4-18 are predicted to have an average density near the density of iron at melt-process temperatures. Thus, the particles should remain suspended during processing. In each of the compositions 15-18, the rare-earth oxides and the metal phase each independently has an average density near the density of iron at melt-process temperatures, such that the material should remain suspended during processing even if not associated as heterogeneous particles of rare-earth oxides and metal.
Compositions are formed as described in any of Examples 1 through 3, with the rare-earth oxides and metal phase shown in Table 2 forming the particles. The steel alloy comprises one or more of carbon, nickel, chromium, molybdenum, manganese, and silicon, with the balance iron. The concentrations shown in Table 2 are in percent based on the total mass of the composition (i.e., composition #19 has an iron concentration of 85.40%-85.65%).
The particles formed of each of the compositions 19-23 are predicted to have an average density near the density of the steel alloy at melt-process temperatures. Thus, the particles should remain suspended during processing.
While the present invention has been described herein with respect to certain illustrated embodiments, those of ordinary skill in the art will recognize and appreciate that it is not so limited. Rather, many additions, deletions, and modifications to the illustrated embodiments may be made without departing from the scope of the invention as hereinafter claimed, including legal equivalents thereof. In addition, features from one embodiment may be combined with features of another embodiment while still being encompassed within the scope of the invention as contemplated by the inventors. Further, embodiments of the disclosure have utility with different and various compositions.
