Patent Application
20130183188
The present disclosure is related generally to nickel-titanium alloys including a rare earth element, and more particularly to powder metallurgical processing of nickel-titanium alloys including a rare earth element.
Nickel-titanium alloys are commonly used for the manufacture of intraluminal biomedical devices, such as self-expandable stents, stent grafts, embolic protection filters, and stone extraction baskets. Such devices may exploit the superelastic or shape memory behavior of equiatomic or near-equiatomic nickel-titanium alloys, which are commonly referred to as Nitinol. As a result of the poor radiopacity of nickel-titanium alloys, however, such devices may be difficult to visualize from outside the body using non-invasive imaging techniques, such as x-ray fluoroscopy. Visualization is particularly problematic when the intraluminal device is made of fine wires or thin-walled struts. Consequently, a clinician may not be able to accurately place and/or manipulate a Nitinol stent or basket within a body vessel.
Current approaches to improving the radiopacity of nickel-titanium medical devices include the use of radiopaque markers, coatings, or cores made of heavy metal elements. In addition, noble metals such as platinum (Pt), palladium (Pd) and gold (Au) have been employed as alloying additions to the improve the radiopacity of Nitinol, despite the high cost of these elements. In a more recent development, it has been shown (e.g., U.S. Patent Application Publication 2008/0053577, “Nickel-Titanium Alloy Including a Rare Earth Element”) that rare earth elements such as erbium can be alloyed with Nitinol to yield a ternary alloy with radiopacity that is comparable to if not better than that of a Ni—Ti—Pt alloy.
Ternary nickel-titanium alloys that include rare earth or other alloying elements are commonly formed by vacuum melting techniques. However, upon cooling the alloy from the melt, a brittle network of secondary phase(s) may form in the alloy matrix, potentially diminishing the workability and mechanical properties of the ternary alloy. If the brittle second phase network cannot be broken up by suitable homogenization heat treatments and/or thermomechanical working steps, then it may not be possible to find practical application for the ternary nickel-titanium alloy in medical devices or other applications.
It has been discovered that, by using preferred combinations of starting powders in conjunction with appropriate sintering conditions, sintered Ni—Ti—RE alloys that exhibit good workability along with a desired austenite finish (Af) temperature may be produced.
A mixture of powders for preparing a sintered nickel-titanium-rare earth (Ni—Ti—RE) alloy includes Ni—Ti alloy powders comprising from about 55 wt. % Ni to about 61 wt. % Ni and from about 39 wt. % Ti to about 45 wt. % Ti, and RE alloy powders including a RE element.
A method of forming a sintered nickel-titanium-rare earth (Ni—Ti—RE) alloy comprises adding Ni—Ti alloy powders and RE alloy powders to a powder consolidation unit including an electrically conductive die and punch connectable to a power supply. The Ni—Ti alloy powders comprise from about 55 wt. % Ni to about 61 wt. % Ni and from about 39 wt. % Ti to about 45 wt. % Ti, and the RE alloy powders include a RE element. The powders are heated to a sintering temperature of from about 730° C. to about 840° C., and a pressure of from about 60 MPa to about 100 MPa is applied to the powders at the sintering temperature. A sintered Ni—Ti—RE alloy is formed.
As used in the following specification and the appended claims, the following terms have the meanings ascribed below:
Martensite start temperature (Ms) is the temperature at which a phase transformation to martensite begins upon cooling for a shape memory material exhibiting a martensitic phase transformation.
Martensite finish temperature (Mf) is the temperature at which the phase transformation to martensite concludes upon cooling.
Austenite start temperature (As) is the temperature at which a phase transformation to austenite begins upon heating for a shape memory material exhibiting an austenitic phase transformation.
Austenite finish temperature (Af) is the temperature at which the phase transformation to austenite concludes upon heating.
Radiopacity is a measure of the capacity of a material or object to absorb incident electromagnetic radiation, such as x-ray radiation. A radiopaque material preferentially absorbs incident x-rays and tends to show high radiation contrast and good visibility in x-ray images. A material that is not radiopaque tends to transmit incident x-rays and may not be readily visible in x-ray images.
Workability refers to the ease with which an alloy may be formed to have a different shape and/or dimensions, where the forming is carried out by a method such as rolling, forging, extrusion, etc.
Cold working or cold forming is plastically deforming a component without applying heat to alter the size, shape and/or mechanical properties of the component.
Hot working or hot forming is plastically deforming a component at an elevated temperature (typically at or above the recrystallization temperature of the component) to alter the size, shape and/or mechanical properties of the component.
The term “themomechanical processing” may refer to hot and/or cold working.
Percent (%) cold work is a measurement of the amount of plastic deformation imparted to a component, where the amount is calculated as a percent reduction in a given dimension. For example, in wire drawing, the % cold work may correspond to the percent reduction in the cross-sectional area of the wire resulting from a drawing pass.
The term “prealloyed” is used to describe powders that are obtained from an ingot of a particular alloy composition that has been converted to a powder (e.g., by gas atomization). Such powders may be referred to as “prealloyed powders” or “alloy powders” in the present disclosure.
Sintering temperature refers to a temperature at which precursor powders may be sintered together when exposed to an applied pressure.
Softening temperature, when used in reference to a rare earth element, refers to a temperature at which the rare earth element softens, as determined by hot hardness measurements or melting temperature data.
The terms “comprising,” “including” and “having” are used interchangeably throughout the specification and claims as open-ended transitional terms that cover the expressly recited subject matter alone or in combination with unrecited subject matter.
As noted above, novel combinations of starting powders may be used in conjunction with appropriate sintering conditions to form sintered Ni—Ti—RE alloys that exhibit good workability and ductility along with a desired Af temperature. The starting powders may be selected to overcompensate for the amount of Ni that may react with the RE element during sintering, and thus the sintered Ni—Ti—RE alloy may retain a sufficient amount of Ni in the matrix phase to exhibit an Af temperature below body temperature. The sintered Ni—Ti—RE alloy may thus be superelastic at body temperature. In some cases, the desired Af temperature may be achieved after hot and/or cold working of the sintered alloy. The inventors have recognized that the hot and cold workability of the sintered Ni—Ti—RE is influenced not only by the composition of the starting powders but also by the sintering conditions. For example, an improved result may be achieved by increasing the sintering pressure while decreasing the sintering temperature, as discussed further below.
A mixture of powders for preparing a sintered nickel-titanium-rare earth metal (Ni—Ti—RE) alloy may include Ni—Ti powders and rare earth element-containing powders. The Ni—Ti powders may be prealloyed Ni—Ti powders, which are alternately referred to as Ni—Ti alloy powders, of an appropriate composition that may be substantially equiatomic (i.e., about 50 at. % Ni (about 56 wt. % Ni) and 50 at. % Ti (about 44 wt. % Ti)) or, more preferably, nickel-rich (i.e., greater than about 50 at. % Ni (about 56 wt. % Ni). Alternatively, elemental Ni powders and elemental Ti powders may be used in the same proportions. Throughout this disclosure, powders including the elements Ni and Ti may be referred to as Ni—Ti powders whether they are elemental Ni and Ti powders or Ni—Ti alloy powders (prealloyed Ni—Ti powders).
Several different types of rare earth element-containing powders can be added to the Ni—Ti powders to form the sintered Ni—Ti—RE alloy. The term “rare earth element” is used alternately with “rare earth metal” to refer to elements found in the lanthanide series and/or the actinide series of the periodic table, which include La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, and U. In addition, yttrium (Y) and scandium (Sc) are sometimes referred to as rare earth elements although they are not elements of the lanthanide or actinide series. Typically, the rare earth element is selected from the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Preferably the rare earth element includes erbium.
The powders may be elemental RE powders (including only the rare earth element and any incidental impurities) or RE alloy powders that include, in addition to the rare earth element and any incidental impurities, one or more additional alloying elements and/or dopant elements. Specific examples of these powders are provided below.
According to one embodiment, the mixture of powders for preparing a sintered Ni—Ti—RE alloy may include Ni—Ti alloy powders and RE alloy powders. The Ni—Ti alloy powders may comprise from about 55 wt. % Ni to about 61 wt. % Ni and from about 39 wt. % Ti to about 45 wt. % Ti, or from about 57 wt. % Ni to about 59 wt. % Ni and from about 41 wt. % Ti to about 43 wt. % Ti, and the RE alloy powders include a RE element and may also include at least one additional element.
The at least one additional element may be an additional alloying element or a dopant element selected from the group consisting of B, Al, Cr, Mn, Fe, Ni, Co, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, TI, Pb, Bi, Po, V, other rare earth elements, and Y. The additional element may be present in the RE alloy powder at a concentration that may be as low as parts per million (ppm) levels to as high as about 95 wt. %. When used herein, ppm is in terms of weight. Typically, the additional element has a concentration of no more than about 50 wt. %, no more than about 30 wt. %, or no more than about 15 wt. %, and it may be no more than about 5 wt. % of the RE alloy powder. For example, in the case of a dopant element such as B, the concentration may be at least about 10 ppm, at least about 50 ppm, or at least about 100 ppm. Typically, the concentration of the dopant element is no more than about 1000 ppm, or no more than about 500 ppm, or no more than about 300 ppm. In the case of an additional alloying element, which may be, for example, a transition metal or another metal, the concentration may be at least about 0.1 wt. %, at least about 1 wt. %, at least about 5 wt. %, at least about 10 wt. %, or at least about 20 wt. % of the RE alloy powders.
The Ni—Ti alloy powders mixed with the RE alloy powders may comprise a mixture of first binary alloy powders and second binary alloy powders, where the first binary alloy powders comprise about 54-58 wt. % Ni and about 42-46 wt. % Ti, and the second binary alloy powders comprise about 58-62 wt. % Ni and about 38-42 wt. % Ti. For example, the first binary alloy powders may include about 56 wt. % Ni and about 44 wt. % Ti and the second binary alloy powders comprise about 60 wt. % Ni and about 40 wt. % Ti. A weight ratio of the first binary alloy powders to the second binary alloy powders may be at least about 30:70, at least about 40:60, at least about 50:50, or at least about 60:40. The weight ratio may also be no more than about 50:50, no more than about 60:40, or no more than about 70:30. For example, the weight ratio may range from about 70:30 to about 30:70, or from about 60:40 to about 40:60. Advantageously, the weight ratio is from about 40:60 to about 50:50, as discussed in the Examples.
The Ni—Ti alloy powders may not comprise a mixture of first and second binary alloy powders of different compositions, but rather may include a single binary powder composition. For example, the Ni—Ti alloy powders may comprise from about 58 wt. % Ni to about 59 wt. % Ni and from about 41 wt. % Ti to about 42 wt. % Ti, e.g., about 58.5 wt. % Ni and about 41.5 wt. % Ti.
A weight ratio of the Ni—Ti alloy powders to the RE alloy powders may be at least about 60:40, at least about 65:35, at least about 70:30, at least about 75:25, or at least about 80:20. Typically the weight ratio of the Ni—Ti alloy powders to the RE alloy powders is no more than about 90:10, or no more than about 85:15. For example, the weight ratio may be from about 75:25 to about 85:15, or about 83:17. The desired weight ratio may be determined based on the desired concentration of the rare earth element in the sintered Ni—Ti—RE alloy, while taking into account the concentration of any additional elements in the RE alloy powders. Experiments regarding the radiopacity of Ni—Ti—RE alloys have shown that an amount of from about 10 wt. % RE to about 30 wt. % RE, from about 12 wt. % RE to about 25 wt. % RE, or from about 15 wt. % RE to about 20 wt. % RE, may be advantageous for the sintered Ni—Ti—RE alloy.
Examples of suitable RE-containing powders include, for example: prealloyed RE-Ni alloy (e.g., Er—Ni alloy) powders, optionally with B or Fe doping, that may be produced by gas atomization to achieve a fine particle size (see
Among the possible contemplated powder compositions are the following, in wt. %: Ni55:Ti45, Ni56:Ti44, Ni57:Ti43, Ni58:Ti42, Ni59:Ti41, Ni60:Ti40, Ni60.5:Ti39.5, and Ni61:Ti39; Er98.5:Fe1.5, Er(balance):Fe1.5:100 ppm B, Er(balance):100 ppm B, Er(balance):Ni25.74:Fe1, Er(balance):Ni25.74:Fe1:100 ppm B, Er(balance):Ni26:100 ppm B, assuming +/−5 wt. % Ni, +/−1 wt. % Fe or +/−0.5 wt. % Fe, and +/−50 ppm B.
The average particle size of the powders may be small, e.g., a D50 size of about 50 microns with a distribution of from about 10 microns to about 100 microns. (D50 refers to a median particle size where about 50% by weight of the particles are smaller and 50% by weight are larger than the indicated size.) The D50 size of the particles may be from about 10 to about 100 microns, or from about 30 to about 70 microns, or from about 40 to about 60 microns. However, at smaller particle sizes, the ratio of surface area to volume rises and the oxide/oxygen content may increase accordingly. Consequently, atomizing, sieving, shipping, storing, mixing and sintering is advantageously carried out in a controlled vacuum or inert gas (e.g., argon) environment if possible to minimize oxygen content.
The aforementioned powders may be obtained from commercial sources or produced using powder production methods known in the art (e.g., gas atomization, ball milling, etc.). Ni—Ti alloy powders can be atomized by most commercial gas atomization processes, including gas atomization of a super heated melt stream from a graphite crucible, cold crucible gas atomization, electrode induction-melted atomization etc. Extreme care is advisable when atomizing rare earth metals and alloys as pure rare earth metal and some high rare earth content alloys are pyrophoric when powdered. When melted at superheated temperatures, the metal is highly reactive and may attack graphite and ceramic crucibles. Pure rare earth metal and some high rare earth content alloys can be atomized via electrode induction-melted atomization and through cold crucible gas atomization. Gas atomization of a super-heated melt stream from a ceramic crucible is safe for rare earth alloys for non-reactive compositions. Extreme care is also advisable when further handling rare earth alloy powders and mixing with Ni—Ti powders. Dust clouds and increases in temperature are advantageously avoided. When mixed with Ni—Ti powders, the rare earth powders are effectively diluted and safer to handle.
The use of high purity elemental powders or RE alloy powders including a dopant element in the sintering process may be referred to as “reactive” sintering due to the proclivity of the powders to react with Ni. The scavenging of nickel from the Ni—Ti matrix by the RE element may be a downside of reactive sintering using high purity elemental RE powders, since reduced Ni levels may raise the transformation temperatures (e.g., Af) of the alloy to a level at which superelasticity is not obtained at body temperature. This problem may be diminished or avoided altogether by using fully dehydrogenated HDH RE powder or by using prealloyed RE-Ni powders having a composition that compensates for the scavenging of the nickel, as set forth in the Examples below. Full dehydrogenation of HDH Er powders can be achieved by heating the powders in a furnace with at a temperature of about 900° C. under a vacuum of 10−10 bar.
Reactive sintering may be advantageous in part because the rare earth particles may reduce in size during sintering due to their reaction with the NiTi particles. This may result in either many finer particles replacing the starting rare earth particle or a halo of finer particles surrounding the now smaller initial rare earth particle. If the formation of Ti rich regions within these alloys can be eliminated and the transformation temperatures (e.g., Af) controlled, this route may be very attractive in a production environment, as the ramp rate can be increased (e.g., to about 35° C./min).
A challenge with using prealloyed RE-Ni powders is that, for a given atomic percentage of the rare earth element, a larger percentage of second phase inclusions may be obtained than if an elemental rare earth powder is used; this means the superelastic matrix accounts for a smaller proportion of the alloy and the recoverable strain or the upper and lower loading plateaus may be reduced. Using a ductile and radiopaque alloy such as ErAg or other ductile rare earth intermetallic compounds, such as yttrium-silver (YAg), yttrium-copper (YCu), dysprosium-copper (DyCu), cerium-silver (CeAg), erbium-silver (ErAg), erbium-gold (ErAu), erbium-copper (ErCu), holmium-copper (HoCu), neodymium-silver (NdAg), may be a way around this (e.g., see Gschneidner Jr. K. A. et al. (2009) “Influence of the electronic structure on the ductile behaviour of B2 CsCl-type AB intermetallics,” Acta Materialia 57, 5876-5881, which is hereby incorporated by reference), with some of the intermetallics reported to achieve >20% strain after heat treating and hot rolling.
According to one embodiment, the RE alloy powders may be RE-Fe alloy powders that include iron (Fe) in addition to the rare earth metal (RE). For example, the Fe may be present in the RE alloy powders at a concentration of from about 0.5 wt. % Fe to about 2.5 wt. % Fe, or from about 1 wt. % to about 2 wt. %, e.g., about 1.5 wt. % Fe. The balance of the RE-Fe alloy powders may be the RE element and any incidental impurities. The RE element may be selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, U, Y and Sc. Typically, the RE element is selected from the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. In one example, the RE element is Er and the Er—Fe alloy powders may comprise about 1.5 wt. % Fe. In some embodiments, the RE-Fe alloy powders (which may be Er—Fe alloy powders) may further comprise B in addition to any incidental impurities. For example, the RE-Fe alloy powders may be RE-Fe-B powders including B at a concentration of from about 50 ppm to about 150 ppm.
According to another embodiment, the RE alloy powders may be RE-Ni—Fe alloy powders that include iron and nickel in addition to the rare earth metal. For example, the RE-Ni—Fe alloy powders may comprise from about 21 wt. % Ni to about 31 wt. % Ni, from about 0.5 wt. % Fe to about 1.5 wt. % Fe, and the balance (remainder) may be the rare earth element and any incidental impurities. As above, the RE element may be selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, U, Y and Sc. Typically, the RE element is selected from the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The RE-Ni—Fe alloy powders may comprise about 26 wt. % Ni and/or about 1 wt. % Fe. The RE-Ni—Fe alloy powders may further comprise B at a concentration of from about 50 ppm to about 150 ppm, e.g., about 100 ppm. In one example, the RE element may be Er and the RE-Ni—Fe alloy powders may include about 26 wt. % Ni and about 1 wt. % Fe.
According to another embodiment, the RE alloy powders may be RE-Ni—B powders that include nickel and boron in addition to the rare earth metal. For example, the RE-Ni—B alloy powders may comprise from about 21 wt. % Ni to about 31 wt. % Ni, B at a concentration of from about 50 ppm to about 150 ppm, and the balance may be the RE element and any incidental impurities. As above, the RE element may be selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, U, Y and Sc. Typically, the RE element is selected from the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. In one example, the RE element may be Er and the concentration of B may be about 100 ppm. The RE-Ni—B alloy powders may comprise about 26 wt. % Ni.
According to another embodiment, the RE alloy powders may be RE-B alloy powders that include boron in addition to the rare earth metal. For example, the RE-B alloy powders may comprise B at a concentration of from about 50 ppm to about 150 ppm, and the balance may be the RE element and any incidental impurities. As above, the RE element may be selected from the group consisting of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Ac, Th, Pa, U, Y and Sc. Typically, the RE element is selected from the group consisting of La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. In one example, the RE element may be Er and the concentration of B may be about 100 ppm.
A sintered Ni—Ti—RE alloy prepared from any of the above-described mixtures may include from about 5 wt. % RE to about 35 wt. % RE, from about 10 wt. % RE to about 30 wt. % RE, from about 12 wt. % RE to about 25 wt. % RE, or from about 15 wt. % RE to about 20 wt. % RE. The sintered Ni—Ti—RE alloy may also include from about 45 wt. % Ni to about 50 wt. % Ni and from about 33 wt. % Ti to about 38 wt. % Ti. The sintered Ni—Ti—RE alloy may include a NiTi matrix phase and a second phase comprising discrete regions dispersed in the matrix phase, where the second phase comprises the RE element. The second phase may also include an additional element selected from the group consisting of B, Al, Cr, Mn, Fe, Ni, Co, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, TI, Pb, Bi, Po, V, other rare earth elements, and Y. There may be more than one second phase in the sintered Ni—Ti—RE alloy. The NiTi matrix phase may comprise a Ni:Ti weight ratio of at least about 55:45, or at least about 56:44. The Ni:Ti weight ratio is typically no greater than 60:40, and may be no greater than 58:42.
The sintered Ni—Ti—RE alloy has a phase structure that depends on the composition and processing history of the alloy. The RE element, which is present in the second phase, may also be in solid solution with the NiTi matrix phase containing Ni and Ti. The second phase comprising the RE element may include Ni and/or Ti. For example, the RE element may form an intermetallic compound phase with Ni and/or with Ti. In other words, the RE element may combine with Ni in specific proportions and/or with Ti in specific proportions to form the compound phase. The RE element may substitute for Ti and form one or more intermetallic compound phases with Ni, such as, for example, NiRE, Ni2RE, Ni3RE2, Ni3RE7 or another phase, e.g., NixREy, where x and y may have integer values or fractional values typically from 1 to 20. Alternatively, the RE element may substitute for Ni and combine with Ti to form a solid solution or a compound such as TixREy. The Ni—Ti—RE alloy may also include one or more other intermetallic compound phases of Ni and Ti, such as NiTi, which may be the matrix phase, Ni3Ti and/or NiTi2, depending on the composition and heat treatment. The RE element may form a ternary intermetallic compound phase with both Ni and Ti atoms, such as NixTiyREz. The RE element may also form a quaternary intermetallic compound phase, such as NixTiyREzMm, that includes at least one additional element (represented by M) in addition to the rare earth metal. Some exemplary phases in various Ni—Ti—RE alloys are identified below in TABLE 1, where x, y, z and m may have integer or fractional values typically from 1 to 20.
The one or more additional elements that may be present in the sintered Ni—Ti—RE alloy (in addition to the RE element) may be in solid solution with the NiTi matrix phase and/or may form one or more second phases with Ni, Ti, and/or the RE element. Accordingly, the second phase may include the additional alloying element in addition to the rare earth element. The second phase may also or alternatively include nickel (Ni) and/or titanium (Ti). The discrete particles of the second phase may have an average size of from about 1 to about 500 microns, and preferably from about 1 to about 150 microns. The matrix phase may comprise NiTi.
The one or more additional alloying elements present in the sintered Ni—Ti—RE alloy may be selected from the group consisting of Al, Cr, Mn, Fe, Ni, Co, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, TI, Pb, Bi, Po, V, other rare earth elements, and Y. The sintered Ni—Ti—RE alloy may also or alternatively include small amounts (e.g., hundreds of ppm or less) of non-metallic elemental additions, such as, for example, B, C, H, N, or O, although non-metallic elements are generally not included in the summation of alloying elements used to specify the composition of the alloy. B may be considered to be a dopant element added intentionally to the alloy to improve workability and/or ductility, and may be present in amounts of from about 10 ppm to about 300 ppm, from about 20 to about 200 ppm, or from about 50 ppm to about 150 ppm. Preferably, the amounts of C, O, and N are consistent with the American Society of Testing and Materials (ASTM) standard F2063, so as to avoid forming a high number density of and/or large-size carbide, oxide, nitride or complex carbonitride particles, which may affect the mechanical properties of the Ni—Ti—RE alloy. H is preferably controlled per ASTM standard F2063 to minimize hydrogen embrittlement of the alloy. The aforementioned ASTM standards are hereby incorporated by reference.
In one example, the sintered Ni—Ti—RE alloy may be a sintered Ni—Ti—Er alloy that includes from about 45 wt. % to about 50 wt. % Ni, from about 33 wt. % to about 38 wt. % Ti, and from about 15 wt. % Er to about 20 wt. % Er, or from about 16 wt. % Er to about 17 wt. % Er. The sintered Ni—Ti—RE alloy may further comprise an additional element, which may be Fe and/or B. For example, the Ni—Ti—Er alloy may include from about 0.1 wt. % Fe to about 0.3 wt. % Fe. The Ni—Ti—Er alloy may also or alternatively include B in an amount of about 100 ppm or less. The sintered Ni—Ti—Er alloy may include a NiTi matrix phase and a second phase comprising discrete regions dispersed in the matrix phase, where the second phase comprises Er. The second phase may further comprise Ni. For example, the second phase comprising Er and Ni may be an erbium-rich phase including at least about 50 wt. % Er. The NiTi matrix phase may comprise the intermetallic compound NiTi.
The sintering may be carried out using a spark plasma sintering (SPS) process, which entails forming a dense compacted speciman from metal and/or alloy powders by passing a pulsed electrical current though the powders while applying a pressure thereto. A low voltage, high pulsed current may generate a spark plasma at high localized temperatures throughout the compact, generating heat uniformly through the powder.
In contrast to conventional melting techniques (e.g., vacuum induction melting (VIM) or vacuum arc melting (VAR)) for Ni—Ti—RE alloy fabrication, SPS may result in fine dispersion of the rare earth element or a secondary phase within the alloy microstructure, and thus the billet or compact produced by SPS may not need to undergo a homogenization heat treatment prior to hot or cold working. Sintering also may permit a dense ternary alloy compact to be formed at a much lower temperature (e.g., <850° C.) than a typical melting process, which is typically carried out at a temperature in excess of 1350° C., and the sintering temperature can be further reduced if desired by using smaller starting particle sizes and a higher sintering pressure. Another advantage of SPS compared to conventional melting processes and other powder metallurgy methods is that the powder particles may be purified during sintering, thereby minimizing contaminants in the resulting ternary Ni—Ti—RE alloy. It is possible to obtain extremely low oxygen and acceptable carbon contents independent of the impurity level in the starting powder. For example, the oxygen content of alloys sintered via SPS may be as low as about 0.007 at. % O, whereas an oxygen content of about 0.03 at. % 0 is typical of VIM melted Ni—Ti alloy specimens.
To form the sintered Ni—Ti—RE alloy, Ni—Ti alloy powders and RE alloy powders are added to a powder consolidation unit which may include an electrically conductive die and a punch connectable to a power supply. The Ni—Ti alloy powders may comprise from about 55 wt. % Ni to about 61 wt. % Ni and from about 39 wt. % Ti to about 45 wt. % Ti, and the RE alloy powders comprise a RE element and may also comprise an additional element. The RE element and the additional element may be selected as set forth above.
A pulsed electrical current may be passed through the powders and they may be heated to a desired sintering temperature, which may be from about 730° C. to about 840° C. The powders may be heated to the sintering temperature at ramp rate of about 35°/min or less, and the ramp rate is preferably about 25°/min or less. Pressure is applied to the powders at the sintering temperature, and the sintering temperature is maintained for a hold time sufficient to form a sintered Ni—Ti—RE alloy having a density of at least about 95% of theoretical density.
An advantage of the sintering process compared to melt processing is that the sintered Ni—Ti—RE alloy may be formed in a fairly short time. For example, for a 10 mm-diameter billet, the hold time employed to produce the sintered alloy typically takes from about 15 min to about 25 min, depending on the material being sintered. Generally speaking, the hold time may be at least about 1 min, at least about 10 minutes, or at least about 15 minutes, e.g., from about 1 min to about 60 min, from about 10 min to about 20 min, or from about 5 min to about 15 min. Accordingly, the sintering process may have a total time duration of about 72 minutes or less, which is significantly shorter than the time required for other sintering routes, despite the low ramp rates employed here.
In general, a low sintering temperature (e.g., <850° C.) and low ramp rate (≦35° C.) can be utilized along with an appropriate sintering pressure to successfully form a sintered Ni—Ti—RE alloy of the desired density. Higher sintering pressures, for example, at least about 50 MPa, at least about 60 MPa, at least about 70 MPa, or at least about 85 MPa, may be advantageous. Typically, the sintering pressure is no higher than about 110 MPa. For example, the pressure applied at the sintering temperature may range from about 45 MPa to about 110 MPa, or from about 60 MPa to about 100 MPa.
The pressure during sintering can be increased to compensate for a reduction in sintering temperature, and/or the average particle size of the powders can be decreased. Advantageously, the sintered alloy achieves a density of at least about 98% of theoretical density as a result of the sintering process. The density may also be at least about 95% of theoretical density, or at least about 90% of theoretical density.
As discussed in U.S. patent application Ser. No.13/656,151, entitled “Method of Forming a Sintered Nickel-Titanium-Rare Earth Alloy,” which is hereby incorporated by reference in its entirety, the sintering temperature of the Ni—Ti—RE alloy may coincide with a softening temperature of the rare earth element. The softening temperature may be the temperature at which the rare earth element has a Rockwell (E) hardness of from 17 to 20, or from 16 to 21. The softening temperature may also be related to the absolute melting temperature (Tm) of the rare earth element. For example, the softening temperature may be from about 0.50·Tm to about 0.55·Tm. Accordingly, the desired sintering temperature may be from about 650° C. to about 850° C., or from about 700° C. to about 825° C. When the rare earth element is Er, the sintering temperature is preferably from about 730° C. to about 840° C., 740° C. to about 840° C., or from about 750° C. to about 800° C.
The sintered Ni—Ti—RE alloy may be prepared in a die having a desired final shape, so that the sintered alloy may be used in the as-pressed form as a net-shape or near net-shape component. Alternatively, the sintered Ni—Ti—RE alloy may take the form of a billet or a button and may undergo further thermomechanical processing after sintering in order to obtain a desired shape for a specific application. The mechanical and/or superelastic properties of the sintered Ni—Ti—RE alloy may also be altered or improved by thermomechanical processing, which may include one or more—e.g., a series of—hot working and/or cold working steps. A series of hot or cold working steps may be at least 3, at least 5, at least 10, at least 20, or at least 40 and typically no more than 100 hot or cold working steps carried out sequentially. The hot working may entail rolling, extrusion, forging, drawing, and/or another mechanical process carried out at an elevated temperature and resulting in plastic deformation of the sintered Ni—Ti—RE alloy. The cold working may entail rolling, extrusion, forging, drawing, and/or another mechanical process carried out at room temperature to further plastically deform the alloy. Typically, hot working is performed prior to cold working. As would be known by one of ordinary skill in the art, interpass annealing steps may be carried out between cold working steps or passes, in order to reduce strain and to increase the workability of the alloy for subsequent cold working steps. What may be referred to as interpass annealing or re-heating steps may also be be carried out between the hot working steps or passes.
In one example, the sintered Ni—Ti—RE alloy may undergo up to 60 hot rolling passes to form a 5 mm-diameter rod from the as-sintered billet, which may be about 25 mm in diameter, followed by sequential cold working (e.g., rolling and/or drawing) and interpass annealing steps in order to form an even smaller-diameter rod or wire (e.g., less than about 5 mm, less than about 3 mm, or less than about 1 mm in diameter). The hot rolling and interpass annealing steps may be carried out at a temperature in the range of from about 550° C. to about 750° C., from about 600° C. to about 750° C., or from about 630° C. to about 730° C. An area reduction of at least about 3% per pass and generally from about 5% per pass to about 30% per pass may be achieved. The area reduction may also be from about 5% per pass to about 15% per pass, or from about 5% per pass to about 10% per pass. The final cold worked form, which may be a rod or wire, may be annealed at a temperature below about 550° C., for 2-10 minutes. The annealing may be done in air, in vacuum, or in a gas environment that includes one or more of air, Ar, N2 or He. A gas environment including Ar and air is preferable to prevent deterioration of the alloy due to oxidation.
Thermomechanical processing equipment known in the art may be employed for the hot and/or cold working. Advantageously, the sintered and optionally thermomechanically processed Ni—Ti—RE alloy component may have an austenite finish temperature of 37° C. or less. Due to deformation caused by hot and/or cold working, the discrete regions of the second phase(s) may comprise an elongated shape. Following cold working of the Ni—Ti—RE component, an overall % reduction in cross-sectional area of at least about 30%, at least about 50%, at least about 70%, or at least about 90% may be achieved. The % reduction per pass is typically at least about 3%, at least about 5%; at least about 10%, or at least about 20%, and is typically no higher than about 30%.
The sintering method and optional thermomechanical processing described here are believed to be particularly advantageous for forming Ni—Ti—RE alloys suitable for various applications, including use in implantable medical devices. Ni—Ti—RE alloys are described in detail in U.S. Patent Application Publication 2008/0053577, “Nickel-Titanium Alloy Including a Rare Earth Element,” filed on Sep. 6, 2007, and in U.S. Patent Application Publication 2011/0114230, “Nickel-Titanium Alloy and Method of Processing the Alloy,” filed on Nov. 15, 2010, both of which are hereby incorporated by reference in their entirety.
The sintering method set forth herein may be carried out using a spark plasma sintering apparatus such as, for example, Dr. Sinterlab SPS 515S (Sumitomo Coal Mining Co. Ltd., Japan). The SPS die in this case is made from high grade graphite and the sintering is performed in vacuum (˜10−3 Torr). In a typical SPS run, a powder sample is packed into the high strength graphite die and placed between the upper and lower electrodes, as shown schematically in
Over 75 experiments were carried out to sinter mixtures of Ni—Ti alloy powders and RE alloy powders using different starting powder compositions and various sintering parameters, followed by hot and cold working steps. The process parameters and results are summarized in Tables 2A-6B below. The sintered samples had the physical form of small disks of 25 mm in diameter and about 4 mm in thickness.
In each experiment, a mixture of first binary alloy powders (“Ni56Ti”) comprising about 56 wt. % Ni and about 44 wt. % Ti and second binary alloy powders (“Ni60Ti”) comprising about 60 wt. % Ni and about 40 wt. % Ti was sintered with RE alloy powders comprising Er and Fe. Different weight ratios of the first and second binary alloy powders (Ni56Ti and Ni60Ti) were employed in the experiments. In each experiment, the Er—Fe alloy powders included 1.5 wt. % Fe. The balance (remainder) of the Er—Fe alloy powders was Er and any incidental impurities.
Table 2A shows results for samples 1-15, which included a 70:30 weight ratio of Ni56Ti to Ni60Ti powders, and Table 2B shows the composition of the sintered alloy corresponding to samples 1-15; Table 3A shows results for samples 21-35, which included a 60:40 weight ratio of Ni56Ti to Ni60Ti powders, and Table 3B shows the composition of the sintered alloy corresponding to samples 21-35; Table 4A shows results for samples 41-55, which included a 50:50 weight ratio of Ni56Ti to Ni60Ti powders, and Table 4B shows the composition of the sintered alloy corresponding to samples 41-55; Table 5A shows results for samples 61-75, which included a 40:60 weight ratio of Ni56Ti to Ni60Ti powders, and Table 5B shows the composition of the sintered alloy corresponding to samples 61-75; Table 6A shows results for samples 81-95, which included a 30:70 weight ratio of Ni56Ti to Ni60Ti powders, and Table 6B shows the composition of the sintered alloy corresponding to samples 81-95.
For each set of samples, sintering and hot rolling were carried out at temperatures of 760° C., 800° C., and 840° C. using hold times of 5 min, 30 min, or 60 min. A sintering pressure of either 60 or 70 MPa was employed in each experiment. In some cases, sintering was followed by a heat treatment at a temperature of 760° C., 800° C., or 840° C., with a heat treatment hold time of 24 min or 48 min. After sintering, hot working and then cold working were carried out, and the results are evaluated on a scale from 0 (=poor) to 3 (=superb), as indicated in the tables below. The hot working entailed hot rolling at a temperature of 760° C. or hot extruding at a temperature of 760° C-800° C. with a short soak (about 30 min) at temperature, and the cold working entailed multiple cold rolling passes (e.g., 20-60 passes), with interpass annealing treatments at about 760° C. or less, preferably. The samples were evaluated in terms of their ability to be hot and cold worked. The best thermomechanical processing results were obtained from Ni—Ti—RE alloy samples sintered and hot rolled at a temperature of about 760° C. or less and at a pressure of about 70 MPa or higher.
The microstructure of a number of samples in the as-sintered state and after thermomechanical processing was investigated using scanning electron microscopy (SEM). The SEM images of
Another thermomechanically processed sample is shown in the SEM image of
An additional set of experiments labeled N16-N20 and N36 (“N-series”) is summarized in Table 7. In this series of experiments, high sintering pressures (100 MPa, with one exception of 70 MPa) were employed to enhance the workability of the Ni—Ti—RE alloys. In addition, no post-sintering heat treatments were employed, as it was found from prior experiments that the heat treatments dramatically reduced rollability due to sample grain growth, and Af was also undesirably increased. It is believed that any needed homogenization occurs during the interpass annealing steps involved in hot working and cold working without incurring significant, if any, grain growth.
The experiments carried out on the N-series of samples employed weight ratios of Ni56Ti to Ni60Ti powders of 70:30 and 60:40. Each sample was sintered at 760° C., 730° C. or 700° C. for a hold time of 30 minutes. Ramp rates to the sintering temperature were 25° C./min, 38° C./min or 50° C./min. After sintering, the N-series of samples were hot rolled (760° C.) and then cold rolled with the maximum reductions possible on the rigs. While all of the samples were successfully processed, a combination of 50:50 weight ratio and 760° C. sintering temperature was found to be best from a cold rolling point of view.
Table 8 shows the cold rolling reductions (in terms of height since the specimens were flat rolled) and interpass annealing treatments for several exemplary samples that received a score of 3 (“superb”) for the hot and/or cold rolling results.
Additional sintering and thermomechanical processing experiments were carried out on an second set of powder mixtures comprising Ni—Ti alloy and RE alloy powders. As in the above-described experiments, the RE alloy powders were Er—Fe alloy powders including about 1.5 wt. % Fe, with the balance being Er and any incidental impurities. In several experiments (Samples S1-S10), Ni—Er alloy powders were used instead of the Er—Fe alloy powders. Cylindrical billets or ingots of about 30-35 mm in length and 25 mm in diameter were formed in the sintering experiments (in contrast to the disks formed in Example 1).
In some of the experiments, a mixture of first binary alloy powders (“Ni56Ti”) comprising about 56 wt. % Ni and about 44 wt. % Ti and second binary alloy powders (“Ni60Ti”) comprising about 60 wt. % Ni and about 40 wt. % Ti was sintered with Er—Fe or Ni-Er alloy powders. Different weight ratios of the first and second binary alloy powders (Ni56Ti and Ni60Ti) were used in the mixtures. In other experiments, only Ni56Ti powders or Ni60Ti powders were sintered with the Er—Fe alloy powders. In the case of samples S18-S20 (see Table 9 below) the particle sizes of the powders were as follows: for the Ni56Ti powders, the d50 size was 18.8 μm; for the Ni60Ti powders, the d50 size was 25-50 μm; and for the Er—Fe alloy powders, the d50 particle size was 25-50 μm. The sintering was carried out at a temperature ranging from about 760° C. to about 880° C. and at a pressure of about 50 MPa to about 85 MPa, as summarized in Table 9 below. All ramp rates were about 25° C./min or less. No homogenization heat treatments were carried out.
As in the experiments of Example 1, 760° C. was found to be a preferred sintering temperature to produce a sintered Ni—Ti—RE alloy with a good capacity to be hot and cold worked. Also, a sintering pressure of at least about 85 MPa and a sintering time of about 15 min or less have been identified as preferred process conditions.
After sintering, the sintered samples, which may be referred to as ingots or billets, were hot and cold worked. The hot working entailed canning (containing) and then hot rolling the ingots down to a diameter of about 3 mm. A square rolling rig was used for the hot rolling. First, the sintered ingots were hot rolled down to an 8 mm rod using all 12 grooves. The hot rolled samples were then decanned and recanned with thicker cans and then passed through 11 of the grooves. Interpass annealing or re-heating was carried out at 760° C. for 3 mins before each single pass. The samples were successfully hot rolled down to 3 mm-diameter rods.
The hot rolled ingots were cold drawn to diameters of 2 mm or less and in some cases less than 1 m (e.g., about 0.8 mm). A 3 mm to 0.5 mm die with a 10% area reduction in each pass was employed for the cold drawing. Interpass annealing steps were carried out between cold drawing steps at a temperature of about 760° C. for 3 min before each single pass. The interpass annealing steps were done in air.
After cold drawing, the Af of the drawn wire is in the range of from about 40° C. to about 50° C., as measured for the 1.71 mm diameter cold drawn wire and the 0.8 mm diameter cold drawn wire after annealing for 3 min at 500° C. Bend and free recovery tests were performed on the cold drawn 1.71 mm diameter wire and the 1.46 mm diameter wire. As shown in the x-ray image of
Although the present invention has been described in considerable detail with reference to certain embodiments thereof, other embodiments are possible without departing from the present invention. The spirit and scope of the appended claims should not be limited, therefore, to the description of the preferred embodiments contained herein. All embodiments that come within the meaning of the claims, either literally or by equivalence, are intended to be embraced therein. Furthermore, the advantages described above are not necessarily the only advantages of the invention, and it is not necessarily expected that all of the described advantages will be achieved with every embodiment of the invention.
It is to be understood that the different features of the various embodiments described herein can be combined together. It is also to be understood that although the dependent claims are set out in single dependent form the features of the claims can be combined as if the claims were in multiple dependent form.
The present patent document claims the benefit of the filing date under 35 U.S.C. §119 to U.S. Provisional Patent Application No. 61/587,919, filed Jan. 18, 2012, and which is hereby incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61587919 | Jan 2012 | US |
