High-Hardness Precious Metal Alloy and Method for Producing the Same

The present invention relates to a high-hardness precious metal alloy where Pt, Au, and Pd as precious metals are essential constituent metals, particularly to a Pt—Au—Ni—Pd quaternary alloy which achieves increased hardness comparable with or more than conventional one, by spinodal decomposition and/or ordering.

Precious metals such as Pt (platinum) and Au (gold) are metals not only excellent in chemical stability/corrosion resistance, but also favorable in electric characteristics such as conductivity. Therefore, precious metals and alloys thereof are utilized in various fields, for example, in the electric/electronic field and in the medical field. Examples of use of precious metal alloys in the electric/electronic field include probe pins incorporated in probe cards for inspection of semiconductor devices or the like, and electric contacts (sliding contacts/switching contacts) such as brushes for motors, relays, and switches. Use in the medical field has recently attracted attentions, and precious metal alloys are used as constituent materials of various medical instruments. Examples of such medical instruments include various types of medical instruments, such as embolization coils and embolization clips, guide wires, stents, and catheters. Such medical instruments are tools to be directly contacted with the human body and embedded in the human body and thus are required to have biocompatibility and chemical stability. Such medical instruments are also required to have X-ray visibility in consideration of use in surgery/diagnosis with X-ray. Precious metal alloys are also favorable in such biocompatibility and X-ray visibility.

Precious metal alloys, if subjected to various applications described above, are required to be enhanced in mechanical properties such as hardness and strength. For example, probe pins are to be repeatedly contacted with mating materials for a long period, and thus are required to have wear resistance. In particular, higher-hardness probe pins are needed to be developed in order to address recent high integration of various devices and recent high performance of motors. Medical tools, which are tools to be travelled in pulsing/beating vessels and then embedded, such as guide wires and embolization coils, are required to have mechanical properties such as hardness and spring properties so that operations of such tools are made without any failures.

Precious metal alloys are also metals, and thus common strengthening mechanisms for metal materials can be applied for an enhancement in hardness of such alloys. In other words, conventional precious metal alloys have been tried to be enhanced in hardness by application of any combination of strengthening mechanisms including work hardening (dislocation strengthening), solid solution strengthening, and precipitation hardening (dispersion strengthening). Examples of an enhancement in hardness of the above precious metal alloys as probe pins or contact materials include an increase in hardness by not only solid solution strengthening for alloying of Pt with Ni or the like, but also work hardening for an increase in final rate of working, as in a Pt—Ni alloy described in Patent Document 1. In Patent Document 2 (Ag—Pd—Cu-based alloy) and Patent Document 3 (Pt—Cr—Ni-based alloy), a high-hardness precious metal alloy is obtained with not only solid solution strengthening and precipitation hardening with additive elements, but also work hardening where the rate of working is adjusted.

As described above, precious metal alloys are required to be enhanced in mechanical properties such as hardness in various applications. In order to respond to such requirements, it is also deemed that there is a need for further strengthening with various strengthening mechanisms described above. However, solid solution strengthening and precipitation strengthening, while are tried to achieve selection of an additive element and optimization of the amount of such an additive element, and also optimization of a production process such as a heat treatment, have limitations to the amount of hardening with these trials. For example, the amount of hardening by precipitation hardening of the precious metal alloys of Patent Documents 2 and 3 is about 150 Hv, and no sufficient hardness is achieved with only precipitation hardening. These precious metal alloys actually make up hardness with not only precipitation hardening, but also work hardening.

If work hardening is excessively applied, there is a problem to be concerned. Work hardening, while provides a large amount of hardening and is deemed to be a useful strengthening method, sometimes causes embrittlement of a material. Such material embrittlement can be a factor of disconnection during wire drawing, or of breakage or fracture in secondary working (pressing, coiling, bending, or the like) or in actual use. Electric materials such as probe pins and medical tools such as guide wires and embolization coils are produced by working of thin wires, and thus it is necessary to ensure workability in thin-wire working. Therefore, there are also naturally limitations to the amount of hardening by work hardening, in consideration of the problems of material embrittlement and workability.

The present invention has been made in view of the above circumstances, and provides a precious metal alloy increased in hardness by application of a strengthening mechanism different from a method which has been routinely used. In view of such an object, the present invention provides a material-strengthening method which does not depend on work hardening and which is based on a process involving mainly a heat treatment.

In order to solve the above problems, the present inventors have focused on spinodal decomposition as a strengthening method different from any strengthening mechanism noted above. Spinodal decomposition corresponds to one mode of phase separation in a material texture, and is a phenomenon where decomposition progresses due to a continuous increase in variation in concentration without a nucleation/growth process to be applied to precipitation hardening. A material texture generated by spinodal decomposition due to the variation in concentration exhibits a very fine periodic structure of several nanometers to several tens of nanometers, called modulated texture. A modulated texture generated by spinodal decomposition is periodically varied in concentration of a solute atom in a crystal, as a function of location, and is also periodically changed in lattice constant. Thus, a periodic internal stress field is generated on a sliding surface, and the field interacts with dislocation.

While such a strengthening mechanism by spinodal decomposition is deemed to be similar to precipitation strengthening due to nucleation/growth, both are different in that the change in lattice constant, imparted with not a precipitate but the modulation in concentration, contributes a hindrance to dislocation movement. Such strengthening by spinodal decomposition provides a high amount of hardening due to a fine modulated texture as described above, and thus is deemed to be useful as a procedure for enhancing hardness without causing any material embrittlement as in work hardening.

Herein, a phenomenon called spinodal decomposition and a fine texture formed with this phenomenon are known, and a strengthening mechanism with this phenomenon is also clarified to some extent. A Pt—Au alloy is then known as an alloy generating spinodal decomposition, among precious metal alloys.illustrates a Pt—Au system phase diagram. A Pt—Au alloy is also revealed from thermodynamic calculation, with respect to a region (chemical spinodal curve) indicating composition and temperature regions which allow for the occurrence of spinodal decomposition.

Even if a Pt—Au alloy is hardened by spinodal decomposition, the amount of hardening is at most about 160 Hv and the resulting hardness is at most about 500 Hv. While spinodal decomposition is known about its phenomenon and mechanism, there are a few application examples, in particular, application examples to precious metal alloys. The present inventors have determined that there is a room of improvement in strengthening by spinodal decomposition, as a method for hardening/strengthening a precious metal alloy, and have decided to make further considerations. As a result, the present inventors have considered that composition optimization within a binary alloy has limitations to maximum exhibition of the hardening ability of a precious metal alloy by spinodal decomposition and a ternary or higher alloy is to be applied. The present inventors have then made intensive studies, and as a result, have found that an increase in hardness by spinodal decomposition can be achieved by not only optimization of the composition of a quaternary alloy of Pt, Au, Ni, and Pd, as a configuration of a precious metal alloy, but also an appropriate heat treatment.

The present inventors have also found in the course of the foregoing studies that a Pt—Au—Ni—Pd quaternary alloy having the predetermined composition can express the ordering of constituent elements singly or together with spinodal decomposition. Such expression of the ordering results in generation of an ordered phase of a predetermined structure and accordingly the action of an increase in hardness. The present inventors have perceived that hardening with such an ordered phase can act on a Pt—Au—Ni—Pd alloy singly or compositely with hardening by spinodal decomposition.

If a Pt—Au—Ni—Pd alloy is a precious metal alloy hardenable by spinodal decomposition and ordering, there should be, of course, a composition range where such ordering is expressed. The present inventors have made further studies, namely, have searched the constitution range of an alloy to be increased in hardness by spinodal decomposition or the like, and as a result, have found that definition of such a range requires not only specification of the composition range of each metal element (Pt, Au, Ni, Pd), but also introduction of a parameter (hereinafter, this parameter is referred to as “compositional parameter”) associated with interaction among such composition ranges. The present inventors have made optimization of an alloy composition range and two compositional parameters which allow for the occurrence of an increase in hardness by spinodal decomposition and ordering, and thus have conceived the present invention.

In other words, the present invention is drawn to a precious metal alloy including 7.5% by atom or more and 72.5% by atom or less of Pt, 5.5% by atom or more and 62.5% by atom or less of Au, 3% by atom or more and 62.5% by atom or less of Ni, and 0.15% by atom or more and 38% by atom or less of Pd, wherein, when respective concentrations (% by atom) of Pt, Au, Ni, and Pd are designated as C, C, C, and C, a value of a first compositional parameter z1 represented by the following expression is 0.5 or more and 2.88 or less, and furthermore the concentration Cof Pd satisfies C≤z2 with respect to a second compositional parameter z2 represented by the following expression:

The present invention is also drawn to a precious metal alloy including 10% by atom or more and 67.5% by atom or less of Pt, 5.85% by atom or more and 40% by atom or less of Au, 10% by atom or more and 60% by atom or less of Ni, and 0.2% by atom or more and 34% by atom or less of Pd, and fulfilling requirements with respect to the first and second compositional parameters.

The present invention is further drawn to a precious metal alloy including 17.5% by atom or more and 60.5% by atom or less of Pt, 6.25% by atom or more and 30% by atom or less of Au, 15% by atom or more and 57.5% by atom or less of Ni, and 0.75% by atom or more and 24.5% by atom or less of Pd, and fulfilling requirements with respect to the first and second compositional parameters.

Each of the Pt—Au—Ni—Pd alloys having the above three composition ranges includes a modulated texture by spinodal decomposition, and/or an ordered phase.

The present application provides a method for producing the above precious metal alloy. In other words, a present inventive method for producing the precious metal alloy is a method for producing the precious metal alloy, including a step of providing a precious metal alloy including 7.5% by atom or more and 72.5% by atom or less of Pt, 5.5% by atom or more and 62.5% by atom or less of Au, 3% by atom or more and 62.5% by atom or less of Ni, and 0.15% by atom or more and 38% by atom or less of Pd, a solution treatment step of heating the precious metal alloy at a temperature of 850° C. or more and 1350° C. or less and then quenching the precious metal alloy, and an aging treatment step of heating the precious metal alloy after the solution treatment, at a temperature of 300° C. or more and 700° C. or less.

Another present inventive method for producing the precious metal alloy is a method for producing the precious metal alloy, including a step of providing a precious metal alloy including 7.5% by atom or more and 72.5% by atom or less of Pt, 5.5% by atom or more and 62.5% by atom or less of Au, 3% by atom or more and 62.5% by atom or less of Ni, and 0.15% by atom or more and 38% by atom or less of Pd, and a heat treatment step of heating the precious metal alloy at a temperature of 850° C. or more and 1350° C. or less and then cooling the precious metal alloy, wherein the cooling in the heat treatment step is a treatment involving quenching in a temperature region of not more than a melting point and 600° C. or more and cooling at a cooling rate of 2.5° C./s or less in a temperature region of less than 600° C.

As described above, the present invention is drawn to a precious metal alloy where material strengthening is made with a modulated texture by spinodal decomposition and an ordered phase, instead of solid solution strengthening, precipitation strengthening, or work hardening which has been widely used as a material-strengthening method. According to the present invention, a high-hardness precious metal alloy with unprecedented strengthening ability can be obtained without any work hardening (dislocation strengthening) which may cause material embrittlement.

Hereinafter, embodiments of the present invention are described. As described above, the precious metal alloy of the present invention is a Pt—Au—Ni—Pd quaternary alloy, and a hardening factor of the precious metal alloy includes at least any of a modulated texture by spinodal decomposition and an ordered phase by ordering. The following description also provides the description of each strengthening mechanism in the present invention, and the descriptions of constituent metals of the precious metal alloy of the present invention and their composition ranges and also two compositional parameters, as well as material texture features and the hardness of the precious metal alloy of the present invention. The method for producing the precious metal alloy of the present invention (heat treatment step) is also described.

As described above, a texture formed by spinodal decomposition is referred to as so-called modulated texture. The modulated texture is periodically varied in concentration, and circumferentially forms an internal stress field and thus contributes to an increase in hardness. The resistance force (critical shear stress) of dislocation motion in such a periodic internal stress field is expressed by the following expression, and the lattice strain (e), the elastic coefficient (Y), and the modulation amplitude in concentration (A) are considered to be control factors (examples of specific reference documents include Masaharu KATO, Introduction to the Theory of Dislocations (issued on August, 1999, publication: Shokado)).

It is considered in studies based on Expression 3 described above that, roughly, the elastic coefficient is proportional to the Young's modulus of each constituent metal and the lattice strain ε is proportional to the difference in lattice constant between the constituent metals. It is also considered that the modulation amplitude in concentration A in Expression 3 indicates a larger value as the mixing enthalpy between metal elements is larger. Values in Table 1 below are known with respect to the lattice constants of Pt, Au, Ni, and Pd constituting the Pt—Au—Ni—Pd alloy of the present invention. Values in Table 2 below are known with respect to values of mixing enthalpy (Reference Document: Akira Takeuchi, Akihisa Inoue, “Classification of Bulk Metallic Glasses by Atomic Size Difference, Heat of Mixing and Period of Constituent Elements and Its Application to Characterization of the Main Alloying Element”, Materials Transactions, vol 46 (2005), p 2817-2829).

With reference to Table 2, the mixing enthalpy is positive in combinations of Au—Pt, Au—Ni, and Pt—Pd. It is understood from binary phase diagrams of Au—Pt-based and Au—Ni-based alloys that these alloys are characterized by providing a single phase at a high temperature, but being very high in value itself of mixing enthalpy. Expression of spinodal decomposition and remarkable hardening with the modulated texture in the precious metal alloy of the present invention can be presumed in consideration of Expression 3 with reference to the lattice constant in Table 1 and the mixing enthalpy in Table 2, based on each binary phase diagram between the constituent metals. This point is described below in more detail.

An ordered phase generated by ordering contributes to an increase in hardness of the alloy due to each factor: (i) the Burgers vector of dislocation increases, (ii) an antiphase boundary can occur in the ordered phase, and (iii) the change in volume due to ordering results in distortion of the lattices in the interior of the ordered phase and the exterior of the ordered phase and acts on suppression of dislocation motion.

The precious metal alloy of the present invention includes both metals constituting a Pt—Ni-based alloy known as a combination of metals which generate ordering. The ordering in the Pt—Ni-based alloy is expressed by a solution treatment and an aging treatment, and is known to be achieved by an aging treatment in an order-disorder transformation region or air cooling or the like started from a single phase region to result in hardening. The ordering in the Pt—Ni-based alloy can provide hardening with an ordered phase of an L1-type structure or an L1-type structure.

The precious metal alloy of the present invention can achieve increased hardness with an ordered phase generated by ordering as in the above Pt—Ni-based alloy. Although the configuration of the ordered phase in the present invention is not necessarily completely clear, the configuration is considered to correspond to a phase having a crystal structure which is the same as or similar to the ordered phase generated in the above Pt—Ni-based alloy. In other words, such a phase is a phase including at least Pt and Ni and having a fcc structure and/or a fct structure. The ordered phase in the present invention is presumed to be preferably a phase of an L1-type structure or an L1-type structure, or a crystal structure similar thereto.

The precious metal alloy of the present invention is a Pt—Au—Ni—Pd quaternary alloy. With respect to a binary phase diagram constituted with metal elements among Au, Ni, Pd, and Pt, Au—Ni-based, Au—Pt-based, and Pt—Pd-based alloys are two-phase separation type alloys. It can be seen from Table 2 above that there are many combinations of elements, in which the mixing enthalpy between metal elements among Au, Ni, Pd, and Pt is positive. It is thus considered that the Pt—Au—Ni—Pd-based alloy is highly liable to have a high mixing enthalpy and exhibit phase separation in a low temperature region. Therefore, it is considered that the Pt—Au—Ni—Pd quaternary alloy highly possibly expresses spinodal decomposition and the concentration amplitude (A) due to such expression is also large. Pt and Ni are relatively high in Young's modulus and thus are considered to be also high in elastic coefficient (Y). Furthermore, Ni is large in difference in lattice constant from those of Au, Pt, and Pd and thus is considered to be also high in lattice strain (s). In consideration of these and Expression 3, the constituent metals of the precious metal alloy of the present invention are considered to correspond to a suitable combination for achieving expression of spinodal decomposition and an increase in hardness due to spinodal decomposition. Hereinafter, the action of each metal constituting the present invention is described.

Pt is an essential element for spinodal decomposition in the alloy system of the present invention. Spinodal decomposition is not expressed at a too high or too low Pt concentration, and the concentration range of Pt, necessary for expression, is present. Pt can be taken with Ni to form an ordered phase, contributing to an increase in hardness. The Young's modulus of Pt is as relatively high as 169.9 GPa. As can be seen from Expression 3 described above, Pt can be expected as a metal which allows for an increase in amount of hardening of the alloy in expression of spinodal decomposition.

Au is also an essential element for expression of spinodal decomposition in the alloy system of the present invention. Spinodal decomposition is not expressed at a too high or too low Au concentration, and the concentration range of Au, necessary for expression, is present. If the Au concentration is out of an optimal range, usual nucleation/growth easily occurs and no suitable increase in hardness can be obtained.

Ni acts as a strengthening factor in expression of spinodal decomposition, in the precious metal alloy of the present invention. Ni is higher in Young's modulus than Au, Pt, and Pd. As can be seen with respect to Table 1 above, Ni is large in difference in lattice constant from each metal of Au, Pt, and Pd, and increases the lattice strain F. Accordingly, Ni acts to increase strengthening ability due to spinodal decomposition, as can be seen from Expression 3. Furthermore, Ni and Pt are metals forming an ordered phase, and also act to contribute to an increase in hardness by ordering.

Ni is a congener with and is similar in electron structure to Pt and Pd, and thus can constitute an alloy without any losses in corrosion resistance and oxidation resistance of a precious metal as much as possible. Thus, Ni also has a secondary effect of reducing the price of the entire precious metal alloy.

Pd acts to not only extend the solid solubility limit of each element constituting the precious metal alloy of the present invention and expand the concentration region which allows for expression of spinodal decomposition in the precious metal alloy of the present invention, but also promote spinodal decomposition, and has an effect of enhancing the amount of hardening due to spinodal decomposition. Herein, if Pd is excessively added, the spinodal decomposition temperature is excessively reduced and thus spinodal decomposition is liable to be inhibited on the contrary. Furthermore, excess addition of Pd is also liable to suppress ordering, leading to a reduction in amount of hardening of the alloy system as a whole. Accordingly, Pd also has an optimal concentration range as described above in order to optimize the amount of hardening of the precious metal alloy. As described below, the Pd concentration is controlled by a compositional parameter associated with the Au concentration.

The composition range of each metal element of Pt, Au, Ni, and Pd in the precious metal alloy of the present invention is defined so that the above-described actions are exerted. The composition range is as follows: Pt: 7.5% by atom or more and 72.5% by atom or less, Au: 5.5% by atom or more and 62.5% by atom or less, Ni: 3% by atom or more and 62.5% by atom or less, and Pd: 0.15% by atom or more and 38% by atom or less. This corresponds to a concentration range defined for expression of spinodal decomposition and ordering effective for an increase in hardness of the precious metal alloy. Hereinafter, the composition range is sometimes referred to as “composition range A1”.

While the detail of the method for producing the precious metal alloy of the present invention is described below, spinodal decomposition is expressed in a solution treatment for quenching a solid solution alloy having the composition and an aging treatment step, and high hardness can be obtained. The precious metal alloy of the present invention, while exhibits a region allowing for a whole solid solution widely extending in a high temperature region, has a miscibility gap in a low temperature region. Therefore, it is considered that the precious metal alloy of the present invention, having the above composition range, can be subjected to a solution treatment in a high temperature region and then quenched to form a supersaturated solid solution and be subjected to a subsequent aging treatment to generate spinodal decomposition. It is here also effective to utilize a CALPHAD method (Calculation of Phase Diagrams method) with respect to thermodynamic behaviors (transformation point, phase equilibrium, solid solubility limit, melting point, and the like) of the precious metal alloy of the present invention. Calculation by a CALPHAD method is preferably made by use of commercially available thermodynamic calculation software (for example, Thermo-Calc (ITOCHU Techno-Solutions Corporation)) and precious metal alloy database (for example, TCNOB1 (ITOCHU Techno-Solutions Corporation)).

It is necessary for the precious metal alloy of the present invention to not only have the above composition range of each constituent metal element, but also satisfy z1 and z2 as first and second compositional parameters associated with correlation of the concentration of each metal element. Such two compositional parameters are defined as follows, when the respective concentrations (% by atom) of Pt, Au, and Ni in the precious metal alloy are designated as C, C, C, and C.

The first compositional parameter z1 is defined by the following expression with the concentrations (C, C, C) of Pt, Au, and Ni.

In the present invention, the value of the first parameter z1 is needed to be 0.5 or more and 2.88 or less. If the value of z1 is less than 0.5, spinodal strengthening ability is low and no sufficient hardness is obtained even by the aging treatment. On the other hand, if the value of z1 is more than 2.88, the solid solubility limit of each element is low and two-phase separation is much more liable to occur. Therefore, no sufficient supersaturated solid solution is obtained even by the solution treatment and the hardness after the aging treatment is insufficient. The value of the first compositional parameter z1 is more preferably 1.0 or more and 2.7 or less, further preferably 1.1 or more and 2.6 or less.

The second compositional parameter z2 is defined by the following expression with the concentration (C) of Au in the precious metal alloy.

The second compositional parameter z2 defines the upper limit of the Pd concentration in the precious metal alloy (the Pd concentration defined with the compositional parameter z2 is sometimes referred to as “critical Pd concentration”). The Pd concentration in the precious metal alloy of the present invention is needed to be z2 or less. If the Pd concentration is more than z2, spinodal decomposition and/or ordering of the precious metal alloy are/is suppressed and hardening as a whole is insufficient. The second compositional parameter z2 is a compositional parameter thus provided.

The second compositional parameter z2 is defined with coefficients a, b, c, and d in the above expression. The coefficients a, b, c, and d are as follows: a=0.00077, b=−0.102, c=3.607, and d=1.722. The parameter z2 determined from these coefficients is preferably a value calculated with a=0.00143, b=−0.155, c=4.739, and d=−10.201. A value calculated with a=0.00310, b=−0.255, c=6.047, and d=−18.974 is more preferably applied to the compositional parameter z2.

The composition of the precious metal alloy of the present invention is needed to satisfy the above composition range of each metal element and fulfill both requirements defined based on the two compositional parameters z1 and z2.

The present invention is here drawn to a precious metal alloy including Pt, Au, Ni, and Pd in the above ranges, preferably a precious metal alloy including Pt, Au, Ni, and Pd in the above ranges and an inevitable component. The inevitable component is an unavoidable component included in impurities in a raw material or included due to a production step or the like. Specific examples of the inevitable component include Ag, Rh, Ir, Ru, Al, Mg, Ca, Fe, Mn, Sc, Y, Zr, Zn, Re, Mo, Cr, Nb, Ta, V, Hf, Ti, W, Co, Si, Sn, Cu, Th, B, C, N, S, P, O, H, and rare-earth elements. Such inevitable impurities are incorporated from a raw material, and an apparatus and the like in melting and casting. The content of such inevitable impurities is preferably within a range not inhibiting characteristics of the precious metal alloy of the present invention, the content per element is preferably 0.1% by atom or less, and the total is preferably 0.5% by atom or less, particularly preferably 0.1% by atom or less. Herein, when the inevitable component is included in the precious metal alloy, it is difficult to clearly distinguish whether the inevitable component is a component inevitably included or a component willingly added. In the present invention, as long as the component does not modify characteristics of the precious metal alloy, the component is considered to be an inevitable component without any distinction of the object of such incorporation.