Au-Ni Alloys Research Articles

Formation of icosahedral quasicrystalline phase in Zr70Ni10M20(M=Pd, Au, Pt) ternary metallic glasses

An icosahedral quasicrystalline phase was confirmed as a primary precipitation phase in melt-spun Zr70Ni10M20 (M=Pd, Au and Pt) ternary metallic glasses with two-stage crystallization process. The onset temperature of the transformation from amorphous to icosahedral phase are 687 K for Pd, 754 K for Au and 783 K for Pt at a heating rate of 0.67 K s−1. The size of the icosahedral particles is in the range of 5–20 nm. The second crystallization reaction results in the formation of Zr2Ni+Zr2Pd and Zr2Ni+ZrPt phases in the Zr–Ni–Pd and Zr–Ni–Pt alloys, respectively, and Zr3Au phase in the Zr–Ni–Au alloy through a single exothermic reaction. The formation of nanoscale icosahedral phase indicates the possibility that icosahedral short-range order exists in the glassy state.

Mathematical model of interdiffusion in multi-component open systems showing concentration dependent intrinsic diffusivities

The generalized Darken model of interdiffusion is presented. This model is valid for an open and closed system and when intrinsic diffusivities vary with composition. The method of calculation of the intrinsic diffusivities is presented. The practical applications of the model are presented. The intrinsic diffusion coefficients in ternary Cr-Fe-Ni system are determined. Then the evolution of the Cr-Ni steels because of its oxidation is presented, i.e., the interdiffusion in open system due to reaction at the boundary. The computer simulation of interdiffusion process in binary Au-Ni alloy system in which diffusivities markedly vary with composition is shown.

Surface segregation in ternary alloys

Surface segregation profiles of binary (Cu–Ni, Au–Ni, Cu–Au) and ternary (Cu–Au–Ni) alloys are determined via Monte Carlo–Metropolis computer simulations using the BFS method for alloys for the calculation of the energetics. The behavior of Cu or Au in Ni is contrasted with their behavior when both are present. The interaction between Cu and Au and its effect on the segregation profiles for Cu–Au–Ni alloys is discussed.

Microstructural investigation of oxidized Ni/Au ohmic contact to p-type GaN

The microstructure of oxidized Ni/Au films on p-GaN was examined to elucidate the formation of a low resistance ohmic contact to p-GaN with a field-emission gun transmission electron microscope in conjunction with composition analyses. The p-GaN/Ni/Au samples were heat treated at 500 °C in air mainly composed of a mixture of crystalline NiO, Au, and amorphous Ni–Ga–O phases. Small voids adjacent to the p-GaN film were also observed. The as-deposited Au film converted into discontinuous islands containing small amounts of Ni that connect with p-GaN. NiO formed a continuous film at the surface that covers the Au islands and the amorphous Ni–Ga–O phases. Moreover, NiO partially contacts p-GaN as well as Au islands and the amorphous Ni–Ga–O phase. The orientation relationship of the crystalline NiO, Au-rich islands, and p-GaN film was identified as NiO(111)//Au(111̄)//GaN(0002) and NiO[11̄0]//Au[11̄0]//GaN[112̄0]. The results suggested that Ni atoms diffuse through the Au layer onto the surface and react with oxygen to form NiO, whereas Au atoms diffuse towards the inside to form a Au–Ni alloy. The microstructural examination indicated that the crystalline NiO and/or the amorphous Ni–Ga–O phases may significantly affect the low resistance ohmic contact to p-GaN.

Influence of epitaxial stress on spinodal decomposition in binary alloys with large size effects: application to the Au–Ni system

Within the framework of Cahn’s pioneering theory of spinodal decomposition, the effect of the epitaxial stress on the stability of a binary alloy solid solution against infinitesimal compositional fluctuations is revisited. It is shown that the elastic energy term due to a coherent lattice mismatch with a substrate modifies the linearised diffusion equation. This term is particularly high when size effects between the two constituents are strong. It is then necessary to take into account anharmonic terms in the expression of the elastic energy. From the modified diffusion equation, a new spinodal curve, called the epitaxial coherent spinodal, may be calculated for different substrate lattice parameters. The results of the model are compared with recent experiments on coherently grown (001) Au–Ni alloys. A modulated structure has been evidenced upon annealing which is in good agreement with our calculations.

The Influence of the Rhodium Content in Platinum Catalysts and of the Gold Content in Nickel Catalysts on the Course of the CO + NO Reaction

With ever-increasing demands linked with decreasing sulphur levels in gasoline fuels, base metal catalysts have become attractive as alternatives. The Ni-Au system is an interesting alloy for use in the reaction between CO and NO, partly because of the large difference between the heats of chemisorption of NO and CO on the two metals. This paper presents a study of the course of NO reduction by CO which additionally made it possible to trace changes in the activity and selectivity of the process for alloys of varied Ni-Au composition and alloys of varied Pt-Rh composition together with the effects of Pt and Rh dispersion and the concentration of B5 sites on the surface of the Pt and Rh catalysts.

Anisotropic modulated structures upon annealing of epitaxial AuNi ultrathin films on Au(001)

This letter presents experimental evidence of an anisotropic compositional modulation upon annealing in (Ni/Au)n multilayers and Au–Ni alloy ultrathin films embedded in Au layers. These materials were grown by molecular beam epitaxy, on a (001)Au buffer layer. The influence of temperature, local composition, and strain has been investigated by high-resolution electron microscopy and x-ray diffraction.

Cu-Au, Ag-Au, Cu-Ag, and Ni-Au intermetallics: First-principles study of temperature-composition phase diagrams and structures

The classic metallurgical systems -- noble metal alloys -- that have formed the benchmark for various alloy theories, are revisited. First-principles fully relaxed general potential LAPW total energies of a few ordered structures are used as input to a mixed-space cluster expansion calculation to study the phase stability, thermodynamic properties and bond lengths in Cu-Au, Ag-Au, Cu-Ag and Ni-Au alloys. (i) Our theoretical calculations correctly reproduce the tendencies of Ag-Au and Cu-Au to form compounds and Ni-Au and Cu-Ag to phase separate at T=0 K. (ii) Of all possible structures, Cu/sub 3/Au (L1/sub 2/) and CuAu (L1/sub 0/) are found to be the most stable low-temperature phases of Cu/sub 1-x/Au/sub x/ with transition temperatures of 530 K and 660 K, respectively, compared to the experimental values 663 K and 670 K. The significant improvement over previous first-principles studies is attributed to the more accurate treatment of atomic relaxations in the present work. (iii) LAPW formation enthalpies demonstrate that L1/sub 2/, the commonly assumed stable phase of CuAu/sub 3/, is not the ground state for Au-rich alloys, but rather that ordered <100> superlattices are stabilized. (iv) We extract the non-configurational (e.g., vibrational) entropies of formation and obtain large values for the size mismatched systems: 0.48 k/sub B//atom in Ni/sub 0.5/Au/sub 0.5/ (T=1100 K), 0.37 k/sub B//atom in Cu/sub 0.14/Ag/sub 0.86/ (T=1052 K), and 0.16 k/sub B//atom in Cu/sub 0.5/Au/sub 0.5/ (T=800 K). (v) Using 8 atom/cell special quasirandom structures we study the bond lengths in disordered Cu-Au and Ni-Au alloys and obtain good qualitative agreement with recent EXAFS measurements.

Effects of anharmonic strain on the phase stability of epitaxial films and superlattices: Applications to noble metals

Epitaxial strain energies of epitaxial films and bulk superlattices are studied via first-principles total energy calculations using the local-density approximation. Anharmonic effects due to large lattice mismatch, beyond the reach of the harmonic elasticity theory, are found to be very important in Cu/Au (lattice mismatch 12%), Cu/Ag (12%) and Ni/Au (15%). We find that <001> is the elastically soft direction for biaxial expansion of Cu and Ni, but it is <201> for large biaxial compression of Cu, Ag, and Au. The stability of superlattices is discussed in terms of the coherency strain and interfacial energies. We find that in phase-separating systems such as Cu-Ag the superlattice formation energies decrease with superlattice period, and the interfacial energy is positive. Superlattices are formed easiest on (001) and hardest on (111) substrates. For ordering systems, such as Cu-Au and Ag-Au, the formation energy of superlattices increases with period, and interfacial energies are negative. These superlattices are formed easiest on (001) or (110) and hardest on (111) substrates. For Ni-Au we find a hybrid behavior: superlattices along <111> and <001> like in phase-separating systems, while for <110> they behave like in ordering systems. Finally, recent experimental results on epitaxial stabilization of disordered Ni-Au and Cu-Ag alloys, immiscible in the bulk form, are explained in terms of destabilization of the phase separated state due to lattice mismatch between the substrate and constituents.

Deformation Behavior of FCC Crystalline Metallic Nanowires Under High Strain Rates

AbstractWe used molecular dynamics (MD) methods to study the deformation behavior of metallic alloy crystal nanowires of pure Cu, NiCu alloy and NiAu alloy, under high rates of uniaxial tensile strain, ranging from 5* 108/s to 5* 10/s. These nanowires are just about 2 nm thick and hence cannot sustain dislocations, instead we find that deformation proceeds through twinning and coherent slipping mechanisms. NiAu has a 13% size mismatch whereas NiCu only 2.5%. As a result the critical strain rate at which the “nanowire crystals” flow like a “liquid” is 100 times smaller for NiAu. We also calculated the elastic constants at each strain state for all strain rates to identify the relation between mechanical “shear” instability and deformation process.

Ni-Au: A testing ground for theories of phase stability

The theory of phase stability in the Ni-Au alloy system is a popular topic due to the large size mismatch between Ni and Au, which makes the effects of atomic relaxation critical and also to the fact that Ni-Au exhibits a phase separation tendency at low temperatures, but measurements at high-temperature show an ordering-type short-range order. We have clarified the wide disparity which exists in the previously calculated values of mixing energies and thermodynamic properties by computing ‘state-of-the-art’ energetics (full-potential, fully-relaxed LDA total energies) combined with ‘state-of-the-art’ statistics ( k-space cluster expansion with Monte Carlo simulations) for the Ni-Au system. We find: (i) LDA provides accurate mixing energies of disordered Ni 1− x Au x alloys ( H mix ≤ + 100 meV/ atom) provided that both atomic relaxation (a ~ 100 meV/atom effect) and short-range order (~ 25 meV/atom) are taken into account properly. (ii) Previous studies using empirical potentials or approximated LDA methods often underestimate the formation energy of ordered compounds and hence also underestimate the mixing energy of random alloys, (iii) Measured values of the total entropy of mixing combined with calculated values of the configurational entropy demonstrate that the non-configurational entropy in Ni-Au is large and leads to a significant reduction in miscibility gap temperature, (iv) The calculated short-range order agrees well with measurements and both predict ordering in the disordered phase, (v) Consequently, using inverse Monte Carlo to extract interaction energies from the measured/calculated short-range order in Ni-Au would result in interactions which would produce ordering-type mixing energies, in contradiction with both experimental measurements and precise LDA calculations.

Diffusion and phase diagram in binary alloys

The relationship betwwen diffusion and phase diagram is discussed in more detail for AgAu and AuNi alloys. For each alloy tracer diffusion and interdiffusion data have been compared with the corresponding phase diagram. Tracer diffusion is related to the solid-liquid phase transition. Ag and Au tracer diffusivities D∗(x) in AgAu alloys and melting temperature T m( x) of AgAu show opposite curvatures. The same holds for D∗ of Au and Ni in AuNi alloys and indicates a vacancy diffusion mechanism. Interdiffusion depends on the critical temperature of the solid-solid phase transition of the phase diagram. The curvature of interdiffusivity D( x) is determined by the sign of T c( x) or the interaction energy ε of the regular alloy. The calculations agree well with the experimental data.

Effect of Stress on Coherently Grown Au-Ni (001) Solid Solutions

AbstractThe effect of epitaxial stress on the stability of Au-Ni solid solutions grown by MBE on Au(001) is investigated. Experimental studies (TEM, GIXD) have shown that the elastic energy of the AuNi alloys is relaxed via two competing ways: twinning and an ordering process in which pseudo-periodic antiphase structures based on the L10 unit cell are found. The stabilisation of a metastable ordered L10 phase in the stressed Au-Ni system is explained using thermodynamic calculations based on the minimisation of free energy including elastic contributions.

Mechanical behavior and microstructure of Au-Ni brazes

Thin Au-Ni alloy (57.5 at. pct Au and 42.5 at. pct Ni) interlayer brazes between strong base metals were found to have very high ultimate tensile strengths (UTSs) as a result of the triaxial stress state within the braze (reducing the effective, or von Mises, stress) and the high strength of the braze alloy. This high strength can be rationalized by the observed very refined two-phase microstructure. The yield strength and quasi-steady-state flow stress of the braze alloy was found to be relatively strain-rate insensitive. These results explain the absence of time-dependent, or delayed, tensile failures that have been observed in other braze interlayers. The fracture process appears to be one of cavity nucleation by microplasticity, followed by cavity coalescence or impingement once the required nuclei density is achieved. Substantial cavity growth does not appear to occur. This failure process contrasts with that of other interlayer metals, in which the failure mechanism appears to be unstable cavity growth.

Characteristics of microstructures of NiAu and PdAu alloy films prepared by an evaporation method

The influence of film preparation process on phase separation or interdiffusion has been studied for NiAu alloy films and Au/Pd bilayer films. Microstructures of the films were observed by a transmission electron microscope. The microstructures of the NiAu alloy films which were annealed after removal from substrates were consistent with those of bulk specimens, while the NiAu alloy film which was epitaxially grown on a substrate exhibited a different microstructure from that expected from the equilibrium state. Moiré patterns were always seen for all the as-deposited Au/pd epitaxial bilayer films, and they disappeared gradually with annealing, showing that Au/Pd bilayer films were transformed to be a homogeneous single phase. The analysis of the changes in moiré fringe spacing during annealing showed that the diffusion took place more rapidly in the Au(001)/Pd(001) films than in the Au(111)/Pd(111) films. This is not explained by a concept that diffusion takes place equivalently in any orientations. It is shown that the characteristics of phase separation or interdiffusion in alloy films prepared by evaporation are not always the same as those of bulk alloys.

Initial growth of Au on Ni(110): Surface alloying of immiscible metals.

Atomic resolution scanning tunneling microscopy of Au deposited on Ni (110) shows that even though Au is completely insoluble in bulk Ni, it replaces Ni in the first surface layer forming a surface Au-Ni alloy and the squeezed out Ni atoms agglomerate in Ni islands on the surface. This picture is supported by total energy calculations within the effective-medium theory, which also provide a detailed understanding of this surprising phenomenon.

Quantitative Auger electron spectroscopy analysis of the Au-Ni system

Quantitative AES analysis was carried out for the Au-Ni system. The aim is to discuss whether we can use different-energy Auger transitions to determine the changes of composition in depth and how we can optimize the results. Using nine in-situ-scraped standard samples with compositions from 10 to 90 at% Au, eleven calibration curves, corresponding to eleven combinations of Au and Ni Auger transitions with energies from 43 to 2111 eV, were obtained. Particularly discussed was the method by which reliable calibration curves were obtained. This was especially true for the Auger transitions of Au(95 eV) and Ni(102 eV): one of which may have a very weak intensity at corresponding extremes of composition. These calibration curves were then used to detect surface composition changes in five alloys (Au90Ni10, Au70Ni30, Au50Ni50, Au30Ni70 and Au10Ni90) after bombarding them with 1 keV Ar ions. The results suggest that a bombardment-induced Gibbsian segregation (BIGS) existed in the bombarded Au-Ni alloys and the BIGS ratio is strongly dependent on the bulk composition. This article demonstrates how to resolve complicated near-surface profiles.

Search for positron localization near transition-metal solutes of negative effective charge in Ni and Cu

Results of an early (1973) angular correlation (ACAR) study of dilute (0.5% at.%) Cu based alloys by a Japanese group were interpreted in terms of an attraction of e + by transition metal solutes of effective negative charge. Doppler Broadening (DB) measurements reveal no such an effect for Cu(Mn) and Cu(Ni) solid solutions as well as for Ni alloys with 3d, 4d and 5d transition metal solutes (0.1 to 1.5 at.%) i.e. no evidence of e + localization near these impurities is seen. Our results strongly suggest that the ACAR results are due to the metallurgical state of the samples. In contrast, significant DB lineshape parameter variations, observed for our Ni(Zr) alloys, are attributed to positron trapping in and near Ni 5 Zr precipitates. Our DB results for a series of Ni(Au) alloys are understood in terms of a combination of the effect of an overall lattice expansion and a positron preference for clusters of Au atoms. The above comparison between DB and ACAR results is supported by our «spin polarized» DB results for a (001) Ni single crystal which resemble those obtained by other groups using a «spin polarized» 2D-ACAR technique

Surface composition changes caused by Ar ion bombardment of Au-Ni alloys

Surface composition changes caused by 1 keV Ar ion bombardment of a series of Au-Ni alloys at room temperature were studied. Using 11 combinations of different energy Auger spectra, the variation of the composition with depth from ∼ 0.4 to 1.5 nm in a bombarded Au-Ni alloy was obtained. We found that a drastic variation in concentration took place within the first few layers. This is evidently indicating that a bombardment-induced Gibbsian segregation exists in the Au-Ni system. We also found that the degree of gold depletion at the second layer depended strongly on the bulk composition, thus this depletion became slight as the gold content in alloy was high. In addition, we have roughly estimated the concentration of Au at the topmost layer of a bombarded Au-Ni alloy and found that this value tentatively might be smaller than the bulk one. This result differs from two known examples: C(Au, top) > C(Au, bulk) in a bombarded Au-Cu alloy and C(Cu, top) = C(Cu, bulk) in a bombarded Cu-Ni alloy. Comparing our experimental results with a recent theory presented by Kelly, we suggest that a more sophisticated theory should take this new phenomenon into account.

Ni-Au合金薄膜の相分離における微細組織の特徴

Ni-Au合金薄膜の相分離における微細組織の特徴