Au Ni Solid Solutions Research Articles

Microstructural evolution and mechanical characterization of Nb-interlayer-inserted Ti–6Al–4V/Si3N4 joints brazed with AuNiTi filler

The interfacial microstructure evolution and mechanical behavior of Ti–6Al–4V/Si3N4 joints with insertion of a 2 mm-thick Nb interlayer were studied by brazing with Au96.5Ni3Ti0.5 (mass%) alloy as a heat- and corrosion-resistant filler. When the joints were brazed at 1323 K with holding times of 0, 5, and 10 min, multilayered Au4Ti, Au2Ti, AuTi, AuTi3, and Au2Nb3 intermetallic compounds were formed at the Ti–6Al–4V/Nb interface, whereas two distinct regions consisting of Au2Nb3 layer and Au–Ni solid solution were predominantly developed at the Nb/Si3N4 interface. Joints brazed with a 0 min holding time fractured, with initiation at the reaction-layer/Si3N4 interface and propagation into the Si3N4 with a concave morphology because of large residual stress; the joints exhibited a maximum average room-temperature bending strength of 53 MPa.

Local melting of Au/Ni thin films irradiated by femtosecond laser through GaN

We found appropriate femtosecond laser irradiation conditions to locally melt Au/Ni double-layered thin films deposited on a GaN layer. The laser irradiation of the Au/Ni films was carried out through GaN and transmission electron microscopy revealed that GaN acted as a tough window to introduce laser pulses without suffering laser-induced damages. The Au/Ni double-layered thin films melted along the laser-irradiation lines and re-solidified into coarse particles containing an Au–Ni solid solution phase. On the other hand, in the area between laser-modified regions, the Au/Ni thin films retained the same morphology as the as-deposited state. The transition from the melted region to the unaffected area occurred in a narrow intermediate region. The present results show a possibility of a novel technique to site-specifically melt thin films to form a solid solution phase by femtosecond laser irradiation.

Facet-controlled phase separation in supersaturated Au-Ni nanoparticles upon shape equilibration

Solid-state dewetting is used to fabricate supersaturated, submicron-sized Au-Ni solid solution particles out of thin Au/Ni bilayers by means of a rapid thermal annealing technique. Phase separation in such particles is studied with respect to their equilibrium crystal (or Wulff) shape by subsequent annealing at elevated temperature. It is found that {100} faceting planes of the equilibrated particles are enriched with Ni and {111} faces with Au. Both phases are considered by quantum-mechanical calculations in combination with an error-reduction scheme that was developed to compensate for a missing exchange-correlation potential that would reliably describe both Au and Ni. The observed phase configuration is then related to the minimization of strongly anisotropic elastic energies of Au- and Ni-rich phases and results in a rather unique nanoparticle composite state that is characterized by nearly uniform value of elastic response to epitaxial strains all over the faceted surface. The same conclusion is yielded also by evaluating bi-axial elastic moduli when employing interpolated experimental elastic constants. This work demonstrates a useful route for studying features of physical metallurgy at the mesoscale.

Solid-state dewetting of Au/Ni bilayers: The effect of alloying on morphology evolution

The solid-state dewetting of thin Au/Ni bilayers deposited onto SiO2/Si substrates is investigated. A rapid thermal treatment is used to induce the dewetting process by an increase in temperature. The evolution of the (111) peaks of X-ray diffraction reveals a characteristic change due to mixing of Au and Ni. At low temperature, the Au-Ni thin film is found to break up at the phase boundaries and growing voids are shown to be surrounded by a Ni-rich phase. Branch-like void growth is observed. Upon annealing at increasing temperatures, Au-Ni solid solutions are formed well above the bulk equilibrium solubility of Au and Ni. It is found that this metastable phase formation makes the Au-Ni thin film less vulnerable to rupturing. Moreover, growth mode of still evolving voids changes into a more regular, faceted one due to alloying. Finally, it is shown that annealing above the miscibility gap forms supersaturated, well-oriented Au-Ni solid solution agglomerates via dewetting.

Dewetting of Au/Ni bilayer films on prepatterned substrates and the formation of arrays of supersaturated Au-Ni nanoparticles

Au/Ni bilayer thin films are studied with respect to their dewetting behavior on prepatterned SiO2/Si substrates (inverted pyramidal pits). Dewetting of Au-Ni solid-solutions is controlled by a rapid thermal treatment at temperatures above the miscibility gap followed by a fast cooling procedure. Upon annealing, it is observed that some Au-rich film material separates on the mesa in case of still coalescent (i.e., incomplete dewetting) nanoparticles. Moreover, arrays of supersaturated Au-Ni solid-solution nanoparticles are formed out of the bilayer film after templated dewetting. Faceting of the nanoparticles indicates textural effects. In order to prove that these findings are caused by the prepatterned substrate, a comparison with the bilayer dewetting on a flat SiO2/Si substrate is provided.

Effect of current density on composition and structure of electrodeposited Au–Ni alloy coatings

The effect of current density on composition and structure of galvanic Au–Ni alloy coatings electrodeposited from weakly acidic additive free electrolyte was investigated. At low current density (up to ∼10 mA cm−2), light yellow coloured alloys enriched with Au are deposited. At higher current density the Ni content is abruptly increased and the coatings become coarse and dark. Deposits with an average content of Ni up to ∼50 at-% were obtained. Only a small amount of Ni (up to 20 at-%) takes part in the formation of non-equilibrium super saturated Au–Ni solid solution. The remainder of the Ni is deposited in the interglobular space as an amorphous Ni oxide–hydroxide or as Ni bearing hydroxide salts. Besides Ni, the alloy coatings contain C, N, O, K and H.

Formation of supersaturated Au–Ni nanoparticles via dewetting of an Au/Ni bilayer

Supersaturated Au–Ni solid-solution nanoparticles are formed via dewetting of a nanometer Au/Ni thin bilayer using rapid thermal treatment. Annealing at temperatures well below the melting temperatures but above the miscibility gap induces the dewetting and involves the formation of Au–Ni solid-solutions. When cooling down to room temperature, particles formed are usually expected to decompose. When this is kinetically hindered, the supersaturated solution remains in dewetted state. These supersaturated particles are found to lose the ferromagnetic properties of Ni.

The intermetallic compound Ni3Au and gold-nickel solid solutions in metalliferous sediments of the triassic chert formation in Sikhote-Alin in the Far East

The intermetallic compound Ni3Au and Au-Ni solid solutions (native nickel Au and Au-bearing native Ni) were found in the contact metamorphosed metalliferous sediments of the Triassic chert formation in Sikhote-Alin. The metalliferous rocks are characterized by high contents of Au, Ag, and PGE, as well as the presence of diverse minerals of precious metals. Nickel gold (Au0.91-0.88 Ni0.09-0.12) is found as grains and crystals (3 to 4 im in size) in Au-bearing cherts with hematite, which are conventionally defined as “itabir-ites” and in the altered siliceous rocks of the Dal’nerechensky district (the upper reaches of the Gornya River). The nickel gold is associated with copper Au, pure native Au (Au1.00), and Au-Ag and Au-Ag-Pb solid solutions (Au0.86/0.84Ag0.14_0.16 and Au0.78Ag0.19Pb0.03, respectively). The Au-bearing Ni is found in the metamorphosed carbonaceous mudstones and Au-bearing “itabirites” of the Shirokopadninsky area (Olgin-sky district). The Au content varies from 6.09 wt % (Ni0.98Au0.02) to 11.30 wt % (Ni0.96Au0.04) in some Au-bearing Ni grains (about 10—15 im in diameter) taken from the metamorphosed mudstones. The grains of Au-bearing Ni (Co0.001_0.00Au00.2-0.17 Ni 0.98_0.83) in the “itabirites” are also characterized by their heterogeneous composition and the fine impregnation of the Ni3 Au intermetallic compound (Ni2.99Au1.01 based on the microprobe analysis).

Brazed joints evaluation and computer modelling of mass transport in multi-component systems in the Au–Ni solder-14-5 PH joints

Vacuum brazing tests were carried out on the age-hardened steel of the 14-5 PH type using the 82Au–18Ni solder. It has been found that structure of the joint consisted of two kinds of Au–Ni solid solutions: one is rich in gold and the other rich in nickel. The influence of roughness of the surfaces to be joined and the distance between them as well as duration of the brazing on the joint structure and properties have been determined. The maximum tensile strength ( R m = 1260 MN/m 2) was obtained for the joints of polished surfaces without any distances elements, brazed in temperature 1273 K for 7.2 ks and aged in temperature 723 K for 7.2 ks. In the further part of this paper the interdiffusion in the 14-5 PH steel/Au–18Ni solder system is analyzed. A special attention is put on the Ni–Au system which is a base of the brazed joints. The objective of this study is to calculate the intrinsic diffusivities of Au and Ni which have been used for modelling of mass transport during vacuum brazing. The generalized darken method (GDM) of interdiffusion and the inverse method (IM) have been used for the calculations of the effective intrinsic diffusivities of Au, Ni and other elements. These diffusivities have subsequently been employed to compute the concentration profiles in the 14-5 PH steel/Au–18Ni solder system and been compared with the experimental results.

How fast are the ultra-fast nano-scale solid–liquid phase transitions induced by energetic particles in solids?

We study the thermodynamic forces acting on the evolution of the nanoscale regions excited by collisions of energetic particles into solid targets. We analyze the role of diffusion, thermo-migration, and the liquidus–solidus two-phase field crossing, as the system cools down from the collision-induced melt under different conditions of energy deposition. To determine the relevance of these thermodynamic forces, solute redistribution is evaluated using molecular dynamics simulations of equilibrium Au–Ni solid solutions. At low collision energies, our results show that the quenching of spherical cascades is too fast to allow for solute redistribution according to equilibrium solidification as determined from the equilibrium phase diagram (zone refining effect), and only thermo-migration is observed. At higher energies instead, in the cylindrical symmetry of ion tracks, quenching rate is in a range that shows the combined effects of thermo-migration and solute redistribution that, depending on the material, can reinforce or cancel each other. These results are relevant for the interpretation of the early stage of radiation damage in alloys, and show that the combination of ultra-fast but nano-scale characteristics of these processes can still be described in terms of linear response of the perturbed system.

Producibility of brazed high-dimension centrifugal compressor impellers

The centrifugal compressor impeller is a high technology machine part and its manufacture is a demanding process. The paper shows that it is possible to join high-dimensional impellers parts by brazing. Various aspects of the construction and the brazing process of high-dimensional impellers have been considered and some modifications of the brazing have been proposed. The method was applied in industry with a good result. Vacuum brazing tests were carried out on age-hardened steel of the 14–5 pH type using 82Au–18Ni solder. It was found that the joint has a multi-layer structure consisting of two kinds of Au–Ni solid solutions: one is rich in gold and the other is rich in nickel. The influence of the roughness of the surfaces to be joined and the distance between them on the joint structure and properties have been determined. The maximum tensile strength (Rm=1265MN/m2) was obtained for joints of polished surfaces without any distance elements, brazed at a temperature of 1273K for 7.2ks (2h) and aged at a temperature of 723K for 7.2ks (2h).

On the stability of (0 0 1) Au/Ni artificially modulated structures grown by MBE

We report on the structural stability upon annealing of (0 0 1) Au/Ni artificially modulated structures (AMS) grown by molecular beam epitaxy (MBE). The structural evolution of samples with a nominal periodicity of 2, 6 and 7 monolayers (ML) and Ni atomic fractions of 0.4 and 0.5 has been investigated by in situ X-ray Diffraction and compared with what occurs for AuNi solid solutions of the same compositions, for which a modulated structure with a periodicity between 3 and 4 ML along the growth direction has been reported previously. The effect of the epitaxial strain imposed by the buffer layer on which the AMS is deposited has also been studied. It is shown that, whatever the nominal period is, the AMS evolve towards a modulated structure having a ∼3 ML period, provided that the epitaxial strain is high enough. These experimental results are explained in terms of an epitaxial-strain-stabilized ordering, as supported by numerical simulations.

Stress-induced compositional modulations in ultrathin Au–Ni solid solutions grown on Au(001) by molecular beam epitaxy: a temperature X-ray diffraction study

Ultrathin Au 1− c Ni c alloys have been grown at room temperature by molecular beam epitaxy on Au(001) buffer layers. The Ni concentration c, ranging from 0.2 to 0.4, is much higher than the bulk solubility. The structural evolution with temperature has been investigated by X-ray diffraction measurements. The AuNi layer is coherently strained on the Au buffer and a modulated structure develops along the growth direction upon annealing at 220°C. The period of the modulation is around 6–8 Å and gives rise to satellite peaks. A simulation has shown that these peaks may be reproduced by a superlattice cell in which monoatomic Ni planes are separated by three or four Au atomic planes. At higher temperatures, there is a diffusion of Ni in the surrounding Au regions, leading to a homogeneous and dilute AuNi solid solution.

Embedded-atom-method effective-pair-interaction study of the structural and thermodynamic properties of Cu-Ni, Cu-Ag, and Au-Ni solid solutions.

The structural and thermodynamic properties of Cu-Ni, Cu-Ag, and Au-Ni solid solutions have been studied using a computational approach which combines an embedded-atom-method (EAM) description of alloy energetics with a second-order-expansion (SOE) treatment of compositional and displacive disorder. It is discussed in detail how the SOE approach allows the EAM expression for the energy of a substitutional alloy to be cast in the form of a generalized lattice-gas Hamiltonian containing effective pair interactions with arbitrary range. Furthermore, we show how the SOE-EAM method can be combined with either mean-field or Monte Carlo statistical mechanics techniques in order to calculate short-range-order (SRO) parameters, average nearest-neighbor bond lengths, and alloy thermodynamic properties which include contributions from static displacive relaxations and dynamic atomic vibrations. We demonstrate that the contributions to alloy heats of mixing arising from displacive relaxations can be sizeable, and that the neglect of these terms can lead to large overestimations of calculated phase-transition temperatures. The effects of vibrational free-energy contributions on the results of composition-temperature phase diagram calculations are estimated to be relatively small for the phase-separating alloy systems considered in this study.It is shown that within the SOE approach displacive effects can act only to displace the peak in the Fourier-transformed SRO parameter away from Brillouin-zone-boundary special points and towards the origin. Consistent with this result, we show that the unusual SRO observed in diffuse scattering experiments for Au-Ni solid solutions can be understood as arising from a competition between chemical and displacive driving forces which favor ordering and clustering, respectively. \textcopyright{} 1996 The American Physical Society.

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.

Tetragonal Au-Ni (001) Solid Solutions Grown by MBE

A study has been made of metastable Au-Ni thin films codeposited by MBE on Au(001). The in-plane relaxation of the deposit was measured with RHEED during growth and the tetragonality of the whole Au-Ni layer was measured with non-symmetric X-ray scans and GDCD. Plan view and cross-sectional TEM show that the alloys relax via twinning on {111} planes. In addition, diffraction patterns show the presence of diffuse rings about points in the reciprocal lattice associated with the Lio structure. These are attributed to the presence of an antiphase structure which forms in response to the very high stresses in the Au-Ni layer. An Lio phase having a high degree of order has been grown by alternating single Au and Ni atomic layers.

Au-Ni solid solutions studied by numerical relaxation

A semi-empirical potential based on the tight-binding second moment approximation is used to study Au-Ni solid solutions at 0 K. The energy of formation, the topological disorder, and the elastic constants are analysed within the whole concentration range. The results coincide very well with available experiments and may be explained by a frustration of the interatomic bondings by the fcc lattice. The size effect predominates over the chemical one and the analysis of interstitial sites shows that the local chemical surroundings lead to a splitting of the fcc lattice tetrahedral volumes.

Single-Crystal AU-NI Solid Solutions Grown by MBE

ABSTRACTEpitaxial solid solutions Au1-x Nix (100) have been obtained at room temperature on Au(100) substrates by the MBE technique. The layers are 10 nm-thick and the composition x has been varied up to 0.37. A large strain (2–3%) has been measured by X-ray diffraction. The lattice parameters have been measured in two perpendicular directions, perpendicular and parallel to the surface. In the latest case, the grazing-incidence technique has been used. Because this technique is very sensitive to the surface, the strain results may be re-interpreted if, the upperlOO nm are enriched in Au.

Equilibrium decomposition curve of AuNi solid solutions

The equilibrium decomposition curve of AuNi solid solutions has been obtained using high-temperature X-ray diffractometry. The empirical rule of the straight line diameter (the rule of the average composition) was not fulfilled for this curve. The temperature dependence of the average composition of the two-phase solid solution showed considerable deviations from the straight line behaviour normally observed in the temperature region from 300 to 500 °C. We have related this anomaly to the presence of a magnetic phase transition in one of the components — the Curie point of nickel. The equilibrium mutual solid solubilities of gold and nickel have been found to be smaller than the presently accepted values.

Studies of lattice distortions in AuNi solid solutions by EXAFS and computer simulations

We report on EXAFS analysis and computer simulations of the topological disorder in concentrated AuNi solid solutions, in relation to the marked difference in size between Au and Ni atoms. The mean nearest - neighbours interatomic distances were derived in a large composition range. It is shown that the atoms are strongly displaced with respect to the average lattice. The three partial pair correlation functions were found to be asymmetric.