Interdiffusion study in the Pd–Pt system

Interdiffusion, intrinsic, tracer and impurity diffusion coefficients are calculated in the Pd–Pt system. Interdiffusion coefficients are more or less insensitive to composition change. Activation energy varies in the range of 324–353 kJ/mol. Impurity diffusion coefficients calculated in this study and available tracer diffusion coefficients in pure elements indicate that Pd has higher diffusion rate compared to Pt in pure Pd, whereas, both the elements have similar diffusion rates in Pt. Kirkendall marker experiments indicate that Pd has much higher diffusion rate in Pd3.5at.%Pt compared to Pt.     

Interdiffusion studies in bulk Au–Ti system

The performance of the contacts, where Au/Ti layers are used in the metallization scheme, largely depends on the product phases grown by interdiffusion at the interface. It is found that four intermetallic compounds grow with narrow homogeneity range and wavy interfaces in the interdiffusion zone. The presence of wavy interfaces is the indication of high anisotropy in diffusion of the product phases. This also reflects in the deviation of parabolic growth from the average. Further, we have determined the relevant diffusion parameters, such as interdiffusion coefficient in the penetrated region of the end members and integrated diffusion coefficients of the intermetallic compounds.     

Interdiffusion inα solid solutions of the Cu-Zn-Sn system

The interdiffusion coefficients in the f c c phase of Cu-Zn-Sn alloys, , have been determined at 1073 K. The concentration profiles indicate that the diffusion rate of tin is greater than that of zinc in the Cu-Zn-Sn alloy. The diffusion paths show the typical S-shaped curves. All of the four interdiffusion coefficients are positive and they are very sensitive to the solute concentration. The atomic mobilities of the three diffusing elements in Kirkendall planes increase in the order of Cu, Zn, Sn. The interaction energy of the Cu-Sn bond is much larger than that of the Zn-Sn bond. From the results of the present work it seems that the Onsager reciprocal relation holds in the a phase of the Cu-Zn-Sn system.     

Kinetics of the eutectoid transformation in the Cu–In system

The present study concerns a detailed investigation of the kinetics of the eutectoid transformation in the Cu–In system based on both the isothermal growth rate of the eutectoid colony (monitored by microstructural change) and enthalpy changes during non-isothermal heating (determined by differential scanning calorimetry) of solution-treated and quenched samples. The maximum growth distance of the eutectoid cells and the equilibrium interlamellar spacing have been determined by optical and scanning electron microscopy in the temperature range 600–825 K. The reaction front velocity was observed to increase with the isothermal ageing temperature in the temperature range studied. A detailed analysis of the isothermal growth kinetics through the models available in the literature has yielded an activation energy of 125–127 kJ mol^-1 for the operating diffusion process, which is comparable with that for discontinuous precipitation in Cu–In or for grain boundary tracer diffusion of ¹¹⁵In in Cu, but significantly lower than that for volume diffusion of In in the β Cu–In alloy. A subsequent differential scanning calorimetric study has indicated a similar activation energy of 133 kJ mol^-1 for the concerned eutectoid transformation. It is thus concluded that the eutectoid transformation in the Cu–In system is a boundary-diffusion-controlled process. This revised version was published online in September 2006 with corrections to the Cover Date.     

Metallic glass formation in the binary Cu–Hf system

Glass formation, structure and thermal properties of alloys in the binary Cu_100?x Hf _x alloy system, where x = 25–50 at.%, are reported and discussed. This work also presents a comparison between copper casting techniques, from thick melt-spun ribbons to suction cast cylindrical rods, and the prediction of critical diameter, d _c, based on maximum ribbon thickness, x _c. Ribbons of Cu₆₀Hf₄₀ and Cu₆₅Hf₃₅ exhibited a fully glassy phase up to a thickness of 170 μm. Suction casting lead to an increase in the largest diameter over which both alloys could be cast, in comparison to melt-spun ribbons, and remain amorphous, with Cu₆₅Hf₃₅ showing a large critical diameter of 1 mm. This result is rationalised by a lower liquidus temperature, T _l, which maximises the reduced glass transition temperature, T _rg, and also correlates closely with the eutectic point. Finally, there were remarkable similarities between the Miedema model and the efficient packing model for predicting the range for metallic glass formation in this binary system.     

Experimental investigation of phase equilibria in the Cu–Ni–Zr system

Phase equilibria in the Cu–Ni–Zr ternary system have been measured through alloy sampling combined with diffusion couple approach. According to the phase relations identified with electron probe microanalysis and X-ray diffraction techniques, isothermal sections at both 1073 and 1293 K were constructed. It is evident that remarkable ternary solubility occurs in almost all binary intermetallic phases at both temperatures. The formerly reported ternary compounds T1 (Cu_20–40Ni_40–60Zr₂₀) and T2 (Cu_20–25Ni_60–65Zr₁₅) were not verified in this work. No other ternary compound was detected. In addition, continuous dissolution between Cu₁₀Zr₇ and Ni₁₀Zr₇ at 1073 K was observed.     

Age-hardening by miscibility limit of Au–Pt and Ag–Cu systems in an Au–Ag–Cu–Pt alloy

The age-hardening by miscibility limit of Au–Pt and Ag–Cu systems in an Au–Ag–Cu–Pt alloy was examined by characterizing the hardening behavior, phase transformations and changes in microstructure, and elemental distribution during aging. The hardness increased by the transformation of the parent α₀ phase into the α₁ and metastable AuCu I′ phases, but not by the further transformation of the metastable AuCu I′ phase into the stable AuCu I phase due to the simultaneously initiated lamellar-forming grain boundary reaction. The replacing ratio of matrix by lamellar structure was not directly proportional to the AuCu I phase formation. The relatively high Pt content caused the severe exclusion of Au from the Cu-rich layer of the lamellar structure due to the overlapped miscibility limit of both Au–Pt and Ag–Cu systems.     

Microstructure development in the directionally solidified Al–4.0 wt% Cu alloy system

The effect of convection on microstructure formation is examined experimentally and theoretically for the vertically upwards-directional solidification of Al–4.0 wt% Cu alloys. In this alloy system, the rejected solute is heavier than the solvent so that fluid flow occurs due to the presence of radial gradients in temperature and composition. A numerical model is presented which shows that convection effects cause the composition to vary along the interface such that the composition increases from the center to the periphery of the sample. This composition variation causes the macroscopic interface to become convex, and give rise to a systematic variation in microstructure along the interface. Critical experiments have been carried out to examine planar to cellular (and cellular to dendritic) transition in a given sample due to the increase in concentration along the interface, and the experimental results are analyzed through the measurements of interface composition and thermal gradient. In addition, the variation in local primary spacing with interface composition is also characterized and compared with the results of the numerical model. It is shown that microstructure transitions and microstructural scales can be correlated quantitatively with the theoretical results based on interface composition and on temperature and solute gradients at the interface.     

Experimental investigation and thermodynamic calculation of the phase equilibria in the Cu–Mo–Ni ternary system

The phase equilibria at 900 °C, 1000 °C, 1100 °C, 1200 °C and 1300 °C in the Cu–Mo–Ni system were experimentally determined by means of optical microscopy (OM), electron probe microanalyzer (EPMA) and X-ray diffraction (XRD) on the equilibrated alloys. The experimental results firstly found that the fcc-type miscibility gap exists at 900 °C, 1000 °C, 1100 °C and 1200 °C in the Cu–Mo–Ni system, and the solubility of Cu in the MoNi phase at 900 °C, 1000 °C, 1100 °C and 1200 °C are about 0.5 at.%, 1.5 at.%, 1.7 at.% and 4.0 at.%, respectively. The as-cast Cu₂₀Mo₂₀Ni₆₀ (at.%), Cu₂₀Mo₃₀Ni₅₀ (at.%), Cu₁₀Mo₆₀Ni₃₀ (at.%), Cu₇₀Mo₁₀Ni₆₀ (at.%), Cu₂₀Mo₆₀Ni₂₀ (at.%) and Cu₈₀Mo₁₀Ni₁₀ (at.%) alloys appear the separated macroscopic morphologies, which are caused by the liquid phase separation on cooling, while the as-cast Cu₁₀Mo₂₅Ni₆₅ (at.%), Cu₃₂Mo₅Ni₆₃ (at.%) and Cu_30.7Mo_6.3Ni₆₃ (at.%) alloys show the homogenous microscopic morphologies. On the basis of the experimental data investigated by the present and previous works, the phase equilibria in the Cu–Mo–Ni system were thermodynamically assessed by using CALPHAD (Calculation of Phase Diagrams) method, and a consistent set of the thermodynamic parameters leading to reasonable agreement between the calculated results and experimental data was obtained.     

Experimental determination and thermodynamic modeling of phase equilibria in the Cu–Cr system

Liquidus temperatures in the Cu–Cr system at compositions of 10.0–72.7 at.% Cr were determined using electromagnetic levitation melting. The present data agree with the prediction of a recent thermodynamic study of the system for compositions up to 20.0 at.% Cr. However, they show large and positive deviations for other compositions. Microscopic studies reveal that compositions between 10.0 and 50.5 at.% Cr solidified into a dendritic microstructure, whereas those between 55.9 and 72.7 at.% Cr solidified into a droplet-shaped microstructure. The microstructure of the latter type provides direct evidence for the existence of a stable miscibility gap over Cr-rich compositions. Phase equilibria in the Cu–Cr system were calculated using the CALPHAD method. A novel phase diagram was proposed for the Cu–Cr system, which shows a monotectic reaction between compositions of 50.8 and 83.2 at.% Cr at an invariant temperature of 2020 ± 22 K. The novel phase diagram has reduced the discrepancies between the literature data.     

Synthesis of a decagonal phase in the Al–Cu–Fe–Cr system by mechanical alloying

Quasicrystalline Al–Cu–Fe–Cr alloys have been prepared by mechanical activation. The morphology of powder particles has been investigated after thermomechanical processing under various conditions. We have identified the sequence of phase transformations in the quaternary alloys in the stability region of quasicrystalline phases and optimized conditions for obtaining a maximum fraction of a decagonal state in powder materials.     

Study on the reaction mechanism of self-propagating high-temperature synthesis of TiC in the Cu–Ti–C system

The mechanism of the self-propagating high temperature synthesis (SHS) processing in the Cu–Ti–C system was investigated. The reaction sequence and mechanism were explored using combustion front quenching method. The SHS reaction in the Cu–Ti–C system starts with the solid diffusion reaction between Cu and Ti particles, subsequently, the Cu–Ti liquid forms and spreads over C particles. The C particles dissolve into the Cu–Ti liquid, leading to the formation of the Cu–Ti–C ternary liquid, as a result, TiC particulates are gradually precipitated out of the liquid.     

Bimodal growth of the nanophases in the dual-phase composites produced by mechanical alloying in immiscible Cu–Ag system

Annealing was carried out to produce Ag–Cu dual-nanophase composite by decomposing Ag–Cu supersaturated solid solution prepared by ball-milling. Differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy were used to characterize the growth kinetics of the nanophase composite upon heating. The results show that bimodal growth kinetics is obeyed in the dual-nanophase system. The coarsening of the second phase of small volume fraction follows the cubic law of LSW theory, while the grain growth of matrix phase follows the kinetic equation of the grain growth of pure nanocrystalline metals. In both cases, the growth is controlled by grain boundary diffusion.     

Contact angles by the solid-phase grain boundary wetting (coverage) in the Co–Cu system

The microstructure of binary Co–13.6 wt% Cu and Cu–4.9 wt% Co alloys after long anneals (930–2,100 h) was studied between 880 and 1,085 °C. The contact angles between (Co) particles and (Cu)/(Cu) grain boundaries (GBs) in the Cu–4.9 wt% Co alloy are between 50° and 70°. In the Co–13.6 wt% Cu alloy, the transition from incomplete to complete wetting (coverage) of (Co)/(Co) GBs by the second solid phase (Cu) has been observed. The portion of completely wetted (Co)/(Co) GBs increases with increasing temperature beginning from T _wss = 970 ± 10 °C and reaches a maximum of 15% at 1,040 °C. This temperature is very close to the Curie point in the Co–Cu alloys (1,050 °C). Above 1,040 °C, the amount of completely wetted (Co)/(Co) GBs decreases with increasing temperature and reaches zero at T _wsf = 1,075 ± 5 °C. Such reversible transition from incomplete to complete wetting (coverage) of a GB by a second solid phase is observed for the first time.     

Microstructure development in the directionally solidified Al–4.0 wt% Cu alloy system

The effect of convection on microstructure formation is examined experimentally and theoretically for the vertically upwards-directional solidification of Al–4.0 wt% Cu alloys. In this alloy system, the rejected solute is heavier than the solvent so that fluid flow occurs due to the presence of radial gradients in temperature and composition. A numerical model is presented which shows that convection effects cause the composition to vary along the interface such that the composition increases from the center to the periphery of the sample. This composition variation causes the macroscopic interface to become convex, and give rise to a systematic variation in microstructure along the interface. Critical experiments have been carried out to examine planar to cellular (and cellular to dendritic) transition in a given sample due to the increase in concentration along the interface, and the experimental results are analyzed through the measurements of interface composition and thermal gradient. In addition, the variation in local primary spacing with interface composition is also characterized and compared with the results of the numerical model. It is shown that microstructure transitions and microstructural scales can be correlated quantitatively with the theoretical results based on interface composition and on temperature and solute gradients at the interface.     

Phase Relations in the Cu–Sb–O System

The Cu–Sb–O system was studied by x-ray diffraction and thermal analysis between 700 and 1000°C. The compositions of copper antimonates were refined. Sb₂O₄ was found to exist in two polymorphs above 800°C: -Sb₂O₄ (dominant phase) and -Sb₂O₄. The evolution of phase equilibria with increasing temperature was examined. The isothermal sections of the Cu–Sb–O phase diagram were mapped out using new and earlier reported results.     

Effects of copper addition on the in-situ synthesis of SiC particles in Al–Si–C–Cu system

In this research work, SiC particles have been successfully in-situ synthesized in Al–Si–Cu matrix alloy utilizing a novel liquid–solid reaction method. The effect of copper addition on the synthesis of SiC in Al–Si–C–Cu system was investigated. The composites mainly contain spherical SiC particles and θ-Al₂Cu eutectic phases, which are embedded in the α-Al matrix. Results indicated that the temperature for forming in-situ SiC particles significantly reduced from 750 °C to 700 °C with the copper addition. The size of in-situ synthesized SiC particles can be as low as 0.2 μm. Further study found that the addition of 10 wt.% copper into Al–Si–C alloy causes its solidus temperature to decrease by about 65 °C. Additionally, the Rockwell hardness value of SiCp/Al–18Si–5Cu composites has an average of 92, which is 50% higher than that of the sample without copper addition.     

Solute–solute and solute–solvent interactions in transition metal alloys: Pt–Ti system

Abstract

The activity of Ti in solid Pt has been measured as a function of composition at 1573 K using a metal oxide–gas equilibration technique under controlled oxygen partial pressures. Thin foils of Pt were equilibrated with TiO_x at constant oxygen chemical potentials. Oxygen partial pressures were established using Ar–H₂–H₂O gas mixtures of controlled composition. A solid state cell, based on yttria doped thoria as the solid electrolyte, independently determined the chemical potential of oxygen in the gas phase. The concentration of Ti in solid Pt was determined using spectrophotometric methods. The activities of Ti were computed from the oxygen potentials established by the gas phase coupled with independent data on the thermodynamic properties of titanium oxides. The excess chemical potential of titanium in solid Pt at 1573 K in J mol^-1 can be represented as Δμ^E_Ti = —83 940 — 214 140 (1 — X _Ti )² with an error of ±2800 J mol^-1. The activity coefficient of Ti at infinite dilution determined from this study and that of other elements of the first transition series in solid Pt obtained from previous work confirm the trend predicted by both Miedema’s model and Engel–Brewer theory. The attractive interaction between the solute and the solvent (Pt) increases with decreasing atomic number of the solute. The self-interaction parameters of the first transition series elements in solid Pt indicate an increase in solute–solute repulsion with decrease in 3d electron concentration of the solute. The standard Gibbs energy of formation of TiPt₃ is —282·57 ± 4 kJ mol^-1 at 1573 K. The large negative values of the Gibbs energy of formation of phases in the system Pt–Ti indicate that Pt is not phase compatible with nitrides and carbides of Ti at high temperatures.