Enhanced growth of intermetallic phases in the Ni–Ti system by current effects

The effect of direct current upon interfacial reactions in the Ni–Ti system was investigated. Isothermal diffusion couple experiments were conducted under varying current densities to de-couple Joule heating from intrinsic effects of the current flux. Current densities of up to 2546 A cm⁻² were used in the temperature range of 625–850 °C. All of the intermetallic compounds (NiTi, Ni₃Ti and NiTi₂) present in the equilibrium phase diagram were identified in the product layer. In addition, β-Ti solid solutions formed in samples annealed above the α→β temperature, 765 °C. The growth of all product layers was found to be parabolic and the applied current was found to significantly increase the growth rate of the intermetallic layers. Using Wagner’s analysis the present results were compared to published results on current-free diffusion couples. The intrinsic growth rate constant of the NiTi₂ intermetallic was found to be 43 times higher under the influence of 2546 A cm⁻² than that obtained without a current at 650 °C. The effective activation energy for the formation of all phases was found to decrease with increasing current density. The effect was strong for all phases but the decrease was most marked for Ni₃Ti. In this case, the activation energy decreased from 292 kJ mol⁻¹ under the influence of a current density of 1527 A cm⁻² to 86 kJ mol⁻¹ when the current density was 2036 A cm⁻². The results are explained in terms of current induced changes in the growth mechanism arising from changes in the concentration of point defects or their mobility.     

Phase equilibria and thermodynamic calculation of the Co–Ta binary system

Experimental investigation and thermodynamic evaluation of the Co–Ta binary phase diagram was carried out. Equilibrium compositions obtained in two-phase alloys and diffusion couples were measured by electron probe microanalyzer (EPMA). A very narrow λ₃(C36) + λ₂(C15) two-phase region is confirmed to be present around 26.5 at.% Ta at temperatures between 950 °C and 1448 °C. Equilibrium relationships above 1500 °C among the liquid, Laves (λ₁(C14), λ₂ and λ₃, whose stoichiometry is described by Co₂Ta), μ(D8_b) and CoTa₂(C16) phases were investigated by microstructural examination in as-cast Co-(24–60 at.%)Ta alloys. The solvus temperature of the γ′ Co₃Ta (L1₂) phase precipitated in the 5.8 at.%Ta γ(Co) and the peritectoid temperature of the Co₇Ta₂ phase in an 8.5 at.%Ta alloy were determined to be 1013 °C and 1033 °C, respectively, by differential scanning calorimeter (DSC). Fine precipitates of the γ′ phase precipitated in the γ (A1) matrix were observed by transmission electron microscope (TEM). Analyzing the present experimental results synthetically, the γ′ Co₃Ta phase was identified to be a metastable phase, of which the γ/γ′ transition temperature of the stoichiometric Co₃Ta alloy was estimated to be 2000 °C. Thermodynamic assessment of the Co–Ta binary system was carried out based on the present results as well as on experimental data in the literature. Calculated results of not only stable but also metastable equilibria were found to be in good agreement with the revised phase diagram. The evaluated stability of the metastable γ′ Co₃Ta coincides with the enthalpy of formation (ΔH(γ'Co₃Ta) = −23.44 kJ/mol) calculated by the ab initio method.     

Experimental investigation and thermodynamic assessment of the C–Co–Fe–Ni–W system

The liquid phase formation temperatures of WC–(Co,Fe,Ni) alloys with free carbon and M₆C phase, respectively, were determined by means of Differential Scanning Calorimetry (DSC) analysis. Based on the experimental results, a thermodynamic modelling is conducted on the C–Co–Fe–Ni–W quinary system using the CALPHAD (CALculation of PHAse Diagram) technique. Temperature-composition sections and projections concerning the sintering areas of the WC–(Co,Fe,Ni) hardmetals are calculated to verify the rationality of the present modelling. Comprehensive comparisons between the experimental and calculated results show that all the experiments can be well reproduced by the present modelling.     

Diffusion kinetics,mechanical properties,and crystallographic characterization of intermetallic compounds in the Mg–Zn binary system

Pure Mg was diffusion bonded to pure Zn at 315 °C for 168 h to produce equilibrium intermetallic compounds of the Mg–Zn system. All equilibrium phases at 315 °C, Mg₂₁Zn₂₅, Mg₄Zn₇, MgZn₂, Mg₂Zn₁₁, were observed to develop. Concentration profiles by electron probe microanalysis, electron diffraction patterns by transmission electron microscopy, and load–displacement curves by nano-indentation were examined to characterize the phase constituents, crystal structure, diffusion kinetics, and mechanical properties. Mg₂₁Zn₂₅ with trigonal, Mg₄Zn₇ with monoclinic, and Mg₂Zn₁₁ with cubic structures were found and their lattice parameters were reported herein. Mg₄Zn₇ and Mg₂Zn₁₁ were observed to have a range of solubility of approximately 2.4 at% and 1.6 at%, respectively. Interdiffusion in MgZn₂ occurred most rapidly, was an order of magnitude slower in Mg₄Zn₇ and Mg₂Zn₁₁, and was the slowest in Mg₂₁Zn₂₅. Composition-dependence of interdiffusion within each intermetallic phase was negligible. The intermetallic phases exhibited insignificant creep, but evidence of discontinuous yielding was observed. The average hardness and reduced moduli were similar for Mg₂₁Zn₂₅, Mg₄Zn₇, and MgZn₂ phases, ∼5 GPa and ∼90 GPa, respectively. However, the Mg₂Zn₁₁ phase had lower hardness of 3.76 GPa and higher modulus of 108.9 GPa. The mechanical properties in the characterized intermetallic phases, exclusive of Mg₂₁Zn₂₅, were strongly concentration-dependent.     

The homogeneity ranges of the delta (δ) and gamma (γ) phases of the Ni-Zn binary system grown by the reaction couple method

When investigating the diffusion layers grown by the reaction couple method at the Ni-Zn interface at 350 and 400 ‡C, the homogeneity ranges of the 5 and γ phases were found to be 87.5 ± 0.2 to 89.0 ± 0.2 at. % Zn and 73.1 ±03 to 86.2 ± 0.3 at. % Zn, respectively. Chemical compositions of the phases investigated can therefore be expressed in the simplest form as δNiZn_{7 to 8} and γNiZn_{3 to 6}.     

Reaction-sintering of intermetallic alloys of the Ni–Al–Mo system

A powder metallurgy route has been used for producing binary and ternary alloys of the Ni–Al–Mo system. Elemental powder mixtures were compacted and, then, sintered in a dilatometer. In this way the dimensional changes involved with thermally induced transformations could be followed during continuous heating runs up to the sintering temperatures. Sintering was assisted by the formation of a liquid phase, promoted by the heat output coming from the intermetallic phase formation reactions. The amount of liquid phase and the efficiency of sintering was highly dependent on the heating rate. A threshold value for optimal densification was identified for some compositions. The effect of other processing parameters, such as pre-sintering compaction pressure and sintering atmosphere has been considered too. The characterisation of the final products was mainly based on X-ray diffraction analyses. The microstructural parameters and the phase composition of the sintered materials were evaluated. On the basis of these results it is possible to draw some conclusions concerning the main phenomena occurring during the sintering process.     

Growth mechanism of phases by interdiffusion and atomic mechanism of diffusion in the molybdenum–silicon system

Tracer diffusion coefficients are calculated in different phases in the Mo–Si system from diffusion couple experiments using the data available on thermodynamic parameters. Following, possible atomic diffusion mechanism of the species is discussed based on the crystal structure. Unusual diffusion behaviour is found in the Mo₅Si₃ and Mo₃Si phases, which indicate the nature of defects present on different sublattices. Further the growth mechanism of the phases is discussed and morphological evolution during interdiffusion is explained.     

A positron study of the defect structures in the D03 and B2 phases in the Fe–Al system

The positron annihilation technique was used to identify the nature of the vacancy-type defects in the D0₃ and B2 phases of the Fe–Al system. Seven alloys with Al concentrations in the range 22.7–48 at.% Al and with different thermal treatments were examined. Positron lifetime calculations for the expected defects in the two phases were also performed in order to facilitate the defect identification. In the B2 phase, two types of defects were identified: a thermal complex formed by a Fe-divacancy and an Al antisite, and a Fe-vacancy. No constitutional vacancies were found in the D0₃ phase.     

Phase equilibria in the Ti–Al binary system

New experimental results on the phase equilibria between the β-Ti (A2), α-Ti (A3), α₂-Ti₃Al (D0₁₉) and the γ-TiAl (L1₀) phases in the Ti–Al system using specimens with low levels of oxygen are presented. The results obtained on the α/γ and the α₂/γ equilibria are in good agreement with the previous experimental and calculated phase boundaries, while the ones obtained on the α/β equilibrium deviate significantly from the previously proposed phase diagram. It is confirmed that the β phase field extends to higher aluminum contents and that the width of the α+β two-phase region is very narrow, less than 1 at.% Al. The presence of the A2/B2 order–disorder transition in the β phase is also confirmed by a combination of differential scanning calorimetric (DSC) analysis and extrapolation of ordering phase boundaries from the Ti–Al–X (X=Cr, Fe) ternary systems. A thermodynamic analysis has been carried out taking into account the ordering configurations in the β-Ti (A2)/β₂-TiAl (B2), f.c.c.-Al (A1)/γ-TiAl (L1₀) and α-Ti (A3)/α₂-Ti₃Al (D0₁₉) equilibria. It is proposed that the anomalous α/β equilibrium is due to the A2/B2 ordering reaction.     

Compton profile and charge transfer study in intermetallic Ti–Al system

The electronic structure of three intermetallic alloys namely Ti₃Al, TiAl and TiAl₃ in terms of Compton profiles is reported in this work. Directional Compton profiles are calculated for all the three alloys employing CRYSTAL code within the framework of density functional theory. The spherically averaged theoretical values are compared with the measurements made using 59.54 keV gamma-rays from Am²⁴¹ source. The calculations are in overall agreement with measurements in all cases. The measurements are also compared with the superposition of LCAO profiles of elemental solids. For Ti₃Al and TiAl₃ the LCAO values show better agreement whereas for TiAl the synthesized LCAO values are closer to the experiment. Effect of titanium 3d electrons is clearly visible in intermediate range of momentum in the Ti rich alloy. Charge transfer in the three alloys has also been estimated following the superposition of experimental profiles of Ti and Al metals. Comparison of Compton spectra of Ti₃Al, TiAl and TiAl₃ with the superposition of the Compton spectra of elemental constituents suggests a charge transfer of 2.8, 0.9 and 0.6 electron per Al atom, respectively. Such large values seem unreasonable and, therefore, this approach cannot be used for any reliable determination of the charge transfer in this system.     

First-principles phase stability and elastic properties of Al–La binary system intermetallic compounds

The phase stability and the elastic properties of Al–La binary system intermetallic compounds were thoroughly investigated using first-principles calculations. Firstly, the 0 K phase diagram for this system was calculated using the formation enthalpy convex hull construction, which indicates three metastable phases, namely Al₄La (I4/mmm), Al₄La (Imm2), and AlLa₃ (Pm-3m). Then, the stability of Al₁₁La₃ was examined at temperatures lower than 1000 K compared with the two Al₄La allotropes and the Al + Al₂La two-phase equilibrium. The results demonstrate that the needlelike phase in Mg–Al–La based alloys should be indexed as Al₁₁La₃, which is thermodynamically stable, with no decomposition under aging. Thirdly, AlLa₃ (Pm-3m) is more stable than AlLa₃ (P63/mmc) at temperatures higher than approximately 590 K, which well agrees with the experimental results. Finally, the elastic properties and vibrational properties for the stable Al–La intermetallic phases were calculated in this work.     

Experimental phase diagram determination and thermodynamic assessment of the Gd2O3–CoO system

New phase diagram data and a thermodynamic assessment of the Gd₂O₃–CoO system using the CALPHAD approach are presented, giving liquidus data and mutual solid solubilities of Co in Gd₂O₃ and Gd in CoO. The thermodynamic model parameters for the ternary Gd–Co–perovskite phase and for the mutual solid solubilities of Co in Gd₂O₃ and Gd in CoO are optimized to reproduce these new experimental data, as well as phase diagram data from literature. The Gd₂O₃–CoO phase diagram is refined based on the results of experiments using combined differential thermal analysis and thermogravimetry, scanning electron microscopy and X-ray diffraction techniques.