Papers by Keyword: Diffusion Couple

Abstract: High entropy alloys (HEAs) are equimolar multi-principal-element alloys (MPEAs) that are different from traditional solvent-based multicomponent alloys based on the concept of alloy design. Based on initial work by Yeh and co-workers, HEAs were postulated to exhibit four “core” effects: high entropy, sluggish diffusion, lattice distortion, and cocktail effect. Out of these four proposed core effects, “high entropy” and “sluggish diffusion” effects were most debated in the literature as these core effects directly affect the thermodynamic and kinetic understanding of HEAs. The initial work on HEAs by several researchers utilized these effects to indirectly support the experimentally observed “unique” properties, without independent investigation of these core effects. The presumed implications of these core effects resulted in justification or generalization of properties to all HEAs, e.g., all HEAs should exhibit high temperature stability based on high entropy effect, high temperature strength owing to limited grain growth, good diffusion barrier application due to sluggish diffusion kinetics, etc. However, many recent studies have challenged these core effects, and suggested that not all HEAs were observed to exhibit these core effects.

Abstract: CoSb based compounds have gained much importance in the fields of thermoelectric devices. In this work, we have conducted the solid–state conventional bulk diffusion couple experiments. To study the phase evolutions, Co/Sb diffusion couples are annealed at 450–550 °C. The interdiffusion zone is analysed using field emission gun equipped scanning electron microscope and the composition measurements are done in electron probe micro−analyser to confirm the growth of various product phases. The marker experiment indicates that the CoSb₃ phase grows mainly by diffusion of Sb in the binary Co–Sb system. Growth of the CoSb₃ phase is discussed based on assessment correlating the difference in mobilities of species with the high homologous temperature, crystal structure of the phase, and the concept of sublattice diffusion mechanism in line compounds.

Abstract: nterdiffusion coefficients in α2-Ti3Al and γ-TiAl of conventional TiAl and TiAl-8Nb alloy were measured at the temperature ranging from 1273K to 1523K. Single-phase diffusion couples were employed, and the concentration profiles of Al after annealing were measured by an electron probe microanalyzer (EPMA), and the interdiffusion coefficients were calculated according to the Boltzmann-Matano method. The results showed that there was no significant concentration dependence of interdiffusion coefficients for all the alloys with various phases, and the values of interdiffusion coefficients covered three orders of magnitude (E-17-E-14) with the increase of temperature according to Arrhenius law. In α2-Ti3Al and γ-TiAl phase of conventional TiAl alloys, the pre-exponential factor and activation enthalpy were D0=3.95×10-5m2s-1,Q=276KJmol-1 ;D0=7.26×10-5m2s-1,Q=275KJmol-1 respectively. The pre-exponential factor and activation enthalpy were D0=4.54×10-6m2s-1, Q=244KJmol-1 in γ-TiAl phase of TiAl-8Nb alloys. However, the temperature dependence of interdiffusion coefficients in α2-Ti3Al of TiAl-8Nb alloys did not follow Arrhenius laws very well. With the addition of Nb, the interdiffusion coefficients increased significantly in α2-Ti3Al, but changed slightly in γ-TiAl at high temperature.

Abstract: The structures of AlSc with 50at.% and 55at.% Sc were investigated. It was shown that orthorhombic AlSc is formed at Sc 50at.%, and B2 AlSc at Sc 55at.%. After anealing above 1100 °C, the orthorhombic AlSc transited into B2 structure, which indicated that B2 AlSc can also be a metalstable phase in the alloy with Sc less than 50at.%. The effects of these two structures on the phases formation via inter-diffusion were also studied using diffusion couple with titanium and γ-TiAl. It was found that the interdiffusion coefficients are the main factors for Ti/AlSc and structures are the main factors for γ-TiAl/AlSc.

Abstract: Pure titanium and aluminum sheets were used to prepare titanium-aluminum diffusion couples. The diffusion couples were heated to temperatures 550, 575, 600, 625, 640, 650 and 700°C. SEM observations of the titanium-aluminum interface and EPMA results revealed that a TiAl₃ intermetallic layer has formed and thickened between the layers. Grain boundaries of the TiAl₃ compound, which were revealed by back-scattered electron imaging, indicated a size distribution across the layer. Finer grains which were located close to the titanium showed that TiAl₃ has nucleated at the Ti-TiAl₃ interface. Thus, the former grains which had formed close to the aluminum-rich side have grown and coarsened during annealing at high temperatures. Grain coarsening of TiAl₃ decreased the kinetics of the layer thickening.

Abstract: The study of influence of Fe and Ti alloying elements in the Ni3Al intermetallic alloy on diffusion feature in Ni/Ni3Al-Me (Me = Fe or Ti) joints was performed using the diffusion couple technique. Four sets of diffusion couples of diverse compositions prepared by means of resistance welding or electron beam welding in vacuum were used. The concentrations profiles and diffusivities of welded Ni/Ni3Al-Fe and Ni/Ni3Al-Ti joints were determined after annealing at 1050°C for 100 hours. The concentration profiles were smoothed using suitable types of polynomials. The Kirkendall voids in the area between the Matano plane and the new  interface created after annealing were observed. Metallographic study and qualitative evaluation of voids by means of AFM method revealed different feature of Kirkendall regions in Ni/Ni3Al-Fe couples unlike Ni/Ni3Al-Ti joints.

Abstract: The phase equilibria at 900 °C and B2/L21 order-disorder transition in the Ni-Mn-Ga ternary system were investigated by analyzing the equilibrated alloys and diffusion couples using a combination of techniques. It was confirmed that a bcc single phase region exists in a wide composition range at 900 °C, and the critical temperatures of B2/L21 order-disorder transition were determined in Ni-50 at.% section, which exhibits a maximal ordering transition temperature of 796 at Mn content of 25 at.%. The obtained results will be helpful for the preparation and annealing of Ni-Mn-Ga alloys in the specific temperature.

Abstract: The thermodynamic stability ’- Co3(Al,W) phase (L12) in the Co-Al-W ternary system at 900 °C was investigated through microstructure and EPMA analysis on a heat-treated bulk alloy. To promote microstructural evolution, the bulk alloy was cold rolled before heat treatment. By heating at 900 °C, the ’ phase was formed discontinuously in contact with the -Co (A1) phase. With increasing heat treatment time, however, the fraction of ’ phase decreased while that of , CoAl (B2) and Co3W (D019) phases increased. These results are consistent with our previous work with a diffusion-couple method, indicating that the ’ phase is metastable, and the three phases of, CoAl and Co3W are thermodynamically stable at 900 °C.

Abstract: We developed a method of rigorous solution of the Onsager’s flow equations using moments of the interdiffusion-parameter integrands for the determination of average ternary interdiffusion coefficients. The analysis developed by Dayananda and Sohn [1] is the basis for this refined approach. Average main and cross interdiffusion coefficients are determined over selected regions in the diffusion zone using the diffusion-distance moments of the interdiffusion flux flow equations. Thermodynamic stability of solid solutions in the light of interdiffusion phenomenon is taken as validation criteria to identify accurate and reliable values of the ternary interdiffusion coefficients. Regulations are proposed for successful application of the analysis method to various ternary diffusion couples in Ni- and Fe-based intermetallics.

Abstract: Interdiffusion coefficients of Al replacing elements in Ni-Al-X (X=Ti, V and Nb) were estimated by a series of experiments using diffusion couples of Al rich pseudo-binary systems at three different temperatures of 1423, 1473 and 1523K. In order to obtain interdiffusion coefficients of the pseudo-binary systems, the experimental data was analyzed by the Sauer and Freise method, and also impurity diffusion coefficients of Ti, V and Nb in Ni3Al were estimated by applying the Darken-Manning equation. The magnitude of interdiffusion coefficient decreased in order of V, Ti and Nb at all three temperatures. Impurity diffusion coefficients were described by the expressions: , , . The activation enthalpies obtained from the experimental data confirmed the retardation of Ti, V and Nb diffusion in Ni3Al by the anti-site diffusion mechanism. These results are consistent with our previous work on diffusion of Re and Ru in Ni3Al .