Ikuo Ohnuma

Phase equilibria among α (hcp), β (bcc) and γ (L10) phases in Ti–Al base ternary alloys

Phase equilibria between the α(A3), α2 (D019), β(A2 or B2) and the γ(L10) phases in the Ti–Al base ternary systems were investigated over the temperature range 1000–1300°C. The tie lines and the phase boundaries were determined by electron probe microanalysis using multiphase alloys. It was established that almost all the elements except Zr tended to partition into the β phase rather than into the α, α2 or the γ phase, while Zr mostly partitioned into the γ phase. At 1000°C, in the equilibrium state between the α2 and the γ phases, V, Cr, Mo, Ta and W partitioned to the α2 phase rather than to the γ phase, whereas Mn, Fe, Co, Ni, Cu and Zr tended to concentrate into the γ phase. The partition coefficients for the alloying elements were only slightly dependent on their concentration. Based on these data, the relative stabilizing effects of alloying elements on the α, α2, β and γ phases are discussed.

Superelastic Effect in Polycrystalline Ferrous Alloys

A superelastic alloy formed from common elements operates over a wide temperature window. In superelastic alloys, large deformation can revert to a memorized shape after removing the stress. However, the stress increases with increasing temperature, which limits the practical use over a wide temperature range. Polycrystalline Fe-Mn-Al-Ni shape memory alloys show a small temperature dependence of the superelastic stress because of a small transformation entropy change brought about by a magnetic contribution to the Gibbs energies. For one alloy composition, the superelastic stress varies by 0.53 megapascal/°C over a temperature range from –196 to 240°C.

Phase equilibria in the Fe–Co binary system

Abstract α (A2)/γ (A1) phase equilibria of the Fe–Co system between 400 and 800°C were determined by means of lattice parameter measurement using thin film specimens. Bulk specimens were also analyzed to compare the extent of attainment to the equilibrium. The thin film technique was found to be greatly advantageous for obtaining the phase equilibria at lower temperatures where solid-state reactions are too slow to reach the equilibrium state in the conventional methods using bulk specimens. It was confirmed that the α+γ two-phase region extends below the temperature at which the α (A2)/α′ (B2) transus meets the α/α+γ boundary. Thermodynamic analysis was also conducted by taking the magnetic and chemical ordering contributions of the B2 structure into account, findings of which confirmed the extension of the α+γ two-phase region below the α/α′ ordering temperature.

Phase equilibria and stability of ordered BCC phases in the Fe-rich portion of the Fe–Al system

Abstract Phase stability of the BCC phases with A2, B2 and D03 structures and phase equilibria among these phases in the Fe-rich portion of the Fe–Al binary system were examined using diffusion couples, transmission electron microscopy (TEM), energy dispersion spectroscopy (EDS) and Vickers hardness (VH) measurement. TEM observations of the diffusion couples annealed at the temperatures between 300 and 700 °C were performed to identify the crystal structure of the ordered phases and to measure the average size of the B2 and D03 ordered domains. Phase boundaries between A2 and B2 or D03 phases were then evaluated by comparison between the concentration profile and the profiles of the ordered phase domain size or Vickers hardness. It was clarified that the present results in the high temperature range over 450 °C are in agreement with the previous data, while the A2+D03 two-phase field extends beyond the A2/D03 phase boundary in the currently available phase diagram into a lower Al range at temperatures below 450 °C. The A2→B2, B2→D03 and para→ferro continuous ordering temperatures were also measured by differential scanning calorimetry (DSC).

Phase equilibria in the Cu-rich portion of the Cu-Al binary system

Abstract The phase equilibria in the Cu–Al binary system over the temperature range 500∼1000°C and the composition range 15∼60 at.% Al have been determined using diffusion couple technique, differential scanning calorimetry (DSC) and high temperature X-ray diffraction (XRD) methods. While the results from this study pertaining to the phase equilibria α/β and ϵ1(ϵ2)/liquid are in agreement with those reported in previous works, significant differences have been found between the earlier results and the present work in the composition range 25∼40 at.% Al. They are: (a) In the composition range 32∼38 at.% Al only a second order reaction, γ1 (D83)→γ0 (D82), is seen to occur and not a two-phase equilibrium γ0/γ1 reaction as reported before (b) The β0 phase is absent at high temperature near 1000°C and compositions near 30 at.% Al. (c) The equilibrium eutectoid γ0→β+γ1 and peritectoid γ0+ϵ1→γ1 reactions do not occur in this system.

Phase Equilibria and Heusler Phase Stability in the Cu-Rich Portion of the Cu-Al-Mn System

Abstract Results pertaining to the phase equilibria between the phases α (A1), β (A2, B2 or L2 1 (D0 3 )) and γ (γ-bronze type), and the two-stage order–disorder transition and decomposition reaction A2–B2–L2 1 in the ternary system Cu–Al–Mn are reported. Ternary isothermal section diagrams at 800, 700, 600 and 550°C have been constructed using Energy Dispersion X-ray Spectrometry (EDX) analysis results, and it is found that the β single-phase region in the Cu–Al system is very significantly widened on increasing the Mn content. The critical temperatures ( T c ) of the A2–B2–L2 1 order–disorder transitions, determined by Differential Scanning Carolimetory (DSC) analysis are found to be strongly dependent on the Al content rather than on the Mn content. It is confirmed by DSC measurements and TEM-EDX analysis that a miscibility gap island between Cu 3 Al and Cu 2 AlMn phases exists in the L2 1 phase region. The second order ordering reaction between D0 3 and L2 1 structures has also been detected by X-ray diffraction. The stability of the bcc β phase is discussed in terms of atomic and magnetic ordering.

Ordering and phase separation in the b.c.c. phase of the Fe-Al-Ti system

Ordering and phase separation in {alpha}-Fe alloys of the Fe-Al-Ti system have been investigated by differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and electron-probe microanalysis (EPMA). The results indicate that the partial replacement of Fe with Ti in the Fe-rich Fe-Al binary alloys results in drastically stabilizing the B2 (FeAl) and D0{sub 3} (Fe{sub 3}Al) phases and increasing the A2-B2-D0{sub 3} successive order-disorder transition temperatures. It is confirmed that there are two kinds of phase separations of the BCC phase, (A2 + D0{sub 3}) and (B2 + D0{sub 3}) in the composition range below 25 at.% Al. The width of the two-phase region drastically increases with decreasing Al composition. The two-phase regions close at ternary tricritical points. Calculations of BCC phase equilibria have been performed with the cluster variation method (CVM) using the irregular tetrahedron approximation. First and second nearest pair interactions as well as tetrahedron interactions are taken into account. Starting from binary and ternary phase equilibria the numerical values of these interactions have been determined and an excellent agreement between calculation and experiments was obtained.

Pb-free high temperature solders for power device packaging

Abstract Reliabilities of joints for power semiconductor devices using a Bi-based high temperature solder has been studied. The Bi-based solder whose melting point is 270 °C were prepared by mixing of the CuAlMn particles and molten Bi to overcome the brittleness of Bi. Then, joined samples using the solder were fabricated and thermal cycling tests were examined. After almost 2000 test cycles of −40/200 °C test, neither intermetallic compounds nor cracks were observed for CTE (Coefficient of Thermal Expansion) matched sample with Cu interface. On the other hand, certain amount of intermetallic compound such as Bi 3 Ni was found for a sample with Ni interface. In addition, higher reliability of this solder than Sn-Cu solder was obtained after −40/250 °C test. Furthermore, an example power module structure using double high temperature solder layers was proposed.

Thermodynamic assessment of the phase diagrams of the Cu-Sb and Sb-Zn systems

Thermodynamic assessments have been made for the Cu-Sb and Sb-Zn binary systems by means of the CALPHAD technique. The Gibbs energies of the liquid, bcc, and fcc phases are described by a substitution solution model and a Redlich-Kister formalism. All of the compounds were treated as stoichiometric compounds. Moreover, the liquidus temperatures of the Zn-rich portion in the Sb-Zn system were measured to check the unusual shape reported by previous work. It was confirmed that the liquidus line is not peculiar but smooth. A consistent set of the thermodynamic parameters was optimized to obtain a better fit between calculated results and experimental data including phase diagram and thermodynamic quantities.

Abnormal Grain Growth Induced by Cyclic Heat Treatment

Making the Grain Most metals contain a large number of ordered crystalline regions that are separated by disordered grain boundaries. If the material is annealed at elevated temperatures, the larger grains will grow uniformly at the expense of the smaller ones. This process slows down over time, making it hard to create very large grains. Abnormal grain growth, in which a few of the crystalline regions grow much faster and larger than the others, can occur if the material is put through a complex annealing process involving straining of the samples. Omori et al. (p. 1500; see the Perspective by Taleff and Pedrazas) find that a much simpler and shorter annealing process can trigger abnormal grain growth in copper-based shape-memory alloys. Thermal cycling between a high-temperature single-phase region and a lower-temperature two-phase region generated dislocations at low temperatures and grain growth on heating. Because this method does not require external straining of the sample, it is not limited to thin sheets or wires. Thermal cycling of a copper-based shape-memory alloy leads to abnormal grain growth and very large grains. [Also see Perspective by Pedrazas] In polycrystalline materials, grain growth occurs at elevated temperatures to reduce the total area of grain boundaries with high energy. The grain growth rate usually slows down with annealing time, making it hard to obtain grains larger than a millimeter in size. We report a crystal growth method that employs only a cyclic heat treatment to obtain a single crystal of more than several centimeters in a copper-based shape-memory alloy. This abnormal grain growth phenomenon results from the formation of a subgrain structure introduced through phase transformation. These findings provide a method of fabricating a single-crystal or large-grain structure important for shape-memory properties, magnetic properties, and creep properties, among others.