Satoru Kobayashi

The Stability of γ ’-Co3(Al,W) Phase in Co-Al-W Ternary System

The thermodynamic stability uf067’- 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 uf067’ phase was formed discontinuously in contact with the uf067-Co (A1) phase. With increasing heat treatment time, however, the fraction of uf067’ phase decreased while that of uf067, CoAl (B2) and Co3W (D019) phases increased. These results are consistent with our previous work with a diffusion-couple method, indicating that the uf067’ phase is metastable, and the three phases ofuf020uf067, CoAl and Co3W are thermodynamically stable at 900 °C.

Relative Importance of Nucleation vs. Growth for Recrystallisation in Particle-containing Fe3Al Alloys

Recrystallisation behavior was studied in two Fe3Al-based alloys containing both large and fine particles with a different fine particle dispersion level using high-resolution SEM and EBSD. High misorientation of 15-30° was created around large particles after a hot rolling process in the two alloys. The kinetics of recrystallisation were, however, considerably retarded in the alloy containing dense fine particles. It was observed that the growth of subgrains created around the large particles was inhibited by the presence of the fine particles. This result clearly suggests that when the particle density (Ns) is high relative to local stored energy (E) around large particles, nucleation can be completely hindered. As the Ns/E level decreases, nucleation may occur and the kinetics of recrystallisation might be determined by both the nucleation rate and the growth of nuclei into a matrix with fine particles.

Microstructure and Texture Evolution during Cold Rolling and Recrystallization of Ni3Al Single Crystals

We have studied the texture and microstructure evolution during cold rolling of Ni3Al single crystals as a function of the initial crystal orientations and revealed that the cold rolling behavior of the single crystals are strongly dependent on the initial crystal orientations, especially on the initial rolling direction (RD). An optimum condition for thin foil fabrication is determined that the initial RD is close to <001>. According to the conditions we have successfully fabricated the wide and thin foils of binary Ni3Al by cold rolling the single crystalline ingots. The thinnest foils obtained so far are about 20µm in thickness and 50mm in width. This document reviews the current status of our research on the thin foils of intermetallic compound Ni3Al.

Microstructure Control Using Recrystallisation in Particle-Containing Fe3Al Alloys

A fine grained and recovered structure with densely formed fine particles has been created by means of a thermomechanical process (TMP) in Fe3Al (bcc derivative structure)-based alloys. The TMP consists of the following two parts; the first part is grain refinement and the second the creation of recovered structure stabilized by fine particles. It was found in the first part that coarse particles are required in the process of deformation to refine grain size by particle stimulated nucleation (PSN) mechanism. Fine M2C type carbide particles were densely precipitated in the second part. These particles were observed to inhibit the growth of subgrains formed around coarse particles during annealing at 700°C. This result suggests that if the fine particle density remains high, the recovered structure can be maintained at 700°C.

The Effect of Ti Addition on Phase Equilibria among Ni (A1), Ni3Al (L12) and Ni3V (D022) Phases

The effect of Ti addition on phase equilibria among Ni (A1), Ni3Al (L12) and Ni3V (D022) phases at 950 °C was investigated through TEM/EDS analysis on heat-treated alloys. The three-phase coexisting region of A1 + L12 + D022 was found to exist around the composition of Ni-4Al-19V (at. %) in the Ni-Al-V ternary system. With addition of Ti to the ternary system the three-phase coexisting region was shifted to the Ni-rich side. Ti partitioned most into the L12 phase and least into the A1 phase. These results suggest that the addition of Ti stabilizes the L12 and D022 phases against A1 phase, and raises the temperature of eutectoid reaction in the Ni3Al-Ni3V pseudo-binary system: A1 → L12 + D022.

Mechanical Properties of Zr-Added Fe-Al Intermetallic Alloys Containing Large Second Phase Particles

Mechanical properties of thermomechanically fabricated D03 Fe-33Al and B2 Fe-38Al intermetallic alloys containing Zr were investigated by means of tensile test and microhardness measurement. The Zr-added ternary alloys showed fine-grained microstructure containing large (Fe,Al)12Zr τ1 phase particles, while the binary alloy showed a single-phase microstructure consisted of coarse recrystallized grains. By introducing the large τ1 phase particles to Fe-Al matrix, tensile strength at room temperature as well as at high temperature (873K) was enhanced but tensile ductility at both temperatures decreased. On the other hand, it was found that vacancy hardening which was significant in the alloys with high contents of Al (i.e., Fe-38Al) was reduced by the large τ1 phase particles.

Effect of the Degree of Order and the Deformation Microstructure on the Kinetics of Recrystallisation in a Fe3Al Ordered Alloy

A Fe3Al based single phase alloy was rolled at 600 °C and 400 °C and the kinetics of recrystallisation in A2 and B2 phase regions was determined in conjunction with the kinetics of the A2-B2 ordering and recovery. At 700°C and 800°C the recrystallisation kinetics depends strongly on the temperature of rolling while at 900°C only a small influence is visible. These results are attributed to the high stored energy in the sample rolled at 400 °C and its high recovery rate at 900 °C, leading to a similar level in stored energy as for the sample rolled at 600 °C in the beginning of annealing. At lower temperatures, in contrast, the difference in stored energy is conserved over longer times in the annealing process. Almost half the apparent activation energy was observed in the ordered state compared to that in the disordered state.

Microstructure and High Temperature Strength in Fe3Al Base Alloys Containing Fine Carbide Particles

The precipitation of carbide particles and high temperature strength in a Fe3Al base alloy with C, Cr and Mo were investigated. The uf06b-Fe3AlC (E21) and M2C (B81) phases precipitated in the temperature range between 1000 °C and 600 °C in periods up to 10 h. The uf06b-Fe3AlC phase tended to precipitate in a film-like morphology on grain boundaries by annealing at relatively high temperatures. The M2C phase was dispersed in a fine needle shape within grains by annealing at 700 °C. The film-like uf06b precipitates covering grain boundaries as well as finely dispersed M2C particles was suggested to contribute to strengthening at 600 °C.

Grain Refinement for Strengthening in Fe 3 Al-based Alloys Through Thermomechanical Processing

A thermomechanical process (TMP) for grain refinement was performed in bulk Fe3Al-based alloys containing ˜10% volume fraction of κ-Fe 3 AlC precipitates. In the TMP, κ particles play an important role in reducing the inhomogeneity of recrystallization due to the matrix orientation. The grain size was refined to ˜20 μm by optimizing the κ particle size. A fine-grained and pancake/recovered microstructure fabricated by the TMP showed more than 1200 MPa tensile strength and 8% tensile ductility at room temperature in air. The tensile strength of this material was higher than those of conventional wrought Fe 3 Al alloys at temperatures between room temperature and 500 °C, and the specific tensile strength was as high as that of the Ti-6Al-4V alloy at temperatures above 400 °C.

The Effects of Annealing Temperature on Tensile Properties in a Fine-grained Fe3Al-based Alloy Containning κ-Fe3AlC Carbide Particles

The effects of annealing temperature on the room temperature tensile properties were investigated in a fine-grained Fe 3 Al based-alloy, and the correlation between microstructure, texture and tensile properties was discussed. Tensile elongation showed a peak as a function of annealing temperature. The highest elongation was obtained in the recrystallized samples annealed at 700°C. The decrease in the elongation with increasing annealing temperature above 700°C is attributed to the increase in the fraction of oriented grains with respect to the tensile direction. A high sensitivity to environmental embrittlement was observed in the partially recrystallized samples annealed at 650°C.