Effect of Microstructure on Cold Workability of Ni 3 Si base Multi-Phase Intermetallic Alloys
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Effect of Microstructure on Cold Workability of Ni3Si base Multi-Phase Intermetallic Alloys Yasuyuki Kaneno, Yasuyuki Matsuoka and Takayuki Takasugi Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan ABSTRACT The effect of microstructure on cold-rolling workability and tensile properties of Ni3Si (L12)íNi3Ti (D024)íNi3Nb (D0a) multi-phase intermetallic alloys was investigated. The cast alloys with different microstructures containing the D024 phase and/or the D0a phase particles in the L12 matrix were homogenized and then cold rolled. For the alloys with the microstructure consisting of coarse plate-like D024 particles in the L12 matrix, serious cracks initiated at the coarse D024 particles in the early stage of the cold rolling process and then propagated, resulting in failure of the rolled plate. On the contrary, for the alloys with the microstructure consisting of fine needle-like D024 precipitates and/or granular-shaped D0a particles, these second phase particles did not spoil the cold workability, leading to successful cold rolling to 90 % reduction. After 90 % cold rolling, the rolled sheets were fully recrystallized at 1173 K for 1 h, resulting in the formation of a fine-grained microstructure. The room-temperature tensile strength and the yield stress of the recrystallized sheet were remarkably enhanced compared with that of the unrolled alloys, possibly due to the fine-grained microstructure as well as the particle hardening. Also, the high-temperature tensile strength and the elongation were improved in the recrystallized sheets compared to an L12 single-phase Ni3(Si,Ti) alloy sheet. Consequently, it was found that the cold rolling and annealing process was beneficial to improve the tensile properties for the present multi-phase intermetallic alloys. INTRODUCTION Geometrically close packed (GCP) Ni3Si (L12), Ni3Ti (D024) and Ni3Nb (D0a) intermetallic compounds have been of great interest not only from fundamental but also from practical point of view. They generally show high thermal, chemical and microstructural stabilities. However, these monolithic intermetallic compounds show a poor ductility at room temperature and also insufficient strength at high temperature. Among these three intermetallic compounds, the room-temperature ductility of the Ni3Si compound has been markedly improved by Ti addition [1]. It has been demonstrated that an L12 Ni3(Si,Ti) intermetallic alloy in which the propensity for intergranular fracture was suppressed could be heavily cold rolled to thin foils, and also that the obtained thin foils showed an extremely high tensile strength of over 2 GPa [2]. However, there remains room for improvement in high temperature strength and ductility for the Ni3(Si,Ti) alloy. On the other hand, it has been found that the multi-phase intermetallic alloys based on the Ni3Si-Ni3Ti-Ni3Nb pseudo-ternary alloy system showed a high tensile strength accompanied with certain levels of tensile ductility ove
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