Microstructures and Mechanical Properties in Ni 3 Si-Ni 3 Ti-Ni 3 Nb-Based Multi-Intermetallic Alloys
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Microstructures and Mechanical Properties in Ni3Si-Ni3Ti-Ni3Nb-Based Multi-Intermetallic Alloys T. Takasugi, K. Ohira and Y. Kaneno Department of Metallurgy and Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan ABSTRACT Microstructure, high-temperature tensile deformation and oxidation property of Ni3Si-Ni3Ti-Ni3Nb multi-phase intermetallic alloys with a microstructure consisting of L12, D024 and D0a phases were investigated. The tensile stress as well as the tensile elongation of these multi-phase alloys increased with increasing Si content, i.e. the volume fraction of L12 phase in the wide range of test temperatures. 50-ppm boron addition to these multi-phase intermetallic alloys resulted in increased tensile stress and tensile elongation. The multi-phase intermetallic alloy with a high Si content had good oxidation resistance, and also the boron addition to this alloy resulted in enhanced oxidation resistance. From an overall evaluation of the properties examined, it was shown that the multi-phase intermetallic alloy, which has a high Si content and is composed of L12 matrix dispersed by D024 and D0a phases, had the most favorable properties as high-temperature mechanical and chemical materials. INTRODUCTION High-temperature structural materials (e.g. super alloys) that are widely utilized today are composed of a two-phase microstructure consisting of γ (Ni solid solution) and γ’ (L12 phase). Also, recent study suggests that multi-phase intermetallic alloys can provide a good balance of room-temperature ductility, high-temperature strength and oxidation resistance, as recognized from recent development of e.g. TiAl-based intermetallic compounds consisting of γ and α2 phases [1,2]. A group of Ni3Si, Ni3Ti and Ni3Nb which are called as geometrically close packed (GCP) structures (phases) have been of great interest as a constituent phase in advanced intermetallic compounds as well as a strengthener in Ni-based superalloys. They generally show high thermal, chemical and microstructural stabilities, and also attractive mechanical properties such as strength anomaly at high temperatures as well as reasonable deformability at low temperatures owing to their relatively simple crystal structures [3-9]. Ni3Si, Ni3Ti and Ni3Nb have L12 structure with a lattice parameter of a=0.3497 nm, D024 structure with a tetragonal structure with a lattice parameter of a=0.5101 nm and c=0.8307 nm, and D0a structure with an ordered structure based on h.c.p. lattice with a parameter of a=0.5106 nm, b=0.4251 nm and c=0.4553 nm, respectively. An isothermal Ni3Si-Ni3Ti-Ni3Nb pseudo-ternary phase diagram at 1323K has been reported by the present authors [10]. The recent revised pseudo-ternary phase diagram keeping Ni content 79.5at.% (Fig. 1) showed that there was a three-phase region of Ni3Si(L12)-Ni3Ti(D024)Ni3Nb(D0a) [11]. The present study focuses on the three-phase microstructure composed of Ni3Si(L12)-Ni3Ti(D024)-Ni3Nb(D0a). We investigate the relation be
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