Influence of Ni Addition on Mechanical Properties and Fracture Behaviors of Ir-15Nb Two-Phase Refractory Superalloys
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Influence of Ni Addition on Mechanical Properties and Fracture Behaviors of Ir-15Nb Two-Phase Refractory Superalloys Yuefeng Gu*, Y. Yamabe-Mitarai, Y. Ro, T. Yokokawa and H. Harada National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibarake 305-0047, Japan (*On leave from Shanghai Jiao Tong University, Shanghai 200030, P. R. China) ABSTRACT Ir-based alloys with fcc-L1 2 two-phase coherent structure, which are called "refractory superalloys", have good potentiality as structure materials used at ultra-high temperatures up to 2000 'C. Preliminary results showed that the refractory superalloys failed predominately by brittle intergranular fracture at room temperature even though they showed higher strength at that temperature. This paper will present the influence of nickel (Ni) addition on mechanical properties and fracture behaviors of one of these alloys, Ir-15at%Nb. The results indicated that Ni addition was beneficial to the compression ductility and strength of two-phase Ir-15at%Nb alloy when Ni content was below the optimum content. INTRODUCTION Intermetallic alloys appear to offer the best opportunity for advanced high temperature materials, but the demands of this application require property-performance combinations that can not be satisfied with the available single-phase alloys. There is a growing experience that multi-phase intermetallic alloy and microstructure can provide the necessary performance. Recently, we developed a new class of two-phase superalloys, namely, refractory superalloys, based on platinum group metals[I-5]. These refractory superalloys have a coherent fcc-L1 2 structure that exhibits morphologies similar to 7/y / microstructure in nickel-base superalloys (i.e. the disordered phase is the matrix). Preliminary results showed that, of these refractory superalloys, ft-based fcc-L 12 two-phase alloys, such as Ir-15at%Nb, Ir15at%Hf and Ir-15at%Zr alloys, were superior in high temperature strength and oxidation resistance[4,5]. However, Ir makes this refractory superalloy dense and expensive. The challenge is to replace Ir as much as possible in these alloys while still retaining their desirable properties. The aims of the present study are to examine the influence of Ni content, which is complete miscible with Ir in Ir-Ni binary alloy system[6], on mechanical properties and fracture behaviors of one of these refractory superalloys, Ir-15at%Nb. The results showed that Irs85 xNbisNix two-phase alloys with X • 10 were superior to the binary Ir- 15at%Nb alloy as an ultra-high temperature structure material in terms of strength, fracture behavior, and density. EXPERIMENT Five alloys samples with compositions fr85.xNb15Nix (X = 0, 10.0, 20.0, 30.0, 50.0) were prepared by arc melting using high-purity components under argon in a vacuum furnace. The purities of the components were 99.99% Ir, Nb and 99.9% Ni. The specimens were made by electron-discharge machining each ingot to a cylinder 6 mm high and 3 mm in diameter. These specimens were annealed at vacuum (
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