ＴｉＣ粒子を複合化した（γ′＋β）２相ニッケルアルミナイドの熱調質処理による高温機械的性質の改善

Abstract

Ni3AI (γ')-NiAI(β) two-phase nickel aluminides with or without reinforcement of 10vol.% TiC particles (expressed as 10%TiC and 34Al, respectively) can be fabricated by a wet mechanical alloying and vacuum hot pressing process. Recently, we have reported these two-phase nickel aluminides have very fine grain structure and, thus, show a high strain rate superplasticity. In this study, an effect of two types of heat treatment on the microstructure have been investigated in order to improve a mechanical properties at high temperatures. The microstructure control is on concerning of the change from the fine grain structure suitable for superplasticity to a coarse grain structure or a lamella structure for the strengthening at high temperatures after the superplastic deformation.The heat treatment to obtain a coarse grain structure in the two-phase nickel aluminides was carried out at 1623 K for 43.2ks. The grain coarsening to about 14μm from 1.2μm in 34Al and to about 8μm from 0.9μm in 10%TiC were attained in average grain sizes. The heat treatment to obtain a lamella structure was consisting of oil quenching from 1623 K and tempering at 1073 K or 1273 K. After this heat treatment, the network like precipitation of γ' phase at the prior β-grain boundaries was observed in all intermetallics. A fine lamella structure was only formed in the network of 34Al tempered at 1073 K.After both heat treatments, the bending strength and defection to fracture in three point bending test above 873 K, and 0.2% flow stress in compression test above 973 K for 34A1 and 10%TiC were improved, compared with fine grained intermetallics. Especially, the formation of ductile networks of γ' phase surrounding brittle β phase was considered to be effective for improving both high temperature strength and ductility. The heat treatment for the coarse grain structure showed more advantage for strengthening at temperatures above 1173K.