Discussion on the mechanism of micro-element Hf eliminating PPBS in powder metallurgy superalloy FGH96
ZHANG Yi-wen1, 2, LIU Jian-tao1, 2, HAN Shou-bo1, 2, CHI Yue1, 2
1. High Temperature Material Institute, Central Iron and Steel Research Institute, Beijing 100081, China 2. Beijing Key Laboratory of Advanced High Temperature Materials, Beijing 100081, China
Abstract：The precipitates at prior particle boundary (PPB) are composed of carbide and small amount oxycarbide or oxide in PM superalloys. The composition and structure of carbide inside the powder, on the powder surface and on the PPB were investigated by SEM, TEM, AES techniques, etc. The mechanism of Hf eliminating prior particle boundary structure (PPBS) was discussed according to thermodynamics and diffusion theory. The results show that metastable MC type carbide MC′ rich in Ti, Nb, Cr, Mo and W elements forms on powder surface during atomization, the metastable MC′ carbide will transform into stable MC carbide during hot isostatic pressing (HIP) process. Ti and C inside powder particles will diffuse to sintering neck during HIP process, then stable MC carbide (Ti,Nb)C will form on powder particle boundaries. When micro-element Hf is added in, more and more stable MC type carbides containing Hf precipitate within powder particles, which means C and Ti elements are bound in (Ti,Nb,Hf)C carbides, suppressing the diffusion of C and Ti to sintering neck, and finally the precipitation of MC type carbides is inhibited.
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