Phase stability of Ni-base single crystal superalloys was compared between Ir addition and Ru addition. Investigated alloys were TMS-238 and its derivative alloys. The thermodynamic equilibrium microstructures obtained by a strain aging showed that the volume fractions of γ, γ and topologically close packed (TCP) phases in the alloys were virtually the same to each other without depending on Ir substitution for Ru. On the other hand, the time temperature transformation diagrams obtained by an ordinary aging showed a significant delay in TCP precipitation in the alloys with Ir substitution. The delay in the TCP precipitation might be attributed to the smaller interdiffusion coefficient of Ir-Ni, compared with one of Ru-Ni. The smaller interdiffusion coefficient may affect the kinetics of the TCP precipitation.
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