Effect of Grain-boundary TCP P (oP56) Phase on Creep in Ni-Cr-Mo Alloys at Elevated Temperatures

The grain-boundary TCP Fe₂Nb Laves phase is found to be effective in increasing the long-term creep rupture strength at temperatures above 973 K in novel Fe-Cr-Ni-Nb austenitic heat-resistant steels strengthened by intermetallic phases, because of the “Grain-boundary Precipitation Strengthening (GBPS)” In this study, thus, an attempt has been made to verify if the GBPS effect due to TCP phase can work for creep strengthening in Ni-based alloys.

Two alloys with compositions of Ni-15Cr-15Mo and Ni-15Cr-17Mo (at. %) were selected, based on our phase diagram study on Ni-Cr-Mo system. In this system, fcc γ-Ni phase is in equilibrium with TCP P phase above 1173 K, whereas it can be in equilibrium with GCP Ni₂(Cr, Mo) phase (oP6) bellow 1073 K. Thus, multi-step heat treatments were employed to the alloys by taking the Mo supersaturation into account for the formation of both P and oP6 phases, and the creep specimens having the different ρ of P phase, but with the same grain-interior hardness by the formation of oP6 phase, were prepared.

The specimen having the higher ρ (60%) exhibits the creep rupture time t_r (=3020 h) three times longer than that (t_r =1090 h) of the specimen with the lower ρ (35 %) under 300 MPa at 973 K. The microstructure analyses revealed that the creep cavities preferentially form at the bare grain boundaries and they are easily coalesced each other. Thus, the specimen with high ρ shows longer rupture life. From all these results, it is obviously that the GBPS by thermally stable TCP phase works for creep strength even in Ni-based alloys. Based on this study, novel microstructure design concept for Ni based alloy without relying on Ni₃Al-γ’ phase will be touched.

Part of this work was supported by JST-Mirai Program Grant Number JPMJMI21E7, Japan.