Novel Microstructure Design Approach Using TCP and GCP in Ni-Cr-M Ternary Systems Based on Phase Diagram Study at Elevated Temperature

Strengthening of Ni-based superalloys is in principle designed using GCP (Geometrically Close-packed phase) of Ni₃Al-γ' (L1₂). However, game-changing microstructural design principle without relying on γ' phase will be needed for further development of the alloys. We are currently constructing a novel microstructure design principle, using thermodynamically stable TCP (Topologically Close-packed phase) for grain boundaries, together with GCP other than γ' phase for grain interiors, based on grain boundary precipitation strengthening (GBPS) mechanism. One of the promising systems is Ni-Cr-Mo ternary system, where TCP of NiMo (oP112) phases, μ (hR13) and P (oP56), together with GCP of Ni₃Mo (oP8) and Ni₂Cr (oP6) exists. In this study, thus, phase equilibria among A1 (fcc)/TCP/GCP phases in Ni-Cr-Mo and Ni-Cr-W systems have been examined at temperature range from 973 K to 1073 K, based on experiment and calculation. In Ni-Cr-Mo system, Ni₂(Cr, Mo) with oP6 Pearson symbol, which is stable at about 873 K in Ni-Cr binary system, is formed to exist even at 1073 K. oP6 phase is coherently formed in A1 matrix with a crystallographic orientation of {110}_A1 // (100)_oP6, <001>Α1 // [010]oP6, indicating GCP at composition range around Ni-15Cr-15Mo as island. In Mo-rich region there is Α1/NiMo/oP6 three-phase coexisting region, whereas another three-phase coexisting region of Α1/P/oP6 exists in Cr-rich region. Based on vertical section, it is possible to design microstructure with TCP at grain boundaries, together with oP6 phase within grain interiors by two-step heat treatment.