Experimental and calculation approach for phase equilibria among γ/TCP/GCP oP6 phases at elevated temperatures

Phase equilibria among the A1 (gamma-fcc), Ni2Cr (oP6) and TCP phases in Ni-Cr-Mo system at temperatures above 973 K have been investigated, to evaluate the possibility for utilizing a novel microstructure design principle for Ni-based alloys having TCP phase at grain boundaries and GCP phase other than gamma ' phase within grain interiors. Unlike the phase diagram calculated based on commercially available thermodynamic databases, the Ni2Cr phase in the binary system becomes stabilized by the presence of Mo solute atoms in solution at temperatures greater than 200 K, and the Ni-2(Cr, Mo)-oP6 single-phase region exists as an island at and around the composition of Ni-20Cr-15Mo (at.%) between temperatures of 973 K and 1073 K. Two distinct three-phase regions of gamma + oP6 + P and gamma + oP6 + TCP NiMo (oP56) were found to exist around the oP6 single-phase region. In the calculation the isothermal section is reproduced by thermodynamic calculation, using a regular solution model for liquid, fcc and bcc phases, and sublattice model for compounds. The novel phase transformations and microstructures occurring in this class of alloys may potentially lead to advances in the design of novel Ni-based alloys.