Phase Equilibria Among A1/TCP/GCP Phases and Microstructure Formation in Ni–Cr–Mo System at Elevated Temperatures

Abstract

Phase equilibria among the A1 (γ-fcc), Ni2Cr (oP6) and TCP phases in Ni–Cr–Mo system at temperatures above 973 K have been investigated, in order 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 γ′ 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 Ni2(Cr, Mo)-oP6 single-phase region exists as an island at around the composition of Ni–20Cr–15Mo (at.%) at temperatures above 973 K. The oP6 phase decomposes to γ and P (oP56) phase at temperatures above 1073 K. Two distinct three-phase regions of γ + oP6 + P and γ + oP6 + NiMo (oP56) were found to exist around the oP6 single-phase region. In case of the decomposition of high-temperature γ phase to the three-phase mixture comprised of γ + oP6 + P, very fine coherent particles of oP6 phase that are only a few hundred nanometers in size form in the γ matrix with a tweed-like morphology. These precipitates possess an orientation relationship of {1 1 ¯ 0 }γ//(100)oP6, <001>γ//[010]oP6, just like precipitation behavior of γ′ particles in Ni-base superalloys. In contrast, the TCP phase preferentially precipitates at the γ grain boundaries. 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.