Experimental phase diagram study of the Ni-rich part of the Ni–Cr–Mo ternary system

Abstract

Experimental studies were conducted to investigate the phase equilibria of the Ni-rich portion of the Ni–Cr–Mo ternary system using scanning electron microscopy and electron probe microanalysis of heat-treated alloys. Experimental isothermal sections involving L-liquid, γ-Ni, P-NiCrMo, NiMo, Ni₃Mo, and Ni₂(Cr, Mo)-oP6 phases were constructed at temperatures below 1573 K. Two distinct liquid phase regions with varying compositions were observed at 1573 K, indicating a phase separation of the liquid phase (L → L₁ + L₂). This can result in the presence of two ternary eutectic reactions: L₁ → γ + P + NiMo and L₂ → γ + σ + P. The Ni₂Cr phase in the binary system was stabilized by substituted Cr by Mo at temperatures above 200 K. The Ni₂(Cr, Mo)-oP6 single-phase region existed as an island around the composition of Ni–9Cr–24Mo (at.%) at 1073 K. The experimentally identified γ + oP6 + Ni₃Mo and γ + P + oP6 regions suggest that the oP6 phase is formed by a ternary peritectoid reaction (γ + P + Ni₃Mo → oP6). Based on these results, the reaction pathways related to the liquid and oP6 phases in the Ni–Cr–Mo ternary system were modified.