Phase Stability of A₂B-Type GCP Compound in Ni-Cr-Mo System Alloys at Elevated Temperatures

Most of the geometrically close-packed (GCP) compounds are A₃B type, whereas a very few A₂B type GCP compound exists. The Ni₂Cr (<i>oP</i>6) is one of them. According to the Ni-Cr phase diagram, this phase is a Kurnakov-type compound and is stable up to 867 K. Recently, our systematic phase diagram study on Ni-Cr-Mo ternary system experimentally reveals that a single-phase region of Ni₂(Cr, Mo) with <i>oP</i>6 crystal structure exists as an island with compositions around Ni-15Cr-15Mo (at.%), up to about 1073 K, approximately 200 K higher than the binary Ni₂Cr compound. The reason why Mo in solution in Ni₂Cr phase stabilizes the phase is bonding energy between Cr and Mo in Cr site should be negative, because thermodynamical calculation shows that modifying ternary-interaction parameter <i>L</i>_Cr,Mo:Ni to negative is the most strongly contributes to reproduce the experimental phase diagram in other thermodynamical parameters, <i>e.g.</i> end-members and interaction parameters among binary and ternary elements. This result is very attractive from both fundamental and engineering viewpoints. The Ni₂Cr phase could be an alternative strengthener to Ni₃Al-γ' phase in Ni-based alloys, if we can further increase the phase stability. In this study, then, in order to verify the mechanism of the increase in phase stability of Ni₂Cr compound by Mo addition, effect of 5^th and 6^th group elements on the phase equilibria at elevated temperatures were systematically examined. Thermodynamic calculations to evaluate the phase stability of this phase is also conducted, by taking the interaction parameters among the three elements into account. Two series of alloys were prepared: series I with nominal compositions of Ni-15Cr-15(Mo, M) (at.%) by replacing Mo with M by every 5 at.% and series II with those of Ni-15Cr-15Mo-5M by addition of 5at.%M to Ni-15Cr-15Mo. These alloys were prepared by arc melting in argon atmosphere with non-consumable tungsten electrode as 35 g button ingots. These ingots were firstly cold rolled by 40% in height, and solution treated at 1473 K for 24 h, followed by a water quench, in order to eliminate the micro segregation. Then, they were equilibrated at 1173-973 K for up to 1000 h. It was found that, in any alloys containing M, the observed Ni₂(Cr, Mo) single phase region in the ternary system was reduced, even in the case of V where Ni₂V is also the A₂B type GCP compound with the <i>oP</i>6 structures. Instead, the stable A₃B-type GCP phases of Ni₃V-γ" (D0₂₂), Ni₃Nb-δ (D0_a) and Ni₃Ta-δ (D0_a) appears in the quaternary systems, indicating that the M elements apparently destabilize the <i>oP</i>6 phase with respect to the A₃B-type GCP phases. The A₂B type GCP phase has B-B bonding unlike A₃B type, so the total bonding energy should always be smaller for A₂B than A₃B (Ω_A-B &lt; 0). The thermodynamically stable existence of the Ni₂(Cr, Mo) phase, despite the thermodynamic existence of the Ni₃Mo-δ phase in the binary system, must be caused by not only the interaction among ternary elements but also size effects. Based on these results, together with the thermodynamic calculations on the observed A₂B and A₃B type GCP phases, elements which do not form A₃B type GCP phase would possibly be effective in increasing the stability of Ni₂Cr. The size effect as well as atomic scale considerations on the difference in phase stability between A₃B and A₂B type GCP phases will also be touched in detail. A part of this research was supported by JST-Mirai Program Grant Number JPMJMI21E7, Japan.