Phase equilibria among γ, Ni3Nb-δ and Fe2Nb-ε phases In Ni-Nb-Fe and Ni-Nb-Fe-Cr systems at elevated temperatures

Abstract

Phase equilibria in Ni-Nb-Fe ternary and Ni-Nb-Fe-Cr quaternary systems at elevated temperatures have been examined. In the ternary system, Ni3Nb-δ (DOa) single phase field expands along the equi-niobium concentration up to 10% Fe at 1473 K. The δ phase becomes in equilibrium with (Fe, Ni)2Nb-ε (C14 Laves) phase with a composition of Ni-30Fe-26Nb and γ phase with its composition of Ni-35Fe-10Nb. A new intermetallic phase is found at compositions around Ni-22Nb-20Fe, and the crystal structure is identified to be ordered hexagonal (hP24 with stacking sequence of abcbcb). This phase is in equilibrium with γ and δ phases, and no δ+γ+ε three-phase coexisting region exists in this system. In Fe-20Cr-(25-40)Ni-(2-6)Nb quaternary system, four intermetallic phases of δ, C14 Laves (ε), C15 Laves (Cr2Nb) and (Ni, Fe)6Nb7-μ phases are found to exist in γ matrix, and the isothermal tetrahedron at 1473 K in this system is constructed. The γ-Ni solid solution phase becomes in equilibrium with C14 Laves phase in Ni poor side (less than 30%), whereas it is in equilibrium with C15 Laves phase in Ni rich side (∼40%). At the very limited region between them, there is a small window opened where the γ phase can be in equilibrium with μ phase. However, at 1073 K, only δ and C14 phases become in equilibrium with γ phase. The changes in the phase equilibria with temperature is caused by the occurrence of two transition peritectoid-type reactions: (I) γ+C15→δ+μ, (II) γ+μ→δ+C14.