Selection of the Massive-like δ-γ Transformation due to Nucleation of Metastable δ Phase in Fe-18 mass%Cr-Ni Alloys with Ni Contents of 8, 11, 14 and 20 mass%

Abstract

It has been realized that a massive-like transformation, in which δ phase (ferrite) transformed to γ phase (austenite) in the solid state during and after solidification, was selected in Fe-C steels. X-ray radiography confirmed that the massive-like transformation also occurred in Fe-18 mass%Cr-Ni alloys with Ni contents of 8, 11, 14 and 20 mass%Ni. According to the equilibrium phase diagram, δ phase is the primary phase in 8 and 11 mass%Ni alloys while γ phase in 14 and 20 mass%Ni alloys. Solidification was always initiated by nucleation of δ phase and consequently fine γ grains were formed by the massive-like transformation in 8 and 11 mass%Ni. On the other hand, nucleation of δ phase as a metastable phase was preferably selected at lower undercoolings (<50 K) in 14 and 20 mass%Ni and consequently the massive-like transformation occurred even in 14 and 20 mass%Ni alloys. Solidification of γ phase can be triggered by nucleation of δ phase followed by the massive-like transformation in the Fe-Cr-Ni with lower Cr/Ni values (the primary γ alloys). Moreover, the present study demonstrates that the massive-like transformation will be commonly observed in Fe-based alloys, in which δ and γ phases are competitive each other from a thermodynamic perspective.