Mechanism of austenite recrystallization induced by martensitic reversion in super invar cast alloy

Abstract

Recently, significant grain refinement in super invar cast alloy having a coarse-grained austenitic structure was reportedly achieved through a novel heat treatment consisting of subzero treatment and subsequent annealing. We investigated the austenite grain refinement mechanism by microstructural analysis using electron backscatter diffraction. Lenticular martensite was observed along the dendrite having a lower Ni concentration after the subzero treatment, which then, reversed to austenite via martensitic transformation upon annealing. The bidirectional martensitic transformation led to the formation of duplex austenitic structure consisting of untransformed and reversed austenite having a coarse-grained structure. Furthermore, recrystallized grains were formed due to the bulging of the prior austenite grain boundaries at regions where untransformed austenite was in contact with reversed austenite. The recrystallized austenite grains preferred growing toward the reversed austenite having annealing twins due to the high dislocation density in the reversed austenite, resulting in a randomly oriented fine-grained austenitic structure.