Microstructure and crystallographic features of martensite transformed from ultrafine-grained austenite in Fe24Ni-0.3C alloy

Abstract

We studied microstructure and crystallographic features, especially orientation relationship, of martensite transformed from austenite with mean grain sizes ranging from 35μm to 750nm in an Fe24Ni0.3C alloy. The austenite structures with ultrafine or fine grains were fabricated through intense straining by accumulative roll-bonding process and subsequent annealing. The morphology of martensite transformed from the austenite with various grain sizes was all lenticular type. On the other hand, the martensite plate size decreased with the decrease in the austenite grain size. The orientation relationship of martensite transformed from coarse-grained austenite with mean grain size (d) of 35 μm changed depending on the location within martensite plate: i.e., it was GreningerTroiano relationship at the center part of each martensite plate, while it changed to KurdjumovSachs relationship near the interphase boundary. In contrast, the whole area of each martensite plate, transformed from fine-grained austenite (d = 2.5 μm) and ultrafine-grained austenite (d = 750 nm), satisfied GreningerTroiano relationship. Furthermore, high density of dislocations and low angle boundaries within the ultrafine-grained austenite resulted in a large scatter of observed orientation relationship. © 2011 The Japan Institute of Metals.