2012 Volume 53 Issue 1 Pages 81-86
We studied microstructure and crystallographic features, especially orientation relationship, of martensite transformed from austenite with mean grain sizes ranging from 35 µm to 750 nm in an Fe–24Ni–0.3C 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 Greninger–Troiano relationship at the center part of each martensite plate, while it changed to Kurdjumov–Sachs 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 Greninger–Troiano relationship. Furthermore, high density of dislocations and low angle boundaries within the ultrafine-grained austenite resulted in a large scatter of observed orientation relationship.
