This book investigates the characteristics of martensite (microstructure, martensite transformation starting temperature (Ms), orientation relationship and martensite variant selection rule) transformed from ultrafine-grained (UFG) austenite fabricated by accumulative roll bonding (ARB) process and subsequent annealing were studied in details. The results indicated that the morphologies of martensite transformed from the coarse-grained austenite and UFG austenite were both lenticular types. However, some martensite plates transformed from the UFG austenite contained low angle boundaries exhibited broken and fragmented shape. Evaluation of the Ms for the ARB processed specimens indicated that it did not change so much, even though the austenite grain size greatly decreased to UFG regime. Orientation relationship of martensite transformed from the UFG austenite was the Greninger-Troiano (G-T) relationship, but the observed orientation relationships were significantly scattered between different martensite plates. In UFG austenie, the elongated grain shape plays an important role for the variant selection of martensite.
