Martensite transformation from ultrafine grained austenite in Fe–28.5 at.% Ni

Martensite transformation from ultrafine-grained (UFG) austenite fabricated by the accumulative roll-bonding (ARB) process in an Fe–28.5 at.% Ni alloy was studied. The sheet ARB processed by five cycles (equivalent strain: ɛ = 4.0) showed the ultrafine lamellar austenite grains with the mean lamellar spacing of 230 nm. The martensite transformation start temperature (Ms) of the UFG austenite fabricated by five cycles of ARB process was 215 K, which was lower than that of the coarse-grained austenite (d = 22 μm, Ms = 251 K). The martensite transformed from the UFG austenite showed two kinds of characteristic morphologies. One was the martensite crystal elongated to the rolling direction (RD) of the sheets (type I), which was quite similar to the UFG austenite before transformation. The other was the martensite crystal inclined by about 50° to RD (type II). The mean thicknesses of the types I and II martensite crystals were 146 and 61 nm, respectively.