Martensitic Transformation from nanocrystalline austenite

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We fabricated austenite structures with grain sizes ranging from 34 um to 290 nm in an Fe-22Ni (wt%) alloy by electrodeposition and subsequent annealing, and investigated microstructure and crystallographic features of martensite transformed from nanocrystalline austenite. Typical lath martensite formed from the coarse-grained austenite with a mean grain size of 34 um. With decreasing the austenite grain size below 4.0 um, the morphology of martensite changed from lath to plate shapes. Through crystallographic orientation analysis, we found that several martensite plates formed within one austenite grain even when the austenite grain size was 290 nm. In addition, crystallographic features, such as variant selection, of martensite transformed from the nanocrystalline austenite were completely different from those of the typical plate-type martensite transformed from coarse-grained austenite.