Atomic force microscope study of stress-induced martensite formation and its reverse transformation in a thermomechanically treated Fe-Mn-Si-Cr-Ni alloy

Consecutive observations of the stress-induced martensite formation and its reversion by atomic force microscopy have been carried out for the fcc/hcp transformation in the thermomechanically treated sample of an Fe-Mn-Si-Cr-Ni shape memory alloy. It is found that thin martensite plates of 0.1-0.2 mum thickness, which are the same martensite variant on the same habit plane, are formed one after another at the immediate neighbor of the existing martensite plate. These martensite plates make a group of several plates within the width of 1-2 mum. This formation mode of martensite is compared with those martensite plates observed by high resolution microscopy and optical microscopy and it is concluded that the basic mode of the stress-induced transformation is that each martensite plate is induced to relieve the shape strain of the existing martensite plate for all the observed magnifications ranging from several hundreds to several millions. The first martensite plate formation is presumed to occur at the pre-existing stacking fault in austenite. In the reverse transformation on heating, it is likely that each martensite plate is reverse-transformed one after another by reverse movement of the Shockley partial dislocations residing at the tip of the plate. This seems to be true for every range of the observable magnification from namometres to microns. Such a reverse transformation mode ensures a good shape memory effect in Fe-Mn-Si-based shape memory alloys. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.