The effects of the reaction temperature, a superimposed elastic stress and prior plastic deformation of the austenite on the dimensions of martensitic plates have been measured in an Fe-23.2wt.%Ni-2.8wt.%Mn alloy, which transforms isothermally at subzero temperatures. The mean semithickness-to-radius ratio (c/r)i at about 0% martensite is found to decrease from 0.085 at - 70°C (203 K) to 0.042 at - 196°C (77 K). On the assumption that a net driving stress equal to the yield stress of the austenite is required for the outward movement of the interfacial dislocations and that a fraction of the transformation strain energy is released as plastic work during the growth of the plate, it is possible to account for the observed decrease in (c/r)i. The released strain energy is estimated to be about 50% at - 70°C (203 K) and 4% at - 196°C (77 K). This is consistent with the transmission electron microscopy observation that the extent of the dislocated region in the partially twinned plates decreases with decreasing temperature, the plates becoming fully twinned at - 196°C. Under a superimposed elastic stress, the (c/r) ratio of the plates increases with increasing stress level. This is explained on the basis of the decrease in the stress required to move the interfacial dislocations. With increasing prior plastic strain of the austenite, the plates become thinner. This is due to the work hardening of the austenite, which increases the stress required to move the interfacial dislocations.
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