Characteristic features of shape memory effect and related transformation behavior in Fe-based alloys

Shape memory mechanisms. characteristic of Fe-based alloys, are discussed based on the microscopic observations with various scales ranging from micron meter to sub-nanometer. Most of the Fe-based shape memory alloys that may be technologically applicable in the future are associated with the fcc/bct and fcc/hcp transformations. The modes of the stress-induced martensitic transformation and its reverse transformation in these transformation systems, which are directly related to the shape memory effect, are examined in detail and it is concluded that the following conditions are the most important ones to achieve a good shape memory effect. In the case of the fcc/bct transformation, the tetragonality of the bet martensite must be large so that the twin boundary energy in martensite can be lowered, which results in that very fine transformation twins are completely extended through a martensite plate. A low twin-boundary energy in martensite is an important factor to reduce the density of dislocations residing at the austenite-martensite interface and make easy the reverse movement of the interface. In the extreme case of no dislocations at the interface, a thermoelastic transformation will be realized. A higher yield stress for slip in bet martensite is another factor to bring about the reversibly mobile interface. These are deduced from the detailed analysis of dislocations generated behind the reverse-moving interface on heating. In the case of the fcc/hcp transformation, the formation of extremely thin hcp martensite plates with uniform distribution in a deformed sample is most required to achieve a good shape memory effect. For such thin martensite plates to be produced, high densities of stacking faults must preexist on the primary slip system in austenite when an external stress is applied for shape change. A new mechanism for the formation of very thin martensite plates by an applied stress is presented. A low stress for inducing martensite transformation compared with the yield stress for slip in austenite, which results from the so-called 'training' treatment, may be a necessary but not sufficient condition for obtaining a good shape memory effect. Future directions of the research on Fe-based shape memory alloys are suggested. (C) 1999 Elsevier Science S.A. All rights reserved.