Bulk and thin film microstructures in untwinned martensites

The microstructure in alloys for which the martensite phase is either the 9R or 18R long-period stacking order structure is investigated. A choice of a new unit cell to describe the lattice of the product phase is made, and it is found to give an exact austenite-martensite interface. A comparison with experimental observations for several different material systems supports this choice of unit cell, and the predictions of the shape strain and habit plane normal vectors are the same as those given by a phenomenological calculation. The approach followed here de-emphasizes the role of the internal defects within the unit cell of the martensite lattice in providing the mechanism by which compatibility between the phases is achieved. It is this reason that the name untwinned martensites is proposed to replace the older name faulted martensites. In addition, microstructures in thin film specimens of the alloys exhibiting the untwinned martensite are studied. In particular, a tent microstructure is constructed in a specially oriented film, and such microstructures have potential applications in micro-devices to act as either a pump or an actuator. (C) 1999 Elsevier Science Ltd. All rights reserved.