Bulk and thin film microstructures in untwinned martensites

The microstructure in alloys for which the martensite phase is either the 9 R 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 i t is found to give an exact austenite-martensite interface. A comparison wi th experimental observations for several different material systems support s this choice of unit cell, and the predictions of the shape strain and hab it plane normal vectors are the same as those given by a phenomenological c alculation. The approach followed here de-emphasizes the role of the intern al 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 olde r name faulted martensites. In addition, microstructures in thin film speci mens of the alloys exhibiting the untwinned martensite are studied. In part icular, a tent microstructure is constructed in a specially oriented film, and such microstructures have potential applications in micro-devices to ac t as either a pump or an actuator. (C) 1999 Elsevier Science Ltd. All right s reserved.