Crystallography of martensite

For an evaluation of the crystallography of the martensitic phases three aspects are important: 1. The transformation mechanism: How does the martensite phase nucleate and grow within the matrix phase, taking into account that diffusional processes are absents but that nevertheless large shape changes occur, 2. The influence of the transformation mechanism on the selection of the martensite structures. 3. The relationship between the stability of the martensite and of the equilibrium phases that are obtained when diffusion is permitted. In this paper mainly the first aspect will be discussed, using as prototypes the Cu based noble metal shape memory alloys with a transformation from a bcc based lattice to a close packed one, and the Fe-Pt alloys with the transformation from fec to bcc. It will be shown that the atom movements during the transformation can be described by a combination of two shears, with the possible exception of the bcc to hexagonal transformation. Arguments are presented to substantiate this mechanism, It also permits to discuss the factors which control the energy dissipation during these shears and thus determine the hysteresis between the transformation and the retransformation. It is suggested that in order to get a good shape memory effect with little hysteresis it is necessary to have a long range ordered matrix.