The effect of two types of precipitates upon the structure of martensite in sputter-deposited Ti-Ni thin films

In sputter-deposited Ti-rich Ti-Ni shape memory alloy thin films, there are two peculiar types of precipitates: plate-like GP zones formed along (100) directions of the Ti-Ni phase and spherical Ti2Ni precipitates homogenously distributed within grains. The effect of these precipitates on twinning in stress-induced martensite were studied with the use of a transmission electron microscope and compared with previous observations of twinning in thermally formed martensite of Ti-48.9 at.% Ni and Ti-48.5 at.% Ni thin films. The main substructure of thermally formed martensite in the thin films containing GP zones or Ti2Ni precipitates consists of three orientations of fine (001) twins. On the other hand, the stress-induced martensite consists of domains of lamellar <011> type II twins and fine (001) twins. While the domains of <011> type II twins are preferably oriented in a way characteristic of stress-induced martensite, the (001) compound twins are not. The <011> type II twins are introduced under stress as a lattice invariant shear, in preference to (001) compound twins.