MARTENSITIC-TRANSFORMATION IN NI50TI50 FILMS

The elasticity and anelasticity of Ni50Ti50 films deposited on Si subs trates were studied, yielding information on the damping and modulus s oftening. It was found that the transformation behavior strongly depen ds on the film thickness and approaches bulk Ni50Ti50 behavior as the film becomes a few microns thick. For the same film thickness the tran sformation depends on the film-substrate adhesion. In films with good adhesion, cross-sectional transmission electron microscopy reveals a t hin parent phase layer which does not transform, while the bulk part o f the Ni50Ti50 film transforms. It is thus proposed that interface con straints stabilize the B2 structure. A microscopic interpretation in t erms of transformation strains at the interface is given.