Martensitic transformation in Ni₅₀Ti₅₀ films

The elasticity and anelasticity of Ni₅₀Ti₅₀ films deposited on Si substrates were studied, yielding information on the damping and modulus softening. It was found that the transformation behavior strongly depends on the film thickness and approaches bulk Ni₅₀Ti₅₀ behavior as the film becomes a few microns thick. For the same film thickness the transformation depends on the film-substrate adhesion. In films with good adhesion, cross-sectional transmission electron microscopy reveals a thin parent phase layer which does not transform, while the bulk part of the Ni₅₀Ti₅₀ film transforms. It is thus proposed that interface constraints stabilize the B2 structure. A microscopic interpretation in terms of transformation strains at the interface is given.