Thickness and film stress effects on the martensitic transformation temperature in equi-atomic NiTi thin films

This paper presents the results of an experimental study of the transformation behavior and its thickness dependence of submicron Ni–50.5 at.%Ti thin films on substrates. Measurements were performed on thin films with a microstructure that consisted of pancake-like grains with aspect ratios in excess of 50:1 in order to minimize grain-imposed constraints. The effect of the residual stress on the martensite transformation behavior was evaluated by using different substrate materials covering a range of thermal expansion coefficients. If the thermal stress in the austenite phase exceeds ∼300 MPa, the martensitic transformation occurs in a one-step process from B2 → B19′, while films with a lower stress follow a two-step transformation of B2 → R-phase → B19′. The slope of the stress–temperature curve during the B2 → B19′ transformation is much larger than the equilibrium value and trends up with decreasing film thickness. For films less than 400 nm in thickness, the transformation temperature decreases rapidly with decreasing film thickness. Cross-sectional TEM analysis reveals small composition shifts in the immediate vicinity of the film interfaces, but not in the bulk of the films, ruling out any composition effects on the transformation temperature. A simple micro-mechanics model is used to explain the observed size effects.