Thickness effect on shape memory behavior of Ti-50.0at.%Ni thin film

The thickness effect on the shape memory behavior of Ti-50.0at.%Ni thin films was investigated. It was found that the transformation strain and residual strain under a constant stress are very sensitive to the film thickness when the thickness is less than the average grain size, 5 mum. Cross-sectional observation showed that these strains are affected by two kinds of constraints from surrounding grains and surface oxide layers. With decreasing thickness, the former effect becomes weak, but the latter effect becomes strong. As a result, the transformation strain and residual strain show a maximum around a thickness of 1-2 mum. In addition, the transformation temperatures were also found to be affected by surface oxidation if the thickness is less than 1 mum. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.