Engineering the stress temperature response of shape memory alloy thin films through the development of composite structures

The high work to volume ratio and the stress recovery of Shape Memory Alloy (SMA) thin films with temperature makes them an ideal choice for microactuators. However, these materials have not gained widespread acceptance due to issues linked with the phase transformation. Primary concerns are the rapid change in stress (narrow transformation range) at the transformation temperature and the wide hysteresis associated with shape memory effect. In applications such as MEMS mirrors and microfluidics, rapid changes (deflections, strain etc.) could ruin devices. The present research developed SMA thin film structures with high stress recovery showing close to linear transformation behavior and minimal hysteresis within the temperature range of interest. It was achieved through the deposition of SMA thin films with varying composition in a layered (composite) format on a single wafer.;Three alloy targets (TiNiCu with transformation in the range 35 to 80°C, TiNi: 80 to 110°C and TiNiHf: 110 to 180°C) and a pure Ti target were used for film deposition. Single layer (TiNi, TiNiCu and TiNiHf) films, two layer (TiNi+TiNiCu and TiNiHf+TiNi) and three layer (TiNi+TiNiHf+TiNiCu) composite structures were fabricated and characterized. For identical film thicknesses and anneal conditions, TiNi and TiNiCu films displayed transformation slopes of 12 and 6 MPa/°C respectively. The TiNi+TiNiCu composite exhibited a two step transformation (low transformation slopes of 2.5 and 3.9 MPa/°C) without a significant impact on stress recovery. Displaying identical recovery stresses, the TiNiHf film possessed a 65°C transformation range and the TiNiHf+TiNi composite exhibited a wide range of 120°C. In addition, the hysteresis dropped by 30°C for the composite compared to a TiNiHf film. A three-layer composite structure exhibited a wide transformation range of 130°C with a significant drop in hysteresis values compared to single and two layer films. These SMA films with superior transformation characteristics (high stress recovery, wide transformation range, close to linear transformation) can be used in MEMS, microfluidic devices and vibration damping applications.