Phase transformation, structural and functional fatigue properties of Ti–Ni–Hf shape memory thin films

The shape memory thin film system Ti–Ni–Hf was investigated with regard to its structural, phase transformation and functional fatigue properties by means of combinatorial and high-throughput methods. Temperature-dependent resistance measurements revealed a broad compositional region showing a reversible phase transformation. A ternary Laves phase was identified using X-ray diffraction as a precipitate phase within the transforming composition region. With increasing Ti content, the amount of the Laves phase increases, which results in an increase in the thermal hysteresis and a simultaneous decrease in the transformation temperatures. Shape memory properties were characterized by temperature-dependent stress change measurements using micromachined Si cantilever array wafers coated with Ti–Ni–Hf. The recovery stress was found to increase for small amounts of Laves phase precipitates. Strengthening of the matrix due to the Laves phase precipitates is concluded to be responsible for the observed increase in recovery stress and improved functional fatigue properties for (Ti,Hf)-rich alloy compositions (Ti40.0Ni47.5Hf12.5).

Research highlights
► The complete reversibly transforming region in the Ti–Ni–Hf system was revealed.
► The existence of a Laves phase in the Ti–Ni–Hf system was proven.
► Ti40Ni47.5Hf12.5 shows superior functional fatigue properties.