Compositional and microstructural sensitivity of the actuation fatigue response in NiTiHf high temperature shape memory alloys

Martensitic transformation temperatures and other transformation characteristics are extremely sensitive to composition and microstructural features in shape memory alloys (SMAs), particularly in well-known NiTi based SMAs. Because of this sensitivity, it is challenging to synthesize NiTi based SMAs with the same transformation characteristics and thermomechanical properties repeatedly; making qualification and certification of these materials difficult. The present study analyses variations in actuation fatigue properties of four batches of Ni 50.3 Ti 29.7 Hf 20 (at.%) high temperature SMA (HTSMA), with similar fabrication methods, through microstructural observations, thermal and compositional studies, thermomechanical testing and tensile actuation fatigue experiments. Small differences in the Ni content and volume fraction of non-metallic inclusions of different batches are found to alter the transformation temperatures and actuation fatigue properties significantly. In general, it is observed that higher Ni content and volume fraction of non-metallic inclusions shorten the fatigue life. The results presented in this study draw attention to the difficulties associated with the fabrication of a target NiTiHf HTSMA composition with desired mechanical and shape memory properties, while also presenting a methodology to screen different batches of the alloy for superior actuation fatigue response using conventional microstructural, thermal and mechanical characterization.