Crack growth behavior during actuation cycling of hot extruded and annealed Ni₅₀Ti₃₀Hf₂₀high temperature shape memory alloys

Actuator-type applications of high temperature shape memory alloys (HTSMAs) require better understanding of the crack growth behavior at high temperatures as there are few studies on how cracks propagate and cause fracture with phase transformation via cooling and heating under constant load. In this study, hot-extruded and subsequently annealed Ni50Ti30Hf20 (at.%) HTSMA dog-bone shaped tensile test samples with pre-notches were cooled and heated under uniaxially applied constant load magnitude till fracture. Therefore, the crack growth behavior and effect of annealing heat treatment on the crack growth rate of Ni50Ti30Hf20 (at.%) HTSMA during actuation fatigue were particularly determined. Additionally, fatigue tests were conducted twice on the pre-notched samples to investigate the repeatability of the data obtained from the experiments. It was shown that slower crack growth was observed in the pre-notched annealed samples than in the hot extruded samples because of the possible stress-relieving effect of annealing after the hot extrusion process. Several minor side crack formations and the propagation of these side cracks together with the crack growth behavior of the major crack were realized in the annealed samples. In contrast, the major crack growth rate was found to be higher than that of the growth rate of the minor side cracks. Additionally, the annealing process led to obtain consistent crack growth rate values and actuation behavior in the cooling-heating cycles.