Investigating the effect of hot extrusion and annealing to the functional fatigue behavior of Ni₅₀Ti₃₀Hf₂₀high temperature shape memory alloy

This study reveals the functional fatigue properties of hot extruded and the effect of post annealing process to the cyclic stability and fatigue properties of Ni50Ti30Hf20 (at.%) alloy. Samples were cut from the hot extruded billet and three of them were randomly selected for functional fatigue experiments. Variation in the shape memory properties such as transformation temperatures, actuation and accumulated irrecoverable strain values was observed. Additionally, the samples showed 580, 754 and 1738 thermal cycles under 200 MPa in fatigue experiments. Six more samples were cut through the cross section of another piece of the extruded billet and enumerated for further fatigue testing to investigate the reason of the variation in the properties. The edges of the gauge sections of enumerated fatigue test samples were ground to remove the surface roughness. Although the fatigue life times were increased by 3-6 times with the edge grinding the differences in the values of the shape memory properties were still observed and this observation was attributed to the deformation variation and the residual stresses induced by hot extrusion. To overcome this problem, hot extruded samples were annealed and the fatigue experiments were repeated. All annealed samples exhibited the same transformation temperatures, actuation and accumulated irrecoverable strain values. Moreover, the transformation temperatures were stabilized and the fatigue life was increased to almost 12 000 cycles.