Functional Fatigue Behavior of Severe Plastically Deformed Ni50Ti30Hf20 High Temperature Shape Memory AlloyWednesday (16.05.2018) 11:45 - 12:10 Part of:
Ni50Ti30Hf20 high temperature shape memory alloy (HTSMA) was produced using high purity Ni, Ti and Hf elements via vacuum induction melting under high purity argon atmosphere, then placed in a mild steel can for hot extrusion at 9000C with an area reduction of 4:1. The alloy was further severe plastically deformed via Equal Channel Angular Extrusion (ECAE) following route C for 2 passes at 700°C. Flat dog bone shape tensile specimens were cut using wire electrical discharge machining from extruded and ECAEd materials. Stress free transformation temperature measurements were done using Differential Scanning Calorimetry to determine the upper and lower cycle temperatures that were used in functional fatigue experiments. It was found that all the transformation temperatures of both extruded and ECAEd samples are above 2000C.
A custom-built test setup was utilized to investigate the functional fatigue behavior of extruded and ECAEd samples under 200 MPa constant stress magnitude. Joule heating and forced air convection methods were used and temperature, displacement and number of cycle values were measured and recorded using LabView program. It was found that both the extruded and ECAEd samples showed very good cyclic stability in terms of shape memory properties such as actuation strain, transformation temperatures and termal hysteresis. However, the actuation strain value of ECAEd sample was measured at around 2.2% throughout 1200 cycles and this value was higher than that of the extruded sample. On the other hand, the functional fatigue life of the ECAEd sample was determined as 1200 cycles while the extruded sample resisted to failure for 1700 cycles.