Investigating The Effect Of Aging Heat Treatment On The Functional Fatigue Behavior Of Ni50.3Ti29.7Hf20 High Temperature Shape Memory AlloyWednesday (16.05.2018) 10:40 - 11:05 Part of:
The functional fatigue response of extruded and aged Ni50,3Ti29,7Hf20 high temperature shape memory alloy (HTSMA) samples was investigated in this study. Materials were fabricated using high purity Ni, Ti and Hf elemental materials via vacuum induction melting under high purity argon atmosphere, then hot extruded at 9000C with an area reduction of 4:1. Flat dog bone shape tensile specimens were cut using wire electrical discharge machining from extruded bulk material. Specimens for microstructural investigation and stress free transformation temperature measurement were cut via diamond saw precision cutter. One set of the specimens were aged in a furnace at 5500C for 3 hours under high purity argon atmosphere, and then samples were water quenched. Stress free transformation temperatures of both aged and as extruded samples were measured using Perkin Elmer Differential Scanning Calorimetry (DSC) 800. Transformation temperatures of aged sample are above 1000C and higher than that of the transformation temperatures of as the extruded sample. Microstructural investigation was performed by Transmission Electron Microscope to determine the Ni-rich precipitates in the matrix. Functional fatigue experiments were conducted on a custom-built functional fatigue test setup under 200 MPa constant stress till the fracture was experienced. Joule heating and forced air convection methods were used for heating and cooling, respectively. Temperature, displacement and number of cycle data were measured and recorded using a program which was scripted in LabView. Aged sample showed better cyclic stability than extruded sample in terms of both transformation temperatures and actuation strain during functional fatigue tests. A dramatic increase in transformation temperatures was observed in the extruded sample, which was accompanied with a decrease in actuation strain through 30.000 cycles.