Investigation of the Effect of Aging Under Stress on the Shape Memory Properties of NiRich NiTiHf High Temperature Shape Memory Alloy
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Shape memory alloys (SMAs) are extraordinary materials having very high shape or strain recovery capabilities via martensite-austenite phase transformation. The shape recovery capability of SMAs can be effectively exploited to generate work against applied load. To utilize SMAs as thermally activated actuators for aerospace applications, high transformation temperatures (TTs) that are beyond 100°C, are desired. NiTi based SMAs exhibits promising properties in terms of high strength and longer functional fatigue life with stable actuation properties, however, they cannot be utilized for high temperature application due to their relatively lower TT. TTs higher than 100°C can be attained when NiTi SMAs are alloyed with Au, Pd, Pt, Zr or Hf. Hf is the utmost favourable alloying element to NiTi Alloys to raise the TTs over 100°C without decreasing the strength of the alloy. As the TTs of NiTi alloys are increased via alloying with Hf they are able to be used at higher temperatures and ternary NiTiHf alloys are named as high temperature shape memory alloys (HTSMAs) in the literature. These alloys lose their strength so, resistance against plastic deformation decreases. Thus, the stability of the shape memory properties such as transformation temperatures (TTs), actuation strain (εact) and thermal hysteresis (Thys) are mitigated. The loss of shape memory properties with the thermal or thermo-mechanical cycles is called as Functional Fatigue (FF) in SMA literature. To increase the stability of the functional shape memory properties of HTSMAs deformation and age hardening methods can be applied. In this study, Ni-rich Ni(50.3at%)Ti(30at%)Hf(20at%) HTSMA was used since it can be a deformation as well as an age hardenable alloy. As cast alloy was solutionized to homogenize the chemistry of the materials. Then, thermal and thermo-mechanical treatments were conducted after solutionizing heat treatment. Stress free aging, stress assisted aging and cold rolling with successive aging treatments were applied. Aging time and temperature parameters were applied as 1 hour and 550°C, respectively in all aging treatments. 5% thickness reduction was applied in cold rolling operation. TTs of all thermally and thermo-mechanically processed samples were measured under stress free condition and functional fatigue experiments (FFEs) were run under 200MPa via thermally cycling the samples between the predetermined upper and lower cycle temperatures (UCT and LCT) to determine the evolution of the shape memory properties with the applied processes. Then, all the results were compared by taking the solutionized sample as the base sample. Experiment results showed that highest functional fatigue life was achieved in the sample, which was cold rolled (CR) with 5% thickness reduction and then successively stress aged at 550°C for 1 hour under 200MPa. Additionally, cold rolling with successive stress aging treatment managed to obtain very stable but relatively lower εact values in Ni-rich Ni(50.3at%)Ti(30at%)Hf(20at%) HTSMA.