Thermomechanical training and characterization of Ni-Ti-Hf and Ni-Ti-Hf-Cu high termperature shape memeory alloys
Denowh, Chantz Michael.
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Nickel-Titanium (NiTi) is the most commonly used Shape Memory Alloy (SMA) for actuator applications. Typical SMA actuators use changes in temperature to initiate solid to solid phase transformations resulting in macroscopic material deformations; though NiTi is limited to temperature changes below 100°C. This eliminates high temperature applications of NiTi actuators. To expand the design window of SMAs, many high temperature NiTi based SMAs have been developed by adding ternary elements to the NiTi matrix. The additions result in degradation of the shape memory behaviors and their usefulness as actuators is still in question. The purpose of this research is to characterize and train two recently developed high temperature SMAs, NiTi 29.7Hf 20 and NiCu 5Ti 29.7Hf 20, to determine their effectiveness as linear actuators. Shape memory effect (SME) and superelastic tests were used to characterize the materials behavior followed by thermomechanical training at a constant stress. The SME test resulted in no martensite detwinning plateau and a non-linear stress-strain curve implying the simultaneous occurrence of slip and martensite reorientation. The superelastic tests also show an austenitic yield stress above 600 MPa. Thermomechanical training resulted in small amounts of plastic strain growth, and the development of two-way shape memory (TWSM). The TWSM results were fitted using the Bo & Lagoudas model, and is capable of predicting the actuation strains at other stresses. The results support the conclusion that hafnium distorts martensite slip planes, and (Ti,Hf) 2Ni and (Ti,Hf) 3Ni 4 precipitates form during aging and annealing. The distorted slip planes cause slip and martensite reorientation to occur simultaneously. This develops a strong stress field during training within the first few cycles. The stress field develops TWSM, but limits further plastic growth and TWSM development. The precipitate formation increases material strength, as seen in the superelastic loading, but also embrittles the material. Thermomechanical training of an annealed specimen resulted in a brittle failure after several thermal cycles due to the growth of particulate size during annealing. The two alloys are ideally suited for high temperature actuators. TWSM was trained into the material and the transformation temperatures are higher than that of NiTi, but low enough to avoid annealing and problematic creep temperatures.