Evolution of Two-Way Shape Memory Effect on Ni-Ti-Hf Shape Memory Alloy Torque Tubes Via Thermo-Mechanical Training

Solid state shape memory actuators are promising candidates to replace electric motors or hydraulic systems in extreme environments. Shape memory alloys outperform other alternatives when volume constraints are present since they have the highest energy density among all active materials. Two-way shape memory effect is desirable for most applications to reduce complexity and increase safety. In recent years the interest for high temperature solid state actuators has increased significantly for applications in aeronautics. These actuators are required to have stable actuation response, high actuation strain and high work output. Precipitation hardened NiTiHf shape memory alloys have high transformation temperatures (M_f>130 °C ), stable actuation response, moderate actuation strain and relatively high two way shape memory effect .  Evolution of two way shape memory effect via termomechanical training , i.e. cycling temperature under constant stress, is investigated in this study. Stress was applied in shear mode via torque. Two different specimen shapes both having outer diameter of 9.52 mm, long-thin (140 mm length, 6.86mm inner diameter) and short-thick (120 mm length, 5.78 mm inner diameter) are tested.  Samples are trained under 150 MPa shear stress. Results showed that two way shape memory effect evolves quickly and saturates in the first 600 cycles. Moreover stability of two-way shape memory effect is studied as a function of annealing temperature. It is found that two-way shape memory effect is unaffected after annealing for 30 minutes at temperatures up to 450 °C and it still exists after annealing at temperatures up to 650 °C.