Scale-up of NiTiHf shape memory alloy tubes with high torque capability

High-torque, NiTiHf high-temperature shape memory alloy tubes were examined by means of constant-torque thermal cycling experiments. The alloy, with a Ni-rich composition of Ni_50.3Ti_29.7Hf₂₀ (at%), was hot worked via extrusion from large ingots to produce final rods with an outer diameter of 25.4 mm. After aging at 550 °C for 3 h and air-cooling, the manufactured torque tubes were subjected to shear stresses from 0 to 500 MPa (torques of 0–1400 N m) and thermally cycled 20 times under stress, and twice after stress removal to evaluate the two-way shape memory effect. A transformation shear strain of 6% was obtained, and a maximum work output of 27.5 J cm⁻³. Two-way shear strains approached 3% strain without any extended training or cycling. The strain values obtained for the first time in this large-diameter, high-torque tube consistently matched smaller diameter tubes previously reported. This highlights the successful production and scale-up of both the material and the component level in torque tubes without degrading the actuation response, while providing much higher torque capability. (paper)