High temperature shape memory alloy Ni_50.3Ti_29.7Hf₂₀ torque tube actuators

The torsional behavior of Ni-rich Ni_50.3Ti_29.7Hf₂₀ (at%) high-temperature shape memory alloy tubes was investigated under pure torsion loading. Torque tubes with varying geometry including outer diameter, wall thickness, and length were subjected to constant-torque thermal cycling at stresses ranging from 0 to 500 MPa (0–175 N m). It was found that the wall thickness had a notable effect on the transformation temperatures where thick-walled tubes transformed at lower temperatures when compared to the thin-walled form. In all tube outer diameters, shear strains were found to be in the order of 6% obtained at stresses above 300 MPa. At lower stresses, little to no effect of wall thickness was observed, but the influence increased at higher stresses where thin-walled tubes generated approximately 2% less strain when compared to the solid forms. Two-way shape memory effect was also evaluated after 20 cycles and was found to reach ∼3% strain when cycled at high stresses. (paper)