Influence of Annealing Temperature on the Shape Memory Effect and Superelasticity of NiTi-Hf High-Temperature Shape Memory Alloys for Aerospace Actuators

Nickel-titanium-hafnium (NiTi-Hf) high-temperature shape memory alloys (HTSMAs) have emerged as premier candidates for solid-state actuation in advanced aerospace systems owing to their high transformation temperatures, elevated actuation work output, and cost competitiveness relative to precious-metal-doped alloys. In this study, the influence of post-deformation annealing temperature (ranging from 500 °C to 800 °C) on the precipitation behavior, martensitic transformation characteristics, shape memory effect, and superelastic response of a Ni_50.3Ti_29.7Hf₂₀ alloy was systematically investigated. Transmission electron microscopy revealed that annealing at 550 °C promoted the dense, uniform precipitation of coherent nanoscale H-phase particles (mean diameter ~18 nm), which significantly strengthened the B2 austenitic matrix against dislocation slip. Differential scanning calorimetry confirmed that the formation of these nickel-rich nanoprecipitates depleted the matrix of nickel, shifting the austenite finish temperature (A_f) above 160 °C. Isobaric thermal cycling under a tensile stress of 300 MPa demonstrated an optimal recoverable actuation strain of 3.82% with negligible irrecoverable plastic drift in specimens annealed at 550 °C. Conversely, higher annealing temperatures (700 °C and 800 °C) induced rapid precipitate coarsening, loss of particle coherency, and severe degradation of both cyclic superelastic stability and functional fatigue life. These findings elucidate the critical role of nanoscale structural engineering via controlled thermal processing to tailor high-temperature shape memory alloys for next-generation morphing wings, variable-geometry exhaust nozzles, and active turbomachinery flow control.