Effect of Copper Content and Heat Treatments on Microstructure and Martensitic Transformation in (Ni,Cu)(Ti,Hf,Zr) High-Entropy High-Temperature Shape Memory Alloys

High-temperature shape memory alloys (HTSMAs) capable of stable operation well above conventional NiTi systems are increasingly sought after for aerospace, automotive, and energy applications. Leveraging multi-principal element design principles, Ni₅₀₋ₓCuₓTi₁₆.₆₇Hf₁₆.₆₇Zr₁₆.₆₇ (x = 0–25 at.%) high-entropy shape memory alloys were systematically investigated to elucidate the combined effects of chemical composition and thermal processing on their microstructure and martensitic transformation behavior. All compositions exhibited reversible B2–B19′ transformations, with the martensite-start temperature (Mₛ) decreasing from ~590 °C to ~160 °C as Cu content increased. Transformation hysteresis initially widened from 150 °C to 254 °C at 5 at.% Cu before narrowing to ~50 °C at higher Cu levels. The monoclinic angle (β) of the martensite reached a maximum of 104.8° for the 5 at.% Cu alloy, correlating with enhanced lattice distortion at intermediate Cu contents. Detailed characterization using SEM/EDS, XRD, WDS, neutron diffraction, and TEM revealed the formation of a previously unreported MgZn₂-type hexagonal Laves phase, confirmed to be detrimental to transformation reversibility. To mitigate its formation, secondary heat treatments between 300 °C and 600 °C were explored, clarifying precipitation kinetics and phase stability across the compositional range. These findings establish composition- and processing-dependent design guidelines for optimizing phase stability and transformation performance in next-generation multicomponent HTSMAs.