Microstructure and shape memory behavior of additively manufactured Ti-30Ta high-temperature shape memory alloy fabricated by laser beam powder bed fusion

Abstract

Titanium-tantalum (Ti-Ta) based alloys can show a reversible martensitic transformation well above 100 °C, which renders them attractive for actuator applications at elevated temperatures. The present study reports on additive manufacturing of a binary Ti-Ta high-temperature shape memory alloy (HT-SMA) by laser beam powder bed fusion (PBF-LB/M). Cuboids with near-full density of 99.99 % have been processed from pre-alloyed Ti-30Ta (at%) powder feedstock. While ω-phase formation during processing causes a β-phase stabilized solidification microstructure, an adequate post-process solution-annealing (1200 °C / 0.5 h) followed by water quenching promotes the formation of a non-equilibrium phase constitution consisting of the martensitic α″-phase. For this heat-treated material state, superior functional properties with fully reversible strains of 2.7 % at a bias stress of 350 MPa are shown. However, poor functional stability is observed. In line with findings previously reported for conventionally processed material, formation of ω-phase is found to dominate functional fatigue and eventually results in a complete loss of the shape memory effect under cyclic loading conditions. By employing detailed microstructure analysis and thermo-mechanical testing accompanied by high-energy in situ synchrotron diffraction, the fundamental interrelationships between processing, microstructure evolution and shape memory behavior are explored and rationalized.