Regulation of microstructure, phase transformation behavior, and enhanced high superelastic cycling stability in laser direct energy deposition NiTi shape memory alloys via aging treatment

Abstract

The remarkable mechanical properties, fatigue resistance, wear and corrosion resistance, biocompatibility, shape memory effect, and superelasticity of NiTi shape memory alloys make them applied in diverse engineering areas, including satellite antennas, oil pipelines, and vascular stents. This paper focuses on the preparation of Ni50.8Ti49.2 alloy via laser directed energy deposition (LDED) utilizing pre-alloyed powder and the heat treatment of solid solution at 950 °C for 8 h followed by subsequent aging at 350 °C, 450 °C and 550 °C for 2 h. The influence of varying aging temperature on the microstructures, phase transformation behavior and superelastic cyclic stability of them was comprehensively summarized and discussed. The martensitic transformation temperatures of NiTi alloys after aging decrease as the aging temperature increases. Specifically, NiTi alloys aged at 450 °C for 2 h exhibit excellent superelasticity and high cyclic stability. After an initial loading-unloading cycle with an 8.079 % pre-strain, the alloy achieves 100 % complete recovery. Even after 10 cycles, the recoverable strain remains at approximately 7.374 %, with a recovery rate exceeding 90 %. These outstanding properties of high superelasticity and superior cyclic stability are significantly better than most other LDED NiTi alloys. This exceptional performance is primarily attributed to the synergistic effects of the R-phase and Ti3Ni4 precipitates on the shape memory alloy.