Synchronous optimization of phase transformation hysteresis and superelasticity in NiTiCu shape memory alloys fabricated by laser direct energy deposition with heat treatment

Shape memory alloys (SMAs) used in actuators with high response frequencies are required to combine a narrow phase transformation hysteresis with excellent superelastic properties at room temperature. In this paper, Ni_45.2Ti_49.5Cu_5.3 (at%) SMA was fabricated using Laser Direct Energy Deposition (LDED) technology, followed by a heat treatment process designed to promote the precipitation of Ti(NiCu)₂ phases with different morphologies along grain boundaries and within grains. It is revealed that the sample aged at 650 ℃ exhibits a phase transformation hysteresis of approximately 17 °C, and achieves a shape recovery ratio of over 90 % even under a pre-strain of 8 %. Additionally, the NiTiCu alloy aged at 650 ℃ demonstrates substantial stability after the fifth cycle, with the recovery rate, η, approaching 100 %. These excellent mechanical properties mainly stem from the significant strengthening effect of Ti(NiCu)₂ precipitates on the matrix and their effective pinning of dislocation motion and slip during deformation. This suppresses the likelihood of plastic deformation in the alloy, thereby significantly enhancing its deformation recovery capability. The results obtained provide valuable insights for the further enhancement of the microstructure and mechanical properties of additively manufactured NiTiCu shape memory alloys.