Enhanced superelastic stability in nanocrystalline NiTiCoZr shape memory alloys via grain refinement and solid solution strengthening

NiTi shape memory alloys (SMAs) exhibit superelasticity through stress-induced reversible martensitic phase transformation, making them one of the most widely used smart material in aviation and medical applications. However, the low resistance to dislocation motion of conventional coarse-grained NiTi SMAs leads to poor superelastic stability under cyclic deformation, severely limiting their practical utility. In contrast, nanocrystalline NiTi SMAs demonstrate superior cyclic stability owing to their enhanced resistance to dislocation motion. Yet, achieving an average grain size below ∼15 nm remains challenging due to limitations in cold deformation and low-temperature annealing processes, restricting further improvements in superelastic stability via grain refinement. In this study, nanocrystalline NiTi, NiTiCo, and NiTiCoZr SMAs with a uniform grain size of 15 nm were fabricated through melting, forging, drawing, and annealing. After 20 loading-unloading cycles at a strain of 6 %, the NiTiCoZr alloy exhibited outstanding superelastic performance, including a high superelastic stress (909 MPa), low residual strain (0.57%), and a minimal stress reduction rate (18%), significantly surpassing the performance of NiTiCo and NiTi alloys with identical grain sizes. Notably, its superelastic stress exceeds twice that of grain-size-matched NiTi alloys. High-resolution TEM and geometric phase analysis revealed that Co/Zr doping in nanocrystalline NiTi induces significant local strain, which—combined with the high density of grain boundaries in the nanocrystalline structure—stabilizes and strengthens the parent phase. This synergistic effect enhances transformation stress and dislocation resistance, yielding superior superelastic performance with high stress and low residual strain.