Ultra-high recovery stress in the Ti-Ni-Nb-Co shape memory alloys with coupling effect of high-density nano-precipitation and grain boundary engineering

In shape memory alloys, most research emphasizes recovery strain, whereas both recovery strain and recovery stress are critical for engineering applications. In constrained, load-bearing scenarios, recovery stress is a key metric because it directly quantifies the deliverable load and thus affects structural reliability. In this work, a Ti_44.5Ni_44.5Nb₉Co₂ alloy was designed to notably enhance recovery stress, through integrated multi-factor regulation of nano-precipitation, grain size, and grain boundary character via cold rolling and annealing. After 40% cold rolling, annealing at 650 °C for 2 h, the alloy achieved a peak recovery stress of 584 MPa, approximately 50% higher than that of conventional Ti-Ni-Nb alloys, while maintaining a favorable recovery ratio of 84.5% with 16% pre-deformation. Transmission electron microscopy, transmission Kikuchi diffraction, and tensile and recovery measurements indicate that the optimal recovery response at 650 °C is associated with fine and dispersed (Ti,Nb)₂Co precipitates, a recovered dislocation and subgrain structure, and a high fraction of low-angle grain boundaries. These coupled microstructural features favor both matrix strength and martensitic transformation reversibility. This work proposes a microstructural co-design paradigm centered on recovery stress as the objective function, providing a transferable design pathway and quantitative basis for the engineering development of high-reliability shape memory alloys.