Stable wide-temperature-range superelasticity in a NiTi/NiTiZrCu dual-shape-memory-alloy architecture

In general, NiTi shape memory alloys (SMAs) have an intrinsic limitation of the narrow superelastic temperature window, which prevents them from exhibiting superelasticity at cryogenic temperatures. To circumvent this issue, this work reports a dual-SMA architecture with laser-powder-bed-fusion NiTi SMA honeycomb skeleton filled by NiTiZrCu SMA reinforcement, which was fabricated via low temperature aging of hot-extruded architecture composite. Microstructure analyses reveal that relatively low-temperature aging at 350 °C preserves the extrusion-induced dislocation substructure while inducing uniformly dispersed and coherent Ni 4 Ti 3 nanoprecipitates within the NiTi skeleton. Interestingly, the architecture composite demonstrates stable wide-temperature-range superelasticity, achieving a recovery ratio of nearly 100% during 10 cycles. This is attributed to the strengthening mechanism by high-density dislocations and Ni 4 Ti 3 nanoprecipitates as well as the high lattice compatibility of the R-phase transformation. Most notably, its stable superelasticity exhibits an anomalous wide-temperature-range down to -50 °C, 93 °C below its austenite finish temperature (A f = 43 o C). This cryogenic superelasticity originates from a mechanical synergistic coupling between the dual SMA constituents. Upon unloading, the elastic strain energy stored in the high-strength NiTiZrCu reinforcement is released to the NiTi skeleton through the sound metallurgical interface, which compels the reverse reorientation of the detwinned NiTi martensite. This work demonstrates a novel pathway for designing SMA composites with stable superelasticity in cryogenic environments.