Shape-memory NiTi foams produced by solid-state replication with NaF

A martensitic NiTi foam was produced by hot isostatic pressing a blend of NiTi and NaF powders and subsequent dissolution of the NaF phase. The NiTi foam consists of 40 vol.% near-fully open pores, 240 μm in size, and with ragged surfaces due to incomplete NiTi powder densification. Near linear stress-strain curves are measured in compression with an average loading stiffness of 4 GPa, well below the unloading stiffness of 13 GPa because of detwinning on loading. Shape-memory recovery after unloading corresponds to 85-89% of the unloading plastic strain. After sintering at 1250 °C, the foam exhibits 20% porosity, smaller, smoother and partially-closed pores, and a shift in composition towards a martensite/austenite mixture at ambient temperature. This new composition allows for the activation of the superelastic effect in the austenite during loading and unloading resulting in average stiffnesses of 6-12 GPa, and the shape-memory effect in the martensite with 60-97% of the plastic strain recoverable.