Physical mechanism for nanocrystalline NiTi alloy with ultrasmall-sized grains exhibiting one-way shape memory effect without temperature-induced phase transformation

In the present work, a novel finding is that nanocrystalline NiTi alloy with ultrasmall-sized grains (where the diameter of grains is less than or equal to 10 nm) presents one-way shape memory effect (OWSME) without temperature-induced phase transformation, where the involved physical mechanism is revealed by combining experiments with molecular dynamics simulation (MD). The results show that fraction of grain boundaries (GBs) is so much that temperature-induced martensitic transformation is suppressed, but it leads to a certain unrecoverable strain. The following loading results in increase of strain and decrease of Gibbs free energy, which leads to stress-induced martensitic transition. After unloading, Gibbs free energy is increased by the release of elastic strain, which leads to partial reverse martensitic transition. Consequently, OWSME in nanocrystalline NiTi alloy with ultrasmall-sized grains is aroused by retained stress-induced martensitic transition followed by its reverse phase transition rather than temperature-induced one followed by its reverse phase transition.