Pseudoelastic cycling and ageing effects at ambient temperature in nanocrystalline Ni-rich NiTi wire

It is well known how the features of a stress–strain hysteresis during pseudoelastic pull–pull cycling of NiTi evolve: plateau stresses for the forward (loading) and reverse transformation (unloading) decrease, the width of the stress–strain hysteresis decreases and irreversible strain accumulates. This evolution is strong during early cycling. It is generally assumed that these effects, which saturate after about a hundred cycles, are associated with the elementary processes which govern dislocation plasticity and stress-induced martensitic transformations. Dislocations can do both, stabilize martensite (through their stress fields) and contribute to accumulation of irreversible strain. While it is accepted that dislocations may alter the cyclic stress–strain behavior during stress-induced martensitic transformations in NiTi, it is generally assumed that atomic reordering processes (which are known to significantly affect martensitic transformations in Cu-base shape memory alloys) play no role. In the present investigation we show that hold times in saturated stress–strain cycles show significant effects which are smaller than, but similar in type to those in Cu-base alloys. Further work is required to fully rationalize the microstructural origin of our findings.