Pseudoelastic cycling of ultra-fine-grained NiTi shape-memory wires

In the present study, we investigate pseudoelastic pull-pull cycling of ultra-fine-grained (40 nm) Ni-rich (50.9 at.% Ni) NiTi shape-memory wires at temperatures ranging from 301 to 323 K. Strain-controlled experiments were performed using incremental strain steps and different constant maximum strains. Pull-pull cycling results in decreasing/increasing plateau stresses characterizing the forward/reverse transformations and an accumulation of non-recoverable strain. Saturation is reached after 30 cycles. We interpret our results in terms of a microstructural scenario where dislocations, which are introduced during the martensitic transformation (lattice invariant shear) and during pull-pull cycling (dislocation plasticity), interact with the stress-induced formation of martensite. We show that the slopes of stress-strain curves naturally depend on the total strain imposed in strain-controlled testing. We also provide a dislocation-based explanation for the evolving stress levels of the loading and unloading plateaus during pseudoelastic cycling. And most importantly, we show how dislocations act as microstructural markers which allow the material to remember its previous stress-strain history.