Change of transformation mechanism during pseudoelastic cycling of NiTi shape memory alloys

The nature of the austenite - martensite transformation upon pseudoelastic (PE) cycling under load control changes from highly-localized (presence of Lüders-like bands) to homogeneous with increasing number of PE cycles. The mechanism of this effect is analysed quantitatively in the present study on the basis of microstructural changes as a function of the number of PE cycles, especially the change of the density of (retained) dislocations. Particularly, the changes in the mechanical properties (Young's modulus, strain hardening, accumulated residual strain) and the martensite-nucleation energy are quantitatively correlated for the first time with the increase of the dislocation density. A new phenomenological model is proposed, which can describe the observed evolutions of the dislocation density, the strain hardening and accumulated residual strain with increasing number of PE cycles.