Microstructure and pseudo-elastic low-cycle high amplitude fatigue of NiTi

A binary NiTi alloy consisting of 50.7 at. % Ni was studied under cyclic tension. This alloy can show pseudo-elastic behaviour at room temperature ($M_\delta <\rm RT < M_{\rm d}$). The following microstructural conditions were compared: 1. homogenous $\beta$-phase (SHT: solution heat treated), 2. $\beta$-phase with small coherent particles (AHT: age heat treated), 3. $\beta$-phase with defects (dislocations and particles)(TMT: thermo mechanically treated). All structures show stress induced phase transformation during mechanical cycling at ambient temperature. They exhibit heterogeneous strain. The amount of heterogeneity is reduced by the TM-treatment. The change in the pseudoelastic (PE) stress-strain curves, as well as of the transformation temperatures were analysed as a function of the number of mechanical cycles. The changes were interpreted in terms of formation and accumulation of secondary (i.e. fatigue induced) defects and / or residual martensite. The introduction of primary defects (i.e. as introduced by TMT) has the strongest effect on the stability against mechanical fatigue and change of the transformation temperatures