On plastic strains generated by B2-B19’ martensitic transformations in NiTi proceeding under external stress

In order to reveal the mechanisms by which plastic strains and lattice defects are generated by the B2-B19’ martensitic transformation in NiTi proceeding under external stress, special thermomechanical loading tests were performed on nanocrystalline superelastic and shape memory NiTi wires with identical austenitic microstructures. Recovered and unrecovered strains generated separately by forward and reverse martensitic transformations were evaluated. Martensite variant microstructures and lattice defects created by both transformations were analysed by transmission electron microscopy.

It is found that plastic strains and lattice defects are generated by martensitic transformation proceeding above certain stress limits via dislocation slip and kwinking deformation in martensite. Below the limit, recovered strains increase with increasing stress and unrecovered strains are negligible. Above the limit, recovered strains remain nearly constant while unrecovered plastic strains increase with increasing stress. At comparable applied stresses: i) larger plastic strains are generated by the reverse than by the forward transformation, ii) larger recoverable strains are generated by the superelastic wire. The stress limits are higher for the forward than for the reverse transformation and higher for the superelastic than for shape memory wire.

The mechanisms by which plastic strains and lattice defects are generated is discussed. It is explained: i) why martensitic transformation generates plastic strains and lattice defects via dislocation slip and kwinking deformation in martensite, ii) why larger plastic strain is generated by the reverse than by the forward transformation and iii) why and how the unrecovered plastic strains depend on the austenite texture and tension/compression direction of loading.