B2=>B19'=>B2~T Martensitic Transformation as a Mechanism of Plastic Deformation of NiTi

摘要

It is commonly assumed in the SMA field that the localized superelastic deformation of NiTi wires in tension due to the stress induced cubic to monoclinic martensitic transformation is limited at elevated temperatures by-unrecoverable deformation due to dislocation slip, which is the source of the functional fatigue. Nevertheless, it is not very clear how this dislocation slip takes place. Although the it has been proved that the dislocation slip accompanies the stress induced martensitic transformation to compensate the strain incompatibilities arising at the moving habit planes and stationary grain boundaries in NiTi polycrystals [1,2], recent experimental and theoretical results suggest that it is not the key deformation mechanism limiting the strain recoverability at elevated temperatures [3,4], that this is the stress induced B2=>B19,:=>B2~T transformation into twinned austenite coupled with dislocation slip. These results will be reviewed and their implications for NiTi technology will be discussed.