TEM analysis of permanent lattice defects created by forward and reverse martensitic transformations proceeding under stress in nanocrystalline NiTi wire

The origin of cyclic instability of stress-strain-temperature responses of NiTi shape memory alloy (SMA) in cyclic thermomechanical loads (functional fatigue) represents one of the unsolved problems in the SMA field. In particular, the mechanism by which incremental plastic strains and permanent lattice defects are generated during thermomechanical cycling is not known. To reveal it, we evaluated recoverable strain, plastic strain and permanent lattice defects in austenite created by forward and reverse martensitic transformations (MT) proceeding under wide range of external tensile stress by closed loop thermal cycling under constant stress and transmission electron microscopy (TEM) analysis of permanent lattice defects in austenite consisting of slip dislocations and deformation bands, variation of crystal lattice orientation and elastic strain field in grains. We propose that forward MT proceeding upon cooling under external stress generates plastic strain via slip of [100](001) dislocations that nucleate at habit plane interfaces, glide in martensite across entire grains and disappear within grain boundaries. Reverse MT upon heating under external stress generates plastic strain also via [100](001) dislocation slip but on multiple slip systems within interfacial microstructure layers created by reorientation of martensite that propagate prior habit plane interfaces transforming the oriented martensite into plastically deformed austenite. Both forward and reverse MT proceed via cooperative transformation/twinning/slipping within large number of grains enabling shape strains in grains to remain compatible at grain boundaries. When NiTi wire is subjected to cyclic thermomechanical loading, incremental plastic strains and permanent lattice defects are generated by the proposed deformation mechanisms anytime the forward and reverse MTs occur under external stress above certain thresholds characteristic for each NiTi wire. The plastic strains and permanent lattice defects accumulate upon thermomechanical cycling which brings about functional fatigue. Assuming dislocation slip in martensite as the origin of functional fatigue, it is discussed how NiTi-based alloys can be strengthened against it.