Tensile Deformation of B19‘ Monoclinic Martensite in Nanocrystalline NiTi Wires

When deformed in martensite state, NiTi undergoes pseudoplastic deformation taking place via motion of intervariant interfaces (called martensite reorientation or detwinning), followed by plastic deformation of the B19' monoclinic martensite. The state of the art view is that: (i) the martensite reorientation proceeds via detwinning of <011> type-II twin laminates created by the martensitic transformation upon cooling and (ii) the reoriented martensite deforms plastically via dislocation slip. Although this view might be correct for single crystals and large grain size polycrystals, doubts existed whether it applies also for nanocrystalline NiTi which displays (001) compound twinned microstructures after stress free cooling from the austenite. The authors performed systematic experimental investigations of martensitic microstructures (postmortem TEM) and textures (in-situ HEXRD) evolving during tensile tests on nanocrystalline NiTi wires until fracture. The results indicate that the widespread view of the martensite reorientation as "detwinning" is incorrect. Plastic deformation of martensite proceeds via peculiar deformation mechanism involving (20-1) and (100) deformation twinning assisted by [100]/(011) dislocation slip. It enables the nanocrystalline NiTi wire to deform plastically at ~1 GPa engineering stress up to very large plastic strains ~50% and refines the austenitic microstructure down to nanoscale. Upon unloading and heating, reverse martensitic transformation takes place leaving large recoverable as well as unrecovered strains and high density of {114} austenite twins in the microstructure.