3D Reconstruction of Martensitic Microstructures in Grains of Deformed Nanocrystalline NiTi Wires by TEM

A novel experimental method allowing for 3D reconstruction of martensite variant microstructures evolving during tensile thermomechanical loading test on nanocrystalline NiTi wire is introduced. The method is based on the determination of the location, size and orientation of all martensite variants and interfaces within a selected polycrystal grain using post mortem selected area electron diffraction with dark field image analysis in TEM. It is found that martensitic microstructures in grains of nanocrystalline NiTi wire evolving during tensile thermomechanical loading tests (involve martensite reorientation, stress induced martensitic transformation and plastic deformation of martensite) are rather different than currently assumed in the SMA field. Martensitic microstructures in the NiTi wire deformed up to the end of reorientation (transformation) plateau up to ~7% strain at room temperature (100 °C), respectively, were found to contain single domain (001) compound twinned martensite filling whole grains. This was rationalized by the theoretical treatment of strain accommodation in grains of the fiber textured NiTi wire deformed in tension predicting such singular microstructural state for NiTi wire deformed up to 6.74% strain. Upon reverse martensitic subsequent stress free heating above the Af temperature, this martensitic microstructure retransforms back to the parent austenite yielding recoverable strains typical for NiTi (~6%) accompanied by very small unrecovered strain (~0.6%- 1.5% depending on the test temperature). On further tensile loading up to 15% strain, plastic deformation of oriented martensite starts by coordinated (100) and (201‾) deformation 1 twinning in martensite assisted by [1 0 0](0 0 1) dislocation slip giving rise to characteristic wedge type martensitic microstructure. Upon subsequent stress free heating above the Af temperature, this martensitic microstructure transforms to twinned austenitic microstructure yielding large recoverable strains (~10%) accompanied by large unrecovered strains (~5%). The dislocation slip assisted (100) and (201‾) deformation twinning in martensite renders NiTi excellent combination of strength and deformability and leads to refinement of austenitic microstructure accomplished via introducing {114} austenite twins into it.