Concurrent Martensitic Transformation and Plastic Deformation in Superelasticity of NiTi

Mechanics of simultaneous martensitic transformation and plastic deformation of NiTi polycrystals will be discussed based on the recently obtained experimental evidence through in-situ synchrotron diffraction and electrical resistivity studies during cyclic superelastic deformation of thin NiTi wires in tension and TEM characterization of microstructures in cycled wires.  The results are interpreted based on the simulations of concurrent martensitic transformation and plasticity by micromechanics model. It is concluded that, beyond the characteristics of the B2-R-B19´ martensitic transformation and elastic properties of individual phases, the shape and stability of superelastic stress-stress curve depends critically on the grain interactions – i.e. partitioning of stress, strain and phase fractions among particularly oriented polycrystal grains which develops during superelastic cycling. It is discussed how this information might be used to achieve optimized stable superelasticity of NiTi wires in tension by combination of microstructure control by the cold work/annealing, fine precipitate strengthening and texture control.