Regularities and mechanisms of mechanical twinning in TiNi alloys

We generalize experimental findings and theoretical reasoning for a new mechanism of deformation and crystal lattice reorientation. This is the mechanism of direct plus reverse (B2 → B19(B19′) → B2) martensitic transformations as a mechanism of localized deformation band formation and mechanical twinning in the B2 phase of TiNi alloys. The paper gives atomic models of these transformations, analysis results for the distortion tensors and features of lattice reorientation in the transformation zones. The relation of mechanical twinning to yield stress asymmetry is analyzed for the case of tension and compression of TiNi alloy single crystals in directions of type <001>. To explain this phenomenon, we introduce the notion of the factor of normal stresses reduced to the principal axes of the tensor of uniform transformation strain in the mechanical twinning zones. It is shown that this factor can be used for the quantitative estimation of yield stress asymmetry in [001] single crystals of TiNi alloys.