Dynamic Mechanical Spectroscopy of Nanograined Thin NiTi Wires

Thermomechanical behaviour of NiTi SMA is due to the complex interaction between several deformation mechanisms including lattice elasticity, phase transformations between austenite (A), martensite (M) and R-phase (R), reorientations of R and/or M variants, twinning and detwinning phenomena and the usual plasticity due to dislocation movements. As regards the elasticity, it is essential to point out that, although each individual phase has its own intrinsic elastic properties, their evaluation on polycrystalline NiTi wires is complicated due to the presence of internal interfaces introduced by the martensitic transformation. As both the interphase and martensitic interfaces are responsive to stress, two major problems appear [1]. First, it becomes experimentally difficult to evaluate the intrinsic elastic properties of phases as well as macroscopic elastic moduli by conventional methods. Second, since the motion of internal interfaces upon mechanical loading results in texture evolution, effective elastic properties of phases (e.g.Young's modulus) vary as well - i.e. they would be different e.g. in tension and compression.