Precursors to stress-induced martensitic transformations and associated superelasticity: Molecular dynamics simulations and an analytical theory

Abstract

Precursor phenomena are critical issues for martensitic transformation, as they provide important clues for understanding the origin of the transformation and the structure of the transformation products. Prior to temperature-induced martensitic transformation, it has been recognized for a long time that the basal plane shear modulus ${C}^{\ensuremath{'}}$ ({110} $⟨1\overline{1}0⟩$ shear mode) of the parent phase decreases on approaching the transformation temperature. On the other hand, martensitic transformation can also be induced by stress; but little has been known about whether similar precursor phenomenon also exists prior to such stress-induced martensitic transformation. In the present study, we successfully simulated the stress-induced martensitic transformation and associated superelasticity in a generic martensitic system by means of molecular dynamics method. Through calculating ${C}^{\ensuremath{'}}$ as a function of applied stress, we found a significant softening of ${C}^{\ensuremath{'}}$ prior to the stress-induced martensitic transformation. This is a clear evidence for the existence of lattice softening prior to stress-induced martensitic transformation. Our results suggest that lattice softening is a common feature for both temperature- and stress-induced martensitic transformation. An interesting result is that stress does not soften all the crystallographically equivalent {110} $⟨1\overline{1}0⟩$ shear moduli (all are ${C}^{\ensuremath{'}}$ by definition); it softens some of them while it hardens the rest. This is contrasting with the temperature-induced ${C}^{\ensuremath{'}}$ softening, in which all equivalent {110} $⟨1\overline{1}0⟩$ shear moduli simultaneously soften to the same extent. It corresponds to the well-observed fact for given stress direction only certain martensite variant(s) are induced while other variants are prohibited. We also formulated an analytical theory to investigate the variation of ${C}^{\ensuremath{'}}$ under stress, and obtained similar result as that of our molecular dynamics simulations.