Experimental study of elastic constant softening prior to stress-induced martensitic transformation

There has been a large body of experimental evidence for the existence of precursor lattice softening prior to temperature-induced martensitic transformation, but little is known about whether such softening effect also exists prior to stress-induced martensitic transformation. Recent molecular dynamics simulations suggested such a possibility, but direct experimental evidence remains unclear. In the present study, we established a simple experimental method that can measure elastic constant C′ from zero stress up to the critical stress prior to stress-induced martensitic transformation. By applying this method to a Cu-Al-Ni single crystal, we found C′ of (101) [1̄ 01] shear mode does soften with increasing applied stress along the [001] direction, until a stress-induced martensitic transformation occurs at a critical stress. This is direct evidence for the existence of lattice softening prior to a stress-induced martensitic transformation. Our result completes an important conclusion that the softening of elastic constant C′ is a common feature prior to both temperature- and stress-induced martensitic transformations. Furthermore, we also analyzed the dependence of C′ on uniaxial tensile stress through a theoretical model and obtained similar result as that of our experiment.