Origin of ``aging'' in shape-memory alloys

For more than half a century it has been widely observed that a majority of shape-memory alloys exhibit a gradual change in physical properties with time in the martensitic phase, and this is referred to as ``aging.'' However, its microscopic mechanism has remained controversial due to lack of experiments that can probe atomic level processes. We clarify the atomic mechanism for how shape memory alloys ``age'' in time using a combination of molecular dynamics and Monte-Carlo simulations. Through analysis of the atomic configurations during aging, we find that the observed phenomenon is associated with a gradual change in the short range order of point defects so that the defect short range order tends to adopt the same ``symmetry'' as the crystal symmetry of the host martensite lattice. The results provide atomic-level evidence for the symmetry-conforming short-range order model, and may provide new insight into how to control aging to design aging-free shape memory alloys. Reference: 1). J. Deng, X. Ding, T. Lookman, et al, Physical Review B , \textbf{81}, 220101(R), 2010 2). J. Deng, X. Ding, T. Lookman, et al, Physical Review B, \textbf{82},184101, 2010 3). J. Deng, X. Ding, T. Lookman, et al, Applied Physics Letters, \textbf{97},171902, 2010