Strain glass: Glassy martensite

"Glass", a frozen disordered-state, has been found in areas as diverse as amorphous solids, magnetic alloys, ferroelectrics, superconductors, and even in models of biological evolutions. In the present review we introduce a new class of glass-the "strain-glass", which was discovered very recently. Strain glass is derived from a martensitic system, where the local-strain is frozen in disordered configuration. The first example of strain glass was found in the well-studied Ni-rich Ti_50-xNi_50-x martensitic system in its "non-transforming" composition regime (x>1.5). Contrasting to the familiar martensitic transition, the strain glass transition is not accompanied by a change in the average structure, or a thermal peak in the DSC measurement. It involves a dynamic freezing process with broken ergodicity, during which nano-sized martensite domains are frozen. More interestingly, the seemingly "non-martensitic" strain glass exhibits unexpected properties: shape memory effect and superelasticity, like a normal martensitic alloy. Strain glass bears a striking similarity with other two classes of glasses: cluster-spin glass and ferroelectric relaxor. These ferroic-transition-derived glasses can be considered as a more general class of glass: ferroic glass. The finding of strain glass may provide new opportunities for martensite research from both fundamental side and application side.