The Shape Memory (‘Marmem’) Effect in Alloys

Abstract

AbstractThe shape memory effect, by which a material, apparently plastically deformed, reverts to its original shape upon heating to some higher temperature, has been reviewed and discussed from the point of view of the crystallography of martensitic transformations. Considering the general characteristics of martensitic materials that exhibit the shape memory behaviour, some phenomena peculiar to the prototype NiTi alloy are shown to have no direct relation to the shape memory. It is concluded that the shape memory effect can be universally correlated with a martensitic transformation that is thermoelastic in nature, the thermoelasticity being attributed to ordering in the parent and martensitic phases.Prerequisites for the shape memory behaviour are suggested to be: (1) that the martensitic transformation is thermoelastic; (2) that the parent and martensitic phases are ordered; and (3) that the martensite is internally twinned. The low-temperature deformation process is discussed with reference to the l...