The Shape Memory (‘Marmem’) Effect in Alloys

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Abstract
The 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 last of these. The deformation behaviour depends on the deformation temperature in relation to the M s and M f temperatures and consists of several probable mechanisms: (a) an intramartensite twin-boundary displacement under stress, possibly accompanied by the introduction of additional reversible mechanical twins; and (b) the growth of some existing plates and the shrinkage of others under stress, including stress-induced reversion followed by retransformation to habit-plane variants with more favourable stress coupling. At temperatures well below M f only process (a) is considered likely to occur, and processes (a) and (b) would apply to the martensitic state whether thermally or mechanically induced. Upon heating, the deformed martensite must transform back to the original parent-phase orientation in a thermoelastic manner, and accordingly the martensite exhibits a memory. The term ‘marmem’ is introduced to describe this general behaviour, now common to a number of materials. Related phenomena such as ‘rubberlike’ behaviour and ‘pseudoelasticity’ are also discussed in relation to the shape memory effect. Present uses of marmem alloys are described and some future applications are suggested.

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