Structural Memory in Metals

preservation of the shape and crystallographic orientation of any elements of structure after a forward and reverse polymorphic transformation (upon cooling and heating, respectively). The reverse transformation may restore the contours of the initial crystals (grain boundaries), the lattice orientation of the crystals, and the position of dislocations and packing defects in the crystals, and sometimes also the macroscopic shape of the sample, if a marten-site-phase structure formed during plastic deformation (the “memory effect”).

The structural memory of crystallographic orientation and lattice defects is due to the ordered rearrangement of one type of lattice into another during a shearing polymorphic transformation; restoration of the grain shape is also due to the preservation of chemical heterogeneity (segregation of impurities and inclusions at the site of the old boundaries). Structural memory in alloy steel hinders reduction of grain size during the annealing of castings and forgings. The structural memory of dislocations generated by cold working are used to increase the strength of steel by thermomechanical treatment. Alloys with the memory effect are used in the manufacture of instrument parts that change their shape upon heating.