Transformation plasticity is a weakening of the mechanical properties of a polycrystal while it is undergoing a phase transformation. It must be distinguished from other processes linked to phase transitions, like the so-called ‘Transformation-Induced Plasticity’ which makes steels tougher.

After a brief review of the thermodynamics and kinetics of phase transformations, the experimental evidence for transformation plasticity in metals and ceramics is presented. The macroscopic model of Greenwood & Johnson, inspired from ‘yielding creep’ models, is reviewed: it considers that the internal stresses caused by the volume change of grains overcome the yield strength of the solid and make it flow under small stresses. Microscopic models are presented, in which the internal stress is relaxed by dislocations that move under the applied stress.