Mechanism of martensite aging effects and new aspects

We reviewed recent progress in studying the long-standing puzzle of the martensite aging effect, which includes the rubber-like behavior of martensite and the martensite stabilization effect. Emphasis was laid on a recently proposed general principle called symmetry-conforming short-range order (SC-SRO), which states that there exists interaction between crystal symmetry and distribution of point defects such that the SRO tries to follow the crystal symmetry, if in equilibrium. In fresh (unaged) martensite SRO symmetry inherits the parent symmetry due to the diffusionless nature of the martensitic transformation, thus it does not match the crystal symmetry of martensite right after the transformation. This is an unstable state; as a result atomic diffusion occurs so as to correct the 'incorrect' SRO symmetry during the subsequent aging. This is what occurs during martensite aging. This mechanism fully explained all the known facts about martensite stabilization and rubber-like behavior. We further provided TEM evidence for the SC-SRO principle, and predicted two new effects, which are expected from the SC-SRO principle: (i) microstructure memory, and (ii) two-way shape memory (due to aging). Both were verified experimentally. The symmetry-conforming SRO property, which is a general property of crystalline materials, may also be applied to a wide range of phenomena concerning symmetry change.