We reviewed recent progress in studying the long-standing puzzle of the mar
tensite aging effect, which includes the rubber-like behavior of martensite
and the martensite stabilization effect. Emphasis was laid on a recently p
roposed general principle called symmetry-conforming short-range order (SC-
SRO), which states that there exists interaction between crystal symmetry a
nd distribution of point defects such that the SRO tries to follow the crys
tal symmetry, if in equilibrium. In fresh (unaged) martensite SRO symmetry
inherits the parent symmetry due to the diffusionless nature of the martens
itic transformation, thus it does not match the crystal symmetry of martens
ite right after the transformation. This is an unstable state; as a result
atomic diffusion occurs so as to correct the 'incorrect' SRO symmetry durin
g the subsequent aging. This is what occurs during martensite aging. This m
echanism 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-SR
O principle: (i) microstructure memory, and (ii) two-way shape memory (due
to aging). Both were verified experimentally. The symmetry-conforming SRO p
roperty, which is a general property of crystalline materials, may also be
applied to a wide range of phenomena concerning symmetry change. (C) 2001 E
lsevier Science B.V. All rights reserved.