  Understanding the mechanism of martensitic transformation is of great
importance in developing advanced high strength steels, especially
TRansformation-Induced Plasticity (TRIP) steels. The TRIP effect leads to
enhanced work-hardening rate, postponed onset of necking and excellent
formability. In-situ transmission electron microscopy has been performed to
systematically investigate the dynamic interactions between dislocations and
alpha martensite at microscale. Local stress concentrations, e.g. from notches
or dislocation pile-ups, render free edges and grain boundaries favorable
nucleation sites for alpha martensite. Its growth leads to partial dislocation
emission on two independent slip planes from the hetero-interface when the
austenite matrix is initially free of dislocations. The kinematic analysis
reveals that activating slip systems on two independent {111} planes of
austenite are necessary in accommodating the interfacial mismatch strain. Full
dislocation emission is generally observed inside of austenite regions that
contain high density of dislocations. In both situations, phase boundary
propagation generates large amounts of dislocations entering into the matrix,
which renders the total deformation compatible and provide substantial strain
hardening of the host phase. These moving dislocation sources enable plastic
relaxation and prevent local damage accumulation by intense slipping on the
softer side of the interfacial region. Thus, finely dispersed martensite
distribution renders plastic deformation more uniform throughout the austenitic
matrix, which explains the exceptional combination of strength and ductility of
TRIP steels.
