rate. DP steels with banded martensitic structure or large martensite islands exhibit low elongation to fracture. It was
also observed that a coarse dual phase microstructure exhibits a much lower value of elongation to fracture, but
higher strength than a fine dual phase microstructure [2, 3]. Failure in the coarse structure is due to the initiation of
cleavage cracking of martensite at low strain levels. The failure in fine dispersed structures occurs by void
nucleation and void coalescence with regard to decohesion of interface between the ferrite matrix and the martensite
islands. For TRIP steels the presence of martensite in the initial microstructure can lead to early crack initiation.
Increased stress triaxiality accelerates the TRIP effect and crack growth in the microstructure. The strong
dependence of the rate of martensite transformation on the stress triaxiality originates from the volume expansion
involved in the martensite formation. The volume expansion hinders damage development in the microstructure by
delaying the void nucleation. The influence of the TRIP effect on damage is related to the volume fraction of
martensite and its mechanical properties depending on carbon content [4, 5]. Chatterjee [6] showed that in TRIP
steels small hard martensite does not readily crack, as the load transfer onto the martensite is difficult by straining
the microstructure. Long plates of martensite transformed from the austenite with coarse grain size lead to earlier
cracking. The behavior of void nucleation and void coalescence of TRIP steels was investigated by means of
Electron Backscatter Diffraction (EBSD) in [7]. It was found that the void formation takes place in areas
characterized by high hardness gradient. In fact, the void growth preferentially proceeds along grain and phase
boundaries. When the austenite-martensite transformation in TRIP steel occurs at low deformation, voids will
nucleate due to the newly formed high strength martensite. When first transformation occurs at higher strains, the
voids are more likely to form at the grain boundaries of ferrite or inclusions. Fig. 1 (b) and (c) show the micrographs
and secondary electron images of TRIP steel after tensile test at uniform elongation. Many small voids were
detected exactly in the vicinity of austenitic grains, Fig. 1 (b). The phase transformation leads to volume expansion
and local stress concentration in the microstructure. The increased dislocation density promotes void nucleation.
Some voids were also observed at the grain boundaries of ferrite, Fig. 1 (c).