Damage Initiation and Evolution Behaviors of Ultrahigh-Strength Multi-Phase Steels during Tensile Deformation

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The damage initiation and evolution behaviors of ferrite-martensite dual phase (DP), transformation-induced plasticity (TRIP)-aided dual-phase (TDP), quenched and tempered (QT), and TRIP-aided martensitic (TM) steels during tensile deformation were investigated. Voids were initiated at the phase boundaries and inside the martensite in the DP and TDP steels, whereas fine voids were observed at the prior austenite, packet, and block boundaries in the QT and TM steels. In the DP and TDP steels, the size of the voids remarkably increased with the plastic strain, even though the number of voids increased slightly. By contrast, the QT and TM steels exhibited a drastic increase in the number of voids, whereas a slight increase in the size of the voids was observed. The voids in the TM steel hardly extended as the plastic strain increased unlike those in the QT steel. The extent of voids in the DP and TDP steels might be attributed to stress and plastic strain partitioning between the different phases during tensile deformation. In addition, the promotion of void initiation and suppression of void growth might be attributed to the fine and uniform martensite matrix in the QT and TM steels. The suppression of void initiation in the TDP steel and void growth in the TM steel might be attributed to the stress and plastic strain relaxations at the void initiation site and the vicinity of voids owing to the effective martensitic transformation of retained austenite.