Effects of the Phase Content on Spallation Damage Behavior in Dual-Phase Steel

The dual-phase steel (martensite (M)/ferrite (F)) samples with different phase content were dynamic loaded simultaneously by one-stage light gas gun. The velocity of free surface particles was measured by Doppler pin system (DPS) during the loading experiment. The soft recovered samples were investigated with optical microscopy, nanoindentation, and EBSD techniques to study the effect of the phase content on dynamic damage evolution in dual-phase steel. The results show that the sample two (1000 degrees C/60 min + 780 degrees C/30 min+quenching) has a higher M area percentage (77.2%), larger M size, and smaller number of M and less M/F interfaces compared with the sample one (1000 degrees C/60 min + 740 degrees C/30 min+quenching, with M area percentage of 45.51%). Due to the reflection and transmission of shock wave at M/F interface, tensile stress will be generated inside M with higher shock impedance. Under the same dynamic loading conditions, the more M/F interface means the greater the probability of void nucleation inside M. Thus the sample two with less M/F interfaces has lower nucleation density and lower spallation strength. The microcrack propagation resistance increases with the increase in the area percentage of M and the size of M, which results in the lower damage evolution rate of the sample two. Meanwhile, the size of M will affect the direction of microcracks propagation. Each M with larger size in the sample two is composed of several prior austenite (A) grains, and the orientation of the martensite packets in prior A grains is very different. Therefore, the microcrack propagation in the sample two is limited to different regions, the direction of microcracks propagation is easy to be deflected.