Atomistic simulation of shock compression of bcc molybdenum single crystals: Role of preexisting dislocations and temperature

It is known that plastic relaxation behind the shock wave front in metals and alloys is achieved through intense dislocation multiplication. Most of the molecular dynamics simulations usually consider perfect crystals, in which dislocation needs to be nucleated. The present paper presents the molecular dynamic simulations of shock wave loading in [100], [110], and [111] molybdenum crystals of micrometer length, both perfect and with dislocations, over a wide range of temperatures from 300 to 2100 K. The evolution of the shock wave structure and the Hugoniot elastic limit (HEL) is analyzed for the dependence of temperature and the presence of dislocations. It is found that behind the wave front, preexisting dislocation loops, depending on their orientation, could either multiply on their own or serve as the nucleation sources of new screw dislocation segments. The formation of twin bands is also found in [110] and [100] Mo crystals with dislocations as well as in perfect [110] crystals. In Mo crystals with preexisting dislocations, the HEL decays monotonically, and the decay rate weakly depends between [110] and [111] orientations. The HEL decays much slower at the front of the elastic precursor in the [100] crystal; however, the post-spike HEL values decay with the same exponent as for [110] and [111] Mo crystals. The decay exponents are found to be in range between 0.25 and 0.45, which agree with experiments when the shock propagation distance is above 0.2 mm. The HEL decreases slightly with increasing temperature, which is also in accordance with experiments.