Molecular-dynamic simulations of martensitic transformation of cobalt

With a potential-energy function of Co described by the embedded-atom method (EAM), molecular-dynamics (MD) simulations were performed for a series of initial fee configurations with different types of dislocations or preset hcp embryos. The gliding process of a Shockley dislocation on a close-packed plane has been observed, which starts from the origin of the dislocation and proceeds at a high speed of 280 m/s toward a certain direction. An atom which has been swept by the dislocation line was detected to contribute a displacement close to the Burgers vector of a Shockley dislocation. It is in this way that a new stacking sequence is produced and an hcp lamella grows in the fee structure. A similar gliding process has been observed in the case where an intrinsic stacking fault is preexisting in the fcc structure. The transformation is, again, toward forming a local hcp region. These results prove that a special dislocation in the fcc structure can act as an embryo of the hcp, as described in many dislocation mechanisms of the martensitic transformation. The fcc -> hcp phase-transformation process of Co has been further reproduced by a simulation initiated from an fcc/hcp two-phase configuration. It yields a single hcp crystal as the final transformed product.