THE TRANSFORMATION MECHANISM OF THE SIGMA-5(013) GRAIN-BOUNDARY STRUCTURE IN BCC METALS

A mechanism of the phase transition between two different structures o f the Sigma 5 36.87 degrees[100] {013} symmetrical tilt grain boundary in sec metals has been studied using a molecular dynamics method. For both grain boundary structures, the boundary plane is a plane of mirr or symmetry; however, in one case the atomic density at the boundary p lane is twice that of the {013} planes in the perfect crystal. The int eraction between atoms is described by simple pairwise forces. The pha se transition from the metastable configuration with a higher energy w as thermally activated. It is shown that the phase transformation need not occur by a simple vacancy mechanism but may have a displacive cha racter when it is accompanied by a rigid-body displacement of whole gr ains. In this case, the transformation is not a local event and has fe atures of collective atomic motion.