Analysis of stress degradation of metals by positron annihilation

Since the stress degradation of metals originates from the change of the microstructure induced by glide motion and multiplication of dislocations, fundamental features of this process have been investigated from the viewpoint of the dislocation - dislocation interaction and the consequent vacancy production by using positron technique and computer simulation. Corresponding to the positron trapping at deformation induced defects in metals such as Fe and Ni, positron lifetime calculation has been made for the configurations obtained in model Fe and Ni lattices which contain a dislocation and vacancies produced by dislocation cutting process. The important results are (1) straight dislocation lines usually behave as shallow traps for a positron and the corresponding positron lifetime is close to that at matrix, (2) complex defects consisting of a dislocation line and vacancies usually show intermediate positron lifetimes between matrix and a single vacancy. Vacancy production process by dislocation cutting is simulated in model Fe lattice and it has been clarified that a vacancy is produced at each step of a climbing jog dragged by a screw dislocation line on <111> atomic row in the vicinity of a jog. Another topic of vacancy production process in a small Cu precipitate cut by a gliding dislocation has also been simulated in Fe model lattice, showing a conversion from a straight dislocation line to a jogged line after cutting the Cu precipitates. (author)