On the formation of interstitial loops in b.c.c. metals

Abstract

Two types of interstitial loops with Burgers vector b = a/2 and b = a, respectively, have been observed experimentally in irradiated b.c.c. metals. The present paper discusses how these loops can form from a common loop nucleus consisting of a single platelet of interstitials on a {110} plane. Such a loop contains a stacking fault which is removed by one of two shears described by the following dislocation reactions: It is suggested that the latter higher energy, shear, requires thermal energy to assist it and therefore only occurs at elevated temperatures. Consideration is given to the elastic energy of the loops with particular reference to their shape and orientation. The calculations predict that the loops are initially rectilinear and as they grow they should change to the circular form and rotate into a pure edge orientation. The crystallographic symmetry of the a loops and their growth mechanism probably explains the observed retention of the rectilinear form.