The ductile-brittle transition in the fracture of α-iron: II

The expressions obtained in a previous paper for the fracture-mode transition temperature at a notch are shown to agree well with experimental measurements of its variation with grain size and with the friction on a free dislocation. A calculation is given of the effect of the dislocation locking strength on the transition temperature and this is in reasonable agreement with the observed temperatures for steels with weakened locking due to the presence of manganese. It is suggested that strain and strain-ageing raise the transition temperature because of the increased tensile stress available at fracture for the propagation of a dislocation crack.