A DISLOCATION SHIELDING PREDICTION OF THE TOUGHNESS TRANSITION DURING CLEAVAGE OF SEMIBRITTLE CRYSTALS

An interconnected set of observations assesses current equilibrium models of the ductile-brittle-transition temperature (DBTT). This involves in situ transmission electron microscopy (TEM) studies of crack-tip dislocations in single and polycrystals and bulk fracture toughness tests at various temperatures. Beyond K(I) values of 8 MPa . m1/2 in both iron-base single and polycrystals, large numbers of redundant dislocations are created, as postulated recently by Weertman.[38] Still, the necessary shielding dislocations, as required by equilibrium, can be detected at values as high as 20 and 40 MPa . m1/2 by ex situ TEM and electron channeling, respectively. In addition, the close approach of dislocations to the crack tip in some of the studies, as opposed to others, suggests that large dislocation free zones (DFZ) are a thin-film artifact. However, a failure criterion based partly on the Rice-Thomson model[2] is both consistent with the absence of a large DFZ and observed fracture toughness variations with test temperature. It is emphasized that this toughness transition is entirely in the semibrittle regime where cleavage is the failure mode. Nevertheless, K(Ic) values increase from 3 to 60 MPa . m1/2 with an increase in test temperature.