Book excerpt: A new strain-rate dependent mechanism of dislocation generation that can become active suddenly above a critical temperature has been developed in our research carried out under the present AFOSR support. This mechanism is a thermally driven, stress-assisted cooperative instability of many dislocation loops that leads to an outburst of dislocation activity above the strain-rate dependent critical temperature. The strain-rate dependence originates from the glide of pre-existing and thermally nucleated dislocations below the critical temperature. We have determined theoretically and shown by experiments that the onset of yielding in a crack-free crystal with a very low dislocation content (Si in our study) is remarkably similar to the brittle-to-ductile transition (BDT) in a pre-cracked crystal of the same material. There is significant evidence to show that both processes are controlled by the cooperative process of dislocation generation. As a result, we now have, for the first time, a model that is capable of predicting the brittle-to-ductile-transition-temperature (BDTT) of a material as a function of strain rate and dislocation content.