DISLOCATION ACTIVITY AND BRITTLE-DUCTILE TRANSITIONS IN SINGLE-CRYSTALS

Experimental work on the brittle-ductile transition (BDT) has shown that the activation energy for the BDT is equal to that for dislocation glide in silicon, germanium and sapphire. Simple models, based on the shielding effect of a single evolving array of dislocations, can account for most features of the BDT in such materials: These models are reviewed. Modification of the dislocation sources in test specimens can be used to change the BDT temperature and the ''shape'' of the BDT; thus realistic positioning of dislocation sources in models of BDT behaviour is critical in achieving an accurate simulation of experiments. Magnesium oxide, hydrogen-embrittled iron-silicon and molybdenum exhibit stable crack growth before fracture; the process appears to be by dislocation-enhanced microcleavage. Experimental results are reviewed. New results show that single-crystal TiAl behaves in this way. A model of stable crack growth is outlined.