THE BRITTLE DUCTILE TRANSITION IN SILICON

Experimental and theoretical studies of the brittle-ductile transition (BDT) of precracked single crystals of silicon are discussed. For a given strain-rate the temperature T(c) at which the BDT occurs varies by up to 250 K for similar materials but different test geometries. However, for a given material and geometry the transition is always very sharp (DELTA-T(c) < 5 K) and the strain-rate dependence of T(c) is controlled by the activation energy for dislocation velocity. Computer simulations of the dynamics of the generation and motion of dislocations from crack tips show that a sharp transition arises from the need to nucleate a high density of crack tip sources which emit dislocations which rapidly shield the crack. Sources are thought to be nucleated either by existing dislocations distant from the crack tip moving to the crack tip, or by dislocations emitted from a few preferential sites along the crack tip moving along the tip. The nucleation stage controls T(c), which is structure sensitive, depending on the initial dislocation distribution in the crystal or the spacing of preferred sites along the crack. The model explains the observed variability of T(c), and accounts for the values of T(c) and the strain-rate dependence in cases where the dislocation arrangement is known. The predictions of the computer simulations are in good agreement with the rate of advance of the plastic zones observed by X-ray topography. The model also predicts the density of preferential sites along the crack tips to account for the experimental results.