This is a copy of the slides presented at the meeting but not formally written up for the volume.

Low temperature fracture behavior of multiphase metallic materials is controlled by the microcracks originated in brittle precipitates embedded metallic matrix. Fracture in these materials propagates by the extension of ‘critical microcrack’ situated in the plastic zone of macrocrack ahead of it. The crack-tip plasticity of both microcrack and macrocrack are simulated as dislocation arrays using as discrete dislocation simulation. The analysis reveals the factors that contribute to the exponential increase in fracture toughness with temperature at the brittle - ductile temperature (BDT). They are found to be: (a) the marginal increase in microscopic fracture stress, (b) the increase in crack-tip blunting with increase in plastic-flow with temperature, and (c) the increase in dislocation mobility with temperature. On applying the model to a set of microcrack distributions it has been found that (1) always it is one of the largest microcracks that lead to the fracture (2) the scatter in fracture toughness measurements is due the scatter in the size of the microcracks not their relative position to the macrocrack.