QUASI-STATIC INTERGRANULAR BRITTLE-FRACTURE - DYNAMIC EMBRITTLEMENT

A mode of brittle fracture is described which is fundamentally differe nt from the rapid transgranular cleavage or intergranular decohesion t hat is usually associated with that term. It involves stress-induced d iffusion of surface-adsorbed embrittling elements along grain boundari es, and it occurs by slow, step-wise crack growth, the rate of which c an, in principle, be calculated from the knowledge of the relevant int ergranular diffusion coefficient, the stress profile at the crack tip and the dependence of the stress for grain-boundary decohesion on the concentration of the embrittling element. This mode of fracture is pos tulated to be possible in any high-strength alloy with a low-melting-p oint element adsorbed on the surface if the applied stress is high eno ugh. Known examples include the brittle type of stress-relief cracking in steels, tin-induced cracking of Cu-Sn alloys, oxygen-induced crack ing of iron-, copper-, and nickel-based alloys, and the group of pheno mena known as liquid-metal embrittlement and solid-metal embrittlement .