(ii) Moderate concentration. The element comes from the vapor phase around the alloy. (iii) High concentration. The embrittling species is in a liquid phase. EMBRITTLEMENT FROM SURFACE SEGREGATION The phenomenon was first recognized in the context of stress-relief cracking of alloy steels [3]. This is sometimes found in the coarse-grained heat-affected zone of welds, usually in thick sections. During the welding process, the HAZ reaches temperatures high enough to allow some dissolution of sulfides in the steel. During cooling, the dissolved sulfur segregates to austenite grain boundaries and then re-precipitates as very fine (probably metastable) sulfides in these boundaries. When the structure is later heated to allow relief of the residual stresses by creep, cavities can nucleate around large intergranular particles, like oxides or sulfides. If the stress is high enough, brittle cracks can grow from these cavities. This is caused by the presence of elemental sulfur that is segregated on the surfaces of the cavity and the subsequent crack and is then induced to diffuse into the grain boundary ahead of the crack by the tensile stress normal to the grain boundary, as illustrated schematically in Fig. 1. Sulfur is a very potent embrittling element in iron, and the concentration needed for decohesion at the stresses involved is probably less than one atom in ten [21]. Fig. 1 Schematic representation of (a) surface adsorption of an embrittling element, (b) inward diffusion driven by a tensile stress, (c) decohesion in the diffusion zone. The process depicted in Fig. 1 is analogous to the classical Hull-Rimmer mechanism of diffusive growth of creep cavities [4], which is the reverse of the sintering process. The stress across the grain boundary provides a gradient in chemical potential of atoms on the surface vs. in the bulk solid. That is, the stress does work when surface atoms diffuse into the bulk in exchange for lattice vacancies. Unlike in creep-cavity growth where the diffusing atoms are the same as in the bulk, in dynamic embrittlement the surface atoms are a mobile (i.e., lowmelting) element that is adsorbed on the cavity/crack surface. In a detailed study of stress-relief cracking using a simulated HAZ in a NiCrMnMo steel cooled rapidly from a high temperature [1], the following characteristics were found: (i) The initial crack growth is macroscopically discontinuous, occurring in bursts, as indicated by the results from a compact-tension specimen under a constant load at
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