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Mechanisms for cleavage and intergranular embrittlement in Fe

The mechanism of cleavage in bcc Fe has been investigated with electronic structure calculations and compared to similar calculations modeling cleavage in other bcc materials (Li, Nb, Mo). It has been found in the BCC materials we have studied that those known to undergo (100) cleavage show enhancement of the density of states (DOS) on the virtual surface layer as the material is strained to failure. Using total energy calculations it is shown that there is a point of inflection (corresponding to the point of maximum force versus displacement) in the total energy at nearly the same point as the maximum enhancement in the DOS. We conjecture that this point represents the transition state for the fracture process. Extending these ideas, the electronic structure near a Fe(111) grain boundary has been calculated both with and without segregated P. We find clear evidence for the formation of a P band of states, suggesting the existence of P-P interactions within the segregated layer. A chemical model of strain-induced bond failure is suggested in which bonds parallel to the fracture surface compete with the cohesive Fe-Fe bonds normal to the surface. 7 refs., 5 figs.