Effect of Impurities on the Electronic Structureof Grain Boundaries and Intergranular Cohesionin Tungsten

Abstract

ABSTRACTThe cohesion of a grain boundary (GB) is believed to be the controlling factor limiting theductility of high-strength metallic alloys, and particularly W. Intergranular embrittlement isusually associated with segregation of impurities at the GBs. Impurities present in ppmconcentrations can result in a dramatic decrease in plasticity. This paper reviews recent results onboth semi-empirical and first-principles modelling of the energetics and the electronic structuresof impurities on a Σ3 (111) GB in W. Our calculations have shown that impurities, such as N, O,P, S, and Si weaken the intergranular cohesion resulting in “loosening” the GB. The presence ofB and C on the contrary, enhances the interatomic interaction across the GB. The so-called site-competition effect should play an important role affecting impurity distribution in W GBs.Among the impurities analyzed, B in the GB has the lowest energy, and thus would tend todisplace other impurity atoms from the GB. Microalloying with 10-50 ppm B may be an effectiveway of improving tungsten's ductility. These results are important for understanding thefundamental physics of intergranular embrittlement.