Effect of impurities on the electronic structure of grain boundaries and intergranular cohesion in tungsten

Abstract

The cohesion of a grain boundary (GB) is believed to be the controlling factor limiting the ductility of high-strength metallic alloys, and particularly those containing W. Intergranular embrittlement is usually associated with segregation of impurities at the GBs. Impurities present in ppm concentrations can result in a dramatic decrease in plasticity. This paper reviews recent results on both semi-empirical and first-principles modelling of the energetics and the electronic structures of 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’ of the GB. The presence of B and C on the contrary, enhances the interatomic interaction across the GB. The so-called ‘site-composition 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 to displace other impurity atoms from the GB. Microalloying with 10–50 ppm B may be an effective way of improving tungsten's ductility. These results are important for understanding the fundamental physics of intergranular embrittlement.