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

Abstract

The semi-empirical and first-principles calculations of the energetics and the electronic structures of impurities on a ∑3 (111) grain boundary (GB) in Fe and W have revealed important features of the impurity effects. In both Fe and W, 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. Boron plays a dual role in both Fe and W; not only does its presence at GBs enhance the intergranular cohesion, but it also accomplishes ‘site competition cleansing’ by displacing the other impurity atoms off the GB. Micro alloying with 10–50 ppm B may be an effective way of improving the ductility of both Fe-base alloys and W. Another important result is understanding the decohesion effect of H in Fe. Contrary to general belief, H does not contribute its electron to the Fe d-bands at all. Instead, the electron stays very strongly localized close to its proton. There is also virtually no hybridization with the Fe electrons and virtually no effect on Fe atoms magnetization. As a result the interatomic bonding along the Fe-atom chains via H atoms is very inefficient, thus resulting in significant weakening of the intergranular cohesion. These results are important for understanding the fundamental physics of intergranular embrittlement.