Inclination of self-interstitial dumbbells in molybdenum and tungsten: A first-principles study

Body-centered-cubic (BCC) transition metals are ubiquitous structural materials, and their mechanical degradation under irradiation is significantly influenced by the stability and mobility of the lattice defects. In this study, we analyzed the self-interstitial atoms (SIAs) in BCC molybdenum (Mo) and tungsten (W) in comparison with other BCC transition metals utilizing the first-principles method; particularly, we focused on uncommon dumbbells whose direction are inclined from 〈111〉 toward 〈110〉 on the {110} plane. Such a direction is not stable in the group 5 BCC metals (i.e., vanadium, niobium, and tantalum) or in α-iron. Our first-principles relaxation simulations indicated that inclined dumbbells were more energetically favored than common 〈111〉 dumbbells in Mo, while this is not necessarily the case for W. However, a certain degree of lattice strain, such as shear or expansive strain, could make inclined dumbbells more favored also in W, suggesting that the lattice strain can substantially influence the migration barrier of SIAs in these metals because inclined dumbbells generally have a larger migration barrier than 〈111〉 dumbbells. We also elucidated the mechanism of the inclination using the electronic charge density; the charge density map of the perfect crystals suggested that the anti-bonding state of electrons along the 〈111〉 direction is likely to cause the instability of 〈111〉 dumbbells, and the charge density map near dumbbells suggested how 〈111〉 dumbbells are inclined toward the 〈110〉 direction.