Segregation effects of Y, Ti, Cr and Si on the intergranular fracture of niobium

The intergranular fracture of materials is sensitive to the grain-boundary segregation of solutes, which is not yet clear for niobium alloys, a vital promising high-temperature material in the aerospace industry. In this study, first-principles calculations are performed to study the segregation effects of four mostly-used solutes (Y, Ti, Cr and Si) on the cohesion of two distinct grain boundaries (GBs) of Nb. Based on Rice-Wang Model, it is found that Y embrittles the GB; while Ti and Si strengthen it; the effect of Cr on the GB cohesion is structure-dependent. The strengthening/embrittling of the GBs induced by the segregation are attributed to the local structural distortion and charge redistributions around GBs. The underlying physical origin of different alloying effects on the intergranular fracture are identified to be the variation of bonding characters between the host Nb[sbnd]Nb atoms across the GB regions.