The material class of molybdenum alloys is known for excellent material properties, e.g. high melting point, high strength, low coefficient of thermal expansion, high thermal conductivity and a high Young’s modulus. One drawback of these materials is their high ductile to brittle transition temperature that leads to intergranular fracture at room temperature in recrystallized condition. The intergranular fracture can be traced back to a reduced strength of grain boundaries (GBs), which can be strongly modified by segregation of interstitial solutes like oxygen and carbon.
In this study, we present density-functional theory (DFT) simulations of molybdenum, where we investigate the effect of C and O segregation on the cohesive properties of the GBs. The segregation is first treated separately for the solutes and in a second stage, we also look onto co-segregation phenomena, where solute elements are allowed to interact with each other. These interactions can be either repulsive or attractive, leading to depletion or accumulation of an element at an interface and thereby large changes in GB cohesion are induced. By modeling the interactive co-segregation of C and O, we obtain coverage dependent segregation energies that elucidate processes closer to real alloys compared to previous investigations treating segregation of solute elements independently of each other. The changes in GB cohesion due to different C and O coverages are quantified using ab-initio fracture simulations, where the GB is subjected to uniaxial loading. We find that O weakens the GB cohesion, but by co-segregation of C this effect is compensated. These simulations suggest ways to decrease the detrimental effect of O and other impurities and how to improve the cohesive properties in Mo.