Theoretical study on segregation of Cu, Mo and W impurities and stability of impurity–vacancy pairs in bcc Fe

Reduced activation ferritic steel is one of the leading structural material candidates for a nuclear fusion reactor, in which tungsten is introduced in order to improve the radiation resistance. Since the solute impurities have an effect on the embrittlement through segregation under irradiation, the stability of impurity elements should be elaborated. In the present study the segregation characteristics of tungsten and some general solute impurities in bcc iron were investigated nonempirically by first principles calculations, where the equilibrium segregation was considered via a regular solution model and the change in enthalpy for segregation were directly evaluated for comparison. Subsequently the energetic stabilities of impurity–impurity and impurity–vacancy pair were evaluated. The segregation enthalpy is influenced by the electronic interaction between the d-electron of Fe and the outer electron of the impurity element, and molybdenum and tungsten tend to prevent solute atoms from segregation.