The effect of segregated sp-impurities on grain-boundary and surface structure, magnetism and embrittlement in nickel

We present a detailed theoretical study of segregation and strengthening/embrittling energy of sp-elements from the 3rd, 4th and 5th period (Al, Si, P, S, Ga, Ge, As, Se, In, Sn, Sb and Te) at the Σ5(2 1 0) grain boundary (GB) in fcc ferromagnetic nickel. For comparison, we investigate also the segregation of these impurities at the (2 1 0) free surface (FS). On the basis of ab initio electronic structure calculations, full relaxation of the geometric configuration of the GB and FS without and with impurities is performed and the effect of impurities on the distribution of magnetic moments is analysed. Whereas there is a slight enhancement of magnetization at the clean GB and FS with respect to bulk nickel (3–7% and 24%, respectively), the studied impurities entirely kill or strongly reduce ferromagnetism at the GB and in its immediate neighbourhood so that magnetically dead layers are formed. This effect, which is due to the hybridization of the impurity sp-states and nickel d-states, is even more pronounced at the impurity-decorated (2 1 0) FS. We determine the preferred segregation sites at the Σ5(2 1 0) GB for the sp-impurities studied, their segregation enthalpies and strengthening/embrittling energies with their decomposition into the chemical and mechanical components. We find interstitially segregated Si as a GB cohesion enhancer, substitutionally segregated Al and interstitially segregated P with none or minimum strengthening effect and interstitially segregated S, Ge, As, Se and substitutionally segregated Ga, In, Sn, Sb and Te as GB embrittlers in nickel. As there is very little experimental information on GB segregation in nickel most of the present results are theoretical predictions which may motivate future experimental work.