Site preference of S-doping and its influence on the properties of a Ni/Ni₃Al interface

A first-principles investigation of the doping effects of S on the properties of a Ni/Ni₃Al interface is conducted. S-doping is found to be energetically permissible either at sub-lattice sites or at octahedral interstitial centers, and S atoms prefer to substitute host atoms, especially Ni atoms, at the coherent (0 0 2)γ/γ' layer. Among octahedral interstitial centers, the most favorable condition is the S segregation onto an octahedral interstice bounded by 6 Ni atoms at the coherent interfacial layer. The calculation of Griffith rupture work W and local bond overlap population shows that S-doping not only reduces the rupture strength of the Ni/Ni₃Al interface, especially at preferentially occupied sites, but also causes the inter-phase fracture mode and site to be changed. Doping with the trace element sulfur is indeed deleterious for the strengthening of the Ni/Ni₃Al interface; however, the segregation of S-doping onto the octahedral interstitial sites at the (0 0 1)γ or the coherent (0 0 2)γ/γ' layer is demonstrated to be profitable for improvement of the local toughness of the Ni/Ni₃Al interface to some extent, particularly in their inter-phase fracture regions. The S-induced embrittlement of the Ni/Ni₃Al interface can be attributed to a variation in atomic bonding energy. As S replaces Ni at the (0 0 1)γ layer or located at the octahedral interstices at the (0 0 1)γ layer or the coherent (0 0 2)γ/γ' layer, the large local elastic strain energy in the inter-phase fracture regions should be responsible for the change in the inter-phase fracture sites.