The Pt, Hf dopants in aluminide coating against S impurity: Dependence on grain boundary cohesion and energetic properties

Abstract

The Pt and Hf additions in aluminide coating can pronouncedly change corrosion resistance against sulfur attack. To provide a comprehensive picture of this issue, by first-principles simulations we study the sulfur-induced embrittlement of ∑5(310)[001] symmetrical tilt grain boundary (STGB) in NiAl, with a focus on roles of Pt, Hf dopants played in sulfur-attacked grain boundary cohesion. The results show that the Hf-doping tends to increase GB combination with larger pre-breaker strength and theoretical tensile strength after sulfur introduction, while Pt-doping hardly makes contribution to the GB mechanical properties. The sulfur-induced embrittlement can be attributed to the charge density depletion and breaking of chemical bonding during stretching process, evaluated by the charge density distributions and bond length analysis. By analyzing the sulfur formation energy at NiAl bulk and NiAl(110) surface, we find that the presence of Pt enhances the energy criterion forming S impurity, while the significant reduction in adsorption energy after Hf-doping results from HfS strong atomic bonding. Alternatively, the existence of Pt decreases probability of S occurrence and becomes the barrier against S invasion. Instead, Hf tightly ‘embraces’ S impurity and impedes its diffusion into depth.