Sulfur embrittlement on γ/γ′ interface of Ni-base single crystal superalloys

The impurity sulfur embrittlement on gamma/gamma' interface of Ni-base single crystal superalloys has been investigated by first principles quantum mechanics DMol calculations. Using a local sum of vertical (SigmaBO(v)) and horizontal (71301) Mayer bond orders, we proposed a new method to evaluate the competition between the shear and cohesive strengths of the interface. Coupled with the phenomenological theory of fracture, we define the ratio of R-BO = Sigma(BO)(h)/SigmaBO(v) to assess the embrittlement trend of the interface related to sulfur doping or sulfur doping combined with Re substitution. It is shown that sulfur increases R-BO by 121% relative to the sulfur-free gamma/gamma' interface, which could induce interface embrittlement from the electron bonding point of view. Calculations of both BO and charge density distribution reveal that it is the strong bonds between sulfur and Ni atoms lying within the interface that contribute to the interface embrittlement. The substitution of Re for A] at the gamma/gamma' interface results in reduced R,,, thus relieving the tendency of interface embrittlement. Furthermore, our model on sulfur embrittlement is compared with previous models. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.