Mechanism of Intermediate Temperature Embrittlement of Ni and Ni-based Superalloys

TLDR

Ni‑based superalloys are critical for aircraft engine propulsion, yet they exhibit ductility loss at intermediate temperatures, and literature reveals divergent interpretations of the underlying embrittlement mechanism. The article seeks to confirm the generality of intermediate‑temperature embrittlement in Ni and Ni‑based alloys and to outline future research directions for a comprehensive understanding of the phenomenon. The authors review and discuss existing mechanistic interpretations, drawing on representative investigations to assess their validity. They conclude that the mechanism remains inadequately explained, proposing that nonequilibrium interface segregation of impurities, modulated by strain rate, may underlie the embrittlement.

Abstract

Ni-based superalloys play an important role in aircraft engine propulsion. However, many experiments demonstrated that these alloys as well as Ni always show ductility loss at intermediate temperature, which constrains further development. A comparison of published papers by various authors reveals considerable differences in understanding the mechanism of intermediate temperature embrittlement. To clarify this situation, the present article first confirms the generality of intermediate temperature embrittlement of Ni and Ni-based alloys by the experimental results reported in the literature. The existing interpretations of the mechanism are then outlined. Based on the generality, these interpretations are discussed through the representative investigations on intermediate temperature embrittlement. It is shown that the mechanism of intermediate temperature embrittlement has not been satisfactorily explained yet and "nonequilibrium interface segregation" of impurities taking into account the effect of strain rate may be the origin of intermediate temperature embrittlement of Ni and Ni-based superalloys. Future research directions aiming at the reason of abnormal fracture mode, the effect of the state of applied stress, the influence of strain rate, and the development of the theory nonequilibrium grain boundary segregation, are suggested to provide a complete understanding of intermediate temperature embrittlement.