Trace cerium/ yttrium addition for inhibiting sulfur-induced intermediate temperature embrittlement in ultrahigh-purity copper

Sulfur (S), as the common impurity prevalent in copper (Cu) and Cu-based alloys, often induces the intergranular embrittlement at intermediate temperatures even with a ppm concentration, a phenomenon variously called "intermediate temperature embrittlement" (ITE). However, the inhibition of such an embrittlement has been rarely reported. In this study, we investigate the influence of trace cerium/ yttrium (Ce/Y) addition on the ductility of the ultrahigh-purity Cu containing minute amounts of S impurities at temperatures between room temperature (RT) and 900 degrees C. The results show that both Ce and Y additions can improve the ductility of the Cu-S alloy at any test temperature, which is attributed to the inhibition of Ce/Y on the grain boundary (GB) segregation of S by forming the RE-S precipitates. By comparison, the modification of ITE caused by S impurities is not significant after trace Ce is introduced, while the addition of small amounts of Y can completely eliminate the ITE phenomenon at temperatures of 300-750 degrees C. This results from the size of the RE-S precipitates and the structure of the transition nanolayers between the RE-S precipitates and Cu matrix, which is dependent on the diffusion rates of the dissolved S, Ce and Y atoms in Cu during the tensile deformation. Our findings provide a comprehensive experimental and computational understanding on the fundamental mechanisms of rare earth elements inhibiting the embrittlement induced by impurities in metallic materials.