Trace sulfur-induced embrittlement of ultrahigh-purity copper

Sulfur (S), as an impurity element in copper and copper-based alloys, usually leads to intergranular embrittlement even at ppm concentrations. However, the intrinsic mechanisms of the embrittlement remain largely unclear. In this study, we systematically investigate the influence of trace amounts of S (53 and 300 wt ppm) on the ductility of an ultrahigh-purity copper (99.99999%) in the range of room temperature to 900 °C. Tensile results show an excellent plastic deformation capacity at any test temperature for the ultrahigh-purity copper, while a remarkable reduction in ductility for both S-containing alloys, particularly at 300–750 °C. A microanalysis reveals formation of micro- and nanoscale Cu–S precipitates, mainly distributed at grain boundaries, acting as the nucleation sites for small cavities leading to the crack initiation. This is responsible for the ductility loss in the S-containing alloys in the whole temperature range. The distinct embrittlement behavior between 300 and 750 °C is independent on the crystallographic orientation relationship between the Cu–S precipitates and Cu matrix, but is dominated by the temperature-dependent phase transition behavior of the Cu–S precipitates. Such a structural transformation with the volume expansion accelerates the nucleation and propagation of the microcracks, causing the intergranular embrittlement at intermediate temperatures. Our findings provide a comprehensive experimental understanding on the fundamental mechanisms of impurity-induced the embrittlement in metallic materials. • The effect of trace sulfur on the ductility of ultrahigh-purity copper in the temperature range of RT–900 °C is systematically investigated. • The fracture mechanisms induced by trace S with temperature variations are revealed. • A phase transformation of C u 1.96 S (cubic) .→ C u 31 S 16 / C u 2 S (monoclinic) → C u 39 S 28 (hexagonal) → C u 1.96 S (tetragonal) with increasing temperature is directly visualized.