Disorder-induced melting in nickel: implication to intergranular sulfur embrittlement

Why and how sulfur segregation leads to intergranular embrittlement of nickel has been investigated by a combination of Auger electron spectroscopy, slow-strain-rate tensile tests, ion-implantation, and Rutherford backscattering spectrometry studies. Grain-boundary sulfur concentrations in dilute Ni–S alloys were systematically varied by time-controlled annealing of specimens at 625 °C. The critical sulfur concentration for 50% intergranular fracture of 15.5±3.4 at.% S was found to be, within experimental error, equal to the critical implant concentration of 14.2±3.3 at.% S required to induce 50% amorphization of single-crystal nickel during S+ implantation at liquid nitrogen temperature. This suggests that segregation-induced intergranular embrittlement, like implantation-induced amorphization, may be a disorder-induced melting process, albeit one occurring locally at grain boundaries. In addition, a kinetic model for segregation-induced embrittlement based on Poisson statistics is introduced, and the synergistic effects of hydrogen–sulfur co-segregation on embrittlement are discussed.