Interactions between sulphur and microstructural defects in a Ni-16 wt% Cr-7 wt% Fe alloy and microsulphide nucleation

Abstract

The distribution of sulphur in the form of isolated atoms or microscopic sulphide particles (microsulphides) in the structure of two Inconel 600 alloys of different purity was analysed with the help of two new techniques: high-resolution sulphur autoradiography (with ³⁵S^*) for the segregation of sulphur and high-resolution sulphur microprint to observe large amounts of sulphur. Among the various places where sulphur may segregate (grain boundaries, microprecipitates, slip-planes, isolated dislocations) some are preferred sites for further nucleation and growth of microsulphides. As a result, preferential interactions may be observed between sulphur and chromium oxycarbides, especially in the case of industrial alloys, in as much as they have a higher carbon content. The results have been considered essentially in terms of: (i) a gain in energy during the chemical reaction, which leads to the formation of chromium sulphides from chromium carbides and (ii) a reduction in overcrowding through the formation of microsulphides. The critical free energy W^* for the nucleation of microsulphide is considered with the help of the Burke equation.