In a polycrystal intergranular segregation refers to the gathering of solute atoms at the grain boundaries as a monolayer or a fraction of a monolayer. This phenomenon can dramatically affect the grain boundary cohesion and lead to different types of material degradation. For example, the detrimental effect of sulphur or phosphorus grain boundary segregation on the mechanical properties of nickel alloys and steels has been documented in numerous experimental studies.
The first part of the talk will be an overview of the available experimental techniques to detect and quantify grain boundary segregation: Auger Electron Spectroscopy (AES), Secondary Ion Mass Spectrometry (SIMS), Wavelength Dispersive X-ray Spectroscopy (WDS) and Scanning Transmission Electron Microscopy + Energy Dispersive X-ray Spectroscopy. Possibilities and limitations of the different techniques will be discussed. In addition a focus will be made on the recent efforts in having the grain boundary solute concentrations quantified consistently from different techniques and expressed with the same unity (atoms/m² vs fraction of a monolayer).
The second part of the talk will be devoted to grain boundary cracking in structural materials related to impurity segregation. The first case study will deal with the effect of phosphorus segregation on the onset of grain boundary brittleness in a martensitic stainless steel after long-term ageing (> 5000 h) at low temperature (300°C). Evidence will be given here that grain boundary cracking is due to a combined effect of phosphorus segregation that occurs during the tempering treatment at 600°C and bulk hardening that occurs during ageing at 300°C.
The second example is about grain boundary cracking during multipass welding of nickel base superalloy. Experiments conducted on model alloys of well controlled composition as well as industrial alloys have demonstrated the detrimental effect of impurity sulphur and the beneficial effect of carbon and niobium. Advanced characterisation using WDS and STEM-EDS have shown that the internal surfaces of cracks are covered by sulphur, whereas uncracked grain boundaries show no sulphur enrichment. The propagation of cracks observed during welding can be interpreted here by a local weakening of the grain boundary at the crack tip due to sulphur.