Spatial resolution of EPMA for the characterization of grain boundaries, precipitates and dendrites in Ni-Cr-Fe alloys

A. CARRERAS; G. CASTELLANO

Campinas

Encuentro; XXII Reunião Anual de Usuários do LNLS; 2011

LNLS, Brasil

ERNiCrFe-7 nickel alloy, also known as Filler Metal 52 (FM-52), is widely used for joining structural parts in chemical, nuclear and oil industries. This alloy is susceptible to ductility dip cracking (DDC) when highly restrained components are welded. DDC occurrence depends on the chemical composition, segregation to grain boundaries, precipitation of intermetallic phases at both intragranular and intergranular regions, grain boundary morphology, etc. Previous research suggests that DDC resistance of FM-52 can be improved by optimizing grain boundary morphology and second-phase precipitation during welding. Consequently, it is interesting to know the relationship between chemical composition and grain boundary morphology in as-welded microstructures. In a recent study, it was observed that metallic additions to FM-52 induce changes in the tortuous character of grain boundaries but almost do not alter the crystallite orientation distribution function and the misorientation distribution between grains. In this work the spatial resolution of Electron Probe Microanalysis (EPMA) technique is studied, with the aim of developing a methodology for the characterization of grain boundaries, precipitates and dendrites in Ni-Cr-Fe alloys, in a submicron scale. Monte Carlo simulations are used to analyze the spatial distribution of K- and L-shell ionizations for Ni, Cr and Fe, when pure metals and alloys are irradiated with different beam energies. From this survey, a dependence of the spatial resolution on incident energy is determined. Complementarily, the accuracy of the chemical quantification of the alloys by EPMA is analyzed, in experiments performed with the same incident energies than the simulations. An energy dispersive system is used for the analysis through K lines, whereas a wavelength dispersive one is used in order to perform the analysis by means of L lines. Finally, the optimal experimental conditions are found for an accurate quantification of these alloys with the best spatial resolution possible.