X-ray microanalysis of grain boundary segregation in steels by s.t.e.m

Abstract

Equilibrium segregation of Groups IV-VI residual elements during heat treatment ( ca . 773K) of commercial low alloy steels can significantly modify their toughness, impact properties, corrosion and stress corrosion behaviour and creep life. Since these segregations are highly localized (less than about 10 nm) and do not result in microstructural changes, their direct measurement has proved difficult. The most widely used method to date is Auger electron spectroscopy (A.e.s.). This technique is surface specific, requires uncontaminated intergranularly fractured surfaces and is therefore limited by specimen preparation. The development of scanning transmission electron microscopy (s.t.e.m.) with fine electron probes, ca . 15 nm in diameter, when coupled with an energy dispersive analyser enables high resolution X-ray microanalysis within thin foils. This allows grain boundary chemical analysis for elements with atomic number not less than 12. The application of this technique has been investigated by examining segregation of residual elements in experimental iron-based alloys and commercial ferritic steels.