P. Doig

The spatial resolution of X-ray microanalysis in the scanning transmission electron microscope

Abstract The spatial resolution of X-ray microanalysis within thin foils in the scanning transmission electron microscope (STEM), interfaced with an energy dispersive X-ray spectrometer, has been analysed. The electron beam divergence within the foil has been described analytically and the resulting X-ray intensity profiles have been numerically evaluated for a range of simple elemental composition distributions within the foil. The effects of incident electron probe diameter, electron accelerating voltage and foil thickness on the measured X-ray intensity ratio profiles are considered and discussed.

The Detection of Monolayer Grain Boundary Segregations in Steels Using STEM-EDS X-Ray Microanalysis

The detection and measurement of monolayer embrittling segregations to prior austenite grain boundaries in ferritic steels using scanning transmission electron microscopy combined with energy dispersive X-ray microanalysis are considered theoretically. The influence of experimental variables of electron probe size, electron accelerating voltage, and foil thickness on the detectability limits are considered in relation to electron probe spreading within the foil and the resulting X-ray counting statistics. The analysis predicts that a grain boundary coverage of 0.01 and 0.2 of a monolayer of tin and phosphorus, respectively, may be detected in a ferritic steel using a conventional STEM-EDS system.

A comparison of X-ray (STEM) and Auger electron spectroscopy for the microanalysis of grain boundary segregation

Abstract Auger electron spectroscopy and STEM X-ray microanalysis have been used to measure grain boundary segregations in iron-based alloys. Analyses have been carried out on iron-3 wt.% nickel alloys containing additions of either tin or phosphorous which have been embrittled by heat treatment at 823 K. The results of the two analyses are discussed and used to compare critically this application of the techniques.

Nonequilibrium solute segregation to austenite grain boundaries in low alloy ferritic and austenitic steels

Nonequilibrium segregations of substitutional solute and impurity elements to austenite grain boundaries during cooling from the austenitizing temperature have been measured in thin foil specimens of ferritic 2.25 pct Cr 1 pct Mo, 2.25 pct Cr 1 pct Mo 0.08 pct Sn and austenitic Type 316 steels using scanning transmission electron microscopy combined with energy dispersive X-ray spectrometry (STEM-EDS). The results are discussed and compared with the predictions of a theoretical analysis which describes the segregation in terms of a quench-induced diffusion of vacancy-solute atom pairs to the grain boundaries.

The influence of specimen thickness on X-ray count rates in STEM-microanalysis

Abstract Measurements have been made on the variation with thickness of the ratio between the X-ray count rates for CuKα and AIKα in thin foils of an aged A1 6% Zn, 1.98% Mg, 1.34% Cu alloy using scanning transmission electron microscopy. Accurate microanalysis is prevented by what appears to be a copper-rich surface film, probably formed during electrochemical preparation of the specimen. The film is not removed by ion beam thinning but is reduced by chemical etching. The implication of the results in general for microanalysis of thin films is discussed and the importance of specimen preparation emphasized.

X-Ray microanalysis of grain boundary segregations in steels using the scanning transmission electron microscope

This paper describes an analysis which allows real composition segregation profiles to planar grain boundaries to be determined quantitatively from STEM X-ray microanalysis on thin foils. This analysis is applied to the measurement of such segregations in ironbased alloys. The influence of electron beam spreading within the foil is evaluated and an analysis is developed which allows solute composition profiles to be obtained from measured X-ray intensity profiles. The influence of various experimental parameters on the measured X-ray intensity profile are examined and discussed. The analysis procedure is applied to experimental measurements of tin segregated to prior austenite grain boundaries within the heat affected zone of a ’/2 PGt CrMoV low alloy steel weldment taken from a boiler steam chest.

The X-ray measurement of tangential stress on cylindrically curved surfaces

Abstract The effects of specimen curvature and applied tangential stress on the angular position of diffracted X-ray peaks, recorded using the diffractometer method, have been analysed. This analysis has been used to measure applied tangential stresses on the outer surface of an expanded C-ring specimen. The measured stress values were in good agreement with the applied stresses.

STEM-EDS X-ray microanalysis of small precipitates in iron base thin foils

The experimental conditions for detection of segregated elements in the X-ray spectrum recorded from small precipitates ≦ 100 nm diameter contained within a thin foil of an iron base alloy using STEM-EDS X-ray microanalysis are described. The influence of precipitate size, position and composition, foil thickness, electron accelerating voltage, and X-ray emission intensity on its detectability are evaluated. Optimum experimental conditions for microanalysis of these precipitates are established and the limitations of current techniques discussed.