P. E. J. Flewitt

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.

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 sintering of creep-induced cavities in a low alloy ferritic steel (1Cr1Mo0.75V)

Sintering heat treatments at temperatures in the range 1173 to 1273 K have been undertaken in an atmospheric pressure of argon gas to remove grain boundary cavities within a lCrlMo0.75V ferritic steel (Durehete 1055) following creep rupture after service operation at 838 K. High sensitivity density measurements and optical metallography have been used to monitor the progressive sintering of cavities. The results are discussed in relation to existing models which describe the sintering of cavities by grain boundary diffusion.

The stress corrosion susceptibility of a quenched and tempered 12 pct crmov martensitic stainless steel

The stress corrosion susceptibility of a martensitic 12 pct Cr 1 pct MoV stainless steel in alkaline chloride solution has been measured as a function of tempering heat treatment. The microstructures produced during tempering have been characterized by transmission electron microscopy and related to measured hardness values. In addition, scanning transmission electron microscopy combined with energy dispersive X-ray microanalysis has allowed the distribution of alloying elements within the microstructure to be examined. Electron energy loss spectroscopy was used to establish fully precipitate compositions, and the microanalysis results have been explained in terms of a diffusion controlled growth of grain boundary precipitates. The overall stress corrosion cracking susceptibility has been correlated with the development of chromium solute depletion profiles about prior austenite grain boundaries.

Environmental cracking of type 316 austenitic stainless steel weldments in high temperature co2 gas

The environmental cracking of Type 316 austenitic stainless steel manual metal arc (MMA) weldments in high temperature CO2 has been investigated. The welding thermal transient has been analyzed and used to predict the sensitization of the parent material and resulting residual stresses. Sensitization is considered to result from the local depletion of chromium about grain boundaries and this has been considered theoretically and measured using STEM combined with energy dispersive X-ray analysis. Residual stresses have been measured using the X-ray diffraction method. Flux residues over the HAZ of the weldment have been identified using X-ray diffraction analysis. Chemical changes in these residues which resulted in cracking of test samples exposed to high temperature (673 to 823 K) CO2 gas have been analyzed by X-ray diffraction. The susceptibility of the weldments to cracking was found to be influenced by the carbon content and hardness of the material. In materials of high carbon content and high hardness, the necessary cracking parameters are satisfied by the combined contribution of the microstructural sensitization, tensile residual stresses, and the chemical interaction of weld flux residue with the CO2 gas which provides a molten salt environment capable of allowing rapid transport of aggressive species to active crack sites.

Recovery of creep properties of a vanadium-strengthened steel (1Cr1Mo0.75VTiB) by reheat treatment

A reheat treatment cycle for Durehete 1055 has been developed, the first stage of which removes grain boundary creep cavitation. Subsequent stages control microstructural parameters such as prior austenite grain size and allow in-service degradation of the vanadium carbide precipitate distribution to be reversed. Samples have been examined using a range of optical and electron optical techniques, and the compositions of extracted carbide precipitates have been determined using STEM-EDS X-ray microanalysis. It is shown that by reheat treatment the uniaxial creep life of creep-ruptured, ex-service material can be restored, and the creep ductility of previously coarse-grained material improved. The application of the results to life regeneration of service components is briefly discussed.

Measurement of alloy and impurity elemental distributions in commercial steels using stem

This note describes the application of the STEM X-ray microanalysis technique to the measurement of local composition in commercial alloy ferritic steels. Results of chemical microanalyses obtained from a 12% chromium ferritic stainless steel and 214% chromium-1% molybdenum low alloy ferritic steel serve to illustrate its use and application.

A simple design for a low temperature electron microscope stage

Abstract A simple design for an electron microscope goniometer stage which operates from room temperature down to ∼20 K is described. The stage is cooled by a standard hydrogen ‘Cryotip’ which relies on Joule/Thompson cooling, and consequently only requires the storage of pressurised gaseous nitrogen and hydrogen. Controlled temperatures are obtainable within the ranges 16K to 33K and 66K to 110K. The stage is capable of ±20° tilt together with rotation through 360°. Results on niobium 44.1% titanium alloy illustrate the performance of the stage.

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.