Analytical Electron Microscopy of HSLA Steels Prepared using a Focused Ion Beam System

The work presented in this thesis is an investigation into the use of Focused Ion Beams (FIB) as a novel way of preparing high strength, low alloy (HSLA) steel specimens for analytical electron microscopy investigations.

Traditionally, ferromagnetic steel precipitates have been looked at in the transmission electron microscope (TEM) using either extraction replicas or electro-polished foils. Although electropolished foils cannot be beaten in terms of imaging quality, the large amounts of iron surrounding the specimens make detailed nanoanalysis very difficult. Extraction replicas offer an alternative, but do not give any spatial, in-situ information on the precipitates and their positioning in the steel. There are also uncertainties about the efficiency of the extraction process in isolating very small precipitates.

Focused Ion Beam liftout specimens offer an alternative way of preparing steel specimens for analysis. A liftout specimen is sufficiently small and thin that any magnetic effects should be reduced, allowing easy imaging and analysis.

This research aims to address some of the issues facing FIB specimen preparation of steels, and look at ways of reducing the level of ion damage in the finished specimen. Once precipitates have been identified within the matrix, it is hoped that detailed nanoanalysis of the precipitates can be carried out.

The specimens discussed throughout this research were analysed using an FEI Tecnai TF20 Scanning Transmission Electron Microscope. Basic electron microscopy techniques necessary to the analysis of the steel specimens are discussed in Chapter Two. TEM, STEM, PEELS, EDX and Spectrum Imaging techniques are all discussed, as well as an introduction to basic data processing using Digital Micrograph and other IT packages.

An introduction to the different types of TEM sample preparation techniques that can be used to prepare steel specimens for analysis in an electron microscope can be found in Chapter Three. Electropolishing, and carbon extraction replicas are briefly looked at, before Focused Ion Beam (FIB) specimen preparation methods are studied in detail.

An investigation into the different methods used within the FIB to prepare steel specimens of suitable quality and thickness for detailed nanoanalysis in the Tecnai is presented in Chapter Four. The results prove that FIB milling can be a very effective way of producing HSLA steel TEM specimens for nanoanalysis. Although modifications need to be made to the standard TEMWizard script used to prepare silicon specimen, and thinning the specimens in a reliable manner can often be difficult, it is clear that the FIB liftout method offers a viable alternative to electropolishing and extraction replicas.

Once suitable steel liftout specimens had been prepared, they were subsequently imaged in the Tecnai and analysed using PEELS and EDX. Chapter Five shows that although nanoanalysis can be carried out with relative ease, actually finding the precipitates within the specimen is much more difficult. The specimen surface is marred by a speckled effect that makes imaging and location of the precipitates extremely difficult. Despite this, some precipitates were found, and analysis carried out using EDX and EELS.

The EELS results proved that it was theoretically possible to detect very small precipitates within the matrix using nanoanalytical methods, but that surface damage caused by the FIB preparation limited the easy identification of the precipitates.

Ways of reducing the surface damage were sought. Chapter Six investigates the possibility of removing the surface layers of a FIB specimen using an ex-situ Ar ion mill. Two types of Ar ion mill were investigated: A Gatan Precision Ion Polishing (PIPS) system, and the Technoorg-Linda Gentlemill TM.

Both types of mills successfully removed the damaged layers, but the Gentlemill proved to be more effective across the entire specimen. It proved much easier to identify possible precipitates within the matrix of the low-kV milled specimens, with the surface cleanliness of the Gentlemill specimen almost equivalent to that of an electropolished specimen. Continuation work on Gentlemill specimens by Craven and MacKenzie (based on the research and techniques presented in this thesis) also showed that precipitates as small as 2nm could be identified and analysed with ease.

The Low Loss MLLS Fit method discussed in Chapter Seven also offers a way of searching for precipitates in a wide area, pinpointing precipitates of a specific chemistry, and then analysing those precipitates – all within a single microscope mode (STEM) and with potentially greater accuracy than any other method investigated throughout this thesis.

Low Loss MLLS Fitting also shows great potential as a useful tool for determining the fraction volume and precipitate densities within steels.

General discussions and conclusions, including a discussion of further work are presented in Chapter Eight.