Scanning Electron Microscopy

The Scanning Electron Microscope used in this project was a Leo Carl Zeiss 1530 VP Field Emission Gun Scanning Electron Microscope (FEG-SEM), which enables visualisation of surface features of the specimen with an achievable resolution of 1 nanometre. The FEG-SEM is equipped with a combined analytical system from Oxford Instruments consisting of an X-max 80 Energy Dispersive Spectroscopy (EDS) Silicon Drift Detector (SDD) and a Nordlys F Electron Backscatter Diffraction (EBSD) camera both run using Aztec software. This versatile instrument is capable of examining microstructures and carrying out chemical and crystallographic analysis.

3.5.1 SEM imaging

Three imaging modes in the FEG-SEM, secondary electron, backscattered electron and in-lens, were used in this study dependent on the characteristics of the features being examined.

The secondary electron and backscattered electron modes were commonly used for imaging general microstructures, voids and creep damage, and some of the second phase particles. Backscattered electrons possess higher energy than secondary electrons and therefore can escape from a larger interaction volume in the sample subsurface. This leads to a lower spatial resolution of backscattered electron mode than secondary electron mode. However, the intensity of backscattered electron signal provides benefits in terms of examining high atomic number features such as W rich Laves phase (much brighter appearance than in secondary electron mode) and low atomic number features such as BN, and even voids/damage/cracks (much darker

appearance than in secondary electron mode). The accelerating voltage (EHT) selected for both imaging modes ranges from 5 kV to 20 kV with the aperture size of 30 µm in diameter to yield good atomic number contrast.

The other powerful imaging mode frequently used in this project is called in-lens imaging mode. The annular in-in-lens secondary electron detector is capable of detecting the secondary electrons which develop in the immediate spot centre to form images with good contrast and high signal-to-noise ratio.

One of the advantages of the in-lens detector is its high efficiency with regard to the detection of SE1 electrons, the secondary electrons directly generated by the primary electrons. The position of the in-lens detector within the beam path excludes the detection of backscattered electrons and secondary electrons of the categories SE2, the secondary electrons generated by the backscattered electrons when they pass through the specimen surface, and SE3, the secondary electrons generated by the backscattered electrons when they impinge onto the pole pieces and other parts of the optical system and specimen chamber. Both SE2 and SE3 signals carry information similar to that of backscattered electrons. In-lens technique itself provides an efficient way to significantly reduce the contribution of SE2 and SE3 signals to the collected SE signal, and therefore reveal the pure information on the surface of the sample. The in-lens technique has been developed to be an important method in this project to study inclusion particles especially the BN-type.

3.5.2 Energy dispersive X-ray analysis

Energy dispersive X-ray spectroscopy (EDS) was carried out in SEM examination to obtain chemical information of various phases in Grade 92 steels. This technique allows manual spot analysis or automated scanning of a specified area. The chemical composition is determined by collecting information from X-rays emitted from the sample surface. The X-rays are generated by the bombardment of electron beam and the X-ray signals from the interacted volume (see Figure 3.3) are collected and analysed by the EDS detector attached to the SEM, resulting in a quantified chemical composition.

In this project, the most used parameters for EDS analysis in FEG-SEM were a working distance of 8.5 mm and an accelerating voltage of 10 kV with an aperture size of 120 µm, to yield a good X-ray signal intensity. The parameters in FIB-SEM are usually a working distance of 5 mm and an accelerating voltage of 10 kV with an aperture size of 60 µm. A detailed introduction of the FIB system will be presented later in Section 3.6.

3.5.3 Electron backscatter diffraction analysis

Electron backscatter diffraction (EBSD) is an SEM-based technique applicable to crystalline materials. This technique can be used to index and identify the seven crystal systems to achieve misorientation mapping, phase discrimination, and using complementary techniques (e.g. EDS) to identify intermetallic phases. As shown in Figure 3.4, when an electron beam bombards onto a crystalline material, the electron scattering in the specimen causes electrons to travel in all directions beneath the surface of the incident point. Some of the backscattered electrons will satisfy the Bragg condition (+θ and -θ) and are diffracted into cones of intensity with a semi-angle of (90^o-θ), with the cone axis normal to the diffracting plane. These pairs of flat cones intercept the imaging plane and are imaged as two nearly straight Kikuchi lines separated by an angle of 2θ (Michael, 2005). The angle for diffraction from a given set of planes is called the Bragg angle and is given by:

𝜆 = 2𝑑 sin 𝜃 Equation 3.5 where d is the spacing of the atomic planes, λ is the wavelength of the

electron and θ is the Bragg angle for diffraction.

A typical electron backscatter diffraction pattern consists of a large number of parallel lines of intensity which essentially reflects a map of the angular relationships that exist within the crystal. To record EBSD patterns, a commercial CCD camera with a phosphor screen is installed inside the SEM sample chamber.

In this project, the EBSD analysis was performed using an EDAX Hikari camera attached to an FEI Nova 600 Nanolab FEGSEM/FIB dual beam

system. The pre-tilt angle of the sample with respect to the direction of the electron beam is 70^o. Samples for EBSD analysis were rigorously polished to an extreme fine finish using a 0.02 µm colloidal silica suspension for 30 to 40 minutes. The analysis was conducted using an accelerating voltage of 20 kV with an aperture size of 50 µm.

Figure 3.3 Electron interaction volume generated by high-energy electron beam, after Wittke (2008).

Figure 3.4 Illustration of the formation of Kikuchi bands from backscattered electron diffraction, after Schwartz (2000).