CHAPTER 3 EXPERIMENTAL METHODS
3.7 Transmission Electron Microscopy
Figure 3.9 Typical procedures of the preparation of a TEM lift-out sample: (a) deposit a 2 µm thick Pt-layer onto the interested area; (b) cut trenches in the front and back side of the interested area; (c) U-shape cut and weld the OmniProbe needle tip onto the sample; (d) lift the sample out and weld onto the sample holder grid; (e) finally thin down to 150-200 nm.
3.7 Transmission Electron Microscopy
Two Transmission Electron Microscopes (TEM) were employed in this project.
The first one was a JEOL 2000FX TEM equipped with an Erlangshen ES500W digital camera and an Oxford Instrument Inca Link EDS system. The other one was a FEI Tecnai F20 FEGTEM equipped with a Gatan mulitscan CCD camera, a Fischione high angular annual dark field detector (HAADF), a large area (80 mm2) windowless silicon drift detector (SDD) energy dispersive spectrometer (EDS) and a Gatan ENFINA electron energy loss spectrometer (EELS). The main purpose of using the two instruments in this project is to identify phases both chemically and crystallographically, and to investigate the chemical distribution of precipitates such as MX carbonitrides. Two types of TEM samples were analysed, FIB prepared thin foil lift-outs (described in the last section) and the carbon extraction replica samples.
3.7.1 Preparation of carbon extraction replica samples
Carbon extraction replica samples are useful for the characterisation of small second phase particles. One of the benefits of this technique is to avoid the effect of the ferritic matrix in EDS analysis, since the matrix is fully removed from the particles and therefore the chemical compositions of the particles can
(d) (e)
present a much larger examinable area than thin foil lift-outs. Furthermore, the replicas can preserve and reflect the relative positions of second phase particles.
A schematic illustration of the steps of the preparation of carbon extraction replica samples is shown in Figure 3.10. A typical G92 sample needs to be firstly mounted and polished down to 1 µm surface finish then to apply a light chemical etch to remove a thin layer of the matrix but leave the second phase of interest exposed and intact. To achieve this, a ~10 s chemical etch by Vilella’s reagent was carefully carried out. A carbon film was deposited onto the surface of the light etched sample using a Quorum Q150T ES carbon Evaporator, in which a carbon arc source was operated in a 10-5-10-4 Pa vacuum. A carbon film coating with a thickness of ~20 nm was deposited that had a characteristic dark brown colour. After the deposition of the carbon coating, the coated surface was scored into ~3×3 mm squares using a blade as the diameter of a copper grid is 3 mm. An electrolytic etch was then performed to remove the carbon film off the sample. The coated and scored sample was rinsed into a pre-prepared beaker containing 10% HCl in methanol with a voltage of 1-3 V and a current of 30-50 mA. The sample was taken out after ~15 s etch and rinsed into another beaker with pure methanol to remove the residual electrolyte. Subsequently the sample was carefully immersed into a pre-prepared third beaker containing distilled water at a 45o entering angle. The carbon film broke into squares during the immersion into the distilled water and floated on the water. Those floating carbon film squares were carefully fished out and deposited onto 400 mesh square copper grids using tweezers. After a natural air drying, the carbon extraction replica samples were prepared and stored in a TEM specimen support grids.
3.7.2 TEM imaging and energy dispersive X-ray analysis
In the JEOL 2000FX TEM, bright field (BF) and dark field (DF) imaging modes were both used. In the Tecnai the system was operated in Scanning Transmission Electron Microscope (STEM) mode and bright field (BF) and high angular annual dark field (HAADF) modes were used for imaging. It is
noteworthy that HAADF mode provides strong atomic number contrast and is therefore advantageous for imaging precipitates.
The two TEMs were both equipped with EDS systems. The working principle of TEM-based EDS is similar to the SEM-based ones. The EDS equipped with the JEOL 2000FX TEM is capable of doing spot analysis only due to the fact that the microscope is not fitted with scan coils, whilst the EDS in Tecnai is
Selected area diffraction technique provides crystallographic information of the analysed phase as a complementary method to the chemical information to identify phases in TEM. In this project, all the work on electron diffraction was carried out on the JEOL 2000FX TEM for consistency. Figure 3.11 illustrates the basic principle of the formation of the selected area diffraction pattern. When the incident electron beam hits onto a crystalline area in the sample, the diffracted electrons which satisfy the Bragg condition for the d-spacing of the local crystalline planes become the origin of the diffracted spots in the diffraction pattern. To index the diffraction pattern, the distance between the transmitted spot and the diffracted spots (Rhkl) need to be measured.
Together with the camera length (L) and the wavelength associated with electrons (λ), the spacing (dhkl) for the Bragg hkl indices can be calculated by the equation:
𝑅ℎ𝑘𝑙𝑑ℎ𝑘𝑙 = 𝐿𝜆 Equation 3.6 where Lλ is the camera constant. The wavelength of electrons is related to the accelerating voltage and can be calculated by the equation:
𝜆 = √2𝑚ℎ
𝜀𝑒𝑉 Equation 3.7
where h is Plank’s constant, mε is the electron mass, e is the charge on the electron, and V is the accelerating voltage. In this project, the accelerating voltage of the TEM is 200 kV, and therefore the electron wavelength (λ) is 0.0251 Å. Also a diffraction ‘ring’ pattern of a pure aluminium sample was used as calibration standard to measure the camera length (L).
Figure 3.10 Illustration of the preparation of carbon extraction replica samples: (a) the flat surface of the two phase (or more) material is (b) light chemical etched and (c)
Figure 3.11 A schematic diagram of selected area diffraction formation showing the relationship between the camera length, distance on the diffraction and the Bragg angle.