**5.4. More advanced techniques**

Experimentally resolving and characterising grain boundary structure often requires a host of techniques: X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), X-ray energy dispersive spectroscopy (EDS) and electron energy loss spectroscopy (EELS). XRD or neutron diffraction normally deals with a texture that reflects an average value obtained from many different grains, that is, macrotexture. This chapter will address some techniques to obtain microtex-

Most grain boundary characters can be observed in low-vacuum secondary electron microscope (LV-SEM). In the backscattered electrons (BSE) mode, Z-contrast can assist phase identification. SEM/BSE hardly observe oxide scale and steel substrate without etching, because two parts of oxides and steel hardly to etching both using the same etchant. That is because polishing and etching for sample preparation can bring out the grain boundaries to more easily delineate individual grains. SEM/EDS and scanning transmission electron microscopy (STEM)/EELS help analyse the chemical species present and, combined with elemental mapping, can provide a distribution of the different chemistries in a spot, line or area. Sometimes, SEM can couple focus ion beam (FIB) to observe the grain characters along the cutting surface

Electron backscattered diffraction (EBSD) can perform microstructure, phase identification, the crystallographic texture, and internal stresses, of oxidised sample. Some system can provide a transmission kikuchi diffraction (TKD) mode where the short working distance in backscattered electrons (BSE) detector as a complement. Various professional software suite fully integrated with image collection, versatile EBSD analysis and phase identification, can be used to acquire the online texture information and to analyse the offline scanning maps. Grain boundary mapping shows the crystallographic orientation of individual grains and the microstructure in the oxide scale [9]. Further, grain reconstruction can be carried out to delve

High-resolution transmission electron microscopy (HR-TEM) has improved microscopy resolution, more developed techniques, and coupling with advanced approaches will enable the understanding and engineering of grain boundaries (including twins) and intergranular films. For instance, scanning transmission electron microscopy (STEM) coupled with electron energy loss spectroscopy (EELS) is capable of simultaneously mapping the atomic/electronic structure of light elements such as oxygen at adequate spatial resolution. The electronic state of the elements across the boundary can be identified by STEM-EELS line scan crossing the grain boundary in steps of a few nanometres [32]. TEM imaging further resolves details of the

ture involving some individual grains.

**5.1. Secondary electron microscope**

72 Study of Grain Boundary Character

when preparing for TEM samples.

**5.2. Electron backscattered diffraction EBSD**

**5.3. Transmission electron microscope**

crystal structure of grains and grain boundaries.

the various mechanism associated with individual grains.

Atom probe tomography has some unique virtues for hydrogen detection, such as near-atomic resolution and equal sensitivity to all elements in the periodic table. The advanced technique has been used to investigate hydrogen embrittlement in the oxide scale of two common zirconium alloys [33]. Grain boundaries of oxides can be low-field areas of the APT specimen and then can readily be identified [34]. A combined use of TEM and APT can be used to quantify grain boundary segregation and has been applied to the case of carbon GB segregation in ferrite [24] and intergranular oxidation of a Ni–4Al alloy [35].

For some corrosion environment, electrochemical scanning tunnelling microscopy (ECSTM) has applied to analyse *in situ* the passivation of grain boundaries on microcrystalline copper in 0.1 M NaOH aqueous solution [36]. ECSTM has provided accurate *in situ* topographic information on grain and grain boundary effects on the local active dissolution of microcrystalline copper in acid solution.

Time-of-flight secondary ion mass spectrometry (TOF-SIMS) provides elemental, chemical state and molecular information from surfaces of solid materials. Analogous to SEM/EDS instruments, TOF-SIMS aims to the compositional analysis of ultra-thin layers and nanoscale sample features. The difference is that TOF-SIMS can be used to characterise molecular information from organic materials and tissue sections for medical research.
