A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y / F E B R U A R Y 2 0 2 3 3 4 which help to visualize phases such as a carbides, precipitates, and intermetallics that are only a few micrometers in size. Another benefit of using an electron microscope is that it is usually equipped with an energy dispersive spectroscopy (EDS) attachment, which helps to identify the elemental composition of the phases being visualized in a number of ways such as point scan, line scan, and elemental mapping. ELECTRON BACKSCATTER DIFFRACTION Electron backscatter diffraction (EBSD) is a SEM-based technique useful for phase analysis. This method uses an EBSD detector attached to an SEM instrument to generate information on material microstructure such as grain size and orientation, grain boundary misorientation, andphase composition. This technique is especially helpful for phase analysis where the secondary phase fraction and sizes are very small. In the case of phase transformation induced by aging, EBSD can be used to identify where the new phase is being precipitated. For example, in 17-4 PH stainless steel, austenite formed during heat treatment is known to form preferentially at the martensite lath boundaries. Figure 4 shows the presence of austenite in a martensitic matrix after heat treatment of 17-4PHstainless steels. Although EBSD is a powerful analytical technique, it has some drawbacks. First, for EBSD analysis to be carried out, specific sample preparation requirements must be met, such as electropolishing the sample surface to remove all stresses introduced in the material at subsurface levels during general polishing and grinding. Further, EBSD can only differentiate between two phases if they have different crystal structures. If the two phases present in the material have a different chemical composition but similar crystal structure, EBSD should not be used. TRANSMISSION ELECTRON MICROSCOPY Transmission electron microscopy (TEM) is one of the most widely used techniques when the secondary phases are just a few nanometers in size. TEM uses a very thin sample, often thinned mechanically to about 50 µm followed by ion milling to attain a sample thickness through which the electron beam can be transmitted to generate an image based on electron interaction with the material. TEM is often used to identify secondary phases like carbides and nanoprecipitates, which would otherwise not be detected in SEM or other analytical techniques due to their small size. With the advanced instrument technology found in TEM, it is possible to observe changes happening in the material on the order of a few atomic spacings. Additional information about TEM sample preparation procedures and the TEM technique itself can be found in References 4 and 5. Examining material microstructure with TEM is often combined with EDS spectra generation for chemical analysis and selected area electron diffraction (SAED) pattern analysis to identify phase structure, orientation, and lattice parameters for the phases present in the material. Figure 5 is an example of a TEM micrograph showing fine Y-Ti-O particles present in a ferrite matrix. As can be seen, there are Y-Ti-O phases as small as 10 nm, which would not be visible with scanning electron microscopy. ATOM PROBE TOMOGRAPHY Atom probe tomography (APT) is widely used for phase identification. However, compared to SEM, XRD, or TEM—which are primarily based on the interaction of electrons and x-rays with the sample surface—the APT technique uses a completely different approach to generate an elemental profile that can be used for phase analysis. APT samples are generally very small needle-shaped specimens generated from bulk material using techniques such as focused ion beam milling. The tip radii of the needle- shaped specimen can be about 100 nm. Once an electric field is applied, the atoms present at the tip of the specimen are removed from the specimen and passed through a spectrometer where the atoms are characterized. This process is repeated and finally the data for all the elements present in the material are collected and reconstructed into data such as the example shown in Fig. 6. This technique is helpful in Fig. 6 — Atomprobe tomography results showing distribution of Cu atoms in a test steel at different annealing durations. ©ASM International. Reused with permission[8]. (a) (b) (c) (d) Fig. 5 — TEMmicrograph showing the presence of Y-Ti-O particles in the ferrite matrix and the associated SAED pattern. ©ASM International. Reused with permission[6]. (a) (b)
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