Scanning Electron Microscopy

The scanning electron microscope is a useful tool for examining metal powders, fracture surfaces, as-pressed and sintered surfaces formed by dies and punches, and, potentially, the as-polished sections used for optical microscopy. Scanning electron micrographs of iron, prealloyed steel, and stainless steel powders are shown in Fig. 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, and 24 in the series of representative micrographs in this article. In addition, Fig. 25 and 26 show the smooth outer surfaces of parts that were contacted by punches or dies during consolidation. Figure 26 illustrates the unsintered view of the side of a part that contacted the die. The powders have been pressed into close contact, and the boundaries between particles are readily visible. Sintering, which completes the bonding of adjacent powder particles, is traced in the accompanying optical micrographs (Fig. 27, 28, 29, 30, 31) that illustrate the disappearance of the particle boundaries and the rounding of pores in a diffusion-alloyed steel.

The development of bonding between metal particles may also be followed by examining their fracture surfaces. As bonds develop, the fracture shows cup-and-cone or dimpled-fracture regions where the bonds have been torn apart. The regions between the ductile cups and cones are the smooth surfaces of the original particles, which were not bonded to the adjacent particles. The progress of sintering may be followed through the increasing number and area fraction of the ductile torn regions. This is well illustrated in the scanning electron micrographs shown in Fig. 32, 33, 34, 35, 36, 37, and 38. The energy spectra of the x-rays generated by the electron beam striking the atoms of the specimen can be analyzed to determine which elements are present at the fracture surface. The fracture surface may be scanned for the wavelength characteristic of a particular element and to record the intensity or concentration of the element as a function of location. This concentration appears on an x-ray map as a collection of dots. That display shows if the element is uniformly distributed or somewhat segregated, which is useful for monitoring the dissolution of copper or nickel in steel, or for checking for oxides of manganese or silicon.

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