GRAIN STRUCTURE AND BOUNDARY

One of the original applications of AES, and still one of the most important, is the analysis of grain boundaries in metals and ceramics. Very small amounts of impurity or dopant elements in the bulk material can migrate under appropriate temperature conditions to the boundaries of the grain structure and accumulate there. In that way the concentration of minor elements at the grain boundaries can become... 

Figure 93. Cubic stabilized ZrOi ceramic, unetched, BF. Preparation procedure I. Grain structure and pores at grain boundaries and within the grains. Not all grain boundaries are visible. Subsurface pores have a bright appearance.

Fig. 1.20. A schematic representation of several stages in the formation of a thick polycrystalline film on a substrate. Cluster formation and growth leads to impingement of cluster to form grain boundaries. The development of grain structure and subsequent film growth is determined largely by surface mobility of adatoms relative to the deposition rate and by the substrate temperature relative to the melting temperature of the film material. Adapted from Thompson (2000).

Palumbo, G. and Aust, K.T. (1992) Special properties of S grain boundaries, in Materials Interfaces Atomic-level Structure and Properties, eds. Wolf, D. and Yip, S. (Chapman Hall, London) p. 190. 

To further characterize the event it is first necessary to identify critical features of the initial configuration that will strongly influence the process. For powder compacts, the most obvious features are the morphological characteristics of the powders, their microstructures, and the porosity of the compact. For solid density samples, the grain structure, grain boundaries, defect level, impurities, and inclusions are critical features. 

It was pointed out by Nieh and Wadsworth [5] that fine grain size is a necessary but insufficient condition for HSRS. This conclusion resulted from the observation that many fine-grained composites are not superplastic at high strain rates. Evidently, in addition to grain size, microstructural factors, such as detailed structure and chemical composition at the reinforcement-matrix interfaces and grain boundaries, may play important roles. 

Pressure controls the thickness of the boundary layer and consequently the degree of diffusion as was shown above. By operating at low pressure, the diffusion process can be minimized and surface kinetics becomes rate controlling. Under these conditions, deposited structures tend to be fine-grained, which is usually a desirable condition (Fig. 2.13c). Fine-grained structures can also be obtained at low temperature and high supersaturation as well as low pressure. 

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