Migration of grain boundaries

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... 

Fig. 8 reveals that under the assumption of reasonable values for the adjustable parameters in Eq. (7) the theoretical predictions compare well to the experimental data. The very different behaviour of special and non-special boundaries reflects an influence of grain boundary structure on grain boundary migration mechanism. In particular, impurities not only may have an effect on migration by impurity drag, but also by changing grain boundary structure itself. This was shown recently by Udler and Seidman in a Monte Carlo... 

H. Gleiter. The mechanism of grain boundary migration. Acta Metall., 17(5) 565—573, 1969. 

H. Ichinose and Y. Ishida. In situ observation of grain boundary migration of silicon E3 boundary and its structural transformation at 1000 K. J. Phys. Colloq. (Paris), 51(suppl. no. 1) C1 185—190, 1990. 

R.J. Jahn and P.D. Bristowe. A molecular dynamic study of grain boundary migration without the participation of secondary grain boundary dislocations. Scripta Metall., 24(7) 1313—1318, 1990. 

Thus far, the discussion has been conhned to the static properties of grain boundaries. However, grain boundaries are metastable conhgurations and, as such, in response to external forces (e.g. thermal, mechanical), they exhibit dynamical behavior. Two of the more important ones are briefly mentioned here - grain-boundary migration and sliding. 

For various mechanisms of grain boundary migration, the mobility wQl have various definitions, as given in Table 16.8. The driving force for grain-boundary migration is assumed to be due to the difference in grain curvature ... 

Plot of grain boundary displacement versus anneal time for growth of a-plane sapphire seeds into 500 ppm Ti-doped and undoped aluminas at 1600°C. In comparison to results obtained previously with a lower purity powder, mobilities in undoped material are increased. However, the most striking feature is the increased migration rate for the Ti-doped material. 

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