DETERMINATION OF GRAIN-BOUNDARY VOLUME EXPANSION BY HREM

High resolution electron microscopy (HREM) has provided invaluable insight into the structure of grain boundaries at the atomistic level. Quantitative HREM methods are now in development which, combined with atomistic simulations, can provide further insight into grain boundary structure-energy correlations. For example, the volume expansion (or excess free volume, delta), a thermodynamic parameter directly related to the grain boundary energy, is represented by the normal component of the rigid body translation. Although its magnitude is small, it can be determined experimentally using statistical techniques which locate and fit the peak and valley positions in an experimental HREM image, i.e., a direct measurement of the lattice fringe displacements is made. We have modified the lattice fringe displacement technique such that a measurement accuracy of better than +/- 0.002a(0) can be achieved. Precise knowledge of the position and intensity of the image contrast and a detailed understanding of the sources of error are required. This paper provides a detailed description of the lattice fringe displacement technique as well as a detailed assessment of the sources of error in the measurement.