ON THE SEARCH FOR EXPERIMENTALLY OBSERVED GRAIN BOUNDARY PHASE TRANSITIONS

The phase space for a heterogeneous system containing a grain boundary involves a relatively large number of variables (i.e., at least six plus the number of components), and it is therefore conceptually possible to induce a large variety of grain boundary phase transitions by selectively varying these parameters. Despite this, a review of the literature reveals that there have been virtually no clear-cut experimental observations of transitions reported in which the boundary structure has been observed as a function of time under well defined conditions. In current work, we are searching for roughening/faceting transitions and melting transitions for boundaries in Al by hot stage transmission electron microscopy. A clear example of a reversible roughening/faceting transition has been found. No evidence for melting has been found for temperatures as high as 0.96 Tm (by monitoring GBD core delocalization in several special boundaries with Σ≤ 13) or 0.999 Tm (by observing the local diffraction contrast at general boundaries in polycrystalline specimens)