Bericht des Präsidiums 2000 - 2001

The measurement of grain boundary migration in pure Al bicrystals by X-ray continuous interface tracking is introduced. This method provides information on the mobility of specific grain boundaries without interfering with the process of migration. Moreover, the effect of hydrostatic pressure on grain boundary migration was investigated. Several topics of grain boundary motion relevant to microstructure and texture evolution by recrystallization and grain growth were addressed. It was found that the maximum growth rate misorientation changes with temperature from the exact ∑7 orientation relationship to a 40.50°<111> rotation. This behavior is of concern for recrystallization texture evolution. The effect of material purity on grain boundary migration is shown not to be confined to drag effects but also to involve changes of grain boundary structure. From the activation volume of grain boundary mobility it has to be concluded that at least <110> tilt boundaries move by cooperative motion (group mechanism) of atoms in the boundary

THE DYNAMIC PROPERTIES OF TWO-PHASE ALUMINA/GLASS CERAMICS

Les propriétés dynamiques des céramiques monolytiques se dégradent habituellement en présence d'une seconde phase. La différence d'impédance des constituants est le principal facteur responsable des dommages microstructuraux aux interfaces de phase. Des essais de restauration sous chocs ont montré que pour les alumine/verre, les microfissures étaient initiées à un niveau de contrainte inférieur à celui de la contrainte à rupture dynamique. Les premières microfissures ne sont pas liées entre elles et l'échantillon conserve sa rigidité élastique en dépit de sa perte de résistance à l'écaillage, qui, elle, est associée au dommage microstructural. La microfissuration a pour conséquence des concentrations locales de contraintes résiduelles et des échauffements locaux qui accompagnent la relaxation de l'enérgie élastique. La multiplication des dislocations et des glissements, une déformation plastique et une redistribution des différentes phases vitreuses, ainsi qu'une recristallisation partielle de la phase vitreuse sont parmi les conséquences de cet échauffement local.The dynamic properties of monolithic ceramics are in general degraded by the presence of a second phase. Impedance mismatch is the primary factor responsible for microstructural damage in the region of the phase boundaries. Planar shock recovery experiments have shown that microcracks in alumina/glass are generally initiated at a stress level below the observed dynamic failure stress. The initial microcracks are unconnected and the sample retains elastic rigidity, in spite of the loss of spall strength associated with the microstructural damage. Microcracking results in local residual stress concentrations and local heating which accompany the elastic energy release. Dislocation multiplication and glide, the flow and redistribution of any glassy phase, and partial crystallisation of the glass are among the possible consequences of this local heating