SINGLE-PROCESS THEORIES OF GRAIN BOUNDARY MIGRATION IN THE LIGHT OF RECENT EXPERIMENTAL RESULTS

Il faut connaître la structure atomique et électronique des joints de grains pour discuter, d'un point de vue théorique, les mécanismes associés à la migration des joints de grains activée thermiquement. En ce qui concerne la structure atomique, beaucoup d'informations nouvelles furent acquises ces dernières années (les travaux présentés à ce colloque en témoignent) ; mais ces informations ne peuvent qu'étendre qualitativement les concepts heuristiques existants (dans certains cas déjà bien anciens) des mécanismes de migration intergranulaire. Par contre, l'effet de la structure électronique des joints de grains n'a pas été du tout considérée jusqu'à présent. L'idée que cet aspect est également important, est assez nouvelle. A cause de cela, et puisqu'il existe déjà de nombreux articles de revue sur la migration des joints de grains, cet exposé-ci est limité essentiellement à la discussion de quelques questions restées sans réponse, importantes pour l'interprétation théorique de la migration des joints de grains, spécialement dans les métaux de haute pureté. Seul le cas des joints de grand angle est traité ; l'effet des différences d'orientation sera discuté dans d'autres communications.Theoretical treatment of the mechanisms associated with thermally activated grain boundary migration requires a knowledge of the atomistic and electronic structure of the grain boundary. Although in recent years a whole range of new data has been acquired concerning the atomistic structure of grain boundaries (see for example the relevant papers presented at this colloquium) they can do no more than qualitatively extend the existing, in some cases really old, heuristic concepts of grain boundary migration mechanisms. The effect of the electronic structure on grain boundary migration has not been considered at all till now. The idea that this aspect also is of importance is rather new. In view of this, and considering that there are already a number of recent review articles on the subject of grain boundary migration [1-4], the present paper is restricted essentially to the discussion of several unanswered questions which are of general importance for a deeper theoretical understanding of grain boundary migration in very pure metals. The discussion is also restricted to large angle grain boundaries. Orientation effects will not be included because they are discussed in another of the papers to be presented

Recrystallization : the Actual Topics with Particular Focus on the Investigation of Transformation Kinetics

An attempt will be made to give an overview of the main areas of actual interest with respect to recent progress and to future prospects in the respective fields. In this connection the following areas are treated : The kind of material and the initial thermodynamical unstable state ; the basic processes during heating up of a sample : recovery, nucleation, grain boundary migration ; the end product, which is the result of an interaction of the basic processes. From these different areas attention is turned to the topic of "kinetics", focussing particularly on what we can learn about recrystallization- and recovery-kinetics in metals as a result of modern calorimetric investigation