Influence des ségrégations intergranulaires sur les processus d’accommodation des dislocations extrinseques

Influence of intergranular segregations on the accommodation processes of extrensic dislocations

Laboratoire de métallurgie physique associé au CNRS n° 177, Université Paris-Sud, Orsay

Abstract

Extrinsic dislocations result of two types of interactions between the matrix dislocations and the grain boundaries: crossing through the boundary, dissociation in the boundary. Their study rests on the geometrical approach of Bollmann which provides their Burgers vectors, either the crystal ones or those of the DSC lattice. These dislocations usually have a sessile component in the grain boundary plane.

By thermal activation, dissociation of the extrinsic dislocations can occur, giving rise in the case of the general boundaries to a Spreading of the dislocations. The dissociation products can move, interact, annihilate and, finally, incorporate the intrinsic network. All these processes constitute the accommodation of the extrinsic dislocations in the grain boundary.

The kinetic of the accommodation processes are most probably governed by the intergranular diffusion. It is highly likely that segregated species must influence the extrinsic dislocations reactions. The purpose of this work is to analyze the conditions of stability of extrinsic dislocations after recrystallization in various ferritic alloys.

Our results point out two conditions of stability of extrinsic dislocations: fistly, the necessity of a “cosegregation” of elements having a strong affinity for carbon as it is the case for chromium, molybdenum and vanadium, secondly a minimum level of segregated species which depends on the “special” or “general” character of the boundary. The precipitation which can occur on the dislocations lines decreases the quantity of intergranular segregated elements and involves a destabilization of the extrinsic dislocations.

The accommodation possibilities of extrinsic dislocations have important consequences on the mechanical properties of the grain boundaries, their creep and migration behaviors. When these processes are impeded, the extrinsic dislocations maintain an important elastic stress field and the grain boundary may be considered as a strong one; on the contrary, when the accommodation takes place, the matrix dislocations may continuously incorporate the grain boundary which are a ductile one. Finally, at the end of the accommodation, the strains are very localyzed in the grain boundary core (< 5 A), we get an equilibrated boundary. There is a relation between the microstructure of the material and the presence or the lack of linear defects in the grain boundaries structures.