Effet de la structure des joints de grains sur la déformation plastique des polycristaux

The effect of grain boundaries structure on the plastic deformation of polycrystals

Institut de sciences et technologie des matériaux. Ecole Polytechnique, Varsovie

Abstract

The interaction of lattice dislocations with high angle grain boundaries is considered from the point of view of recently discovered process of spreading of extrinsic grain boundary dislocations (EGBDs). The Lojkowski and Grabski model for the spreading kinetics is explained and the new experimental evidence is provided, showing that the spreading rate is a most sensitive available indicator of the state of grain boundary structure, allowing for the accurate determination of g.b. self diffusion coefficients and g.b. energy. With use of spreading kinetics measurements it is shown that as a result of spreading the grain boundary structure deviates from the equilibrium state, and that high temperature annealing transforms the structure back towards the equilibrium one. This process of transformation is called the recovery of grain boundary structure.

The proof is delivered that the changes of grain boundaries structure produced by the spreading process exerts a strong effect on the value of the yield stress, and that the annihilation of dislocations through the spreading or EGBDs is an important mechanism of dynamic recovery, affecting the plastic flow at high temperatures. The recent results on Hall-Petch relation in stainless steel and aluminium and on Piobert-Luders bands formation in pure aluminium are presented and discussed in terms of changes in grain boundaries structure. It is concluded, that the changes in grain boundary state can influence the mechanical properties of polycrystal much more strongly than changes in grain size.