1. Context

This article is dedicated to the perfect grain seal, the various descriptions of which are presented in historical terms. These have developed separately, sometimes in contradictory ways. Finally, the divergences have been overcome, with all the approaches converging towards a coherent theory of grain boundaries.

First, the joint is defined and described geometrically using the tools of bicrystallography (the extension of crystallography to the bicrystal).

In the case of a defect in the single crystal, a joint is formed by the introduction of local deformations, the organization of which leads to a second "mechanical" description of the grain boundary in terms of dislocations.

The atomic description is the subject of the third approach, closely associated with the previous one, given that a crystalline defect is...