Considering the interfacial plane, two types of twin boundaries must be distinguished: coherent twin boundaries lying in the facets, is schematized on Figure 1(a).Microstructures of the as-prepared Cu sample with extremely fine grains. Noteworthy within this context, not only the fraction and misorientation of the different types of boundaries matter, but their plane, their network, and their connectivity must also be taken into account. Most of published GBE works actually aim at controlling the amount and connectivity of twin boundaries and other special boundaries arising from the intersection of the latter. The interest has been renewed and amplified with the emergence of the Grain Boundary Engineering (GBE) concept by which some properties of a given material can be improved by controlling the grain boundary network. Since their first observations, twins in FCC materials have motivated constant efforts within the physical metallurgy community to elucidate the question of their formation. The low energy of twin boundaries is indeed the established reason why they can be formed and kept in the microstructure while grains evolve. Regarding the abundancy of annealing twins, the amount of stored energy and the relative energy of twin boundaries compared to random boundaries have been pointed out as the main controlling factors by Charnock and Nutting, who gathered data from different materials. In his early paper, Burke reported that annealing twins are prominent after recrystallization and that «the twins found in coarse grained metals are all formed after recrystallization is complete». But there are several mechanisms by which grain boundaries may migrate during annealing, the two main ones being (i) the migration of a recrystallization front driven by the consumption of the energy stored in the form of defects (mostly dislocations) induced by plastic deformation, and (ii) the migration of a grain boundary to reduce its curvature or to adopt a lower energy plane, within the so-called grain growth regime, with no stored energy involved. As pointed out already 70 years ago, annealing twin formation is a process that accompanies grain boundary migration.

One reason might be that the term annealing twins is too general, and actually covers different types of twins arising by distinct mechanisms. Accounting for twins is a requirement since they not only play a role in microstructure evolution upon thermomechanical processing but also affect the in-service material behavior, positively or negatively depending on the involved properties.Īnnealing twins are known since long to be very common in Face-Centered Cubic (FCC) metals and alloys with low-to-medium stacking fault energy, but, quite surprisingly, the exact mechanisms by which they appear and evolve during thermomechanical processing remain poorly understood. Finally, the state of the art of mesoscopic models and simulations able to account for twin boundaries is presented.

Because twins are at the core of most grain boundary engineering approaches, the mechanisms by which an interconnected network of twin and related boundaries can be formed are discussed, in the light of the current knowledge on annealing twin formation mechanisms. Twin topologies are so much different after recrystallization and after grain growth that distinct names would be better suited than the general term of annealing twins. However, incoherent twin boundaries are very frequent as well, notably in recrystallized microstructures and would definitely deserve more specific attention. Most of them focus on coherent twin boundaries. The different mechanisms proposed through the years are reviewed. The question of the formation mechanism of annealing twins in face-centered cubic metals and alloys, which is still not resolved in spite of the fact that the existence of these defects is known for long, is addressed in this paper.

Polycrystal annealing singlecrystal