Recrystallization textures

6.8.6 Recrystallization textures

The preferred orientation developed by cold work often changes on recrystallization to a totally dif- ferent preferred orientation. To explain this observation, Barrett and (later) Beck have put forward the ‘oriented growth’ theory of recrystallization textures, in which it is proposed that nuclei of many orientations initially form but, because the rate of growth of any given nucleus depends on the orienta- tion difference between the matrix and growing crystal, the recrystallized texture will arise from those nuclei which have the fastest growth rate in the cold-worked matrix, i.e. those bounded by large-angle boundaries. It then follows that, because the matrix has a texture, all the nuclei which grow will have orientations that differ by 30–40 ◦ from the cold-worked texture. This explains why the new texture in fcc metals is often related to the old texture, by a rotation of approximately 30–40 ◦ around axes, in bcc metals by 30 ◦ about

◦ about

undoubtedly true that oriented growth provides a selection between favorably and unfavorably ori- ented nuclei, there are many observations to indicate that the initial nucleation is not entirely random. For instance, because of the crystallographic symmetry one would expect grains appearing in an fcc texture to be related to rotations about all four possible rotations about each of the four observed.

To account for such observations, and for those cases where the deformation texture and the annealing texture show strong similarities, oriented nucleation is considered to be important. The oriented nucleation theory assumes that the selection of orientations is determined in the nucleation stage. It is generally accepted that all recrystallization nuclei pre-exist in the deformed matrix, as sub-grains, which become more perfect through recovery processes prior to recrystallization. It is thus most probable that there is some selection of nuclei determined by the representation of the orientations in the deformation texture, and that the oriented nucleation theory should apply in some cases. In many cases the orientations which are strongly represented in the annealing texture are very weakly represented in the deformed material. The most striking example is the ‘cube’ texture, (1 0 0) [0 0 1], found in most fcc pure metals which have been annealed following heavy rolling reductions. In this texture, the cube axes are extremely well aligned along the sheet axes, and its behavior resembles that of a single crystal. It is thus clear that cube-oriented grains or sub-grains must have a very high initial growth rate in order to form the remarkably strong quasi-single-crystal

Mechanical properties I 361 cube texture. The percentage of cubically aligned grains increases with increased deformation, but

the sharpness of the textures is profoundly affected by alloying additions. The amount of alloying addition required to suppress the texture depends on those factors which affect the stacking-fault energy, such as the lattice misfit of the solute atom in the solvent lattice, valency, etc., in much the same way as that described for the transition of a pure metal deformation texture.

In general, however, if the texture is to be altered a distribution of second phase must either be present before cold rolling or be precipitated during annealing. In aluminum, for example, the amount of cube texture can be limited in favor of retained rolling texture by limiting the amount of grain growth with a precipitate dispersion of Si and Fe. By balancing the components, earing can be minimized in drawn aluminum cups. In aluminum-killed steels A1N precipitation prior to recrystallization produces

a higher proportion of grains with {1 1 1} planes parallel to the rolling plane and a high R-value suitable for deep drawing. The A1N dispersion affects sub-grain growth, limiting the available nuclei and increasing the orientation selectivity, thereby favoring the high-energy {1 1 1} grains. Improved R-values in steels in general are probably due to the combined effect of particles in homogenizing the deformed microstructure and in controlling the subsequent sub-grain growth. The overall effect is to limit the availability of nuclei with orientations other than {1 1 1}.