Abstract The influence of recrystallization on crystallographic textures is explored. The long-range motion of grain boundaries during primary recrystallization has the potential to substantially change the texture. In face-centered cubic (fcc) metals the 001<100> or cube texture frequently strengthens from negligible levels to become a strong component. Although there has been much debate about mechanisms, detailed measurements, especially with electron backscatter diffraction (EBSD), show that the final recrystallized texture is often present at the early stages of the process. This shows that oriented nucleation is the dominant effect in a mesoscopic sense. The cube-oriented nuclei are most likely favored through more rapid recovery compared to other texture components. Texture changes also occur in body-centered cubic (bcc) metals; in single-phase materials recrystallization and subsequent grain growth favor the γ-fiber (111//ND) at the expense of the α-fiber. This change is associated with technologically useful increases in drawability and again is largely explained by more rapid nucleation in the γ-fiber component. Texture change in hexagonal metals is more subtle because primary recrystallization often results in negligible texture change, depending on the prior strain. Nevertheless, during grain growth there is often a texture change that is effectively a 30 degrees shift around the c-axes. Where coarse second-phase particles are present, orientation gradients develop in their vicinity, which promotes the formation of new grains ahead of the rest of the material, also known as particle-stimulated nucleation (PSN). Broadly speaking, this phenomenon results in weaker textures.

Abstract The recovery, recrystallization, and grain growth behavior of ordered alloys shows many general similarities to disordered metals. If deformed in the ordered state, ordered compounds recrystallize more rapidly than if deformed in the disordered state, and this is attributable to the larger stored energy. Grain boundary mobility is severely reduced by ordering, thus retarding both recrystallization and grain growth. Deviations from stoichiometry generally increase diffusivity and increase boundary mobility. During recovery of a weakly ordered alloy the hardness will usually decrease if T > Tc and increase if T < Tc. Permanently ordered alloys tend to soften on recovery. In some weakly ordered alloys, a temperature range may exist where recrystallization does not occur, even though it occurs at higher or lower temperatures. Weakly ordered alloys contain antiphase boundaries (APBs) after recrystallization, whereas strongly ordered alloys do not.

Abstract Plastic deformation is accommodated by shear strains resulting from dislocation glide and, in some cases, mechanical twinning. These shear strains result in lattice rotation and the combination of the type of strain and the crystallographically constrained deformation systems results in preferred orientations aka texture (or, in geology, fabric). The most successful theory of texture development is the Taylor model, which assumes that the strain character at the grain level matches that of the polycrystal and that the imposed strain is accommodated by multiple deformation systems. Modifications of this approach improve upon this by allowing for local deviations in strain from the average. For maximum fidelity, finite element simulations with crystal plasticity, or equivalent, can be employed. Fcc and bcc metals have had the most attention but hexagonal metals also develop distinctive textures. Texture in geological materials often provides clues to the history of the rock and similar inferences can be made in any material. Variations in composition often result in texture changes and the most notable example is the change from copper-type textures in pure fcc metals to brass-type textures in alloys. This topic is important because, as with most aspects of recrystallization, the textures found after recrystallization are strongly dependent on, or even similar to those existing at the beginning of the process.

Abstract Various processes of microstructural evolution that occur during annealing are introduced, with a focus on changes in the grain structure. Grain growth is driven by the excess free energy associated with grain boundaries. This may occur uniformly or heterogeneously, in which case it is known as abnormal grain growth. Recrystallization is driven by the excess free energy associated with dislocations stored during plastic deformation. Depending on the circumstances, this is variously known as primary recrystallization, secondary recrystallization, dynamic recrystallization, or meta-dynamic recrystallization. Recovery is the process by which dislocation density decreases without long range motion of boundaries and is often accompanied by the formation of subgrain networks that replace the tangled dislocation structures formed during deformation. Dynamic recovery is the set of processes that decrease dislocation density during the deformation, as opposed to subsequent annealing. The historical development of recrystallization as a topic of research is briefly reviewed along with relevant literature.

Abstract The processes of microstructural evolution during hot deformation are reviewed. These include dynamic recovery and dynamic recrystallization, where the latter occurs via several different mechanisms such as classical nucleation and continuous geometric rearrangement. Dislocations typically adopt cellular or subgrain structures thanks to rapid dynamic recovery. Constitutive descriptions of mechanical behavior such as the Zener-Hollomon equation are discussed because of the definite relationship between subgrain size and flow stress, for example. Texture development during hot deformation is often distinct from cold deformation because it is affected by both the concurrent microstructural evolution and by changes in deformation mechanisms. The latter includes disappearance of twinning at high temperature and the appearance of new slip systems such as non-octahedral slip in cubic metals. Processes of microstructural evolution in minerals and rocks are closely related to those in metals.

Abstract The use is discussed of severe plastic deformation to produce fine-grain microstructures via accumulative roll bonding (ARB), equal channel angular extrusion (ECAE), high pressure torsion (HPT) and friction stir processing (FSP). By contrast to typical grain sizes between, say, 5 and 200 microns, sub-micron grin sizes are feasible. Stability in such structures depends on the formation of a high density and large fraction of high-angle boundaries. The near-linear increase in misorientation with strain described elsewhere is the reason for the need for large monotonic strains. Local relaxations driven by local equilibrium along triple lines in the boundary network roughens ( corrugates ) the boundaries that simple geometric progression would predict to be straight. This process is termed continuous recrystallization, also known as continuous dynamic recrystallization (at high temperatures). The stability of such structures is enhanced by the presence of a dispersion second phase particles at a number density that effectively pins a high fraction of the boundaries. Such stabilization is essential for imparting a superplastic property because this depends on maintaining a fine enough grain size for boundary sliding to contribute to strain and strain-rate sensitivity.

Abstract This chapter reviews the process of recovery, which includes all processes that restore a deformed material to an annealed state but that do not involve long-range motion of boundaries. Dislocations can climb and annihilate to reduce the overall dislocation density and the hardness generally follows a logarithmic decay law. Often the heterogeneity of the deformed microstructure results in dislocation cells becoming more sharply defined such that subgrains form with low angle grain boundaries, i.e., only the geometrically necessary dislocations persist. This process is also known as polygonization. Once a subgrain structure has formed, it can coarsen in much the same way as a high angle grain boundary structure. However, the strong variation in mobility with misorientation, allied with the dispersion in misorientation resulting from deformation, means that such coarsening can be heterogeneous, i.e., abnormal subgrain growth occurs, which provides a nucleation mechanism for recrystallization itself. Particle pinning can stabilize subgrain structures, and thus the Smith-Zener theory is relevant.

Abstract The characteristics of recrystallization in single phase alloys are reviewed. The effect of prior strain level is strong because orientation gradients that accumulate during plastic strain influence the density of new grains. The temperature and strain rate of the prior deformation also influences the process of recrystallization through variations in stored energy. The annealing temperature used for recrystallization affects the kinetics strongly via the grain boundary mobility but typically has little influence over the texture evolution or grain size. In general, the prior plastic deformation controls primary recrystallization. The kinetics of the process are well described by the Johnson-Meh-Avrami-Kolmogorov equation although the theoretical expected exponents are rarely observed through a combination of non-random nucleation, variable stored energy and concurrent recovery. In low stacking fault energy materials, twin formation during recrystallization is extensive and sometimes exploited to perform grain boundary engineering. Solutes exert a drag on moving grain boundaries and thus slow down recrystallization.

Abstract This chapter reviews the recrystallization of two-phase alloys. The main emphasis is systems with stable dispersions of particles. Fine particles below about one micron in size generally inhibit the nucleation step of recrystallization resulting in large recrystallized grain size and slow kinetics. Coarse particles above a micron, however, often have the opposite effect because deformation concentrates around them, gives rise to large orientation gradients and thus accelerates recrystallization, which is known as Particle Stimulated Nucleation (PSN). In general, recrystallization textures are weaker as a consequence of PSN. Particle dispersions are also important for stabilization of grain size after recrystallization, which is known as Smith-Zener pinning. Precipitation during recrystallization strongly inhibits recrystallization, which is associated with heterogeneous nucleation on boundaries.