Dynamic and Static Recrystallization

Abstract

Thermomechanical treatments basically aim at refining the grain size and controlling the crystallographic texture of metallic materials. First developed for the austenitic rolling of C±Mn steels, they are now being extended to a wide range of metals, e.g., aluminum, titanium, and nickel-based alloys. Dynamic or static recrystallizations are certainly the main mechanisms responsible for the microstructure and texture changes occurring during or after deformation. It is therefore not surprising that several teams of the Rhône-Alpes Federation are currently involved in parallel or converging investigations in that field. These researches take advantage of the availability of new experimental devices to produce large strains (Equal Channel Angular Extrusion, ECAE) and investigate the microstructures (orientation mapping by EBSD (Electron Backscattering Diffraction)), as well as the development of analytical modeling and numerical simulations. Some recent results are briefly summarized in this paper.Dynamic recrystallization (DRX), which occurs during straining, has long been considered to be restricted to low stacking fault energy materials, such as copper, c-iron, and the austenitic steels. In high stacking fault energy metals (aluminum, a-iron, ferritic steels), where the dislocation mobility is much larger, dynamic recovery was assumed to be the only operating mechanism. However, it is now well established that two types of DRX can take place: (i) discontinuous or ªclassicalº dynamic recrystallization (DDRX), occurring by nucleation and growth of new grains consuming rapidly the surrounding strain hardened matrix, and (ii) continuous dynamic recrystallization (CDRX) involving the generation of new grain boundaries by the progressive misorientation of neighboring subgrains.The main differences between DDRX and CDRX are the following: ± While the flow curves associated with DDRX switch from single to multiple peak behavior with increasing temperature and/or decreasing strain rate, such transition has never been observed for CDRX.± CDRX leads to strong crystallographic texture at large strains, whereas texture formation is hindered by DDRX. ± The temperature dependence of the flow stress (ªapparent activation energyº) is generally reported to be larger for DDRX, with some exceptions. Recent investigations involving large to very large strains imposed by torsion (i.e., monotonic simple shear) or ECAE (i.e., ªcrossedº simple shear) have shown that DDRX and CDRX combine the same elementary mechanisms, but with different kinetics. When grain boundary migration is slow, CDRX takes place; by contrast, in the case of large grain boundary velocity, the subgrain boundaries are swept by moving boundaries before they themselves transform into large angle grain boundaries. When DDRX takes place, the steady state microstructure is established quite rapidly (e<1). It displays strong heterogeneities, due to the coexistence of ªyoungº and ªoldº grains exhibiting low and high dislocation densities, respectively (Fig. 1). [1] CDRX is ...

Microstructure and Low-Cycle Fatigue of a Friction-Stir-Welded 6061 Aluminum Alloy

“…In high SFE metals, e.g., aluminum, [39] where the dislocation mobility was much larger, dynamic recovery (DRV) was assumed to be the only operating mechanism. [40] However, recent studies indicated the presence of fine dynamically recrystallized grains possessing predominantly HAGBs and a low dislocation density in the friction-stir-welded aluminum alloys. [6,8,41] In the present study, the fine and equiaxed grains in the NZ of the friction-stir-welded 6061Al-T651 alloy were achieved and they were considerably smaller than those in the BM.…”

Plastic Deformation Behavior in Steels during Metal Forming Processes: A Review

“…Since the rate of annihilation due to dynamic recovery is not sufficient to complete with the strain hardening rate in low SFE materials, the dislocation density increases continuously in this case. Contrary, high SFE materials act in favor for higher mobility of dislocation, and consequently dynamic recovery becomes involved as an operating mechanism [49]. The details of materials and the type of possible phenomena are illustrated in Table 2.…”

“…The details of materials and acting mechanisms during the hot deformation process [34,49,50].…”

Enhancing Axisymmetric Compression Testing: Modeling and Addressing Flow Curve Errors

“…The hot flow behavior of materials, commonly obtained through axisymmetric compression testing (ACT), is essential to characterize and study related phenomena such as microstructure development, phase change, [1,2] material recovery, static, dynamic and metadynamic recrystallization (SRX and (M) DRX [3] ). The effective stress-strain curve provides insight into the activation energy required for deformation processes in the material.…”