Thermomechanical Processing Map in Retaining {100}//ND texture via Strain-Induced Boundary Migration Recrystallization Mechanism

The feasibility of establishing thermomechanical conditions to promote {100}//ND fiber texture via strain-induced boundary migration (SIBM) recrystallization mechanism in a non-grain oriented (NGO) electrical steel was investigated. Single-hit uniaxial compression at various temperatures and strains has been applied on Fe-6 wt pct Si to establish the relationship between stored energy and the softening mechanisms. Recovery only and recrystallization by SIBM or by subgrain growth (SGG) have been observed depending on the stored energy level. A strong {100}//ND fiber recrystallization texture, i.e., 45 pct area fraction, was seen in the sample which was deformed to 0.2 strain at 650 °C and then annealed at 1000 °C for 15 minutes, whereas only 13 pct {100}//ND fiber component was observed after 0.4 strain at 500 °C followed by the same annealing treatment. By examining the same microstructural region before and after annealing via EBSD, it has been shown that {100}//ND textured recrystallized grains were formed adjacent to the {100}// ND textured deformed matrix. Low stored energy has been shown to favor the formation of {100}//ND texture recrystallized grains via SIBM recrystallization mechanism attributed to its slow recrystallization nucleation rate. The results from the deformation studies have been used to suggest a processing window map concept to define the recovery, SIBM, and SGG regions for the starting as-cast columnar microstructure.

Microstructure Evolution of the as Tempered P92 Steel at High Temperature Exposure

The microstructure evolution during aging at high temperatures is usually used to thermodynamically simulate those cases of aging at low temperatures but for a very long time for P92 steel, because high temperature can accelerate the microstructure process. Therefore, in the present research, in order to comprehensively understand the microstructure evolution mechanisms during aging at especially high temperatures, the as-tempered P92 steel was exposed at 790 °C. Optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to characterize the microstructures. The dominant mechanisms at the four stages in microstructure evolution process during thermal exposure are (I) dislocation annihilation, (II) lath broadening and equiaxed subgrain nucleation, (III) equiaxed subgrain growth, (IV) recrystallization nucleation and growth. The martensitic lath broadening is dominated by both the motion of “Y”-type lath boundary and the combination of parallel lath boundary. The subgrain growth is by virtue of both the combination of the equiaxed subgrain and the bowing out of subgrain boundary.

Orientation Independent and Dependent Subgrain Growth During Iso-Thermal Annealing of High-Purity and Commercial Purity Aluminium

The orientation dependence on recovery has been studied in cold-rolled and annealed polycrystalline high-purity aluminium (99.99 wt%), binary Al-0.25Mn and commercial purity aluminium. The growth mechanisms were found to be independent of the alloy system and the microchemistry only influences the coarsening kinetics. Orientation-dependent subgrain growth, mainly studied in high-purity aluminium and measured in lamellar bands of uniform orientation, occurs in three distinct ways, depending of the size of the local orientation gradients. Following the evolution in average subgrain size and boundary misorientation by detailed electron backscatter diffraction (EBSD) characterization during annealing, it was found that the rate of subgrain growth in Cube- and Goss-oriented grains were faster than in the typical deformation texture components, particularly after an incubation time when discontinuous subgrain growth occurs. In commercial purity aluminium, general orientation-independent subgrain growth is faster than the orientation-dependent growth because more growth occurs in regions near high-angle grain boundaries separating differently oriented lamellar bands. It appears as if subgrains misoriented by more than 3.5° have a growth advantage over less misoriented subgrains, typically in the interior of lamellar bands. While the average boundary misorientations are decreasing, the individual boundary misorientations are increasing.

Subgrain Structures Characterized by Electron Backscatter Diffraction (EBSD)

Subgrain structures are frequently characterized by the electron backscatter diffraction (EBSD) method, which is both accurate and provides good statistics. This is essential to better understand the subgrain growth mechanisms and e.g. establish the driving forces and motilities for comparison with physically based models. However, there is no commercially available software which can provide adequate subgrain boundary maps necessary for e.g. size and misorientation analysis. Here, a method that produces such maps utilizing only commercially available software is presented. The clue is to provide the EBSD-software with a parameter that can be used to identify all subgrains. By combining various maps exported from the EBSD-software into photo editing software, a new map is made in which all subgrain boundaries are identified. Missing and incomplete boundaries are traced manually before a reconstructed subgrain map is generated and imported back into the EBSD-software. With this method, the built-in algorithms in the EBSD-software can be readily used to e.g. characterize subgrain growth in aluminium with respect to orientation, size and misorientation.

Modelling Time-Dependent Nucleation of Recrystallization in Aluminium Alloys

The basic equations and mathematical framework of a mean-field model for recovery and recrystallization, the latter based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) approach, capable of handling time-dependent nucleation of recrystallization, is presented. Different approaches to account for time-dependent nucleation are discussed. A physically-based nucleation model where “nucleation” of recrystallization is brought about by “abnormal” subgrain growth seems most appealing, in terms of realism and mathematical convenience. Its implementation and effects on the recrystallization behavior are demonstrated through an example of back-annealing after cold deformation of a generic aluminium alloy case