Materials Science Forum Vols. 558-559

Abstract: The stored energy during cold working has been estimated by two approaches. In the first approach, line broadening measurements were determined by neutron diffraction. The second approach is based on the model developed by Dillamore et al. [1]. Therefore, great attention has been paid to the influence of the deformation cell morphology, cell size and the eventual presence of orientation gradient inside the grains according to their orientation. Experimental results show this hierarchy E{111} <112> >E{111} <110>>E{001} <110>.

Abstract: Spot heating system utilizing a fiber-laser has been developed in order to control recrystallization, grain growth process and texture development. The newly developed system enables the laser beam to be focused onto the surface of specimen with a minimum diameter of around 10 m. The maximum power of laser in the present system is 100W. Specimens of cold rolled pure aluminum plates of 1mm thick are used for local recrystallization treatment by the spot heating. After laser-spot heating, preferentially recrystallized grains appeared in the irradiated and heat affected region with a diameter depending on the laser power density. The present system enables spot heating on any critical points of the specimen surface. Some laser-heated specimen show locally recrystallized regions occupied by almost single grain having similar crystallographic orientation as that of cold rolled matrix. But the orientation of recrystallized grain is not always the same as that of the matrix. Selective spot heating on grains having peculiar orientation would bring about a possibility of texture control that has not been achieved by means of conventional materials processing.

Abstract: The static and metadynamic recrystallisation characteristics of a 9SMn28 (EN 1.0715) free cutting steel have been evaluated by employing the stress relaxation technique. In the steel, the sulphur was bound in the form of numerous MnS inclusions. Fractional softening laws stating the kinetics of static and metadynamic softening behaviour were experimentally determined and compared with the existing literature data for C/C-Mn steels. The analysis of the static recrystallisation data suggested the powers of the strain and strain rate to be -2.7 and -0.13, respectively, and the apparent activation energy was estimated as 177 kJ/mol. The power of grain size was taken from a regression model developed previously that is able to predict the static recrystallisation kinetics of vast number of carbon and microalloyed steel grades. Even though a fraction of Mn was out of the solid solution in the form of sulphides, the predictions by the regression model as accounting the balance Mn in the solid solution were quite close to the experimental data, confirming the applicability of the model. As expected, the metadynamic recrystallisation behaviour showed a strong dependence on the strain rate, the power being -0.78 and the apparent activation energy 57 kJ/mol.

Abstract: The distribution of nucleation sites in 90% cold rolled aluminium (AA1050) annealed for 1h at 280°C has been determined by serial sectioning the sample and characterizing each section using the electron back scattering pattern (EBSP) method in the SEM. In total 1844 nuclei are identified. It was found that the nuclei are not randomly distributed but clustered preferentially along TD-RD planes in the sample. The results are presented and implications of clustered nucleation on recrystallization kinetics are discussed.

Abstract: Grain boundary mobility in preferential growth of cube grains ({100}<001>) was evaluated by in-situ electron back scattering diffraction pattern (EBSP) analysis in order to clarify the fundamental mechanism of primary recrystallization in pure aluminum foils of 99.9% purity thermo-mechanically processed in the industrial production route for aluminum foils for electrolytic capacitors. We have carried out the continuous EBSP measurements during recrystallization of the aluminum foils heated to various temperatures in the chamber of scanning electron microscopy (SEM). We have succeeded in dynamic observation of the preferential growth of cube grains by the in-situ EBSP analysis. The in-situ EBSP analysis could reveal the migration rate of grain boundaries surrounding the cube grains. It was clarified that the proportional relation between migration rate and annealing time was satisfied. The stored energy providing the driving force for the grain boundary migration during primary recrystallization could be estimated from the misorientation within the deformed grains. The mobility of the grain boundary could be evaluated using the measured grain boundary migration rate and stored energy. Then the activation energy could be estimated by the in-situ EBSP analysis at various temperatures ranging from 270°C to 310°C. The obtained activation energy was 124 kJ/mol, which approximately corresponded to that for the diffusion of impurity such as iron or silicon in aluminum. This suggested that the rate-determining process of the grain boundary migration of cube grains was impurity diffusion in the pure aluminum foils.

Abstract: Recrystallization behavior of SPD processed high purity iron was studied. The 99.95% iron sheet was deformed by the accumulative roll-bonding (ARB) process up to 8 cycles (equivalent strain of 6.4) at ambient temperature. Subsequently, the ARB-processed specimens were annealed for 1.8ks at various temperatures from 300°C to 500°C. The microstructures of these specimens were characterized by TEM and SEM/EBSP. The microstructure of the as-ARB-processed specimens showed the lamellar boundary structure elongated along RD, which was the typical microstructure of the ARB-processed materials. The mean interval of the lamellar boundaries was about 100 nm. After annealing at 400°C, the ARB specimen showed a partially recrystallized microstructure composed of equiaxed grains with grain size larger than 10 5m and the recovered lamellar boundary structure. After annealing above 500°C, the microstructures were filled with equiaxed recrystallized grains. These results suggest that conventional discontinuous recrystallization characterized by nucleation and growth occurs during annealing at annealing temperature above 400 °C. In previous work reported about the annealing behavior of the low carbon IF steel ARB processed, the continuous recrystallization occurred during annealing at annealing temperature above 600 °C. The recrystallization temperature of the pure iron was much lower than the IF steel and the recrystallization process were significantly different. This difference was suggested to be caused by inhomogeneous microstructure in the pure iron ARB-processed.

Abstract: Novel technological processes for conceptually new integrated deformation-thermal production of flat rolled stock with enhanced physical-mechanical properties and minimized alloying system have been devised on the basis of recently discovered metallophysical laws of influence, through hot plastic deformation by rolling, upon microstructure-phase conversions and states of steel in metallurgical products.

Abstract: A good combination of strength and toughness in HSLA steels can be achieved by the addition of microalloying elements such as Nb. Nb can retard the static recrystallization of austenite at low temperatures by either a solute drag or by a precipitation pinning (when bonded to C or N) effect. Both mechanisms result in improved mechanical properties due to grain size refinement of the transformed ferrite. In this study, 3 Nb-microalloyed model alloys were designed to investigate the solute drag and the precipitation effect separately. The first alloy, containing a stoechiometric ratio of Nb and C, was designed to study the retarding effect of NbC on the recrystallization behavior. A second alloy, containing Nb and only few ppm C, was casted in order to study the effect of Nb in solid solution. The two alloys were compared with a C-Mn reference alloy. The recrystallization behavior of the three alloys were compared by multi-hit torsion tests and double hit compression tests. The Nb-C and the Nb-very low C showed small differences in recrystallization behavior. These results show that Nb delays the recrystallization by a solute drag effect or by the formation of a very small amount of precipitates.

Abstract: The subgrain structure of hot rolled aluminum alloy AA 5005 has been characterized on as-received samples using Electron Backscatter Diffraction (EBSD). Based on the OIM scans of RD-ND and TD-ND, 3 dimensional microstructures of subgrains are built up using the 3D Microstructure Builder, which is a method for developing statistically representative digital representations of microstructures. Following the generation of microstructure, different textures were fit to these reconstructed 3D microstructures, based on individual components such as Brass and S textures. For this study, the Brass texture was chosen as an exemplary case. Monte Carlo simulation was used to model subgrain coarsening and visualization was a key to detecting abnormal grain growth. The main objective is to understand the circumstances under which we can expect abnormal (sub-)grain growth to lead to nucleation of recrystallization.