Materials Science Forum Vols. 558-559

Abstract: Modeling and simulation of recrystallization, grain growth, and related phenomena are important tools for the fundamental understanding of microstructural evolution and prediction of engineering properties. In particular for ultra fine grained and nanocrystalline materials proper account of microstructural evolution is essential for the optimal processing of these materials. It is shown that for modeling of softening phenomena it is important to discriminate between discontinuous primary recrystallization and discontinuous grain growth owing to their quite different underlying physics. Recent developments in recrystallization modeling and simulation of grain growth are addressed, in particular nucleation of recrystallization and junction effects in grain growth. Major progress is also expected from atomistic modeling and quantum-mechanical computations for making available specific material properties.

Abstract: The paper surveys various types of dislocation substructure that are created by plastic deformation in metals. Special reference is made to those substructures that accommodate sharp misorientations as these are of fundamental importance to the nucleation of recrystallisation. Several different mechanisms can give rise to high misorientations; these are discussed in terms of the factors that control them and their relationships to orientation and texture. Different mechanisms for nucleation of recrystallisation may occur depending on the type of substructure, allowing some practical control over the final recrystallised texture.

Abstract: The formation of ultrafine-grained structure in steels by various thermomechanical processings is reviewed from a metallurgical point of view. In the recent new type TMCP, ultrafine ferrite grains with a grain size of about 1μm are obtained when the austenite is heavily deformed at lower temperatures. In this case, dynamic phenomena such as dynamic recrystallization become prominent in the process. In the aging after heavy cold rolling of supersaturated matrix phase in two-phase alloys, the competition between the recovery or recrystallization of matrix phase and the precipitation of second phase occurs, resulting in various types of two-phase structures including microduplex structure. Microduplex structure is also obtained by annealing after heavy cold rolling of coarse two-phase structure in duplex stainless steel and high carbon steel. Recently, various severe plastic deformation processings, in which very large plastic strain over 4 is applied to the materials, have been developed to produce ultrafine grained materials with nanocrystalline and/or submicrocrystalline structures.

Abstract: The study of microstructural evolution in polycrystalline materials has been active for many decades so it is interesting to illustrate the progress that has been made and to point out some remaining challenges. Grain boundaries are important because their long-range motion controls evolution in many cases. We have some understanding of the essential features of grain boundary properties over the five macroscopic degrees of freedom. Excess free energy, for example, is dominated by the two surfaces that comprise the boundary although the twist component also has a non-negligible influence. Mobility is less well defined although there are some clear trends for certain classes of materials such as fcc metals. Computer simulation has made a critical contribution by showing, for example, that mobility exhibits an intrinsic crystallographic anisotropy even in the absence of impurities. At the mesoscopic level, we now have rigorous relationships between geometry and growth rates for individual grains in three dimensions. We are in the process of validating computer models of grain growth against 3D non-destructive measurements. Quantitative modeling of recrystallization that includes texture development has been accomplished in several groups. Other properties such as corrosion resistance are being related quantitatively to microstructure. There remain, however, numerous challenges. Despite decades of study, we still do not have complete cause-and-effect descriptions of most cases of abnormal grain growth. The response of nanostructured materials to annealing can lead to either unexpected resistance to coarsening, or, coarsening at unexpectedly low temperatures. General process models for recrystallization that can be applied to industrial alloys remain elusive although significant progress has been made for the specific case of aluminum alloy processing. Thin films often exhibit stagnation of grain growth that we do not fully understand, as well as abnormal grain growth. Grain boundaries respond to driving forces in more complicated ways than we understood. Clearly many exciting challenges remain in grain growth and recrystallization.

Abstract: Nucleation and normal or abnormal grain growth of new grains have been observed in several metals and alloys during annealing after various levels of strain (from few percents for abnormal grain growth in steel and aluminium to 90 % strain for static recrystallization of highly deformed copper, zirconium and steel). For each of these cases, the links between the initial deformation state of the polycristalline samples and their final textures after annealing are analyzed in some details, by taking into acount the distribution of orientations and misorientations within the samples, as well as the repartition of stored energy . With the help of polycristalline models for the simulation of deformation and recrystallization processes, some general conclusions regarding the nucleation and growth processes are finally drawn.

Abstract: This paper described new characterization methods and data to quantify the influence of solute atoms on grain boundary and sub-grain boundary mobilities in Al-Mn alloys with a view to their integration into recovery and recrystallization modelling. Detailed SEM measurements of grain boundary mobilities during recrystallization have been made by in-situ annealing experiments on cold deformed Al – 0.1 and 0.3wt.% Mn binary alloys. Stored energies are estimated from the sub-grain sizes and misorientations and the boundary velocities directly measured in the temperature range 200-450°C. It is shown that in many cases good agreement with the Cahn, Lücke, Stüwe model for solute drag is obtained, e.g. the activation energies are intermediate between those of boundary and volume solute diffusion. Some particular cases of rapid growth occur in Al-0.1%Mn indicating boundary breakaway from solute clouds. A complementary study of sub-grain boundary mobilities has started on the same alloys; in this case the average mobilities are estimated from FEG-SEM growth data for the average sub-grain size for temperatures in the range 150-300°C. The results are compared with some previous data on Al- Si and show similar rates.

Abstract: The relationship between the deformation orientation distribution function (ODF) and the primary recrystallised ODF in cold and warm rolled metals, is not a simple mathematical transformation from one to the other, but is through thermally activated processes occurring in the deformation microstructure. In BCC metals the mature rolling microstructure consists of cells, microbands and shear bands on a length scale of fraction of a micron, to deformation and transition bands at the grain scale, when this is of the order of 10 or more microns. There is evidence that grain boundary regions are sometimes distinct from grain interiors. Wherever there is a relatively sharp change in either orientation or microstructure such locations are potential sites of recrystallisation nuclei. In this paper the results of a systematic investigation of the development of microstructure in rolled interstitial free (IF) steel using both transmission and scanning electron microscopy are presented. It is shown how the dislocation mesh structure, formed at the earliest stages of rolling, develops into the mature microstructure consisting of cells, microbands and shear bands. Deformation heterogeneities in the microstructure, known to be of vital significance in the recrystallisation process are associated with the α and γ fibre components of the rolling texture. Shear band thickening and α grain fragmentation are also considered, since both processes can produce recrystallisation nuclei, which in the α fibre case can reduce desirable mechanical properties.

Abstract: Al-Mg-Si alloy is the suitable material for the automotive body application. However, it is found that a rope-like profile (ridging) develops when the Al-Mg-Si alloy sheet is stretched along the transverse direction. In this study, in order to clarify the formation process of ridging developed in Al-Mg-Si alloy, the relation between ridging and texture components of the sheet was investigated by 3D profile microscope and SEM/EBSD method. It was found that the ridging developed remarkably in the hot-rolled (at higher temperature) and annealed sheets. In ridging sample, there was the band of cube oriented grains (cube band). It was also found that the region of cube band corresponded to the ridges and valleys which caused a rope-like profile in the sheets. It could be thought that the difference of plastic deformability between cube-oriented grains and other oriented grains led to the development of ridging. This assumption is supported by the lower Taylor factor of cube oriented grains than other oriented grains. From these results, it was concluded that the development of ridging was strongly affected by the distribution of cube oriented grains.

Abstract: Recovery, recrystallization and the formation of recrystallization textures were investigated in three representative Al-Mg-Si alloys used for car body panels. Commercial hot bands of AA6016, AA6111 and AA6061 Al-Mg-Si alloys finished at low temperatures were cold rolled to a rolling reduction of 95 % in thickness and isothermally annealed at temperatures between 250 and 500 °C. In these alloys, precipitation was completed for the most part during low temperature hot rolling, and the sizes and the amount of fine precipitates formed during this low temperature hot rolling strongly affected recrystallization and the development of recrystallization textures. As a result, in the specimens annealed at 300 °C, quite different recrystallization behavior and recrystallization textures were observed. In the AA6061 alloy, in which, among three alloys, the maximum amount of Mg2Si should be precipitated, recrystallization was significantly suppressed. This resulted in the formation of strong {110} <111> and {100} <013> recrystallization textures. Also in the AA6111 alloy, in which precipitation of a medium amount of Mg2Si was expected, recrystallization was retarded to the same extent. In this alloy, however, recrystallization textures consisted of very strong {100} <001> and rather strong {110} <111> main orientations. In theAA6016 alloy, in which the minimum amount of Mg2Si and a large amount of Si particles should be precipitated, recrystallization occurred very rapidly, forming very weak recrystallization textures. In all alloys, annealing at higher temperatures resulted in the formation of weak textures, since fine precipitates were dissolved during annealing. Thus, the solution treatment, which is a necessary step to induce bake hardening in these alloys, randomizes their recrystallization textures.