Phase Field Modelling Of Texture Evolution And Its Mechanisms During Recrystallization Annealing

Recrystallization annealing is a common method to control microstructures andproperties of plastically processed metallic materials during processing and preparation. It isalso a vital technology to optimize crystallographic texture and thus, to improve mechanicaland physical properties. After long time of experimental research, the basic tendencies ofmicrostructure and texture evolution during recrystallization are well studied. Comparatively,the mechanisms related to the formation recrystallization textures are much less understoodsuch that a systematic or mature theory is still lacking. To aid the development ofrecrystallization texture theories, many computational models have been built and applied tosimulate recrystallization behavior. So far, these models have led to great achievements in theaspects of microstructural evolution and dynamics, and they are also capable of capturingsome experimental observations on the texture evolution. However, these models are far awayfrom quantitative predictions and much more researches need to be done. In the present work,a phase-field (PF) model incorporating the state and structural features of materials (such asgrain appearance, orientation and strain stored energy) and including a module of moleculardynamics (MD) for calculation of grain boundary (GB) energy is developed. An energyprinciple for determining the preferred orientations during nucleation, i.e., minimum grainboundary energy (MGBE) principle, is proposed by analyzing the variation of GB energy withgrain orientation. Meanwhile, an active grid tracking (AGT) method for solving the PFdynamics equations is proposed by analyzing the characteristics of order parameters in the PFmodel; A cold-rolled AA1070sheet is used as a testing material to study recrystallizationdynamics and the law and mechanism of microstructure and texture evolution duringannealing, and to validate the prediction precision of the PF model; mechanism and physicalessence of preferred orientation during nucleation and grain growth are studied throughcomparison of experiment and simulation. These theoretical developments and results areexpected to contribute to the further understanding of effect of various structural andprocessing factors on the evolution of recrystallization texture characteristics, and then toguide the efficient control of microstructure, texture and properties in metallic materials. Themain results and conclusions are summarized as follows:(1) Results from MD and Read-Shockley formula are compared, and the way ofintroducing GB energy accounting for the effect of geometric parameters of GB (relationshipbetween orientations of grains and orientation of GB plane) is discussed. Result shows that the GB energy computed by MD is closely related to the geometric parameters of GB, and thetrend of computed GB energy fits well with experimental results. The GB energy between twograins with same orientation relationship (including misorientation axis and misorientationangle) depends on the structure of GB they form; the Read-Shockley equation can also welldescribe the variation of energy of low angle GBs with misorientation angle, it deviateshowever largely from true values and just describes the relation between GB energy andmisorientation angle when it comes to high angle GBs; the GB energy calculated by the MDmethod used in the recrystallization model can well reflect the dependency of GB energy onthe orientation relation between grains and the orientation of GB plane.(2) The MGBE principle is verified by studying the texture evolution feature ofcold-rolled face-centered cubic (FCC) metals during annealing. Result shows that the nucleiorientations assemble around the cube (C)-and R-orientations in various extent, when thenew GB develops as symmetric tilt grain boundary (STGB) or asymmetric tilt grain boundary(ATGB). The C-oriented nuclei are most frequent in the condition of STGB. When the newGB is of the type of twisted grain boundary (TGB), the nuclei would not assemble around theC-orientation but it still has the possibility to form R-orientation. Meanwhile, the probabilityof nuclei orientations gathering around Bs-or other rolling orientations is also related to thetype of new GB, and the probability of orienting near the Bs-orientation when the new GB isof ATGB is obviously higher than the cases when STGB or TGB is formed.(3) The simulation precision and advantage of computing efficiency are validated andanalyzed by taking the simulation of normal grain growth for instance. It is shown that theefficiency of the AGT algorithm is not obviously affected by the number of order parametersin a single grid, and it is an effective method to solve the PF equations. Compared with that ofthe active parameter tracking (APT), the computational efficiency of AGT is less affected bythe size of simulation system and the advantage of this new method in terms of computationalefficiency increases as the simulation proceeds. Although the AGT algorithm only computesthe active parameters at the active grids, it can effectively avoid grain growth and describe thetopological evolution of grains.(4) Annealing of a cold-rolled AA1070plate is simulated by the PF model developed inthis work. The results show that this model can well reflect the texture development duringannealing, the model embodies features of oriented nucleation and selected growth in varyingdegrees, and the recrystallization texture from simulation matches well with the experimentaltexture. Analyses on the nuclei orientations determined according to the MGBE principleindicate that they are closely associated with the matrix orientations around the nucleation locations, and that the preferred nuclei orientations depict features that would be expectedfrom nucleation by subgrain growth or by GB bulging. The nuclei orientations in thedeformation matrix with high strain stored energy are quite similar or completely identical tothe grain orientations in matrix. When nucleation takes place at high-angled GBs, there arepreferences for the nuclei to orient around the C-and R-orientations, which are typicalrecrystallization grain orientations after annealing.