Phase Field Modeling Of Microstructure Evolution In Metallic Materials

Metallic materials, especially structural metallic materials, as the material basis of national economy, have been widely used in construction, aerospace and other industrial fields. With the development of science and technology, people have higher and higher requirements on metallic materials properties which are closely related to their microstructures. The study on microstructure evolution in metallic materials and the corresponding influencing factors can help microstructure design in order to improve the mechanical properties of metallic materials. At present, the major study approaches to the material microstructure are experimental observation and computer simulation. Only by experimental measures it is difficult to study the characteristics of microstructure evolution in materials from the micro or nano level. With the rapid development of material science and computer technology, the aid of computer simulations to predict microstructure evolution in materials and optimize the material parameter is particularly important. In numerous computer simulation methods for the study of material microstructure, the phase field (PF) method becomes one of the most powerful ones due to its profound physical thought.In this dissertation, the microstructure evolution in metal materials is studied from microscale and nanoscale by the PF method, and the innovation works are summarized as follows:Firstly, aiming at the characteristics of different deformation regions and inhomogeneous distribution of the stored energy model in deformed alloy, a multi-state free energy (MSFE) function is established by introducing a weight factor for the stored energy and a characteristics state factor for different deformed regions. And a multi-state phase field (MSPF) model is proposed. Secondly, the MSPF model is applied to the recrystallization nucleation and growth process of AZ31magnesium alloy and the evolution process of the subgrain structure. Then the way of using the weighted frequency distribution to reflect the bimodal grain size distribution is introduced, which has an obvious effect. Thirdly, the moving hot zone PF model with an infinite temperature gradient is built to study the forming process of columnar grain in single-phase polycrystalline materials during the directional annealing. Finally, the dislocation-grain boundary (GB) interactions in detail in nanocrystalline under the action of external force, and the law of motion are studied as well as the extension of the nanoscale microcracks. Through systematic study and exploration, the main results and conclusions are summarized as follows:(1) The study on the recrystallization process of AZ31magnesium alloy using the MSPF model shows that the dynamic regularity of static recrystallization obtained by simulating is in good accord with the JMAK theory, and the Avrami index decreases with the true strain increasing. The greater the deformation rate for alloy is, the faster the stored energy releases, and the shorter the lasting time of static recrystallization process is. The simulation results here are in agreement with theoretical results and experimental results.(2) The study on the evolution process of subgrain structure shows that in the regions with higher stored energy, for example, around grain boundaries, there are very dense finer subgrains. The phenomenon of subgrains merging and swallowing appears earliestly in the higher density regions, and through the mechanism it makes recrystallization grain nucleation and growth during recrystallization. While the distribution of subgrains inside the deformation grain is relative uniform with low number density and relative large size, the subgrains merge and grow slowly. The distribution of recrystallized grains obtained by the weighted frequency shows that the grain grows faster and takes shorter time to complete recrystallization for the larger deformed alloy.(3) The study on grain growth during directional annealing shows that the ease of forming a columnar grain structure and its continued propagation increases while decreasing hot zone velocity and initial grain size, increasing hot zone width and temperature. Second-phase particles (SPPs) dramatically inhibit the generation of columnar grain structure and the inhibitory effect increased with increasing volume fraction of SPPs and decreasing size of SPPs.(4) In the deformation simulations of nanocrystalline, the simulation results reveal that GBs is the source of dislocation for its production and absorption. The main way of dislocation movement is changed from climb to glide with the decreasing of the temperature. When the climb is blocked, it will readily result in asymmetric stress distribution and discordant dislocation detachment. In the deformation simulations of nano-polycrystalline structure, it confirms the occurrence of various types of deformation behaviors such as GB migration, grain rotation, GB serration due to releasing the elastic strain energy, dislocation nucleation and transmission in the GB. Increasing temperature favors the grain rotation and grain boundary migration while hinders dislocation transmission. It will be difficult for the grain rotation and grain boundary migration when the grain size increases.(5) The study on nanoscale microcracks in ductile metallic materials shows that crack branching will start if only the strain reaches a critical value for biaxial tension. In crack propagation the system energy decreases continuously. Faster crack propagation and more crack branches are observed at higher temperatures. During crack propagation, around the main cracks there are some disconnect isolated small cavities, which will become new cracks.The conclusions in this dissertation have a certain reference value for improving the toughness of metallic materials and revealing the deformation damage micromechanism of metallic materials. And they can play a scientific guiding role in resistance to fatigue fracture and improvement of the service life of metallic materials.