Consistent physically-based modeling of dynamic recrystallization under hot working conditions

深入了解动态再结晶（DRX）过程中屈服应力的演变和微观组织的演变对热加工工艺的设计具有至关重要的作用。在金属成形中，半经验的DRX模型至今仍在使用。这些模型不是基于内部状态变量，因此不能重现DRX底层的物理过程。因此，在模型校准期间，超出实验室应用条件的外推本身就充满了不确定性。尽管基于物理的模型有可能缩小现有的差距，但似乎有必要取得重大进展。现有的DRX模型都没有充分考虑到当前金属物理研究中关于DRX的发生和发展的现状。通常，人们仍然认为DRX的成核需要一个临界位错密度。关于晶界位错密度梯度的形成、晶内细胞结构的形成作为可移动亚晶界形成的前驱、亚晶界与母微观结构的高角度晶界的相互作用作为DRX的成核机制的见解尚未被纳入DRX模型。此外，在现有的模型中，动态恢复没有被建模为层错能的函数。更令人不满意的是，现有的DRX模型缺乏热力学一致性。可以证明，当Avrami指数小于或等于3时，Avrami动力学（常用于DRX模型）与基于热力学的poliak - jonas准则在DRX临界条件下不一致。然而，对于DRX的成核，通常的模型假设得出不一致的指数为3。无论是从金属成形的角度，还是从金属物理的角度，都需要从根本上增强DRX的建模。因此，本研究计划的目标是，扩大对热工条件下DRX的发生和发展的物理理解，开发一个基于物理的、依赖于堆叠故障能量的、热力学一致的动态再结晶模型，以高温材料（Alloy 800H）为例，证明新模型可以定量准确地描述热加工条件下微观组织与屈服应力的耦合演变；在消除微观组织演变的时间步长依赖后，将该模型应用于成形模拟有限元软件中；并通过面向实际的算例对模型进行验证。通过这项研究，将弥补现有的基于物理的动态再结晶建模的知识空白，并创建一个面向实践的金属成形工艺设计模型。

For the design of hot working processes, a thorough understanding of the evolution of the yield stress and microstructure evolution during dynamic recrystallization (DRX) plays a vital role. In metal forming mainly semi-empirical DRX models are still being used today. These models are not based on internal state variables and are therefore not able to reproduce the physical processes underlying DRX. Thus, an extrapolation beyond the conditions applied in the laboratory during model calibration is inherently fraught with uncertainty. Although physically-based models have the potential to close the existing gaps, significant advancements seem necessary. None of the currently available DRX models takes the current state of metal physics research regarding the initiation and progression of DRX sufficiently into account. Often, it is still assumed that the nucleation of DRX requires a critical dislocation density. Insights into the formation of dislocation density gradients at the grain boundaries, into the formation of cell structures in the grain as a precursor for the formation of mobile sub-grain boundaries, and the interaction of sub-grain boundaries and the high angle grain boundaries of the parent microstructure as nucleation mechanism of DRX have not yet found their way into DRX models. Also, in existing models, the dynamic recovery is not modeled as a function of the stacking fault energy. Even more unsatisfactory is the lack of thermodynamic consistency of existing DRX models. It can be shown that Avrami kinetics (which are often used in DRX models) with an Avrami exponent less than or equal to 3 are inconsistent to the thermodynamically based Poliak-Jonas-criterion for the critical conditions of DRX. However, the usual model assumptions for the nucleation of DRX yield an inconsistent exponent of 3. Both from the viewpoint of metal forming as well as from metal physics, the need for a fundamentally enhanced modeling of DRX arises. The objectives of the present research proposal are therefore, to expand the physical understanding of the initiation and progression of DRX under hot working conditions, to develop a physically-based, stacking-fault-energy-dependent and thermodynamically consistent model for dynamic recrystallization, to prove at the example of a high temperature material (Alloy 800H) that the coupled evolution of the microstructure and yield stress can be described quantitatively correct under hot working conditions by the new model, to implement the model in a finite element software for forming simulation after abolishing the time step dependence of the microstructure evolution as well as to validate the model using a practice-oriented example. With this research existing knowledge gaps in the physically-based modeling of dynamic recrystallization shall be closed and a practice-oriented model for the design of metal forming process shall be created.