A new model for dynamic recrystallization under hot working conditions based on critical dislocation gradients
A new model for dynamic recrystallization under hot working conditions based on critical dislocation gradients
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基于临界位错梯度的热加工动态再结晶新模型
DOI：
10.1016/j.proeng.2017.10.1111
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发表时间：
2017
期刊：
影响因子：
--
通讯作者：
Bambach
中科院分区：
文献类型：
--
作者：
Bambach
动态再结晶（DRX）发生在金属和合金的热加工过程中，当堆垛故障能量足够低时。它的特点是新晶粒的形核和长大，这是与塑性变形同时发生的。DRX工艺用于工业热成形工艺，以控制工件的微观结构和性能。考虑热变形过程中微观结构和流变应力耦合演化的各种模型已经被开发出来。人们普遍认为，DRX的发生是以应变硬化率作为应力函数的拐点为特征的。这个所谓的二阶导数准则是由Poliak和Jonas在20世纪90年代根据不可逆过程的热力学结果推导出来的。最近的研究结果表明，如果Avrami指数小于等于3，用经典Avrami动力学来描述再结晶体积分数演化的模型就会违反准则。这种不一致的根源在于对DRX核成核和生长的假设，并被证明会影响用于设计和控制金属成形过程的模型的准确性。本文提出了一种新的、一致的DRX成核模型。典型的成核模型假设在整个母晶中有恒定的位错密度，从而推导出成核的临界位错密度。然而，从晶体塑性可以清楚地看出，晶界处的位错密度高于晶内。为了考虑位错梯度，推导了热工况下可动位错密度和不可动位错密度的耦合演化方程，并对其进行了求解。本文提出了一种新的基于母晶中应变相关位错密度梯度的成核判据。一旦达到临界梯度，就开始成核。将新准则的计算结果与常规模型和实验数据进行了比较。对比表明，与传统模型对临界位错密度的过高估计相比，新准则预测的临界位错密度更为真实。因此，在金属成形过程的设计和控制中，新模型可以实现更准确的预测。
Dynamic recrystallization (DRX) occurs during hot working of metals and alloys with a sufficiently low stacking-fault energy. It is characterized by the nucleation and growth of new grains, which takes place concurrently to plastic deformation. DRX processes are used in industrial hot forming processes to control the microstructure and properties of the workpiece. Various models have been developed that consider the coupled evolution of microstructure and flow stress during hot deformation processes. It is widely accepted that the onset of DRX is characterized by an inflection point of the strain hardening rate as a function of stress. This so-called second-derivative criterion was derived by Poliak and Jonas in the 1990s using results from the thermodynamics of irreversible processes. Recent results show that models which use classical Avrami kinetics to describe the evolution of the recrystallized volume fraction violate the criterion if the Avrami exponent assumes a value of 3 or less. This inconsistency has its root in the assumptions made for the nucleation and growth of DRX nuclei and was shown to affect the accuracy of models used to design and control metal forming processes. This paper presents a new, consistent model for the nucleation of DRX. Typical nucleation models assume a constant dislocation density throughout the parent grains to derive a critical dislocation density for nucleation. However, it is clear from crystal plasticity that the dislocation density is higher at the grain boundaries than inside the grains. In order to involve dislocation gradients, coupled evolution equations for mobile and immobile dislocation densities are derived and solved under hot working conditions in this work. A new nucleation criterion is developed based upon the strain-dependent dislocation density gradient in parent grains. Once the critical gradient is reached, nucleation starts. The results of the newly developed criterion are compared with conventional models and experimental data. The comparison shows that the new criterion predicts more realistic critical dislocation densities in contrast to conventional models which over-estimate the critical dislocation density. As a consequence, more accurate predictions in the design and control of metal forming processes may be achieved with the new model.