Fe-C-Mn合金中奥氏体—铁素体相变的相场模拟

发布时间：2018-04-23 14:35

本文选题：Fe-C-Mn合金 + 奥氏体　；参考：《中国科学技术大学》2017年博士论文

【摘要】：为了满足日益增长的材料性能需求,先进钢铁材料的发展吸引了广泛地关注。钢铁材料的力学性能与其组织状态紧密相关,其中,奥氏体-铁素体相变是钢铁材料加工制备和热处理过程中发生的较为普遍的物理冶金现象。合金中发生的奥氏体-铁素体相变会改变材料的组织状态,从而影响室温下材料的力学性能。研究钢铁材料中奥氏体-铁素体相变的微观组织转变及其动力学,对合金成分设计和加工工艺优化以获得所需要的材料组织和性能具有十分重要的意义。因此,本文结合相场模拟对不同成分及加工工艺条件下Fe-C-Mn合金中发生的奥氏体-铁素体相变进行研究。首先建立Fe-C二元合金的奥氏体-铁素体相变相场模型,模拟了两相区发生的奥氏体-铁素体等温相变过程。模型中考虑了C元素扩散和相界面迁移两个因素的共同作用。通过分析铁素体生长过程中C的扩散行为,发现奥氏体/铁素体相界面处于非平衡状态,相变过程表现为混合控制生长的特征。随着相变的进行,奥氏体-铁素体相变逐渐从界面控制生长转变为扩散控制生长模式。利用该相场模型进一步研究了不同等温温度对奥氏体-铁素体相变中微观组织和C浓度场演化的影响,结果表明:随着等温温度的降低,铁素体形核率增大,铁素体相变平衡体积分数增加,但奥氏体内部C浓度分布的不均匀程度加剧,较低温相变温度下的终态微观组织呈现为不规则细小铁素体晶粒围绕分散分布的残余奥氏体的两相结构。且随等温温度的降低,奥氏体-铁素体相变表现出由扩散控制生长模式向界面控制生长模式转化的趋势。为研究Fe-C-Mn三元合金在临界区等温时发生的奥氏体-铁素体相变,本文针对Fe-C-Mn三元合金建立了相场模型。基于Gibbs自由能守恒理论,在相场模型中考虑了替换型元素Mn在奥氏体/铁素体相界面内扩散所导致的自由能耗散,描述了Fe-C-Mn三元系合金中因Mn在相界面偏聚所导致的相变停滞和相变不完全现象。利用相场模型研究了不同Mn含量对奥氏体-铁素体相变微观组织转变和相变动力学的影响,结果显示:随着Mn含量的增加,铁素体晶粒的生长形貌从椭圆状向扁平状转变,奥氏体内C浓度降低,铁素体相的转变速率和体积分数降低,Mn在奥氏体/铁素体相界面内扩散所导致的溶质拖曳现象越明显,相变不完全程度加剧,奥氏体-铁素体相变过程更易表现出界面控制生长模式。最后,利用Fe-C-Mn三元合金的奥氏体-铁素体相变相场模型,模拟了相变过程中不同晶界特征下铁素体晶粒的形貌与生长动力学。模型中通过能量梯度系数和耦合项系数的协同变化,定量表达晶界能与晶界迁移率的各向异性,同时固定相场界面宽度来保证计算精度。模拟结果显示:随着原奥氏体晶界能与铁素体-奥氏体晶界能比值σγ.γ/σα.γ的增加,三叉相界面处的平衡角β减小,铁素体晶粒沿原奥氏体晶界与垂直于奥氏体晶界方向的生长速率差变大。铁素体与奥氏体晶粒间的晶粒取向越接近,铁素体生长越缓慢。应用该模型模拟了多晶组织的演化过程,不同奥氏体晶界处铁素体晶粒表现出不同的生长形貌,揭示了不同类型晶粒形貌与晶粒间之间的取向关系,并与实际金相组织对比,结果两者吻合较好。
[Abstract]:In order to meet the increasing demand for material performance, the development of advanced steel materials has attracted wide attention. The mechanical properties of steel materials are closely related to the state of their tissues. The austenite ferrite transformation is a common physical metallurgy phenomenon occurring during the process of processing and heat treatment of steel materials. The austenite ferrite transformation will change the microstructure of the material and influence the mechanical properties of the materials at room temperature. The microstructure transformation and kinetics of the austenite ferrite transformation in steel materials are studied. It is of great significance to the design and process optimization of the alloy components to obtain the structure and properties of the materials needed. In this paper, the phase field simulation is used to study the austenite ferrite transformation in Fe-C-Mn alloy under different components and processing conditions. First, the phase field model of austenite ferrite phase transition in the Fe-C two element alloy is established, and the isothermal phase transition process of the austenite ferrite in the two phase region is simulated. The diffusion of C elements is considered in the model and the diffusion of the elements is considered in the model. The interaction between the two factors of phase interface migration. Through the analysis of the diffusion behavior of C during the growth of ferrite, it is found that the austenite / ferrite interface is in a nonequilibrium state. The phase transition process is characterized by mixed control growth. With the phase transition, the austenite ferrite transformation gradually transforms from the interface control growth to the diffusion control student. This phase field model is used to further study the effect of different isothermal temperatures on the microstructure and the evolution of C concentration field during the phase transition of austenite ferrite. The results show that the nucleation rate of ferrite increases and the equilibrium volume fraction of ferrite increases with the decrease of isothermal temperature, but the uneven degree of C concentration distribution in austenite is added to the austenite. The final state microstructure at the low temperature phase transition temperature appears as the two-phase structure of the dispersed retained austenite in the irregular fine-ferrite grain. And with the decrease of the isothermal temperature, the phase transformation of austenite ferrite shows the trend of the transformation from the diffusion controlled growth mode to the interface control growth mode. The study is for the study of the Fe-C-Mn three element combination. The phase field phase transition of gold at the critical zone isotherm. In this paper, a phase field model is established for the Fe-C-Mn three element alloy. Based on the Gibbs free energy conservation theory, the free energy dissipation caused by the diffusion of the substitutional element Mn in the austenite / ferrite phase interface is considered in the phase field model, and the Mn in the Fe-C-Mn three element alloy is described. The phase transition stagnation and incomplete phase transition caused by the segregation of phase interface. The effect of different Mn content on the microstructure transformation and phase transition kinetics of austenite ferrite transformation was studied by phase field model. The results showed that the growth morphology of ferrite grains changed from elliptical to flat with the increase of Mn content, and the concentration of C in austenite decreased. The transition rate and volume fraction of ferrite phase decrease, the solute drag caused by the diffusion of Mn in the austenite / ferrite phase interface is more obvious, the phase transition incompleteness intensifies, and the austenite ferrite transformation process is more likely to show the interface control growth mode. Finally, the phase transformation of austenite ferrite phase of the Fe-C-Mn three element alloy is used. The field model is used to simulate the morphology and growth kinetics of ferrite grains under the characteristics of different grain boundaries in the phase transition process. In the model, the anisotropy of the grain boundary energy and grain boundary mobility is expressed by the synergistic variation of the energy gradient coefficient and the coupling coefficient. At the same time, the interface width of the phase field is fixed to ensure the calculation accuracy. The simulation results show that along with the simulation results: The grain boundary energy ratio between the original austenite and austenite grain boundary can be increased, the equilibrium angle of the interface between the trigeminal phase decreases and the growth rate of ferrite grain along the original austenite grain boundary and the direction perpendicular to the austenite grain boundary becomes larger. The closer the grain orientation between the ferrite and austenite grain is, the slower the ferrite growth is. The evolution process of polycrystalline microstructure was simulated by this model. The ferrite grain at different austenite grain boundaries showed different growth morphology, and revealed the relationship between the grain morphology and grain size between different types of grain, and compared with the actual metallographic structure, the results were in good agreement.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG141

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