奥氏体→铁素体相变新相形貌与生长动力学相场法研究

发布时间：2018-03-28 23:29

本文选题：相场方法　切入点：奥氏体/铁素体相变　出处：《上海交通大学》2015年博士论文

【摘要】：在新一代高性能钢铁材料的研发中,通过在热处理加工过程里发生固态相变时进行组织调控,是提高钢材综合性能的有效方法之一。奥氏体→铁素体相变是钢铁材料冷却过程中最重要的组织转变之一,铁素体晶粒尺寸和形态对钢材的力学性能有重要的影响,基于其理论意义和工业应用价值,该相变一直是材料研究领域中的重点和热点。当铁素体从过冷奥氏体中析出时,它可能会具有不同的形态,包括无定形仿晶界铁素体、块状铁素体、晶内等轴铁素体和针状魏氏铁素体等,材料的组织形态进一步地决定了钢材的各种性能。因此,系统研究奥氏体→铁素体相变的热力学和动力学,深入理解不同铁素体组织的生长行为,这对于分析钢材加工工艺与组织形态间的定量关系、设计合金成分以获得所需的综合性能,都有十分重要的意义。本文,通过建立描述奥氏体→铁素体相变的相场模型,并且考虑因生成环境各异相变主控因素的不同,系统地研究了不同类型铁素体新相的形貌特征和生长动力学行为,其中包括由动力学系数的各向异性主控的沿晶界生长铁素体、由界面能的各向异性主控的晶内形核生长铁素体及由弹性能的各向异性主控的从晶界向晶内生长魏氏铁素体。具体地:(1)模拟研究了在化学自由能和各向同性界面能的共同作用下,于晶界处发生奥氏体→铁素体相变时新相的生长动力学行为和形貌特征。在长程扩散型相变中,揭示了界面处成分满足局域平衡条件,新相生长动力学符合抛物型规律。若初始两个新相核胚同时生长,会发生相互作用,伴随着碰撞加速和碰撞富集等现象。在块状相变中,模拟再现了碳原子仅发生短程扩散行为及界面快速迁移等转变特征。此外,通过引入动力学系数的各向异性特征,计算获得的新相形如扁椭圆状,与实验结果相近,澄清了短路扩散行为是决定晶界铁素体形貌的关键因素。(2)模拟研究了在化学自由能和各向异性界面能的共同作用下,于晶内发生奥氏体→铁素体相变时新相形貌演变的动力学行为。而且,通过开展耦合各向异性界面能和各向同性相变驱动力ΔFmc的Allen-Cahn模型的计算分析,系统地讨论了各向异性收缩作用与各向同性长大作用之间的竞争行为及其对新相形貌的影响,并解释了晶内铁素体稳态形貌与Wulff之间的相似性。当ΔFmc的数量级不断增大,稳态形貌与Wulff之间存在着三种典型的相似关系:与Wulff相似地收缩、与Wulff相似地长大及与Wulff不相似地长大。即当ΔFmc→0时,稳态形貌与Wulff相似,可由与相场模型等价的界面模型推导证明;而随着ΔFmc不断增大,稳态形貌逐渐偏离Wulff,且不再相似时对应的“临界”ΔFmc值是与发生Fisher现象的临界值ΔFFisher相符的。正是由于Fe-C合金的实际相变驱动力远小于由相似向不相似转变对应的理论临界值,故模拟获得的晶内铁素体稳态形貌与Wulff相似,是界面能量最低的构型。(3)模拟研究了在化学自由能、各向同性界面能和各向异性弹性应变能的共同作用下,发生奥氏体→魏氏铁素体相变时新相的形貌特征和生长动力学行为。计算结果揭示了弹性能对新相的核胚形状、生长形貌及不同方向上的长大动力学行为都有重要影响。当体积一定时,椭圆形核胚的形核激活能最低,最易出现。在生长过程中,新相形如针状,与实验观察形貌相一致;且新相沿不同方向上的动力学特征显著不同,即沿水平方向呈抛物型增厚,而沿竖直方向呈线性伸长。这是因为,新相的侧边可视作“平直界面”的推进,增厚行为满足扩散型相变中的抛物型长大规律;而新相的顶部较快地由初始构型演变成有稳定曲率半径的尖端,使得其边缘处浓度梯度保持不变,存在着稳恒质量流。此外,随着过冷度或者过饱和度的增大,相变驱动力增加,新相伸长速度增大。
[Abstract]:In the development of a new generation of high performance steel materials, the organization controlled by heat treatment in the machining process occurred in solid phase, is one of the effective ways to improve the comprehensive performance of steel. The austenite to ferrite transformation is one of the most important cooling process of steel materials in the organization, ferrite has important effects on mechanical properties the grain size and morphology of steel, its theoretical and industrial application value based on the phase change has been an important and hot research in the field of material. When the ferrite precipitated from undercooled austenite when it may have different forms, including amorphous imitation grain boundary ferrite, blocky ferrite in the crystal body, equiaxed ferrite and acicular widmanstatten ferrite materials, the organization further determines the performance of steel. Therefore, the system research of austenite to ferrite transformation thermodynamics and dynamic learning, in-depth Understanding the different ferrite growth behavior, the quantitative analysis on the relationship between steel processing technology and organization form between the design of the components of the alloy to obtain comprehensive performance required, are of great significance. In this paper, through the establishment of a description of the phase field model of austenite to ferrite transformation, and consider the environment due to the formation of different transformation of main control factors, a systematic study of the different types of ferrite phase morphology and growth dynamics, including the main control by the anisotropy of the kinetic coefficients along grain boundary ferrite, the anisotropy dominated by the interfacial energy of intragranular nucleation and growth of ferrite and the anisotropy the main performance of the elastic anisotropy from grain boundary to the grain growth of Widmanstatten ferrite. Specifically: (1) studied in chemical free energy interaction and isotropic interfacial energy, occurs in the grain boundary of austenite to ferrite The growth dynamics of the new phase transformation and morphology. In the long-range diffusion transformation, reveals the composition of the interface to meet local equilibrium conditions, a new phase of growth kinetics with parabolic law. If the initial two new phase and embryo growth, may interact with the collision and collision acceleration enrichment phenomenon in massive transformation, simulation of the carbon atom only short-range diffusion behavior and interface fast migration transformation characteristics. In addition, the anisotropy introduced into dynamic coefficients, obtained the new phase such as flat ellipse shape, similar to the experimental results, clarify the short-circuit diffusion behavior is the key of grain boundary ferrite morphology. (2) studied in chemical free energy interaction and anisotropy of interfacial energy, in the grain dynamic behavior of austenite to ferrite transformation and new phase morphology evolution. And, through the calculation and analysis of Allen-Cahn model of coupled Anisotropic Interfacial Energy and isotropic phase transformation driving force Fmc the competition between anisotropic shrinkage and isotropic growth effect and its influence on the new phase morphology are discussed systematically, and explain the intragranular ferrite between body morphology and steady state similar to that of Wulff. When the number of level Delta Fmc increasing, between the steady morphology and Wulff there are three typical similarity relation: similar contraction and Wulff, similar to Wulff and not grow up grow up with Wulff. That is similar to a Fmc when 0, the steady state morphology similar to Wulff, can be proved by the derivation of the interface model and phase field model equivalent; and with the increase of Fmc, the steady-state morphology gradually deviate from the corresponding Wulff, and no similar "critical" delta Fmc value is the critical value of the phenomenon and the occurrence of Fisher Delta FFisher match. It is from Fe-C The actual transformation of the alloy is much smaller than the critical value of theory driven by similar to similar transitions correspond, so simulation of intragranular ferrite steady-state morphology similar to Wulff, is the lowest energy configuration interface. (3) studied in chemical free energy, isotropic interfacial energy interaction and anisotropic elastic strain energy under the occurrence of austenite to ferrite transformation Widmanstaten new phase morphology and growth dynamics. The calculated results reveal the elastic energy of new phase nucleus shape, have an important impact on the growth kinetics and growth morphologies in different directions. When a certain volume, oval nuclei nucleation minimum activation energy and the most vulnerable. In the growth process, a new phase such as acicular, consistent with experimental observations of morphology; and different dynamic characteristics in the direction of the new phase is significantly different, along the horizontal direction along the parabolic thickening. The vertical direction linear stretch. This is because the new phase side can be regarded as a "flat interface" to promote, thickening of parabolic type diffusion behavior to meet the growth and transformation of new phase; the top quickly evolved into a stable configuration by the initial radius of curvature of the tip of the edge of the concentration gradient remain unchanged there is a steady flow and quality. In addition, with the increase of undercooling or supersaturation, phase change driving force increases, the new phase elongation rate increases.

【学位授予单位】：上海交通大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TG142.1

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