Fe-13Cr-5Ni马氏体不锈钢的逆向奥氏体相变及其相场研究

发布时间：2018-05-04 06:33

本文选题：逆向奥氏体相变 + 组织遗传　；参考：《清华大学》2016年博士论文

【摘要】：本文选取Fe-13Cr-5Ni马氏体不锈钢,作为研究对象,从实验以及相场模拟两个方面着手,研究了高温下的逆向奥氏体相变行为,特别是导致组织遗传现像的相变过程。同时深入讨论了,相场方法在研究逆向奥氏体相变(固态相变)中的数值优势。通过对不同热处理条件下奥氏体的行为研究,揭示了不同加热温度以及不同保温时间对逆向奥氏体相变的影响。结果表明,高温下奥氏体在原奥氏体晶界球状形核,在马氏体板条界针状形核,且针状奥氏体与基体间存在K-S晶体学位向关系。逆向奥氏体相变过程中,存在合金元素的配分行为,尤其是Ni元素。随着退火温度的升高,奥氏体中Ni的含量降低,从而降低奥氏体的稳定性;同时,随着退火温度的升高,奥氏体的转变量增加。两种机制共同影响,决定了室温下残余奥氏体的量,随退火温度升高先增加再减少的趋势。保温时间对室温下残余奥氏体的量的影响,与退火温度一样。为了更直观研究高温下奥氏体的相变行为,使用相场方法,建立描述降温马氏体相变及随后升温逆向奥氏体相变过程的相场模型。在新相与母相间为共格界面,且存在K-S位向关系的前提下,模拟了高温下奥氏体的形核与长大过程。并详细讨论了相变过程中,初始组织、相变诱导应变以及外加力学条件等因素,对相变热力学、动力学以及组织演化过程的影响。结合实验与模拟结果,同一原奥氏体晶粒内形成的,与马氏体板条间存在K-S位向关系的针状奥氏体,沿着马氏体板条界不断长大且保持这一位向关系,由此减小相变引起的体系应变能增量,直致这些针状奥氏体相遇合并,最终导致组织遗传的发生。奥氏体与马氏体板条间的共格界面以及K-S位向关系,是导致组织遗传的关键。模拟结果还表明,通过增加初始马氏体组织的缺陷畸变能,以增加利于奥氏体形核的位置与驱动力;以及在逆向奥氏体相变过程中对试样施加外应力作用,以抑制奥氏体相变长大过程,都有助于消除组织遗传现像的发生,得到细小的奥氏体组织。最后研究了相场方法在研究逆向奥氏体相变中的优势。除了可以弥补实验表征手段在对高温下奥氏体相变过程直观观测中的不足,研究结果还表明,相场方法中使用的基于傅立叶变换的FSIPM算法,在模拟固态相变中具有很高的计算精度、同时具有明显优势的计算效率。
[Abstract]:In this paper, Fe-13Cr-5Ni martensitic stainless steel is selected as the research object. The reverse austenitic transformation behavior at high temperature, especially the phase transformation process leading to the genetic phenomenon of the microstructure, is studied from two aspects of experiment and phase field simulation. At the same time, the numerical advantages of phase field method in the study of reverse austenitic transformation (solid phase transformation) are discussed. By studying the behavior of austenite under different heat treatment conditions, the effects of different heating temperature and holding time on the reverse austenite transformation were revealed. The results show that austenite nucleates at the grain boundary of the original austenite at high temperature and acicular nucleation at the martensite lath boundary, and there is a K-S crystal orientation relationship between the acicular austenite and the matrix. During reverse austenitic transformation, there are alloy elements, especially Ni. With the increase of annealing temperature, the content of Ni in austenite decreases and the stability of austenite decreases, and the amount of austenite transformation increases with the increase of annealing temperature. The two mechanisms affect the amount of retained austenite at room temperature, which increases first and then decreases with the increase of annealing temperature. The effect of holding time on the amount of retained austenite at room temperature is the same as the annealing temperature. In order to study the transformation behavior of austenite at high temperature more intuitively, a phase field model was established to describe the martensite transformation at low temperature and the reverse austenite transformation process after heating up by using the phase field method. The nucleation and growth process of austenite at high temperature was simulated under the premise of the coherent interface between the new phase and the parent phase and the existence of K-S orientation. The effects of the initial microstructure, phase transition induced strain and applied mechanical conditions on the thermodynamics, kinetics and microstructure evolution of phase transition are discussed in detail. Combined with the experimental and simulation results, acicular austenite formed in the same austenite grain with K-S orientation relationship with martensite lath grows up along the martensite lath boundary and maintains this orientation relationship. Therefore, the strain energy increment of the system caused by transformation is reduced, and the acicular austenite meets and merges directly, which leads to the occurrence of microstructure heredity. The coherent interface between austenite and martensite lath and the K-S orientation are the key factors leading to tissue inheritance. The simulation results also show that the position and driving force of austenite nucleation are increased by increasing the defect distortion energy of the initial martensite, and the external stress is applied to the specimen during the reverse austenitic transformation. The inhibition of austenitic transformation can help to eliminate the genetic phenomenon of the microstructure and obtain fine austenitic structure. Finally, the advantage of phase field method in the study of reverse austenitic transformation is studied. In addition to the deficiency of experimental characterization in the direct observation of austenitic transformation process at high temperature, the results also show that the FSIPM algorithm based on Fourier transform is used in the phase field method. In the simulation of solid phase transition, it has high calculation accuracy and obvious advantage of calculation efficiency.
【学位授予单位】：清华大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG142.71

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