【作者】 宋述同；

【导师】 关小军；

【作者基本信息】 山东大学 ， 材料加工工程， 2008， 硕士

【摘要】 进入铁器时代以来,钢铁一直是人类社会所需的重要材料。近年来钢铁材料面临着陶瓷材料、高分子材料、有色金属材料等的挑战,但钢铁在材料领域仍扮演着重要角色,广泛应用于国民经济建设的各个领域。钢铁在材料行业中所占据的统治地位,在今后很长时期内不会改变,对钢铁材料性能的研究仍具有十分重要的意义。对相变的研究在钢铁材料性能研究方面具有极其重要的作用,相变影响着材料的物理、化学等各项性能指标。研究奥氏体-铁素体相变的热力学和动力学,建立材料制备加工工艺与其组织状态间的定量关系,对于设计材料成分和制备加工工艺以获得所需要的材料性能具有十分重要的意义。近年来,相场作为一种模拟材料微观组织演变的方法,越来越受到人们的重视,它是目前较为理想的相变组织模拟手段。本文通过所建立的相变模型成功模拟了奥氏体-铁素体转变中的等温扩散转变和块状转变。在等温扩散转变过程中,铁素体晶核逐渐长大,从铁素体相析出的过饱和碳原子不断向奥氏体相一侧转移,使铁素体-奥氏体界面处有明显的碳堆积,碳浓度不断升高,在奥氏体一侧建立了碳原子的浓度梯度。在此浓度梯度的驱动下,奥氏体相中界面处的碳原子向其内部扩散,导致远离界面的奥氏体相内部的碳原子浓度随相变的进行而不断升高。温度影响单晶核的等温扩散转变过程,同一转变时间时,奥氏体中碳浓度梯度随温度升高而减小;低温范围内,铁素体晶核长大速度随温度升高而加快,而高温范围内恰相反。在所模拟的等温块状转变过程中,碳在奥氏体-铁素体相界面处聚集,并且在界面移动过后,新形成的铁素体碳浓度并没有急剧的降低,而是与母相奥氏体中碳浓度相当,说明转变过程中两相界面处发生了碳的短程扩散,此转变为块状转变。所建相场模型及其应用程序具有较好的物理机制,其模拟结果所揭示的碳浓度变化规律和铁素体晶核长大规律与相变理论一致,为进一步采用相场理论进行奥氏体-铁素体转变的模拟研究奠定了基础。更多还原

【Abstract】 Since coming into steel age, steel has always to be important materials needed by human society. Recently steel has been challenged by ceramics, high polymer and nonferrous metals, but it still plays an important part in material fields and is widely used to various fields in our national economy. In material fields, steel holds the dominant position, which will exist for a long time, and it’s also very important to study the property of steel materials.Research in phase transformation which influences the physical and chemical properties of materials has an important effect on steel properties’ study. To study the thermodynamics and kinetics of austenite-ferrite transformation, establish the quantitative relationship between processing technology and microstructure, has important significance for designing material composition and making processing technology to obtain the needed material properties.In recent years, as one of the methods of simulating materials’ microstructure, phase-field simulation which becoming more and more important is an ideal method to simulate phase transformation at present. This paper successfully simulates the isothermal transformation and massive transformation of austenite-ferrite by establishing the phase-field simulation model.In process of isothermal transformation, the ferrite nuclear grows up gradually. The supersaturate carbon atoms in ferrite phase are rejected to austenite continuously so that they are obvious stacking in the interface of austenite and ferrite where the carbon concentration keeps rising, then it makes a gradient of carbon concentrate in austenite. Under the concentrate gradient drive, the carbon atoms in interface transfer to the inside of austenite so that the carbon concentration of austenite where is far away from the interface increase high continuously with the phase transformation developing.Temperature has some effect on the isothermal transformation of single ferrite nuclear. At the same time, the carbon concentration gradient in austenite decreases with the temperature rising. At the range of low temperature the growth speed of the ferrite nuclear accelerate with the temperature increasing, but contrary at the high temperature.In process of massive transformation, carbon accumulates in the interface of austenite-ferrite and after the interface moving the carbon concentration of newly formed ferrite does not show rapid decrease, but almost equal to mother austenite phase. It shows that shot-range diffusion of carbon happens in the interface during transformation which is massive transformation.The phase-field model which we build in this paper and the application program show a better physical mechanism, and the result of simulation also reveals that the change law of carbon concentration and the growth law of ferrite nuclear accord with the theory of phase transformation. Our research lays the foundation for the further study in simulation of austenite-ferrite transformation using phase-field method.更多还原