【作者】 梁炫；

【导师】 吴开明；

【作者基本信息】 武汉科技大学 ， 材料科学与工程， 2023， 博士

【摘要】 铁素体和合金碳化物是钢铁最核心的基本相,对钢铁的性能有重要的影响。如何正确描述铁基钢铁材料的热力学性质,一个巨大的挑战是如何引入与电子结构紧密相关的内禀磁性及磁场诱导磁性、溶质元素微观偏析等因素对热力学性质的贡献。因此,亟需从电子层面来研究其微观电子结构与相变热力学机制和物性特征之间的关联性,为新型钢铁材料的成分设计、热处理工艺开发和服役性能改进提供理论指导。本文以铁基材料中铁素体及合金碳化物为研究对象,通过扫描电镜、透射电镜等检测仪器对合金碳化物的形貌、分布及元素进行表征。利用基于密度泛函理论的第一性原理,结合准谐近似方法、准谐德拜模型和魏氏分子场理论研究了铁素体及合金碳化物的相变热力学机制及物性特征。本文的主要研究内容和研究结果概括如下:（1）晶格振动和电子对铁素体（铁磁性）和Nb C（无磁性）吉布斯自由能的贡献随着温度的升高而增加,在高温下晶格振动对吉布斯自由能的贡献占主导地位,电子的贡献虽小但不容忽视。（2）对于铁磁性材料,内禀磁性和磁场对其吉布斯自由能也有贡献。随着温度的增加,内禀磁性的贡献在增加,磁场的贡献先增加后减小,在磁转变点趋于零。在T=1043 K附近,四种激励对铁素体吉布斯自由能贡献的排序为:晶格振动>内禀磁性>电子>磁场。（3）随着温度的升高,铁素体的磁矩逐渐降低。磁场不会改变磁矩随温度变化的趋势,但是会导致铁素体高温下的磁矩增加,磁转变点升高,磁热容的峰值向高温偏移。磁场强度越大,影响越剧烈。（4）不同温度下形成Nb C时,电子导致的自由能改变量增加被晶格振动导致的自由能改变量减小所补偿,总吉布斯自由能改变量为负,证实了Nb C优异的热力学稳定性。（5）随着Nb含量的增加,液析碳化物Nb C的形貌由球形向多面体转变,带状分布趋势加强。Nb含量的增加促使Nb C在更低的固相分数开始析出。凝固过程中L+δ向L+γ转变,导致固/液界面液相中C的偏析降低,Nb的偏析升高。（6）采用机械合金化和放电等离子烧结工艺获得了含少量Fe₂Si C的产物,其物性参数的理论预测如下:Si倾向于取代Fe₃C中4a魏科夫位置的Fe原子,得到的Fe₂Si C属于正交晶系。Si原子所在的魏科夫位置、八面体结构单元和Fe-C键键长共同决定了Fe₂Si C的磁性特征。与Fe₃C相比,Fe₂Si C具有更高的弹性模量和剪切模量,其物理机制为Si置换4c魏科夫位置的Fe原子后,使Fe-C之间的共价键增强。研究结果为含硅钢中的析出相及其物性特征提供了理论预测。更多还原

【Abstract】 Ferrite and alloy carbides are the core basic phases of steel,which have an important influence on the properties of steel.In order to correctly describe the thermodynamic properties of iron-based steel materials,a huge challenge is how to introduce the contribution of intrinsic magnetism and magnetic field induced magnetism,solute microsegregation to thermodynamic properties.It is urgent to study the relationship between the microscopic electronic structure and the thermodynamic mechanism and physical properties of phase transformation from the electronic level,so as to provide theoretical guidance for the design of new alloy composition,heat treatment and the improvement of service performance.In this thesis,ferrite and alloy carbides in iron-based materials were studied.The morphology,distribution and elements of alloy carbides were characterized by scanning electron microscopy and transmission electron microscopy.The thermodynamic mechanism and physical properties of ferrite and alloy carbides were studied by using the first-principle based on density functional theory,combined with quasiharmonic approximation method,quasiharmonic Debye model and Weiss molecular field theory.The main research contents and results are as follows:（1）The contribution of lattice vibration and electrons to the Gibbs free energy of ferrite（ferromagnetic）and Nb C（nonmagnetic）increases with temperature.At high temperature,the contribution of lattice vibration to Gibbs free energy is dominant,and the contribution of electrons is small but cannot be ignored.（2）For ferromagnetic materials,intrinsic magnetism and magnetic field also contribute to the Gibbs free energy.With the increase of temperature,the contribution of intrinsic magnetism increases.The contribution of magnetic field increases first and then decreases,and tends to zero at the magnetic transition point.At T=1043 K,the order of the contribution of the four excitations to the Gibbs free energy of ferrite is:lattice vibration>intrinsic magnetism>electron>magnetic field.（3）The magnetic moment of ferrite decreases gradually with temperature.The magnetic field does not change the trend of magnetic moment with temperature.It increases the magnetic moment of ferrite at high temperature and the magnetic transition point.The magnetic field shifts the peak value of magnetic heat capacity to higher temperature.The greater the magnetic field strength,the more severe the impact.（4）The increase in free energy change due to electrons at different temperatures is compensated by the decrease in free energy change due to lattice vibration,indicating that lattice vibration makes a major contribution to the stability of Nb C.The Gibbs free energy change at different temperature is negative,indicating that Nb C is thermostatically stable.（5）The morphology of primary carbides changes from sphere to polyhedron with the niobium content increase.The strip distribution of primary carbides is more obvious.With Nb content increases the solid fraction of phase transition increases.When the phase transition from L+δto L+γoccurs,the mass fraction of solute C decreases and the mass fraction of solute Nb increases at the solidification front.（6）The samples containing a small amount of Fe₂Si C were obtained by mechanical alloying and spark plasma sintering.The theoretical prediction of its physical parameters is as follows:Si tends to replace the Fe atom at the 4a sites in Fe₃C and the obtained Fe₂Si C belongs to the orthorhombic system.The magnetic moment of Fe₂Si C is zero.The magnetic characteristics of Fe₂Si C origin from the Wyckoff position of Si atoms,octahedral structural unit and Fe-C bond distances together determine.The bondings of Fe₂Si C and Fe₃C have a mixture of covalent,metallic and ionic characters.Fe₂Si C has higher elastic modulus and shear modulus.The physical mechanism is that the substitution of Si increases the degree of the covalent bond between Fe-C.Fe₂Si C has lower elastic anisotropy.This investigation provides a theoretical prediction for the precipitated phase and its physical properties in silicon-containing steel.更多还原