【作者】 郑勇；

【导师】 于金； 周雪峰； 梁超；

【作者基本信息】 东南大学 ， 材料工程（专业学位）， 2018， 硕士

【摘要】 高速钢是机械加工制造业的基础原材料之一,具有高硬度、高耐磨性及良好的红硬性等优点,广泛应用于高速切削工具、高载荷精密模具等重要领域。高速钢的优异性能与组织中存在的大量碳化物有关,因此,合金碳化物调控是高速钢性能调控的关键。现有高速钢实验设计研究,主要通过调整材料成分配比,改变碳化物结构及成分,优化碳化物特性,最终提升高速钢性能。但这一方法研发效率低、成本高,已不能满足现代材料设计的要求。本文采用第一性原理计算方法,研究Mo、W、V、Cr、Fe对MC和M₂C碳化物的稳定性及力学性能影响规律,以及对MC和M₂C与α-Fe界面稳定性影响规律,并通过一定的实验设计验证模拟计算结果的合理性。主要结果如下:（V,M）C碳化物的晶格常数计算结果表明,随着Cr和Fe原子数增加,碳化物晶格常数变小,而Mo和W原子数增加,晶格常数变大。结合能、弹性常数和态密度结果显示,Cr和Fe会降低碳化物的稳定性,W则会增加其稳定性,而Mo对稳定性影响不大。力学性质结果表明,随着W原子数增加,碳化物的杨氏模量先增加后减小,并在0.25处取到极值;而随着Mo、Cr、Fe原子数增加,碳化物的杨氏模量不断减小;另外随着W、Mo、Fe原子数增加,碳化物的硬度减小。弹性模量与硬度的计算结果与实验结果相吻合。（Mo,M）₂C碳化物的体积计算结果来看,随着V、Cr、Fe原子数增加,碳化物体积变小,而W原子数增加,碳化物体积无变化。结合能结果表明,Cr和Fe会降低M₂C碳化物的热力学稳定性,而W则会增加其稳定性,V稍微增加稳定性。（Mo,M）₂C的弹性模量结果显示,随着W原子数增加,三种结构的弹性模量变大,碳化物强度增大,这与实验结果相吻合;硬度结果表明,Fe原子会降低六方结构和正交结构M₂C的硬度,V原子会增加六方结构M₂C的硬度,但会降低正交结构M₂C的硬度;对三种不同形貌M₂C的TEM衍射斑点和三种不同结构的模拟衍射斑点进行对比得出,M2纤维状为结构1,M2片状为结构2,M42纤维状为结构3。热分解量和相变温度结果表明,这三种不同形貌M₂C碳化物热稳定性大小依次为:M42纤维状>M2片状>M2纤维状,另外随着Mo/Fe增大,M42纤维状M₂C热稳定性增加。因此热稳定性的实验结果与结合能的计算结果得到相互验证。MC/α-Fe的吸附功和界面能结果表明,Mo和Cr原子的引入,可能会增大MC/α-Fe界面的稳定性;Fe原子含量增多,可能会减小MC/α-Fe界面的稳定性;而W原子对MC/α-Fe界面的稳定性影响不大。M₂C/α-Fe吸附功和界面能结果表明,Cr原子的引入,可能会增大M₂C/α-Fe界面的稳定性;V、W和Fe原子含量增多,可能会减小M₂C/α-Fe界面的稳定性。MC/α-Fe和M₂C/α-Fe界面稳定性计算的结果与实验中不同温度回火下MC和M₂C成分变化基本相符。更多还原

【Abstract】 High speed steel is one of the basic raw material in machinery manufacturing industry,which is widely used in the important field of high speed cutting tool,high load precision mould,air temperature bearing etc,due to the high hardness,good wear resistance and excellent red-hardness.The superior performance of high-speed steel is related to a large amount of carbides in the structure.Therefore,the regulation of alloy carbides is the key to the regulation of high-speed steel performance.The existing high speed steel experimental design research mainly aims at improving the performance of high-speed steel by adjusting the ratio of materials to distribution and changing the structure and composition of carbides,which optimizes the properties of carbides.However,this method has low development efficiency and high cost,and it can no longer meet the requirements of modern material design.In this paper,the effects of Mo,W,V,Cr,and Fe on the stability and mechanical properties of MC and M₂C carbides are investigated with first principles,as well as the influence on the interfacial stability of MC and M₂C andα-Fe.Through a certain number of experimental designs,the rationality of the simulation results is verified.The calculated lattice constants of（V,M）C carbides show that the carbide lattice constant decreases with the increase of the number of Cr and Fe atoms,and the lattice constant increases with the increase of the number of Mo and W atoms.From the results of binding energy,elastic constant and density of states,Cr and Fe reduce the stability of carbides,W increase its stability,and Mo has little effect on the stability.The results of the mechanical properties show that as the number of W atoms increases,the Young’s modulus of the carbide increases first and then decreases,and the extremum is taken out at 0.25,whereas with the increase of Mo,Cr,and Fe atoms,the carbide of Young’s modulus decreases continuously.In addition,as the number of W,Mo and Fe atoms increases,the hardness of the carbide decreases.The calculated results of elastic modulus and hardness agree with the experimental results.（Mo,M）₂C carbide volume calculation results show that as the number of V,Cr,Fe atoms increases,the volume of carbides becomes smaller,while the number of W atoms increases,the volume of carbides does not change.The results of the binding energy show that Cr and Fe reduce the thermodynamic stability of M₂C carbides,while W increases its stability and V slightly increases stability.The results of（Mo,M）₂C elastic modulus show that as the number of W atoms increases,the elastic modulus of the three structures become larger and the carbide strength increases,which are consistent with experimental results.Fe reduces the hardness of the hexagonal structure and the orthogonal structure of M₂C.However,V increases the hardness of the hexagonal M₂C structure,but will reduce the hardness of the orthogonal structure M₂C.The comparison of TEM diffraction spots and simulated diffraction spots of three different structure M₂C shows that M2 fiber,M2 sheet and M42 fiber carbide are structure 1,structure2,and structure 3,respectively.The results of phase transition temperature and thermal decomposition show that the thermal stability of these three different morphology M₂C carbides are:M42 fiber>M2 sheet>M2 fiber.In addition,as the Mo/Fe increases,the thermal stability of the M42 fibrous M₂C carbide increases.Therefore,the experimental results of thermal stability and the calculation results of the binding energy are mutually verified.The results of adsorption work and interfacial energies of MC/α-Fe interface indicate that the introduction of Mo and Cr atoms may increase the stability of the MC/α-Fe interface.The stability of the MC/α-Fe interface may decrease with the increase of Fe atom.W atoms have little effect on the stability of MC/α-Fe interface.M₂C/α-Fe adsorption work and interfacial energies results show that the introduction of Cr atoms may increase the stability of the M₂C/α-Fe interface;however,the increase of V,W,and Fe atoms may reduce the stability of M₂C/α-Fe interface.The calculated results of the interface stability of MC/α-Fe and M₂C/α-Fe are basically consistent with the changes of MC and M₂C compositions at different temperatures during tempering process.更多还原