【作者】 金波；

【导师】 刘树红；

【作者基本信息】 中南大学 ， 材料学， 2023， 博士

【摘要】 纳米尺寸析出相的热稳定性和弥散分布通常是时效强化合金获得优良力学性能的关键。对相关材料开展考虑尺寸效应的相图热力学研究,获得其与尺寸相关的相图信息,对于指导合金的设计研发具有重要意义。CALPHAD(CALculation of PHAse Diagrams)方法将计算模拟与关键实验相结合,指导合金的设计,缩短了材料的研发周期,是集成计算材料工程中的重要组成部分。精确的数据库是开展计算模拟的基石,而目前常用的多元合金热力学数据库未考虑尺寸效应对吉布斯自由能的影响,导致热稳定性判断存在偏差,在进行微观组织演变模拟时无法获得准确的形核驱动力,影响模拟结果。本论文采用集成计算材料工程的思想开展考虑尺寸效应的相图热力学研究并应用于Al-Cu-Ag体系时效过程中析出相的微结构演变模拟,主要研究工作如下:(1)通过第一性原理计算结合特殊准随机结构方法对Al-Cu-Ag体系稳定相和亚稳相的摩尔体积和表面能进行了系统研究。通过将热物理性质与热力学模型分开,有效减少了端际组元的数目,确保热物理性质为摩尔分数的函数,并采用CALPHAD方法构建了Al-Cu-Ag体系含亚稳相的摩尔体积和表面能数据库。考虑过剩摩尔体积的影响并耦合CALPHAD类型摩尔体积数据库,基于Butler公式编写了溶体相表面能计算程序Be ST-Calc(Butler equation Surface Tension Calculation)。结合已有的热力学参数和构建的热物理性质数据库,计算预测了合金的摩尔体积和表面能,并同实验数据进行对比,验证了数据库和程序的可靠性。(2)基于纯元素的表面熔化现象,考虑固液界面对熔化过程的影响,结合吉布斯自由能最小化的热力学原理编写了计算程序Cal Na Phad(Calculation of Nano Phase Diagram),耦合输入参数和文献数据,预测了Al、Cu和Ag元素宏观晶体的表面熔化和纳米晶体的熔化行为。计算的宏观晶体的表面预熔化和熔化温度结果显示,随着温度逐渐接近熔点,表面液相层的平衡厚度趋于无限大。计算的纳米晶体熔化行为及与尺寸相关的相图显示,随着尺寸半径增大,液相线和固相线上升,并且液相线趋近体熔点温度。当尺寸大于临界半径时,随着温度上升固相先转变为固液共存再熔化成液相,当尺寸小于临界半径时,随着温度上升固相直接转变为液相。(3)采用分子动力学模拟综合势能、均方位移、回转半径和径向分布函数等多种判据研究了Al-Ag和Al-Cu体系二元fcc＿A1相纳米颗粒的熔化行为,结果表明合金熔点随着尺寸半径的减小而下降。考虑Al-Cu体系中Cu元素在不同亚稳相中的溶解度,结合第一性原理计算,采用CALPHAD方法建立了亚稳相θ"和θ’的热力学描述。结合摩尔体积和表面能数据库、分子动力学模拟和文献数据,建立了考虑尺寸效应的Cu-Ag、Al-Ag和Al-Cu二元系热力学数据库。基于数据库的计算结果显示,随着尺寸下降,尺寸效应对吉布斯自由能的影响逐渐增大,液相线和固相线随之下降,Cu-Ag体系中共晶点朝着富Ag端移动,Al-Ag体系中L→hcp＿A3+fcc＿A1的共晶点向富Al端移动,Al-Cu体系中L→fcc＿A1+θ的共晶点Cu含量增大。(4)基于上述子二元系的计算结果,结合摩尔体积和表面能数据库构建了考虑尺寸效应的Al-Cu-Ag三元系热力学数据库。通过对三元垂直截面的分析,选择了三元合金成分及其时效热处理工艺,分析了不同时效时间合金的硬度和析出相的微结构信息。将构建的考虑尺寸效应的热力学数据库耦合动力学数据库、热物理性质数据库和实验数据,考虑析出相的形貌信息和长径比,采用KWN模型(Kampmann-Wagner Numerical)模拟了Al-Cu和Al-Cu-Ag体系多个成分合金在不同温度时效过程中的微结构演化,准确预测了形核、长大及粗化过程中析出相的平均尺寸、体积分数、数密度以及屈服强度和硬度。图90幅,表19个,参考文献310篇更多还原

【Abstract】 The thermal stability and dispersive distribution of nanoscale precipitates are usually the key to obtain the excellent mechanical properties for the age-strengthening alloys.The information of sizedependent phase diagram for nanomaterials could be demonstrated by the thermodynamic investigation of phase diagram considering size effect,which is important to design and develop the novel alloys.CALPHAD(CALculation of PHAse Diagrams)combines the computational simulation with the key experiments to design novel alloys,which can obviously cut down the research and development cycle.What’s more,CALPHAD approach is an important part in the ICME(Integrated Computational Materials Engineering)framework.The reasonable databases are the cornerstone and guarantee of the accurate computational simulation.Since the size dependence on the Gibbs energy is not considered in current multicomponent thermodynamic database,it would cause the deviation in the judgment of thermal stability,then be hard to obtain the nucleation driving force and the results of microstructure evolution simulation during the kinetic precipitation simulation accurately.This work aims to investigate the size-dependent thermodynamic database based on the ICME,then apply to the precipitate microstructure evolution of Al-Cu-Ag system during the aging process:(1)The first-principles calculation combined SQS(Special Quasirandom Structures)method was applied to systematically investigate the molar volume and surface energy of the stable and metastable phases in AlCu-Ag system.The endmember could be reduced and the thermophysical property would be the function of mole fraction by separating the description of thermophysical and thermodynamic model.The databases of molar volume and surface energy for Al-Cu-Ag system were constructed via CALPHAD approach.Considering the influence of excess molar volume and coupling the CALPHAD-type molar volume database,a surface energy calculation code,Be ST-Calc(Butler equation Surface Tension Calculation),was developed.Combined with the reported thermodynamic parameters and the constructed thermophysical database,the molar volume and surface energy of alloy were calculated,which the thermophysical database and the code were validated in comparison with the experimental data.(2)Based on the surface melting of pure metal elements,the effect of solid/liquid interface was taken into account during the melting process.Combined the thermodynamic principle of minimization for Gibbs energy,the calculation code,Cal Na Phad(Calculation of Nano Phase Diagram),was proposed and compiled.Coupling with the input parameters and literature data,the surface melting of macro-crystals and the melting behavior of nanocrystal for pure Al,Cu and Ag were predicted.The surface premelting and melting temperatures of macro-crystals were calculated,respectively.The equilibrium thickness of liquid surface layer would tend to the infinity with the increment of temperature.The corresponding melting behavior and size-dependent phase diagram of nanocrystal were predicted.And with the increment of size,the liquidus and solidus would increase and the liquidus would tend to the bulk melting point.Below the critical size,the solid would melt directly to the liquid while the solid would transform to the solid + liquid two-phase region then melt to the liquid with the particle radius larger than the critical size during the heating process.(3)The molecular dynamics simulation combined various criteria,including potential energy,mean square displacement,gyration radius and radial distribution function,was applied to investigate the melting behavior of Al-Ag and Al-Cu binary fcc＿A1 nanoparticles.And the melting temperature of nanoparticle would decrease with the decrease of size.Considering the solvi of Cu related to the metastable precipitates in Al-Cu binary system from the literature review and the formation enthalpy from the first-principles calculation,the thermodynamic descriptions of the metastable phases,θ" and θ’,were proposed by CALPHAD method.Based on the databases of molar volume and surface energy,the size-dependent thermodynamic description of Cu-Ag,Al-Ag and Al-Cu binary systems was developed by considering the available data from the molecular dynamics simulation and the reported experimental results.With the decrease of size,the size effect on Gibbs energy would gradually increase,and both liquidus and solidus would decrease based on the calculated results by the constructed thermodynamic databases.The eutectic points of L → fcc＿A1(Cu)+ fcc＿A1(Ag)in Cu-Ag system,L → hcp＿A3 + fcc＿A1in Al-Ag system and L → fcc＿A1 + θ in Al-Cu system would move to the Ag-rich,Al-rich and Cu-rich corner,respectively.(4)Based on the above binary thermodynamic databases and the parameters of molar volume and surface energy,the thermodynamic database of Al-Cu-Ag ternary system was established by considering the size effect.The composition of ternary alloy was selected and the corresponding heat treatment process was designed through the analysis of ternary vertical section.Subsequently the hardness and the microstructure evolution of precipitate at different aging time were investigated.Based on the constructed thermodynamic description,kinetic parameters,thermophysical database and key experimental data,the KWN(Kampmann-Wagner Numerical)model was applied to simulate the microstructure evolution of Al-Cu and Al-Cu-Ag system considering the precipitate morphology and aspect ratio.The mean length,number density,volume fraction,yield strength and hardness were predicted during the nucleation,growth and coarsening process.更多还原