Microscopic Phase-field Simulation of Competition Mechanism Between Ll_2 and D0_(22) Structure in Ni-Cr-Al Alloy

Simulations are performed on temporal evolution of atom morphology and ordering parameters of Ni-14.5 Cr-16.5 Al alloy during early precipitation process at different temperatures based on microscopic phase-field theory; the relationship between precipitation sequence and mechanism of L12 and D022 structure and precipitation temperature are illuminated. The nonstoichiometric ordered L12 phases appear first with congruent ordering+spinodal decomposition mechanism which is then followed by precipitation of D022 phases at ordering domain boundaries of L12 phases by spinodal decomposition mechanism at 1073 K and 1223 K. The nonstoichiometric Ll2 phases transform to stoichiometric ordering phases gradually. The incubation period of L12 and D022 phases is shorter at 1073 K than that 1223 K, and growth speed is higher at 1073 K. At 1373 K, L12 and D022 phases appear simultaneously by non-classical nucleation and growth mechanism. After that the particles of D022 phases diminish and disappear gradually; L12 phases grow and single L12 phases are remained at last.     

Computer Simulation of the Interphase Boundary Evolution in Ni75AlxV25-x Alloy

The interphase boundary evolution of ordered phase in Ni75AlxV25-x alloy during precipitation was simulated on atomic-scale based on the microscopic phase-field dynamic model. The results show that the second phase precipitated from the interphase boundary formed by the first phase and the disordered matrix at high temperature, and from the interphase boundaries of the first phase at low temperature. L12 phase had obvious selective orientation when precipitated from the interphase boundaries of D022- L12 phase nucleated easily at the interphase boundaries formed by [10] and [01] directions of D022 projection along [001] direction, and grew easily at [10] direction. There was no the selective orientation when L12 phase precipitated from the interphase boundary formed by D022 and the disordered matrix. D022 phase had the selective orientation when precipitated from the interphase boundaries of L12, and grew along the [10] direction.     

陶瓷烧结过程孔隙演化的二维相场模拟

采用二维相场模型模拟陶瓷烧结过程中颗粒间孔隙的演化过程.选取四方堆积颗粒间气孔作为对象.通过连续的密度场和长程取向场（LRO）描述烧结体的微结构,密度场的演化由Cahn-Hillard（CH）方程控制,而颗粒的取向场演化由时间相关的Ginzburg-Laudau（TDGL）方程控制.上述非线性演化方程利用半隐傅立叶频域法求解.模拟结果反映了颗粒间接触,烧结颈生长和气孔球化的微观过程.量化计算烧结颈生长率以及在不同晶界和表面迁移率比值时的烧结率,较好地符合理论分析的趋势.     

过冷熔体凝固行为的自适应有限元相场法模拟

通过自适应有限元法求解相场模型,对过冷熔体中自由枝晶生长行为进行了数值模拟.自适应有限元法可减少计算量,提高求解速度,并同时降低存储空间的需求.模拟结果表明,随着各向异性系数ε4的增加,枝晶生长速度Vtip线性增大,而尖端半径rtip以指数方式减小.进一步研究表明界面稳定性系数σ*随各向异性系数ε4的增大而增大.     

Numerical Simulation of Solidification Microstructure and Effects of Phase-field Parameters on Grain Growth Morphologies

1. Introduction Due to the stunning development of the computer performance and numerical solution techniques, great progress has been made in numerical simulation of the solidification microstructure since the last three decades. Among a variety of numerical techniques used to simu- late microstructure, the phase-field method is considered to be a viable computational tool, by which realistic pat- terns of grain growth can be freely obtained. The ap- peal of the phase-field method can be expr…     

振动系统参数识别的ARMA模型与Ibrahim-Randec模型的关系

本文研究了用多变量ARMA模型描述振动系统的受迫响应进行参数识别的数学理论,给出随机减量函数的某些性质,指出了对受白噪声激励下的ARMA模型同Ibrahim-Randec之间的关系.     

EES在有机朗肯循环系统热力特性仿真研究中的应用

有机朗肯循环（OrganicRankineCycle,ORC）是在传统朗肯循环中采用有机工质（~IRl13,R123等）代替水推动膨胀机做功的循环,本文根据集总参数法和能量守恒定律建立有机朗肯循环系统的数学模型,运用软件工程方程求解器（EngineeringEquationSolver,EES）进行仿真研究,得到不同工况下有机朗肯循环系统热力特性的变化规律。研究结果表明,提高系统蒸发压力、降低系统冷凝压力以及选择效率尽可能高的膨胀机,可以提高有机朗肯系统循环热效率。仿真计算结果与实验数据二者吻合较好,表明所发展的数学模型可以满足有机朗肯循环热力系统仿真的要求。     

高熵合金FeCrCoNiMn热浸铝熔体的界面结构及组织形成机制研究

用真空电弧炉制备了铸态高熵合金FeCrCoNiMn(HEA),将高熵合金在700℃铝熔体中进行了不同时间(0~1h)的保温热浸实验,分析了HEA-Al固/液界面反应的组织演变及形成机理。结果表明,高熵合金在铝熔体中热浸反应时,其界面附近可形成由Al_(86)Cr_(13.5)Fe_(6.5)、Al_(86)Mn_(14)、Al_3Ni和Al_9Co_2多种富铝金属间化合物组成的反应层、富铝块体相以及含Fe和Ni的富铝层状析出相和网状结构组织。热浸初期,界面反应层的形成和长大主要受高熵合金表面元素的溶解和脱嵌过程控制,此时反应层和块体相的形成和长大主要位于铝溶体一侧且反应层界面迁移速率较快;形成反应层后,反应层厚度随热浸反应时间的延长而增大,当反应时间t10min后,反应层厚度基本维持在20μm左右不再变化,此时反应层界面迁移速率有所变缓。     

重膜包装机工位转换机构结构设计及运动仿真

目的设计一种工位转换机构,使重膜包装机实现更加精确平稳的取袋送袋和取包送包。方法通过分析工位转换机构的工作,建立相关数学模型,并应用SolidWorks软件建立简化机构模型,然后利用Motion分析中的"跟踪路径"功能设计凸轮理论轮廓曲线,最后建立机构的三维实体模型,并进行仿真分析。结果相对运动的2个摆臂没有发生干涉,其运动学曲线轨迹过渡平稳、没有突变。结论工位转换机构具有良好的运动特性,能够满足工作要求。     

Study on Modification Mechanism of Rare Earth in ZA27 Cast Alloy with Electronic Theory and Molecular Dynamics Method

A model of liquid ZA27 cast alloy is established according to molecular dynamics theory and an atomic structural model of co-existent a phase and liquid is also presented by means of computer programming. Recursion method is adopted to calculate the electronic structure of RE (rare earth) in grains and around phase boundaries respectively. The calculation shows that RE is more stable around phase boundaries than in grains, which explains the fact that the solution of RE in a phase is less, and RE mainly aggregates in front of phase boundary. The calculations of bonding order integrals also show that RE in front of phases hardly solidify onto the grain surfaces as active element so as to prevent grains growth and refine the grains. As a result, the modification mechanism of RE may be explained from the view of electronic structure.     

晶场劈裂序参量导致稀土-硼十二化合物中磁性和比热反常的理论研究

在低温条件(T_C≈3~22 K)下, 除了4f电子满壳层的LuB₁₂之外, 其他稀土-硼十二化合物(简称RB₁₂, R为稀土元素Tb, Dy, Ho, Er, Tm)存在磁性和比热的反常现象, 该反常现象的机制至今仍存争议。尽管磁化率曲线上的隆起与R³⁺离子的反铁磁序类似, 但是中子衍射谱和莫斯堡尔谱的实验均没有明确给出稀土离子的磁结构。这些化合物磁化率、熵的反常行为和低温中子谱的结果都与RB₁₂具有幅度调制的磁结构一致。本课题组采用模型和数值计算相结合的方法来探究该反常行为的物理机制。研究表明: 由于稀土离子meV量级的晶场劈裂易受热涨落扰动, 晶场劈裂能不再是一个量子力学参量, 而是一个热力学序参量。物理图像中轨道角动量逐渐淬灭很好地解释了磁性和比热的反常现象。结合该类晶体低温下在(111)晶向的二聚化行为和非弹性散射谱的结果, 对中子散射谱中的双峰结构也给予了一个可能的解释。