三元Co-Cu-Pb偏晶合金的快速凝固组织形成规律研究

在自由落体条件下实现了三元Co-Cu-Pb合金的液相分离与快速凝固. 实验发现,随液滴直径减小,Co₅₁Cu₄₇Pb₂合金液滴发生由枝晶→两层壳核→枝晶组织的转变,Co₄₇Cu₄₄Pb₉合金液滴的组织形态由壳核组织演化为均匀组织. 两种合金的快速凝固组织均由α(Co),(Cu)和(Pb)固溶体三相组成,α(Co)和(Cu)相主要以枝晶方式生长,(Pb)相分布在(Cu)枝晶间. 关键词： 液相分离 偏晶合金 快速凝固 自由落体     

快速凝固Cu-Pb过偏晶合金的性能表征

研究了Cu-Pb过偏晶合金的急冷快速凝固组织特征,定量表征了快速凝固Cu-Pb过偏晶合金的电阻率和力学性能,理论分析了冷却速率和组织形态对合金性能的影响规律. 研究结果表明,在急冷快速凝固条件下,Cu-Pb过偏晶合金中的(Cu)相和(Pb)相均以枝晶方式生长,晶体形态以均匀细小的等轴晶为特征. 随着冷却速率增大,一方面,凝固组织显著细化,晶界增多,对自由电子的散射作用增强,合金电阻率显著增大;另一方面,细晶强化作用增强,合金的抗拉强度呈线性升高,同时,伴随着晶体缺陷数量的增多,合金的伸长率降低. 关键词： Cu-Pb过偏晶合金 快速凝固 电阻率 力学性能     

偏晶合金液-液相分离的格子玻尔兹曼方法模拟

本文建立了一个模拟在弥散相液滴的扩散长大、碰撞凝并和Ostwald熟化等因素的作用下偏晶合金液-液相分离过程的二维格子玻尔兹曼方法 (lattice Boltzmann method, LBM) 模型.该模型结合了Shan-Chen的两相流模型和Qin的介观粒子相互作用势模型的优点,并在LB演化方程中引入了反映相变的源项.应用该模型模拟研究了偏晶合金液-液相分离过程中单液滴的生长、两液滴的合并和多液滴的生长规律.结果表明在两液相区中第二相单个液滴的生长是一个通过扩散从非平衡态到平衡态过渡的过程.两液滴合并 关键词： 偏晶合金 液-液相分离 格子玻尔兹曼方法     

偏晶溶液相分离过程的实时观测研究

为了揭示自由落体条件下偏晶合金壳核组织的形成机制，基于相似性原理设计了一种环形温度场，对丁二腈-52.6mol%H₂O偏晶溶液的相分离过程进行了实时观测. 发现在两个不混溶液相的分离过程中，富水相液滴经历了“析出→迁移→凝并→聚集"运动过程，最终形成以富水相为中心的两层壳核组织. 同时测定了液滴的运动速率，并对Marangoni迁移速率进行了理论计算，两者能够较好地符合. 由此证实了在偏晶溶液的相分离过程中，第二液相主要是在温度梯度的驱动下产生Marangoni迁移. 这一实验直观地再现了落管无容器处理过程中液滴内部的相分离过程. 关键词： 相分离 偏晶溶液 Marangoni迁移 微重力     

液态三元Fe-Sn-Si/Ge偏晶合金相分离过程的实验和模拟研究

本文采用自由落体实验技术和格子玻尔兹曼计算方法研究了低重力条件下液态Fe-Sn-Si/Ge合金的相分离过程. 实验发现, 二种合金液滴在自由下落过程中均发生显著的液相分离, 形成了壳核和弥散组织. 当Fe-Sn-Si合金中的Si元素被等量的Ge元素替换后, 壳核组织中富Fe区和富Sn区的分布次序会发生反转. 计算表明, 在液相分离过程中冷却速率、Marangoni对流和表面偏析对壳核构型的选择和弥散组织的形成起决定性作用.     

快速凝固Fe62.1Sn27.9Si10合金的分层组织和偏晶胞形成机理

采用自由落体和单辊急冷技术研究了三元Fe_62.1Sn_27.9Si₁₀偏晶合金的相分离和组织形成规律,理论分析了两种快速凝固条件下合金的传热特性.自由落体条件下,由于Marangoni迁移和表面偏析势的作用,液滴凝固组织主要形成富Sn相包裹富Fe相的两层壳核结构.随着液滴直径减小,冷却速率和温度梯度增大,促进偏晶胞快速生长.在单辊急冷条件下,随着辊速的增大,冷却速率从1.1×10⁷增大至6.5×10⁷ K/s,合金熔体内部的液相流动和相分离受到抑制,凝固组织发生"九层结构→两层结构→无分层结构"的转变.同时,凝固过程中FeSn+L₂→FeSn₂包晶反应受到抑制,形成与自由落体条件下不同的相组成.EDS分析显示,αFe相在快速凝固过程中发生显著溶质截留效应. 关键词： Fe-Sn-Si偏晶合金 相分离 快速凝固 溶质截留     

深过冷Cu60Sn30Pb10偏晶合金的快速凝固

实现了大体积Cu60Sn30Pb10偏晶合金的深过冷与快速凝固.实验获得的最大过冷度为173 K(0.17TL).凝固组织发生了明显的宏观偏析,XRD分析表明,试样上部是由固溶体(Sn),(Ph)相和金属间化合物ε(Cu3Sn)相组成的三相区,下部为富(Ph)相区.在小过冷条件下,三相区中ε(CuSn)相的凝固组织为粗大的枝晶,随着过冷度的增大,ε(Cu3Sn)相细化成层片状组织.且层片间距随过冷度的增大而减小,而(Sn),(Ph)两相始终以离异共晶的方式存在.富(Pb)相区中分布有少量的ε(Cu3Sn)枝晶,枝晶长度随过冷度的增大而增大,且在大过冷条件下发生碎断.(Sn)相在ε(Cu3Sn)相表面形核、长大,其形态类似于包晶凝固组织.     

无容器条件下Cu-Pb偏晶的快速生长

在落管无容器条件下实现了Cu-10%Pb亚偏晶和Cu_374%Pb偏晶的快速生长. 发现随着液滴 过冷度的增大, 亚偏晶中初生(Cu)相的生长形态经历“粗大枝晶→碎断枝晶→等轴晶”的转 变. 偏晶的组织形态从多个偏晶胞组织演化为单个偏晶胞组织. 理论计算表明，直径在1000 —60 μm之间的亚偏晶和偏晶合金液滴, 最大过冷度分别为269 K (02 TL_L )和245 K (02 TM_M). 亚偏晶合金中初生(Cu)枝晶的最大生长速度为24 m/s , 关键词： 无容器处理 深过冷 晶体生长 相分离     

深过冷Cu60Sn30Pb10偏晶合金的快速凝固

实现了大体积Cu₆₀Sn₃₀Pb₁₀偏晶合金的深过冷与快速凝固. 实验获得的最大过冷度为173 K(0.17T_L). 凝固组织发生了明显的宏观偏析,XRD分析表明,试样上部是由固溶体(Sn),(Pb)相和金属间化合物ε(Cu₃Sn)相组成的三相区,下部为富(Pb)相区. 在小过冷条件下,三相区中ε(Cu₃Sn)相的凝固组织为粗大的枝晶,随着过冷度的增大,ε(Cu₃Sn)相细化成层片状组织,且层片间距随过冷度的增大而减小,而(Sn),(Pb)两相始终以离异共晶的方式存在. 富(Pb)相区中分布有少量的ε(Cu₃Sn)枝晶,枝晶长度随过冷度的增大而增大,且在大过冷条件下发生碎断. (Sn)相在ε(Cu₃Sn)相表面形核、长大,其形态类似于包晶凝固组织. 关键词： 深过冷 快速凝固 偏晶合金 层片组织     

电磁复合场制备匀质Zn-Bi偏晶合金的物理模拟

本文通过物理模拟的方法研究了强磁场复合交变电流作用对过偏晶合金凝固组织的影响机理,考察了电磁复合场下第二相初生液滴直径的演变规律,以及洛伦兹力作用下已分层少数相被"打碎"进入基体相的过程.结果表明:电磁体积力的增加,有利于液滴初始直径的减小;而交变电流频率的影响存在最佳值,偏离该值对初生液滴直径的抑制效果下降,这与实际试验规律一致;在电磁复合场作用下,已经分层的两液相可以重新实现均匀混合.这些效应有效缓解了偏晶合金凝固前第二相由于各种因素导致的偏聚,模拟结果直观地揭示了电磁复合场制备匀质偏晶合金的主要机理 关键词： 电磁复合场 偏晶合金 物理模拟     

Metastable phase separation and rapid solidification of undercooled Co-Cu alloy under different conditions

The metastable liquid phase separation and rapid solidification behaviours of Co_61.8Cu_38.2 alloy were investigated by using differential thermal analysis (DTA) in combination with glass fluxing, electromagnetic levitation (EML) and drop tube techniques. It is found that the liquid phase separation process and the solidification microstructures intensively depend on the experimental processing parameters, such as undercooling level, cooling rate, gravity level, liquid surface tension and the wetting state of crucible. Large undercooling and surface tension difference of the two liquids tend to facilitate further separation and cause severe macrosegregation. On the other hand, rapid cooling and low gravity effectively suppress the coalescence of the minority phase. Severe macrosegregation patterns are formed in the bulk samples processed by both DTA and EML. In contrast, disperse structures with fine spherical Cu-rich spheres homogeneously distributed in the matrix of Co-rich phase have been obtained in drop tube.     

Heat and mass transfer characteristics during rapid solidification of Fe-Cu peritectic alloys

The viscose flow and microstructure formation of Fe-Cu peritectic alloy melts are investigated by analyzing the velocity and temperature fields during rapid solidification, which is verified by rapid quenching experiments. It is found that a large temperature gradient exists along the vertical direction of melt puddle, whereas there is no obvious temperature variation in the tangent direction of roller surface. After being sprayed from a nozzle, the alloy melt changes the magnitude and direction of its flow and velocity rapidly at a height of about 180 μm. The horizontal flow velocity increases rapidly, but the vertical flow velocity decreases sharply. A thermal boundary layer with 160–300 μm in height and a momentum boundary layer with 160–240 μm in thickness are formed at the bottom of melt puddle, and the Reynolds number Re is in the range of 870 to 1070 in the boundary layer. With the increase of Re number, the cooling rate increases linearly and the thickness of thermal boundary layer increases monotonically. The thickness of momentum boundary layer decreases slowly at first, then rises slightly and decreases sharply. If Re < 1024, the liquid flow has remarkable effects on the microstructure formation due to dominant momentum transfer. The separated liquid phase is likely to form a fiber-like microstructure. If Re>1024, the heat transfer becomes dominating and the liquid phase flow is suppressed, which results in the formation of fine and uniform equiaxed microstructures. Supported by the National Natural Science Foundation of China (Grant Nos. 50121101 and 50395105)     

Solute redistribution profiles during rapid solidification of undercooled ternary Co-Cu-Pb alloy

The solute redistribution and phase separation of liquid ternary Co-35%Cu-32.5%Pb immiscible alloy have been investigated using glass fluxing method. A bulk undercooling of 125 K was achieved and the macrosegregation pattern was characterized by a top Co-rich zone and a bottom Cu-rich zone. The average solute contents of the two separated zones decreased with the increase of undercooling, except for the solute Pb in Cu-rich zone. With the enhancement of undercooling, a morphological transition from dendrites into equaxied grains occurred to the primary α(Co) phase in Co-rich zone. The solute redistribution of Cu in primary α(Co) phase was found to depend upon both the undercooling and composition of Co-rich zone. Stokes migration is shown to be the main dynamic mechanism of droplet movement during liquid phase separation.     

Phase separation and rapid solidification of liquid Cu<Subscript>60</Subscript>Fe<Subscript>30</Subscript>Co<Subscript>10</Subscript> ternary peritectic alloy

The metastable liquid phase separation and rapid solidification of Cu₆₀Fe₃₀Co₁₀ ternary peritectic alloy were investigated by using the drop tube technique and the differential scanning calorimetry method. It was found that the critical temperature of metastable liquid phase separation in this alloy is 1623.5 K, and the two separated liquid phases solidify as Cu(Fe,Co) and Fe(Cu,Co) solid solutions, respectively. The undercooling and cooling rate of droplets processed in the drop tube increase with the decrease of their diameters. During the drop tube processing, the structural morphologies of undercooled droplets are strongly dependent on the cooling rate. With the increase of the cooling rate, Fe(Cu,Co) spheres are refined greatly and become uniformly dispersed in the Cu-rich matrix. The calculations of Marangoni migration velocity (V _M) and Stokes motion velocity (V _S) of Fe(Cu,Co) droplets indicated that Marangoni migration contributes more to the coarsening and congregation of the minor phase during free fall. At the same undercooling, the V _M/V _S ratio increases drastically as Fe(Cu,Co) droplet size decreases. On the other hand, a larger undercooling tends to increase the V _M/V _S value for Fe(Cu,Co) droplets with the same size. Supported by the National Natural Science Foundation of China (Grant Nos. 50121101 and 50395105) and the Scientific and Technological Creative Foundation of Youth in Northwestern Polytechnical University of China (Grant No. W016223)     

Al-Mg合金熔体快速凝固过程中微观结构演化机理的模拟研究

采用分子动力学方法对Al₅₀Mg₅₀合金熔体的快速凝固过程进行了模拟,并采用双体分布函数、键型指数法和原子团类型指数法等方法,从微观结的不同层面对Al-Mg合金熔体快速凝固过程中微观结构的演化机理进行了深入的分析研究.结果表明：本模拟所获得的Faber-Ziman偏结构因子与实验结果符合较好.Al₅₀Mg₅₀合金熔体具有遗传性,在快速凝固过程形成了非晶态结构,其中二十面体短程序结构对非晶态结构的形成起决定性作用.基于原 关键词： Al-Mg合金熔体 快速凝固过程 分子动力学模拟 微观结构     

急冷快速凝固过程中液相流动与组织形成的相关规律

研究了Fe58wt%Sn过偏晶合金的急冷快速凝固和组织形成特征． 实验发现, FeSn过偏晶合金的急冷快速凝固组织由规则排布的纤维状β-Sn相和分布其间的α-Fe相及少量金属间化合物相组成, β-Sn相的几何排列方向与合金条带表面成0—15°的夹角．根据急冷条件下金属熔体的热传导方程和Navier-Stokes方程, 对过偏晶合金的凝固行为和组织形成过程进行了理论分析, 揭示出熔体内部的动量传输对过偏晶合金的液相分离行为具有显著的影响．两相分离发生于液池底部约200μm的急冷区内, 分离的L2液滴在辊面驱 关键词： 液态 相分离 液相流动 快速凝固 晶体生长