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     

压强对液态InGaAs快速凝固过程中微观结构变化的影响

采用分子动力学方法模拟不同压强下液态InGaAs的快速凝固过程,并采用双体分布函数、键角分布函数、配位数统计以及可视化等方法,从微观结构的不同层面分析了压强对凝固过程微观结构的影响机制。结果表明：对于InGaAs体系,压强对最近邻和次近邻的原子排布都有影响,但对次近邻原子排列的影响更为明显,通过次近邻原子键角的调整,使得原子排列更加紧密,体系的短程有序度增强。在原子的配位数结构上,随着压强的增加,部分三配位向四配位发生转变,从而使整个体系达到致密的结构。     

压强对液态InGaAs快速凝固过程中微观结构变化的影响

采用分子动力学方法模拟不同压强下液态InGaAs的快速凝固过程,并采用径向分布函数、键角分布函数、配位数统计以及可视化等方法,从微观结构的不同层面分析了压强对凝固过程微观结构的影响机制.结果表明:对于InGaAs体系,压强对最近邻和次近邻的原子排布都有影响,但对次近邻原子排列的影响更为明显,通过次近邻原子键角的调整,使得原子排列更加紧密,体系的短程有序度增强.在原子的配位数结构上,随着压强的增加,部分三配位向四配位发生转变,从而使整个体系达到致密的结构.     

快速凝固Co-Cu包晶合金的电学性能

研究了Co-Cu包晶合金快速凝固过程中的相选择和组织形成特征, 探索了冷却速率、组织结构和晶体位向与合金电阻率之间的相关规律.实验发现, 快速凝固可使Co在(Cu)中的固溶度扩展至20%.Cu含量大于80%时, L+αCo→(Cu)包晶转变被抑制, (Cu)可从过冷熔体中直接形核析出.Cu含量在40%—70%范围时, Co-Cu合金的液相分离受到抑制, 凝固组织沿条带厚度方向分为两个晶区.细晶区中αCo和(Cu)相竞争形核并生长, αCo枝晶形态细密,细小的（Cu）等轴晶均匀分布于αCo的基体之中.粗晶区αCo相为领先相, 富Cu相分布于αCo枝晶的晶界处.随着冷速的增大, 合金组织显著细化, 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.当晶界散射系数r＝0996—0999时, 可采用M-S模型综合分析快速凝固Co-Cu合金的电阻率.关键词：电阻率快速凝固相结构晶体生长     

快速凝固Ti-Cu-Fe合金的相组成与组织演变规律

采用落管无容器处理技术实现了Ti_61.2Cu_32.5Fe_6.3三元包共晶合金在自由落体条件下的快速凝固,获得了直径为80—1120μm液滴的凝固组织.实验中获得的过冷度范围为34—293 K,最大过冷度达0.23T_L.研究发现,在自由落体条件下,由于受到无容器、微重力、超高真空等因素的影响,合金熔体的凝固组织中包含Cu_0.8Fe_0.2Ti相、CuTi₂相和CuT₃相,显著偏离了平衡状态.Cu_0.8Fe_0.2Ti为初生相,同时又与CuTi₂相形成两相共晶;CuTi₃相则呈现枝晶形貌,并发生了明显的溶质截留效应.随着过冷度的增大,共晶组织由层片共晶向不规则共晶转变,形貌由长条状共晶团变为椭球状共晶团,最终变为球状共晶胞;Cu_0.8Fe_0.2Ti相枝晶形貌由粗大枝晶变为碎断枝晶,进一步变成不规则的粒状晶粒;CuTi₃相枝晶则由碎块状转变为完整枝晶.     

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     

急冷条件下Cu-Pb偏晶合金的相分离研究

研究了Cu-Pb偏晶合金的急冷快速凝固和组织形成规律,并通过将金属熔体的热传导方程和Navier-Stokes方程相耦合, 理论分析了合金熔体的冷却速率、液固相变时间等物理参量与液相分离之间的相关性. 研究结果表明,在急冷快速凝固条件下,熔体的快速冷却对偏晶合金组织形成的影响要比熔体内部液相流动的影响更为显著. 快速凝固使液相分离受到抑制,Cu-Pb偏晶合金均可获得均匀的微观组织结构. 随着冷速的增大,晶粒尺寸明显减小,凝固组织显著细化,晶体形态由粗大枝晶向均匀细小的等轴晶过渡. 提高冷却速率,缩短液固相变时间是重力场中抑制液相分离、获得均匀偏晶组织结构的重要条件. 关键词：偏晶合金快速凝固液相分离微观结构     

急冷条件下NiAl-Mo三元共晶合金的组织形成机制

采用单辊急冷技术实现了NiAl-Mo三元两相共晶合金的快速凝固, 同时与常规条件下的凝固组织进行了对比研究. 实验发现, 单辊急冷的合金条带与常规条件的凝固样品均由B2结构的NiAl金属间化合物和bcc结构的Mo固溶体两相组成, 两相均具有(110)晶面优先生长的趋势, 并呈现出(110)NiAl//(110)Mo取向关系. 常规条件下得到的微观结构主要由规则的两相共晶组织组成, 形成了类似菊花状的共晶胞. 而单辊急冷条件下形成的组织结构主要是由近辊面的柱状晶区和近自由面的等轴晶区组成的凝固组织. 理论计算发现, 合金熔体的单辊辊速由10 m/s增大至50 m/s后, 其冷却速率从1.01×10⁷ K/s逐渐增大到2.46×10⁷ K/s, 冷却速率明显高于常规铸造过程, 因而形成了差别很大的凝固组织. 随着辊速(冷却速率)的增加, 合金条带的厚度从54.4 μm减小至22 μm, 近辊面柱状晶区的厚度所占比例也逐渐增大, 晶粒发生了明显细化.关键词：快速凝固三元共晶共晶转变冷却速率     

Al-Cu-Ge合金的热物理性质与快速凝固规律研究

Al-Cu-Ge合金是典型的三元共晶体系,在工业上有重要的应用价值,对其进行研究有助于了解该合金的热物理性质和提高该合金的结构性能.本文选择了Al55Cu10Ge35,Al70Cu10Ge20和Al80Cu10Ge10三种成分合金作为研究对象,对合金的固态比热和热膨胀系数进行了测量,并对比分析了合金在近平衡凝固和落管快速凝固条件下的组织特征和凝固路径.研究发现,合金比热随Al含量的增大和Ge含量的减少而增大.这三种成分合金的软化温度均为666 K,物理热膨胀系数α在370—650 K温度范围内基本一致,约为1.5×10-5K-1.近平衡凝固条件下合金凝固过程中最后一步反应生成的均为(Al)+(Ge)二相共晶而不是三元共晶,这表明(Al)、(Ge)和CuAl2相在这三种成分的Al-Cu-Ge合金中难以同时形核并协同生长.然而,在快速凝固条件下,初生相的形核和生成受到抑制,合金中更易于形成二相共晶和三元共晶组织.     

冷速对液态金属Mg凝固过程中微观结构演变的影响

采用分子动力学方法对不同冷速下液态金属镁(Mg)快速凝固过程中的微观结构演变进行了模拟研究.并采用能量-温度(E-T)曲线、双体分布函数、Honeycutt-Andersen键型指数法、原子团簇类型指数法(CTIM-3)以及三维可视化等方法系统地考察了凝固过程中微观结构演变与相转变过程.结果发现:在以冷速为1×10~(11)K/s的凝固过程中,亚稳态bcc相优先形成,随后大量解体,其变化规律符合Ostwald规则,系统最终形成以hcp结构为主体与fcc结构共存,中间还夹杂部分bcc结构的致密晶体结构.在1×10~(12)K/s冷速下,结晶过程呈现迟缓现象,形成bcc结构的初始温度降低,系统形成以hcp居多、与bcc和fcc三相共存的结构,且因相互竞争、相互制约而导致不易形成粗大的晶粒结构.而在1×10~(13)K/s冷速下,系统则形成以1551,1541,1431键型为主的多种非晶态基本原子团组成的非晶态结构.此外,在冷速1×10~(12)与1×10~(13)K/s之间的确存在一个形成非晶态结构的临界冷速.     

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)     

Amorphous-crystalline dual-layer structures resulting from metastable liquid phase separation in(Fe₅₀Co₂₅B₁₅Si₁₀)₈₀Cu₂₀ melt-spun ribbons

(Fe50Co25B15Si10)80Cu20 ribbons are prepared by using the single-roller melt-spinning method.A dual-layer structure consisting of a(Fe,Co)-rich amorphous phase and a Cu-rich crystalline phase forms due to metastable liquid phase separation before solidification.The magnetic hysteresis loops of the as-quenched and annealed samples are measured at room temperature.It is indicated that the coercivity of the ribbon is almost zero in the as-quenched state.The crystallization leads to the increase of coercivity and decrease of saturation magnetization.     

Amorphous-crystalline dual-layer structures resulting from metastable liquid phase separation in （Fe50Co25B15Si10）80Cu20 melt-spun ribbons

（Fe50Co25B15Si10）80Cu20 ribbons are prepared by using the single-roller melt-spinning method. A dual-layer structure consisting of a （Fe, Co）-rich amorphous phase and a Cu-rich crystalline phase forms due to metastable liquid phase separation before solidification. The magnetic hysteresis loops of the as-quenched and annealed samples are measured at room temperature. It is indicated that the coercivity of the ribbon is almost zero in the as-quenched state. The crystallization leads to the increase of coercivity and decrease of saturation magnetization.     

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)     

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.     

液态Ti-Al合金的深过冷与快速枝晶生长

采用电磁悬浮和自由落体两种试验技术研究了液态Ti-25 wt.%Al合金的亚稳过冷能力、晶体形核机制和枝晶生长过程. 试验发现, 即使电磁悬浮无容器状态下仍难以消除润湿角θ ≥60°的异质晶核, 合金熔体过冷度可达210 K (0.11T_L). β-Ti相形核的热力学驱动力随过冷度近似以线性方式增大, 其枝晶生长速度高达11.2 m/s, 从而在慢速冷却条件下实现了快速凝固. 理论计算表明, 随着过冷度的逐步增大, β相枝晶生长从溶质扩散控制转变为热扩散控制. 当过冷度超过100 K时, 非平衡溶质截留效应可使合金熔体发生无偏析凝固. 然而, 单靠深过冷状态不足以抑制β相的后续固态相变. 对于落管中快速凝固的直径77-1048 μm合金液滴, 其冷却速率最高达1.05×10⁵ K/s, 深过冷与快速冷却的耦合作用能更有效地调控凝固组织形成过程.