Ternary Eutectic Growth in a Highly Undercooled Liquid Alloy

The crystal nucleation and growth mechanism during the formation of Ag42.4 Cu21.6 Sb36 ternary eutectic are investigated under substantial undercoo]ing conditions. The x-ray diffraction analysis shows that the solidified eutectic phases are not invo]ved (Ag) within a wide undercooling range of 6-114 K. This indicates that under high undercooling condition, the phase constitution of Ag-Cu-Sb eutectic is different from that in the equilibrium phase diagram. With the increase of undercoo]ing, the crystalline morpho]ogy of Ag42.4 Cu21.6 Sb36 alloy transforms from the mixed structure of primary θ(Cu2Sb), two pseudobinary eutectics and (ε θ Sb) ternary eutectic into a unique (ε θ Sb) ternary eutectic. The calculated results indicate that θ(Cu2Sb) is the leading nucleating phase among the three eutectic phases. In addition, the growth morpho]ogy of primary ε(Ag3Sb) compound in Ag60Cu6Sb34 alloy exhibits the characteristics of solid solution and its orthorhombic dendrite grows along the (111) directions.     

深过冷Ag-Cu-Ge三元共晶合金的相组成与凝固特征

在Ag_38.5Cu_33.4Ge_28.1三元共晶合金的深过冷实验中，获得的最大过冷度为175 K(0.22T_E). XRD分析表明，不同过冷条件下其共晶组织均由(Ag),(Ge)和η(Cu₃Ge)三相组成. 在小过冷条件下，三个共晶相协同生长，凝固组织粗大.随着过冷度的增大，共晶组织明显细化，(Ge)相与其他两相分离，以初生相方式生长，而(Ag)相与η相始终呈二相层片共晶方式共生生长. 当过冷度超过80 K时，初生相(Ge)由小过冷时的块状转变为具有小面相特征的枝晶方式生长. 部分小面相(Ge)枝晶出现规则的花状，花瓣数介于5—8之间，并且过冷度越大(Ge)相越容易分瓣. 花状(Ge)枝晶的晶体表面为{111}晶面簇，择优生长方向为〈100〉晶向族. 关键词： 三元共晶 晶体形核 深过冷 快速凝固     

液态三元Fe-Cr-Ni合金中快速枝晶生长与溶质分布规律

采用落管方法实现了液态三元Fe-Cr-Ni合金的深过冷与快速凝固,合金液滴的冷却速率和过冷度均随液滴直径的减小而迅速增大.两种成分合金近平衡凝固组织均为粗大板条状α相.在快速凝固过程中,不同直径Fe_(81.4)Cr_(13.9)Ni_(4.7)合金液滴凝固组织均为板条状α相,其固态相变特征很明显,随着过冷度增大,初生δ相由具有发达主干的粗大枝晶转变为等轴晶.Fe_(81.4)Cr_(4.7)Ni_(13.9)合金液滴凝固组织由α相晶粒组成,随着过冷度增大,初生γ相由具有发达主干的粗大枝晶转变为等轴晶,其枝晶主干长度和二次分枝间距均显著下降,晶粒内溶质的相对偏析度也明显减小,溶质Ni的相对偏析度始终大于溶质Cr.理论计算表明,与γ相相比,δ相枝晶生长速度更大.在实验获得的过冷度范围内,两种Fe-Cr-Ni合金枝晶生长过程均由热扩散控制.     

Metastable Phase Separation and Concomitant Solute Redistribution of Liquid Fe-Cu-Sn Ternary Alloy

Liquid Fe-Cu-Sn ternary alloys with lower Sn contents are usually assumed to display a peritectic-type solidification process under equilibrium condition. Here we show that liquid Fe47.5Cu47.5Sn5 ternary alloy exhibits a metastable immiscibility gap in the undercooling range of 51-329 K （0.19TL）. Macroscopic phase separation occurs once undercooling exceeds 196 K and causes the formation of a floating Fe-rich zone and a descending Cu-rich zone. Solute redistribution induces the depletion of Sn concentration in the Fe-rich zone and its enrichment in the Cu-rich zone. The primary Fe phase grows dendritically and its growth velocity increases with undercooling until the appearance of notable macrosegregation, but will decrease if undercooling further increases beyond 236 K. The microsegregation degrees of both solutes in Fe and Cu phases vary only slightly with undercooling.     

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.     

Amorphous transformation of ternary Cu45Zr45Ag10 alloy under microgravity condition

The influences of undercooling rate and cooling rate on the microstructural evolution of ternary Cu₄₅Zr₄₅Ag₁₀ alloy using single-roller melt spinning and drop tube are investigated. The rapidly quenched alloy ribbons achieve a homogeneous glass structure. The microstructure of the droplets transforms from the Cu₁₀Zr₇ dendrites plus (Cu₁₀Zr₇+AgZr) eutectic into Cu₁₀Zr₇ dendrite with the decrease of droplet diameter. As the diameter decreases to 180 μm, the Cu₄₅Zr₄₅Ag₁₀ alloy changes from crystal to amorphous structure, showing that the cooling rate is not the only influence factor and the undercooling play a certain role in the forming of the amorphous alloy at the same time under microgravity condition.     

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.     

微重力下Fe-Al-Nb合金液滴的快速凝固机理及其对显微硬度的影响

采用落管无容器处理技术实现了Fe_(67.5)Al_(22.8)Nb_(9.7)三元合金在微重力条件下的快速凝固,获得了直径为40—1000μm的合金液滴.实验中合金液滴的过冷度范围为50—216 K,冷却速率随着液滴直径的减小由1.23×10~3K·s~(-1)增大到5.53×10~5K·s~(-1).研究发现,Fe_(67.5)Al_(22.8)Nb_(9.7)合金液滴的凝固组织均由Nb(Fe,Al)_2相和(αFe)相组成,且随着液滴直径的减小,初生Nb(Fe,Al)_2相由树枝晶转变为等轴晶,共晶组织发生了约3倍的细化且生长特征由层片共晶向碎断共晶转变;硬质初生Nb(Fe,Al)_2相的析出有效提高了合金的显微硬度.与电磁悬浮条件下同过冷合金的凝固组织对比发现,落管条件下的合金液滴凝固组织更细化,使得合金显微硬度提高了2%—6%.     

落管中Ni-Fe-Ti合金的快速凝固机理及其磁学性能

采用落管自由落体方法实现了Ni_(45)Fe_(40)Ti_(15)合金在微重力无容器条件下的快速凝固,获得了直径介于160—1050μm的合金液滴.理论计算表明冷却速率及过冷度随液滴直径减小而增大,并呈指数函数关系,实验获得的最大过冷度为210 K(0.14 T_L).随着过冷度增大,凝固组织中粗大的γ-(Fe,Ni)枝晶逐渐细化,二次枝晶间距减小,溶质Ti在γ-(Fe,Ni)相中的固溶度显著扩展.对不同直径合金液滴的凝固样品进行磁学性能分析,结果表明随着凝固合金液滴直径减小,其饱和磁化强度增大,矫顽力减小,矩形比下降,软磁性能明显提高.