Effect of far-field flow on a columnar crystal in the convective undercooled melt

The growth behavior of a columnar crystal in the convective undercooled melt affected by the far-field uniform flow is studied and the asymptotic solution for the interface evolution of the columnar crystal is derived by means of the asymptotic expansion method. The results obtained reveal that the far-field flow induces a significant change of the temperature around the columnar crystal and the convective flow caused by the far-field flow accelerates the growth velocity of the interface of the growing columnar crystal in the upstream direction and inhibits its growth velocity in the downstream direction. Our results are similar to the experimental data and numerical simulations.

Relationship between columnar crystal spacing and electric current density in unidirectional solidification of monophase Cu-Al alloy

On the basis of previous theoretical inferential relationship between the columnar crystal spacing and the density of electric current applied during unidirectional solidification, the effect of current density on the columnar crystal spacing was discussed and analyzed, and the experiment was made to verify the theoretical relationship. The results show that at fast solidification speed the columnar crystal spacing decreases with increasing the density of electric current, while at slow solidification speed the columnar crystal spacing increases with increasing the density of electric current. The critical conditions for the evolution of columnar crystal spacing were confirmed. The calculated values concerning the spacing and the density are consistent with the experimental results.

Solute Distribution in Columnar Crystal Zone and Influences of Correlative Factors for Continuous Casting Slabs

The expression of the solute distribution in columnar crystal zone was deduced when the solid-liquid interface bended periodically, and the quantitative calculations of macrosegregation were also made in the process of the continuous casting. The solute distribution along the thickness direction of the slabs was obtained, which verified the theoretical calculation. The effect of the bulge size, solidification speed, and solidification shrinkage speed on macrosegregation of the slabs was calculated. It can be concluded that normal segregation and negative segregation alternatively appear as a result of the bulge. The normal segregation exponentially depends on the bulge size, and the negative segregation linearly depends on the bulge size. The extent of the normal segregation is greater than that of the negative segregation when the bulge size is the same. The macrosegregation of the same position along the thickness direction of the slabs changes in a sine wave with increasing the solidification rate, and the amplitude is larger at the casting blank center. The normal segregation linearly decreases with increasing the solidification shrinkage speed, and when the solidification shrinkage speed exceeds a critical value, the segregation appears negatively and increases linearly.

Deformation behavior of Cu-12wt%Al alloy wires with continuous columnar crystals in dieless drawing process

The microstructure and mechanical properties of Cu-12wt%Al alloy wires which are composed of continuous columnar crystals after dieless drawing forming at drawing speed of 1.0–1.4 mm/s and deformation temperature of 600–900°C were analyzed, and deformation behavior of the alloy during dieless drawing forming was experimentally investigated. The results showed that in the above-mentioned conditions, recrystallization phenomenon was not found during dieless drawing forming. When a drawing speed of 1.0 mm/s was used, the grain boundaries were out of straight gradually with increasing deformation temperature from 600°C to 900°C, and tensile strength of the dieless drawn Cu-12wt%Al alloy wires increased while elongations decreased with increasing deformation temperature. At drawing speed of 1.1–1.2 mm/s and deformation temperature of 600°C, the effect of dieless drawing forming process on the microstructure of the alloy was inconspicuous, and when drawing speed was up to 1.3–1.4 mm/s, the grain boundaries of continuous columnar crystals became zigzag while there was little effect of drawing speed of 1.1–1.4 mm/s on the elongation and tensile strength of the alloy wires.

Lyotropic Liquid Crystal Electrochemical Polymerization of Thiophene-Based Monomers: Polymerization in Cholesteric Liquid Crystal and Columnar Phase

Synthesis of conductive polymer poly([thiophene]-[benzo[1,2,3]thiadiazole] [thiophene]) (abbreviated as P(T-Btdaz-T)) was achieved by electrochemical polymerization in hydroxypropyl cellulose (HPC)/N,N-dimethylformamide (DMF) in liquid crystal state. The polymer thus obtained shows fingerprint texture, which is derived from helical structure of the HPC in cholesteric liquid crystal state. Fourier transform infrared spectroscopy measurements revealed that the polymer film is P(T-Btdaz-T)/HPC composite. Circular dichroism optical absorption spectroscopy measurements show that the polymer has the optical activity. Next, electrochemical polymerization of 3,4-ethylene-dioxythiophene (EDOT) was carried out in columnar phase liquid crystal. The polymer transcribes the columnar structure and shows optical structure resembling columnar liquid crystal electrolyte solution.

TiAl金属间化合物定向技术研究进展

TiAl合金具有低密度和良好的高温性能,被视为航空航天领域极具应用潜力的一类新型结构材料。具有柱晶/单晶组织特征的全片层结构TiAl合金展现出极为优异的高温综合力学性能。本文首先介绍了定向TiAl合金的发展背景、定向组织、力学性能各向异性等基本概念;然后,基于工作原理、关键技术、组织特征三个层面,对传统Bridgman法、光学浮区法、电磁约束以及电磁冷坩埚等定向技术进行了论述;进一步,结合最新研究成果,对定向退火制备TiAl合金的工作进行了阐述;最后,展望了TiAl合金定向技术的研究方向,指出需要从优化成分设计、组织/相变可控、大尺度定向晶稳定制备等方面发展新原理与新技术。

柱状晶生长取向对定向凝固Mg-Gd合金拉伸和压缩性能的影响

利用定向凝固技术,制备了生长取向为〈2110〉的柱状多晶Mg-6.3Gd合金和生长取向为〈3214〉的柱状多晶Mg-8.2Gd合金,并研究加载方式(拉伸、压缩)、柱状晶取向与力学性能之间的相关性。结果表明:拉伸载荷下,〈2110〉取向柱状晶的基面〈a〉滑移Schmid因子(Fs)低于0.051、柱状晶内启动{1011}压缩孪生协调应变,因此,Mg-6.3Gd合金屈服强度较高(为176 MPa),但伸长率仅8%;〈3214〉取向柱状晶的基面〈a〉滑移Fs高于0.46、柱状晶内启动{1012}拉伸孪生协调应变,因此,Mg-6.3Gd合金屈服强度较低(为57 MPa),但伸长率达到22%。压缩载荷下,两种取向合金启动{1121}和{1012}两种拉伸孪生协调应变,具有较高的形变硬化能力,抗压强度(分别为309 MPa和301 MPa)较屈服强度分别提高214 MPa和219 MPa,且具有较高的塑性。

基于相场法模拟VAR强迫流场中柱状晶生长

基于相场法建立了流-热-质全耦合模型,模拟了真空自耗电弧重熔(VAR)过程铸锭柱状晶在强制流场中的生长。对比柱状晶在有无流场条件下的生长特征解析熔体流动对柱状晶生长的影响机制。模拟结果表明:凝固前沿流场速度大小为45μm/s,方向与枝晶生长方向垂直时,二次枝晶表现出“迎流”生长,枝晶干受冲刷作用向下游倾斜;相邻枝晶形成细小涡流,枝晶干上游侧溶质和热量随熔体带入下游侧,上游侧溶质和温度边界层厚度减小,由无流场时的51μm降至31.5μm,温度梯度大促进二次枝晶发展;下游侧边界层溶质和热量富集程度大,边界层厚度增至78μm,梯度减小明显抑制二次枝晶发展。下游溶质浓度高于无流场情况,枝晶生长缓慢增大微观偏析风险。

奥氏体不锈钢板焊缝底片影像分析

针对奥氏体不锈钢板对接焊缝射线检测中,底片影像上白色条纹性质的判断问题,通过提高管电压、改变透照角度、解剖截面液体渗透试验进行分析,证明该影像是焊缝中间区域柱状晶产生的衍射斑纹,判定不是缺陷。

Crystallization Behavior of High Purity Fused Magnesia

The chemical composition and microstructure of Tibetan microcrystalline magnesite were studied by means of XRF,XRD and FESEM. It is found that the microcrystalline magnesite has uniformly distributed grains,mostly in the range of 2-4 μm,and the MgO content is47. 80%. In fused magnesia,the druses are colorless transparent periclase megacrysts without impurity,while the crust sand is white,with a high MgO content up to97%-99%. The periclase crystals are big,appearing as faced and step-growth morphology. Massive secondary long columnar periclase is crystallized.

PHASE STRUCTURES AND TRANSITION BEHAVIORS OF A TRIPHENYLENE DISCOTIC LIQUID CRYSTAL

The phase behaviors and structures of a triphenylene-derived discotic liquid crystal （LC） hexa-n-octoxyl- triphenylene （C8HET） were studied using the combined techniques of differential scanning calorimetry （DSC）, wide angle X-ray diffraction （WAXD）, selected area electron diffraction （SAED） and polarized light microscopy （PLM）. Onedimensional （1D） powder WAXD results at different temperatures coupled with DSC and PLM observations revealed that the C8HET compound possessed an LC phase and three different crystalline （K3, K2 and K1） phases below the isotropic （I） melt. The I←→ LC phase transition was thermodynamically reversible and independent of the heating and cooling rates. The development and experimental observation of the three crystalline phases relied on different thermal histories. Among the three crystalline phases in CSHET, the K3 phase is the most stable phase, while the K2 and K1 phases are metastable. Note that the K1 phase only formed via a quenching process. Oh the basis of structure sensitive diffraction experiments such as 2D WAXD of oriented samples and SAED of single crystals, detailed structures and molecular packings of these four ordered phases were identified. The LC phase exhibited a hexagonal columnar phase with 2D lattice dimensions ofa = b = 2.38 nm and γ= 120°. All the three crystalline phases possess monoclinic unit cells, yet the y angle is not 90° in the cases of the K2 and the K3 phases, while in the case of the K1 phase the a angle is not 90°.

飞机后整流罩用3A21F铝合金的GTAW工艺研究

为了验证某型飞机后整流罩用3A21F铝合金板钨极惰性气体保护焊(GTAW)焊接接头的性能,试验采用3A21F铝合金进行对接试验,通过宏观检验、金相组织和硬度拉伸等检测手段,研究不同工艺参数对3A21F铝合金GTAW焊缝组织与性能的影响。结果表明,焊缝边缘为柱状晶,该区域易形成应力集中;焊缝中心多为等轴晶;在外部热源的影响下,靠近3A21F铝合金母材的热影响区底部晶粒比较粗大。随着焊接电流的增大,柱状晶和等轴晶数量逐渐增多,尺寸逐渐增大。氩气流量对焊接接头的组织几乎没有影响。柱状晶区宽度随着焊接电流和氩气流量的增加而增大,柱状晶区的硬度高于等轴晶区的硬度,母材硬度最低。当焊接电流为47 A,氩气流量为6.5 L/min时,焊接接头的抗拉强度最大,为112.89 MPa,达到了母材抗拉强度的76%。

电流密度对定向凝固组织中柱状晶间距的影响

以 Ｍ—Ｓ理论为基础，导出了在定向凝固过程中电流通过固液界面的情况下，电流密度与柱状晶间距的理论关系式．并对其进行了实验研究．结果表明，电流密度增加时，其柱状晶间距近似呈直线下降的趋势。

脉冲电流对Al-4.5Cu合金定向凝固组织的影响

研究了脉冲电流对Al-4.5Cu合金定向凝固过程及组织的影响。结果表明,随着脉冲电流峰值和频率的增大,Al-4.5Cu合金柱状晶被熔断,形态由细条状变为不连续,失去其定向凝固的取向性;且柱状晶间距随电流峰值和频率的增大而减小。由脉冲电流的引入而产生的脉冲磁压力及焦耳热效应在固液界面处造成的强烈扰动是定向凝固组织失去取向性的主要原因。

电子束快速成形TC18钛合金柱状晶组织的拉伸性能

采用电子束快速成形方法,获得了一种TC18钛合金的柱状晶组织,研究了热处理工艺对该类组织析出相和拉伸性能的影响。结果表明,在740～830℃温度范围内固溶、在550℃时效处理,随着固溶温度的升高,快速成形样品沿柱状晶方向的强度呈上升趋势,塑性呈下降趋势;在830℃固溶处理后,在550～650℃时效温度范围内,随着时效温度的升高,沿柱状晶方向的强度呈下降趋势,而塑性呈上升趋势;拉伸性能随热处理工艺的变化主要取决于热处理过程中产生的二次α相;发现了延性韧窝和沿晶+准解理两种不同类型的断口形貌,并对其形成原因进行了分析。

一种热障涂层的形貌和相结构特征研究

研究了一种热障涂层的形貌和相结构组成。溅射NiCrAlY粘结层由γ-N_1相和β-NiAl相组成,是柱状结构,其晶粒度小,属于微晶,有利于形成α-Al_2O_3膜。溅射NiCrAlY粘结层经真空处理后,发生γ和γ′的转变,以及β-NiAl和脱溶。EB-PVD TBC具有开放性的柱状结构,并且存在大量的t′-ZrO_2相,使涂层性能大大优化。

柱状晶向等轴晶转变的数值模拟研究进展

柱状晶向等轴晶转变(CET)的研究具有重要意义,数值模拟是其有效的研究方法之一。重点介绍了3种CET转变数值模拟模型——确定性模型、随机性模型和相场模型。指出了各种模型目前在该领域应用中存在的问题,并展望了其今后的发展方向。

液固相变中的界面形态选择

在凝固过程中，液固界面形态经历了从平面向胞状进而树枝晶，然后又向针状胞直至绝对稳定平界面的演绎过程．本文以理论分析与实验结果为基础，讨论了液固界面稳定性与界面形态的关系和生长速率的影响，阐明亚快凝固段快束生长形成的超细柱晶组织特性及其应用前景．

从晶体取向特点探讨ZnO薄膜的晶体不完整性

用MOCVD法在蓝宝石衬底上得到了ZnO（0002）膜，并用XRD和SEM进行了表征．结果表明，薄膜中沿[0001]择优取向生长的柱状晶垂直于衬底表面，晶柱之间存在着边界和间隙．x射线中扫描实验结果表明．晶柱之间的取向偏差在3°-30°之间，X射线ω摇摆曲线和谱线宽度分析结果表明，薄膜中的晶柱是由多个晶粒堆叠而成，且晶粒之间的平均取向偏差也在2．6°以上．实验结果表明，ZnO大失配度异质外延膜是c轴[001]取向柱状多晶体，ZnO薄膜的结晶不完整性主要是由其柱状晶结构造成的。

连续柱状晶组织纯铜板材的制备

采用真空熔炼?氩气保护下拉式连续定向凝固技术制备纯铜板材,研究了工艺参数对凝固成形过程的影响,分析了连续定向凝固板材的组织和性能。结果表明:当熔体温度1160℃、下拉速度70mm/min、冷却水量900L/h时,可以连续稳定成形宽25mm、厚5mm且表面光亮、具有连续柱状晶组织的纯铜板材,其抗拉强度为146MPa,屈服强度为33MPa,延伸率为54%,电导率为105.9%IACS,具有优良的力学性能和电学性能。