Concentration profile and growth rate studies of In1 − x Ga x P LPE by computer simulation technique

Numerical simulation has been used to construct the concentration profiles of Ga and P atoms in In-rich melt at successive equally spaced layers in front of an InGaP crystal growing under normal conditions of liquid phase epitaxy (LPE). The growth rate has been calculated using the concentration gradient at the interface. The composition and the thickness of the InGaP solid grown as a function of growth parameters, such as cooling rate, system temperature and time, have been studied. It is observed that the thickness grown depends on the cooling rate, whereas the solid composition is independent of the cooling rate. The theoretical findings of our model have been compared with experimentally reported values and the results are discussed in detail.     

Calculation of the temperature gradient in the melt zone during the electric-transport liquid phase epitaxy of silicon carbide based solid solutions

The temperature distribution in a growth cell and the temperature gradient in the melt zone during the electric-field liquid phase epitaxy of silicon carbide based solid solutions (ytterbium-gallium, ytterbium-aluminum) were calculated with an allowance for the growth cell geometry. The analysis was based on a solution of the stationary thermal conductivity equations in all five regions of the standard growth cell. The solution was obtained taking into account the following factors: (i) Joule’s heating; (ii) Peltier’s heating (cooling) at the electrode-source (substrate)-melt zone interfaces; (iii) contact heat liberated at the electrode-source (substrate) interface; (iv) dissolution heat; and (v) crystallization heat. Expressions for the temperature gradient ?T in the melt zone as a function of the current density and the dimensions of regions in the growth cell are obtained.     

超高温度梯度场中的Al－Bi偏晶共生行为

应用高频区熔和液态金属冷却方法，考察了定向凝固速率和温度梯度对Ａｌ－Ｂｉ偏晶共生结构的影响．发现在陡的温度梯度下，仅有很低的生长速度方能形成共生；界面前沿的液相流动破坏了共生藉以维持的稳定的三相交角，从而产生厂共生结构的异常用化与破断，利用原子扩散时间与熔体重构时间的等量关系，推导出规则型偏晶共生的动力学方程．     

Aluminium and Al–4·5Cu alloy end chill: structural observation and heat flow analysis

Abstract

End chill experiments were performed on aluminium and Al–4·5Cu (wt-%) in order to study the effect of melt superheat (20–150 K), chill material (copper, iron, or sand), and specimen length (890–230 mm) on the type and size of macrostructure. Increasing melt superheat increases the length of columnar zone, which is shorter for the alloy than for the commercial purity metal. The columnar fraction increases with the thermal conductivity of the chill material and the heat transfer coefficient. The results are correlated with the temperature gradient, solidification rate, and growth rate obtained from a heat flow model. The columnar to equiaxed transition is found to occur at a critical temperature gradient and growth rate. These critical values differ with alloy composition. The grain size of columnar and equiaxed grains is found to follow a power relationship with solidification rate.

MST/1709     

Theoretische Überlegungen zum Erstarrungsverhalten schnell abgeschreckter Schmelzen

Theoretical Considerations on the Solidification Behaviour of Rapidly Quenched Melts The rapid heating, cooling and solidification processes on laser surface melting are calculated. The influence of the laser operating parameters e.g. beam energy density, melting speed and the resultant depth of melt on the solidification parameters cooling rate, solidification rate and temperature gradient is shown. By combining the cooling curves and TTT (CT) curves an attempt can be made to predict the resulting grain size. The use of the programmes is demonstrated for other quenching methods e.g. melt spinning, powder atomizing.     

Controlled growth of CsI〈Tl〉 single crystals

CsI〈Tl〉 crystals have been grown by the axial-heat-flux-close-to-the-phase-interface (AHP) method in a purpose-designed apparatus at an argon overpressure. The furnace and crystallizer of the apparatus have been designed to ensure considerable (5 to 100 K/cm) axial and low (within 1 K/cm) radial temperature gradients at the growth interface. The effects of the melt layer thickness, temperature gradient, and activator concentration in the melt on the crystal quality have been studied. The results demonstrate that reducing the AHP heater temperature for even a short time (by 2°C in 3 min) markedly raises the actual growth rate, from 2 to 15 mm/h, and leads to entrapment of bubbles of various diameters and TlI inclusions.     

Evaluation of cast-on-strap joints in lead-acid batteries

The effects of various processing parameters on the lug–strap joint quality of lead-acid auto-batteries are investigated using a laboratory scale cast-on-strap (C.O.S.) set-up. The results indicate that while good quality joints are easily obtained at low cooling rates, at high cooling rate all the C.O.S. parameters such as flux type, strap composition and melt temperature must be carefully controlled to achieve an acceptable joint. Surface treatment with commercially available fluxes, strap melt temperature of 460 °C and strap antimony content of 4.6wt.% Sb render good quality joints at a high strap cooling rate. Furthermore, a dimensionless parameter (relative contact length) for evaluation of lug–strap joint quality is introduced which combines the important lug–strap joint features into a single quantitative parameter and could be used for design of experiments (DOE) and process optimization in the industrial scale.     

Numerical studies on columnar-to-equiaxed transition in directional solidification of binary alloys

A numerical study on columnar-to-equiaxed transition (CET) during directional solidification of binary alloys is presented using a macroscopic solidification model. The position of CET is predicted numerically using a critical cooling rate criterion reported in literature. The macroscopic solidification model takes into account movement of solid phase due to buoyancy, and drag effect on the moving solid phase because of fluid motion. The model is applied to simulate the solidification process for binary alloys (Sn–Pb) and to estimate solidification parameters such as position of the liquidus, velocity of the liquidus isotherm, temperature gradient ahead of the liquidus, and cooling rate at the liquidus. Solidification phenomena under two cooling configurations are studied: one without melt convection and the other involving thermosolutal convection. The numerically predicted positions of CET compare well with those of experiments reported in literature. Melt convection results in higher cooling rate, higher liquidus isotherm velocities, and stimulation of occurrence of CET in comparison to the nonconvecting case. The movement of solid phase aids further the process of CET. With a fixed solid phase, the occurrence of CET based on the same critical cooling rate is delayed and it occurs at a greater distance from the chill.     

非均匀温度场Fe-0.06%C合金焊接接头凝固过程模拟分析

利用元胞自动机和有限差分(CA-FD)法,采用宏-微观两种尺度,将宏观温度场与微观枝晶生长过程耦合在一起,再现了Fe-0.06%C二元合金焊接熔池的凝固过程.同时,探讨了边界散热速率对焊接熔池中枝晶生长形貌及晶粒尺寸的影响;分析了形核基底数与枝晶生长间的关系;并用实验对模拟结果进行了验证.结果表明,在熔池凝固过程中,温度梯度沿散热边界向绝热边界方向不断减小,等温线弧度不断增大;熔池散热边界附近的液相中溶质浓度远远高于绝热边附近和模拟区域中心的液相溶质浓度;模拟区域内的温度梯度随着边界散热速率的增大而升高.此外,随着形核基底的增加,柱状晶数量基本不变;而等轴晶数量不断增多,分布范围逐渐扩大,但尺寸有所减小.模拟结果反映了焊接熔池的凝固过程,并与实验结果吻合,为实际焊接工艺的选取提供了一定的参考.     

Effect of Ultrasonic Output Power on Refining the Crystal Structures of Ingots and Its Experimental Simulation

In this study, a series of tests were conducted by using aluminum-based alloy to determine the formation of grain refining structure based on the ultrasonic vibration (UV). Furthermore, the simulation test and effect of ultrasonic output power were studied using ammonium chloride. Finally, the mechanism of grain refinement was investigated. The results show that: (1) By applying the UV to aluminum-base alloy, the grain refining rate of ingots tended to increase with the increase of the output value of UV. (2) It was confirmed that time from after the pour to the beginning of crystallization as well as cloudiness tended to decrease with increasing the ultrasonic power value of UV. Moreover, it can be seen from each cooling curve that a uniform temperature gradient existed in the melt as the power of UV increased, that made the melt strongly stirred. (3) It was also considered that the grain refining effect of ingots, which was observed from the simulation tests, resulted from nucleation action and stirrin     

Characterization of rapidly solidified aluminium-silicon alloy

A metallographic investigation of as-cast LM-13 aluminium-silicon alloy, solidified at different cooling rates (using permanent moulds or a single-roll melt spinner), is presented with special reference to the modification of eutectic silicon. and the refinement of primary aluminium. The refinement of microstructure with the increase in cooling rate is mainly attributed to the limited growth kinetics of the nucleated phase during solidification. The rapidly solidified ribbon was heat-treated in order to determine the microstructural stability at high temperature. The tensile strength, percentage elongation and hardness values of as-solidified and heat-treated samples correlate well with the changes in the microstructure observed.     

多晶硅真空定向凝固过程Marangoni对流对铸锭质量的影响研究

对于多晶硅垂直布里奇曼真空定向凝固过程,熔体自由表面张力引起的Marangoni对流对晶体生长质量有着明显的影响。本文通过建立多晶硅真空定向凝固过程的热场-流场-应力场多场耦合数学物理模型,采用数值模拟和实验研究了Marangoni对流作用下熔体传热特性、熔体流动行为、铸锭热应力等因素对硅晶体生长质量的影响。结果表明：Marangoni对流会增强硅熔体流场的流动强度,使得硅熔体平均流速增大3倍左右,进而影响硅熔体内部的对流换热能力,使硅熔体和硅固体温度分布更加均匀,硅熔体温度梯度降低4.8%-9.9%,硅固体温度梯度降低2.1%-2.6%,使多晶硅铸锭过程产生更小的热应力和更少缺陷。     

Anisotropy of some physical parameters in As-prepared As2S3 bulk glass

When cooling a melt the cooling rate in different layers varies due to differences in heat transfer. These changes in cooling rate were determined experimentally for the As₂S₃ melt in a quartz ampoule with diameter of 17 mm. The cooling rate at air quenching varies from 1.36 K/s in the bulk middle to 2.11 K/s in the border layers, and from 3.54 to 38.80 K/s for water quenching, respectively. Further, the corresponding changes in the density, microhardness, glass-transition temperature and electrical conductivity in dependence on the choice of the measured sample from various parts of the bulk were found. The anomalous behaviour of these parameters in dependence on the cooling rate corresponds to the polymeric structure of the glass.     

温度梯度和生长速率对CdZnTe-VBM生长晶体的影响

计算模拟了半导体材料CdZnTe垂直布里奇曼法(CdZnTe-VBM)单晶体生长过程，分析了炉膛温度梯度和坩埚移动速率对结晶界面形态和晶体内组份偏析的影响。计算结果表明炉膛温度梯度和生长速率的变化明显影响固-液界面前沿对流场的形态和强度。界面凹陷深度随着炉膛温度梯度的增加和生长速率减小而减小。炉膛温度梯度的增加和生长速率的减小虽然均能有效的减小径向偏析，但却增加轴向偏析，减小轴向等浓度区的长度。     

Effect of cooling rates on the dendritic morphology transition of Mg–6Gd alloy by in situ X-ray radiography

The effect of cooling rate on the transition of dendrite morphology of a Mg-6 Gd(wt%) alloy was semiquantitatively analyzed under a constant temperature gradient by using synchrotron X-ray radiographic technique. Results show that equiaxed dendrites, including exotic 'butterfly-shaped' dendrite morphology, dominate at high cooling rate(1 K/s). When the cooling rate decreases in the range of 0.5–1 K/s, the equiaxed-to-columnar transition takes place, and solute segregates at the center of two long dendrite arms(LDA) of the 'butterfly-shaped' dendrite. When the cooling rate is lower than 0.3 K/s, directional solidification occurs and the columnar dendritic growth direction gradually rotates from the crystalline axis to the thermal gradient direction with an increase in cooling rate. Meanwhile, interface moves faster but the dendrite arm spacing decreases. Floating, collision and rotation of dendrites under convection were also studied in this work.     

Effect of gravity on InGaSb crystal growth

The effect of gravity on dissolution of GaSb in InSb melt and growth of InGaSb was experimentally investigated. Experiments were carried out in a GaSb(seed)/InSb/GaSb(feed) sandwich system under an imposed temperature gradient. In the experiments, the GaSb feed crystal dissolved into the InSb melt to supply the required GaSb component for the growth of In_0.1Ga_0.9Sb crystal. Two parameters were considered: (1) the inclination angle (θ) of the sample for gravity as 0° and 53°, and (2) the sample diameter (D) as 9 mm and 5mm. When θ was 0°, the interface was almost flat, indicating that convection was axisymmetric and stable. Whereas the interface was distorted towards gravitational direction when θ was 53°, indicating that solutal convection was dominant. The decrease of growth temperature and sample diameter reduced the distortion of interface and the dissolution amount of GaSb feed. The homogeneous crystals were grown at the initial growth stage by supplying the GaSb component during growth.     

TiO2和O-Al2O3晶体的生长习性

通过水热法制备粉体的实验观察到金红石、锐钛矿和α-Al2O3晶体的生长习性．采用配位多面体生长习性法则合理地解释了Ti O2和α-Al2O3的生长习性．其主要结果为α-Al2O3晶体的生长习性为平板{0001}，其各晶面的生长速度为：V｛0001 }＜V｛1123}；锐钛矿的生长习性为四面体，其各晶面的生长速度为V<010>=V<001>>V<010>>V<111>．而PBC理论很难合理地解释α-Al2 O3晶体的生长习性．     

Microstructural development during transient directional solidification of hypermonotectic Al–Bi alloys

Directional unsteady-state solidification experiments were performed with hypermonotectic Al–5.0 wt%Bi and 7.0 wt%Bi alloys. Thermal parameters such as the growth rate (v) and the thermal gradient (G) were experimentally determined by cooling curves recorded along the casting length. The predominant Bi-rich phase was characterized by droplets embedded in the aluminum matrix. Both the interphase spacing (λ) and the Bi-rich particles diameter (d) were measured along the casting length. These microstructural features were correlated to the solidification thermal parameters: growth rate, cooling rate and thermal gradient. An experimental law expressing λ as a function of both G and v was found to better represent the growth of hypermonotectic Al–Bi alloys. Moreover, it was found that the interphase spacing decreases with increasing alloy bismuth content.     

Undercooling in Co–Cu alloys and its effect on solidification structure

Undercooling behaviour and solidification morphology change of various Co–Cu alloys were examined. Each alloy was melted in an alumina crucible under an argon atmosphere by high-frequency induction, and the cyclic heating and cooling was repeated several times in the temperature range between 1300 and 1850 K. The temperature change during the experiment was analysed under the Newtonian cooling assumption. The temperature curve showed that the undercooling in a first few cycles was negligibly small but it increased remarkably. The alloy was undercooled below the metastable liquid miscibility gap after the next several cycles. In these samples, liquid separation was observed. The homogeneously mixed spherical grains of copper-enriched phase were observed in cobalt-enriched matrix for the samples solidified immediately after the liquid separation. The two melts became coarser after the separation by mutual coalescence. In the case of the slow start of the solidification after the separation, they formed a clear interface between the upper cobalt-enriched layer and the lower copper-enriched layer located in the lower part according to the density difference between the two melts. It depended on the cooling rate after liquid separation. The very fine duplex structure can be obtained by the rapid cooling of the melt at the initial stage of the separation.