A front-tracking model to predict solidification macrostructures and columnar to equiaxed transitions in alloy castings

The solidified grain structure (macrostructure) of castings is predicted by process simulation using a newly extended front-tracking technique which models the growth of solid dendritic fronts through undercooled liquid during metallic alloy solidification. Such fronts are either constrained, as is the case with directed columnar growth from mould walls, or unconstrained, as is the case for multiple equiaxed growth from individual nucleating particles distributed throughout the liquid. Non-linear latent heat evolution is treated by incorporating the Scheil equation. Thermal conductivity changes with the solid fraction. A log-normal distribution of activation undercooling to initiate free growth from equiaxed nuclei is used, and the routines to deal with such growth followed by impingement of dendritic grains upon one another are verified by comparison with the results of analytical studies of simplified systems. The extensions to the model enable the predictions of equiaxed grain structure and, importantly, the columnar to equiaxed transition in inoculated alloy castings. The model is validated via comparison with experimental results. The front-tracking method is proposed as a suitable formulation for modelling alloy castings that solidify with a dendritic structure, either columnar, equiaxed, or both.     

Boundary-conforming mapping applied to computations of highly deformed solidification interfaces

A new boundary-conforming mapping is developed for the calculation of highly deformed cellular solidification interfaces in a model of directional solidification of a binary alloy. The mapping is derived through a variational fomulation that is designed so that the grid penetrates the grooves between cells along the interface without causing a loss of ellipticity of the mapping equations. A finite element/Newton method is presented for simultaneous solution of the free boundary problem described by the solutal model of directional solidification and the mapping equations. Results are compared to previous calculations and demonstrate the importance of accurate representation of the interface shape for understanding the solution structure.     

Solutal convection during growth of organic NLO crystals

Effect of solutal and thermal convection plays very important role when a large thermal gradient is applied during crystal growth. To address this problem, we have purified m.nitroaniline (m.NA) and m. dinitrobenzene (m.DNB) and studied crystal growth and effect of growth parameters on the optical quality. Crystals of pure and binary alloy of m.dinitrobenzene and m.nitroaniline were grown by vertical directional solidification method in a two zone transparent furnace. Effect of doping and temperature gradient on the solid-liquid interface morphology and quality of crystal was determined by studying the bulk transparency and nonlinearity.     

微量Na元素的变质作用对Al—Si合金凝固过程的影响

对不同Na含量的Al-Si共晶合金各相中Na的分布进行了分析,并结合显微结构的变化,用DTA方法研究了合金的凝固过程。结果表明:Si相中的Na含量随着合金中Na的重量百分比的增加而增加,在一定的冷却速度下,加入Na可提高Al-Si共晶合金的共晶析出温度,降低Al-Si共晶合金的生长速度,变质效果越好的样品,这种变化越明显,说明,Na的变质作用不是抑制其成核而是促使其成核。     

掺杂元素对铝-硅共晶合金凝固过程的影响

国内外学者对掺杂元素对Al-Si共晶合金的影响进行了大量的研究,这些研究大都侧重于研究变质后的结果,如变质后的晶体结构形态以及掺杂元素对结构形态的影响,再根据变质结果反过来推测可能的变质机理,但对掺杂元素对Al-Si合金凝固过程本身的影响的研究却很少.本文拟采用差热分析法(DTA)和步冷曲线热分析法研究Na、Sr元素对Al-Si共晶合金凝固过程的影响,以探讨Al-Si共晶合金的成核机理。     

In situ and real-time analysis of TGZM phenomena by synchrotron X-ray radiography

The development of brilliant third-generation synchrotron X-ray sources, together with advances in X-ray optics and detectors, has provided timely efficient tools for in-depth understanding of physical phenomena in a broad spectrum of situations. Synchrotron X-ray radiography enables in situ and real-time observation of microstructure evolution, i.e. a direct access to dynamical phenomena which could not be anticipated from post-mortem analysis. Dedicated experiments are carried out at the European Synchrotron Radiation Facility (ESRF) in Grenoble (France) in Al-based alloys to study the dynamics of temperature gradient zone melting (TGZM) phenomenon. TGZM occurs when a liquid–solid zone is submitted to a temperature gradient and leads to the migration of liquid droplets or channels through the solid, up the temperature gradient. The thorough characterisation of both the initial solid during the thermal stabilisation phase prior to solidification (static TGZM) in Al–3.5 wt% Ni alloy and the dendritic microstructure in the later stage of solidification in Al–7.0 wt% Si alloy is performed. Based on experimental observations, quantitative data (in particular liquid-migration velocity) are measured and a very good agreement is found with theoretical analysis.     

非平衡凝固与固态相变中有关形核和长大的竞争研究

非平衡凝固和固态相变作为同热力学和动力学紧密相关的复杂理论体系,在物理冶金和凝聚态物理研究领域占据重要的地位。上述非平衡过程包含形核、生长和碰撞三个微观过程,其发生和发展均由一些竞争过程伴随、辅助甚至驱动。正确理解这些竞争过程决定了能否准确描述整个非平衡过程;材料制备中,其力学和物理/化学性能的提高也归根于这些竞争的发生与发展。因此,很有必要对非平衡凝固和固态相变中的竞争现象进行研究。本综述第一部分从形核/生长类相变角度对非平衡凝固和固态相变进行了概述,并强调了伴随其发生的竞争现象。第二部分立足于经典形核理论,对有关形核热力学、动力学以及形核方式的竞争现象进行了描述。第三部分对凝固中有关热扩散和溶质扩散以及固态相变中有关界面控制和扩散控制的生长方式竞争进行了描述。第四部分对本综述进行了总结性分析和概括,并对本领域的未来发展方向进行了预测。     

Laser glazing of lanthanum magnesium hexaaluminate

Lanthanum magnesium hexaalumminate (LMA) is an important candidate for thermal barrier coatings due to its thermal stability and low thermal conductivity. On the other hand, laser glazing method can potentially make thermal barrier coatings impermeable, resistant to corrosion on the surface and porous at bulk. LMA powder was synthesized at 1600 °C by solid-state reaction, pressed into tablet and laser glazed with a 5-kW continuous wave CO₂ laser. Dendritic structures were observed on the surface of the laser-glazed specimen. The thicker the tablet, the easier the sample cracks. Cracking during laser glazing is attributed to the low thermal expansion coefficient and large thickness of the sample.     

Oxygen and carbon transfer during solidification of semiconductor grade silicon in different processes

A model is established for comparing the solute distribution resulting from four solidification processes currently applied to semiconductor grade silicon: Czochralski pulling (CZ), floating zone (FZ), 1D solidification and electromagnetic continuous pulling (EMCP). This model takes into account solid–liquid interface exchange, evaporation to or contamination by the gas phase, container dissolution, during steady-state solidification, and in the preliminary preparation of the melt. For simplicity, the transfers are treated in the crude approximation of perfectly mixed liquid and boundary layers. As a consequence, only the axial (z) distribution can be represented. Published data on oxygen and carbon transfer give a set of acceptable values for the thickness of the boundary layers. In the FZ and EMCP processes, oxygen evaporation can change the asymptotic behaviour of the reference Pfann law. In CZ and in 1D-solidification, a large variety of solute profile curves can be obtained, because they are very sensitive to the balance between crucible dissolution and evaporation. The CZ process clearly brings supplementary degrees of freedom via the geometry of the crucible, important for the dissolution phenomena, and via the rotation rate of the crystal and of the crucible, important for acting on transfer kinetics.     

Onset of Buoyancy-driven Instability in Porous Medium Solidified from Above

When a porous melt layer saturated by liquid is solidified from above, convection often sets in due to buoyancy forces. In this study, the onset of buoyancy-driven convection during time-dependent solidification is investigated by using the similarly transformed disturbance equations. The thermal disturbance distribution of the solid phase is approximated by the WKB method and effects of various parameters on the stability condition of the melt phase are analyzed theoretically. For the limiting case of λ → 0 and finite k _r, the critical conditions approach asymptotically and . This study presenting a constant-temperature cooling model predicts greater instability and gives more unstable results than those obtained from the constant solidification rate model.