定向倾斜枝晶生长规律及竞争行为的相场法研究

采用多相场模型对定向凝固过程中倾斜枝晶生长进行了研究, 模拟了单一取向枝晶列的演化规律及不同取向枝晶列汇聚生长竞争淘汰行为. 结果表明, 枝晶尖端过冷度随倾斜角度的增大而增大, 即相同条件下倾斜枝晶尖端位置总是低于非倾斜枝晶; 汇聚生长时择优取向枝晶总是阻挡非择优取向枝晶, 但在抽拉速度较低时, 由于溶质扩散场的相互重叠, 晶界处择优取向枝晶的生长受到相邻非择优枝晶的影响而延缓, 这可能导致非择优取向枝晶淘汰择优取向枝晶.     

分析金属熔滴快速凝固过程的双倒易边界元方法

本文发展了非线性边界条件相变传热过程的轴对称双倒易边界元方法,数值模拟了金属熔滴在快速冷却条件下的快速凝固过程。分别研究了在微重力落管和落塔中及喷射成形过程中金属熔滴的快速凝固过程,得到了过冷度,再辉时间,温度变化及相变界面随时间的变化等数值结果。     

DSC Investigation of some Testing and Calibration Compounds, Particularly During Cooling

Two compounds are described with interesting properties for use in DSC. The first compound is adamantane (C₁₀H¹⁶), with a reversible solid-solid transition at 208.62 K [1], suitable for DSC calibration at this low temperature [2]. The second compound is 4,4'-azoxyanisole (C₁₄H₁₄N₂O₃), with a liquid crystal range between 390 and 407 K [3]. This compound shows two transitions on heating, with a large heat effect at 390 K and a small heat effect at 407 K. For this reason, this substance is well suitable for testing the sensitivity and the resolution of DSC instruments [4]. For both compounds not only the heating, but also the cooling behaviour is investigated.This revised version was published online in November 2005 with corrections to the Cover Date.     

Can clustering of liquid water and thermal analysis be of assistance for better understanding of biological germplasm exposed to ultra-low temperatures

Plausible effect of clustering of undercooled liquid water (pentamer configuration, icosahedra formation) is discussed showing water continuous but non-periodic patterns and its impact to the either formation of ice-crystals or ice-glass, particularly when making contact with plants. These processes are viable for the cryogenic storage of biological germplasm and subject for thermoanalytical studies aimed to the determination of glass transition temperatures.     

Two-Phase Stefan Problem as the Limit Case of Two-Phase Stefan Problem with Kinetic Condition

Both one-dimensional two-phase Stefan problem with the thermodynamic equilibrium condition u(R(t),t)=0 and with the kinetic rule u_ε(R_ε(t),t)=εR_ε′(t) at the moving boundary are considered. We prove, when ε approaches zero, R_ε(t) converges to R(t) in C^1+δ/2[0,T] for any finite T>0, 0<δ<1.     

RAPID DENDRITIC GROWTH INVESTIGATED WITH ARTIFICIAL NEURAL NETWORK METHOD

Rapid dendritic growth of γ-(Ni, Fe) phase, β-CoSb intermetallic compound and α-Fe phase was realized by undercooling Ni-10%Fe single phase alloy, Co-60.5%Sb intermetallic alloy and Fe-40%Sn hypomonotectic alloy to a substantial extent. Their experimentally measured dendrite growth velocities were 79.5m/s, 12m/s and 0.705m/s, corresponding to undercooling levels of 303K(0.18T_L), 168K(0.11 T_L) and 219K(0.15 T_L) respectively. Since the usual dendrite growth theory deviates significantly from reality at great undercoolings, an artificial neural network incorporated with stochastic fuzzy control was developed to explore rapid dendrite growth kinetics. It leads to the reasonable prediction that dendritic growth always exhibits a maximum velocity at a certain undercooling, beyond which dendrite growth slows down as undercooling increases still further. In the case of Fe-Sn monotectic alloys, α-Fe dendrite growth velocity was found to depend mainly on undercooling rather than alloy composition.     

Influence of isothermal approximation on the phase-field simulation of directional growth in undercooled melt

By using the phase-field approach,we have simulated the directional growth of alloys in undercooled moten states under the isothermal and nonisothermal conditions.The influences of the isothermal approximation on simulation results are discussed.We found that for undercooling greater than 25K,the isothermal approximation overestimates the interface growth velocity and reduces a critical velocity for an absolute stable planar interface,thus in this simulation,the uinterface morphology shows the plane-cell-plane transition with increasing initial undercooling of the mele,and the planar interface obtained under a large undercooling is absolutely stable.Whereas in the nonisothermal simulation,only plane-cell transition occures in the same range of the initial undercoolings of the melt,and the planar interface tends to be destabilized and evolve into cells.     

Ni-Nd-B合金的性能研究

采用电弧式熔体快淬/吸铸技术，制备了不同成分的Ni-Nd-B试样，借助X射线衍射、扫描电子显微镜、热分析法、静态磁性测试仪等手段进行了分析。研究发现，用电弧式熔体快淬/吸铸技术可以较成功制备系列直径6 mm的Ni-Nd-B大块非晶，且满足深度共晶成分试样表，表现出较理想的非晶形成能力。Ni4NdB材料具有约79.13 ℃的过冷度，Ni13Nd3B2材料具有约69.37 ℃的过冷度；Ni，Nd成分微量变化引起材料较明显的晶化趋向。此外，Ni，Nd成分微量变化导致材料矫顽力、饱和磁化强度减小。     

液态Bi的过冷及Bi-Ⅱ高压相的形成

 本文研究了常压下液态Bi在落管所提供的高清洁、无容器环境中自由下落和在氦气保护下经循环过热处理后的过冷行为。通过控制液滴的尺寸和冷却速率得到了较大的过冷度。过冷样品具有与常压稳定相Bi-Ⅰ（三角点阵）完全不同的单斜点阵。其点阵参数为a=0.696 8 nm，b=0.628 6 nm，c=0.339 7 nm, β=110.33°；单胞体积为0.148 8 nm³，它与Brugger等人在2.6 GPa下得到的Bi-Ⅱ高压相（单斜点阵）的单胞体积相比，后者有一个9.34%的体积压缩，这与Bridgman的结果符合得很好。     

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.