影响金属匀质形核过冷度的因素

将热力学因素和动力学因素对金属匀质形核过冷度的影响统一起来,建立了匀质形核的过冷度方程。结果表明：金属的匀质形核过冷度与金属的形核驱动力因子、体积及冷却速度有关．对同一金属来说,其匀质形核过冷度随液态金属体积的减小或冷却速度的增大而增大,当液态金属体积与冷却速度的比值减小到一临界值时,金属的匀质形核过冷度增大到其最大值;在液态金属体积与冷却速度的比值一定时,金属的匀质形核过冷度随其形核驱动力因子的增大而增大     

Al－45％Cu合金的熔剂净化与过冷

通过金属熔体过热与氯盐熔剂净化相结合，使Ａｌ－４．５％Ｃｕ合金在金属型锭模中获得了６２℃的形核过冷度．用形核理论对大体积Ａｌ－４．５％Ｃｕ合金极限热力学过冷度进行计算后发现其范围在７４Ｋ到１０７Ｋ之间．     

过冷熔体中的强制枝晶生长

过冷熔体中的强制枝晶生长＊谢发勤①张军②傅恒志③细（或超细）柱晶组织及其叶片类产品，都是在液／固界面前沿熔体处于过热状态下形成的，如传统的快速凝固（ＨＲＳ）、液态金属冷却（ＬＭＣ）等定向凝固工艺。在过冷熔体中，由于形核的不可预测性和晶体生长的随机性，...     

过冷合金熔体非平衡凝固中形核方式的竞争

本文立足于经典形核理论和非热形核理论，分析了过冷熔体非平衡凝固中连续形核同位置饱和的竞争．决定过冷熔体形核方式的关键因素是初始熔体的过冷度与异质核心的尺寸．随着净化的进行，熔体过冷度提高，N1，r^＊以及△G^＊逐渐降低，它们之间的相互竞争使形核方式从位置饱和向连续形核转变，并解释了过冷Ni-Sn与Ni-Si共晶合金中观察到的组织变化．表明低过冷下Ni-Sn合金形核方式为位置饱和；Ni-Si共晶合金中，△T〈180K时形核方式为位置饱和，180K以后，形核方式从位置饱和转变为连续形核．     

过冷纯熔体中球状晶体生长的稳定性分析

过冷纯熔体中球状晶体生长界面上存在动力学的作用,即界面稳定性也依赖于动力过冷。从Mullins-Sekerka理论出发,修改了Cristini和Lowengrub的动力过冷项,考虑界面上动力过冷的影响,应用渐近分析的理论继续分析过冷纯熔体中球状晶体生长液固界面的稳定性。结果表明,稳定性由扰动波数的大小确定。     

Ni-Ni3Sn系近共晶合金深过冷形核规律

采用大体积液态金属的微观净化技术，在Ni-Ni3Sn合金系中使Ni-(2.5-17.5)at%Sn和Ni-(17.8-25.0)at%Sn两组合金分别获得了最大达351K和303．5K的“极限”形核过冷度，并使其极限过冷度保持30个循环过热周期不衰减。实验发现，Ni-Ni3Sn系近共晶合金深过冷凝固时，Ni3Sn总是领取先于α（Ni）相形核。根据实验数据和表面异质形成核速率模型，求得Ni3Sn和α（Ni）相异质形核时的异质形核因子f(θ）分别为0．8103和0．12406；求得过冷熔体中Ni3Si和α（Ni）相等形核速率时的温度－成分曲线完全处于共晶点左边，表明在近共晶成分过冷熔体中，金属间化合物Ni3Sn易于α（Ni）形核。     

高洁净钢的洁净度与过冷度的关系

利用高温差热分析测定了高洁净钢的过冷度,用晶相法测定了高洁净钢激冷试样的晶粒度,研究了高洁净钢洁净度与过冷度和晶粒度之间的关系.计算了高洁净钢临界形核半径和形核率等热力学参数.结果表明,高洁净钢随氧含量降低,洁净度提高,过冷度增加,晶粒细化,而钢中硫含量对过冷度无影响.     

液态金属的深过冷及其凝固组织

深过冷熔体能够在低速冷却的情况下实现快速凝固 ,因而深过冷技术引起了人们的关注 ,并在这方面取得了很多成果 .本文分析了深过冷的热力学及动力学基础 ,综述了近年来深过冷技术的发展以及人们在其凝固特征方面的研究发现 .随着深过冷工艺的提高 ,可以获得的过冷度已大大超过了以前人们普遍认为均质形核所能达到的最大过冷度 0 .2Tm.深过冷的凝固组织使晶粒细化、杂质分布改善 ,普遍具有强韧化效果 .此外 ,利用深过冷技术还能制备常规凝固条件下不能得到的一些具有优异性能的新材料     

负温度梯度熔体中晶体生长取向与控制

研究了63－292K热力学过冷度范围内，Cu-Ni单相合金的凝固组织演化规律，分析了负温度梯度熔体中晶体定向生长的过冷度条件以及晶体生长取向机制与控制方法。结果表明：（1）在临界过冷度△T^*(≈106K）附近，晶粒细化过程受溶质扩散控制，在临界过冷度△T^**(≈200K)附近，晶粒经过程受热扩散控制；（2）在63－△T^*K范围内，晶体的生长方向是[100]，在127－△T^*＊K范围内，晶体的生长方向可以是[100]，也可以是[111]。     

获得Cu—Ni—Fe熔体深过冷的烧结坩埚涂层

利用Ｃｕ－３９％Ｎｉ－６％Ｆｅ和Ｎｉ－３０％Ｃｕ－５％Ｆｅ（ｗｔ％）两种合金研究了在不同配方坩埚烧结涂层中合金液冷却时的过冷度。结果表明,（８０％石英砂＋２０％石英玻璃粉）＋３％Ｈ３ＢＯ３坩埚涂层材料,在以石墨为发热体的高频感应炉中１９２３Ｋ温度下烧结后,可获得３ｍｍ厚表面光洁、无裂纹的玻璃态涂层。在该涂层中熔炼上述两种合金,分别获得了１９０Ｋ和２１０Ｋ的大过冷度,超过了两种合金晶粒骤然细化的临界过冷度ΔＴ＊２。     

Ni_（78.6）Si_（21.4）合金的深过冷与凝固组织演化

深过冷技术研究是凝固科学中重要的研究领域之一.文中采用玻璃熔融净化与循环过热法使Ni78.6Si21.4合金获得了318K的过冷度,研究了其凝固组织随初始过冷度（ΔT）的演化规律,并对凝固组织中的亚稳相进行了分析.研究发现：当ΔT〈193K时,凝固组织为Ni3Si＋（Ni3Si＋α-Ni）非规则共晶;193K〈ΔT〈250K,凝固组织为α？Ni＋（Ni3Si＋α-Ni）非规则共晶;ΔT〉250K时,凝固组织为完全的非规则共晶;随过冷度的增加,过冷合金熔体中依次出现了亚稳相Ni31Si12相和Ni3Si2相.     

Nucleation and undercooling of metal melt

The effects of thermodynamic and dynamic factors on nucleation process have been integrated in a theoretical formula representing the dependence of undercooling on parameters concerned. Moreover, a method to determine the kind and amount of the most effective catalyst in an undercooled melt has been acquired. The results show that the undercooling increases with the decreasing surface area of the most effective catalyst and the increasing cooling rate as the kind of the most effective catalyst is constant. It increases to a maximum value when the ratio of the surface area of catalyst ( Sv V) to the cooling rate of melt ( Rc) decreases to a critical value. The maximum undecooling not only depends on the ratio of non-dimensional factor of activation energy for an atom to diffuse (φ) to non-dimensional factor of driving force for nucleus to form (ψ), but also depends on the contact angle of the most effective catalyst; the smaller the ratio of φ to ψ, the higher the maximum undercooling, but it does not     

三元Pb-Sb-Sn合金的快速凝固及组织特征

实现了三元Pb-12%Sb-4%Sn和Pb-15%Sb-10%Sn合金在深过冷条件下的快速凝固,实验中获得的最大过冷度分别为65 K(0.13TE)和70 K(0.13TL)。XRD分析表明,这两种合金在实验过冷度范围内均由(Pb)固溶体、(Sb)固溶体和SbSn金属间化合物三相组成。在小过冷条件下,Pb-12%Sb-4%Sn共晶合金的三相以层片状交替分布,协同生长。随着过冷度增大,(Sb)作为初生相以小平面方式生长,且三元层片共晶组织显著细化。对于Pb-15%Sb-10%Sn合金,其凝固组织由初生相、二相共晶和三元共晶组成,初生SbSn相以具有小平面特征的枝晶方式生长。在实验基础上,对三元Pb-Sb-Sn合金的形核特征和生长规律进行了理论分析。     

Metastable liquid properties and rapid crystal growth of Ti-Ni-Al alloy investigated by electrostatic levitation and molecular dynamics simulation

Electrostatic levitation technique and molecular dynamics simulation were performed to investigate the thermophysical properties, liquid structure and crystal growth dependence on undercooling of Ti₈₅Ni₁₀Al₅ alloy. The liquid Ti₈₅Ni₁₀Al₅ alloy was substantially undercooled up to 335 K (0.18T_L). As undercooling increased, the potential energy of the liquid alloy decreased and the alloy entered into a high metastable state. At this state, the atoms tended to bond with each other and the clusters were inclined to convert into high-coordinated clusters, as confirmed by the fraction of the high-coordinated clusters variation. The enlarged clusters and enhanced local structure stability contributed to the increase of the thermophysical parameters and crystal growth velocity, and eventually dendrite refinement. The density, the specific heat and the surface tension of liquid alloy exhibited a linear relation with temperature and the shear viscosity of liquid alloy showed exponential variation which showed good agreement with the calculation results by molecular dynamics simulation. The growth velocity first increased slowly and then dramatically once the undercooling exceeded the threshold. The achieved maximum crystal growth velocity was 12.4 ms⁻¹ and it was up to 326 times of the value at 94 K undercooling.

     

净化工艺对铜过冷度的影响

对大体积液态铜在真空熔炼条件下用熔融钠钙玻璃净化后的过冷度进行了实验研究。     

The effect of melt overheating on the melt structure transition and solidified structures of Sn-Bi40 alloy

Evolution of the electrical resistivity of Sn-40wt%Bi melt with time under different overheating temperatures during isothermal experiments has been studied, and the relationship between different melt state, solidification behavior and solidified structure has also been investigated. The results show that the melt structure transition revealed by the abnormal change of resistivity would take place within a certain holding time just when the holding temperature is above a certain critical, and that the higher the temperature above the critical, the shorter the “incubation period” of the melt structure transition, and the faster the transition speed. The results of solidification experiments suggest that the melt structure transition caused by different holding time at the same temperature can lead to a higher solidification undercooling degree, finer grain size and change of microscopic pattern. Further exploration indicates that the solidification undercooling degree can come to a head when the melt is held at the specific temperature for a given time. The functionary mechanism of the phenomena above is also discussed briefly. Supported by the National Natural Science Foundation of China (Grant Nos. 50571033, 50371024) and the Natural Science Foundation of Anhui Province(Grant No.070414178)     

基于元胞自动机法的铝合金定向凝固过程微观组织数值模拟

基于溶质扩散和界面能的作用,考虑成分过冷、曲率过冷和界面能各向异性和界面扰动,并改进了溶质再分配的算法以及界面液相浓度偏导数的离散格式,建立了凝固过程中晶体的生长模型。利用此模型并采用元胞自动机方法对铝合金定向凝固过程中的平界面、胞状和枝晶状三种典型界面形貌演化进行了模拟,并把模拟参数和模拟结果与经典凝固理论进行了比较,发现在G/V值略小于成分过冷判据给出的临界值时就已经获得平界面。并验证了平均一次枝晶间距与温度梯度和抽拉速度的变化关系,与理论结果吻合较好。     

过冷对纯镍熔体凝固过程中树枝晶侧向分枝的影响

在相场模拟中，通过捕获液态界面，系统研究了过冷对纯金属熔体凝固过程中树枝晶侧向分枝形貌的影响．结果表明，随着熔体过冷度的增大，晶体中二次枝晶越来越发达，其百分含量线性增加；当过冷度足够大时，一次枝晶可能发生缩颈，并在对流作用下于缩颈处熔断，导致晶粒的自身细化．     

过冷对纯镍熔体凝固过程中树枝晶侧向分枝的影响

在相场模拟中，通过捕获液态界面，系统研究了过冷对纯金属熔体凝固过程中树枝晶侧向分枝形貌的影响.结果表明，随着熔体过冷度的增大，晶体中二次枝晶越来越发达，其百分含量线性增加；当过冷度足够大时，一次枝晶可能发生缩颈，并在对流作用下于缩颈处熔断，导致晶粒的自身细化.