深过冷Cu-Pb偏晶合金的快速凝固行为和凝固组织

用熔融玻璃净化与循环过热相结合的方法,研究了亚偏晶Cu-25%Pb合金,Cu-37.4%Pb偏晶合金和过偏晶Cu-40%Pb(质量分数)合金过冷熔体凝固行为和凝固组织的演化规律,以及Cu-37.4%Pb偏晶合金的过冷度对磨损率的影响.研究表明:在过冷亚偏晶Cu 25%Pb合金熔体凝固过程中先形成α(Cu)初生相,随着过冷度的增大,凝固组织经历粗大枝晶重熔形成的细化枝晶向准球状晶粒演化的过程;在过冷Cu-37.4%Pb偏晶合金熔体凝固过程中初生相为L2相,当过冷度在20～150 K区间时,得到第二相S(Pb)弥散在α(Cu)枝晶间的凝固组织,并且在该过冷区间内随着过冷度的增加,材料的磨损率也逐渐降低;在过冷过偏晶Cu-40%Pb合金熔体凝固过程中初生相为L2相,在过冷度区间42～80 K时,得到以偏晶胞形式分布的凝固组织.     

Bi含量对Al-Bi偏晶合金显微组织演变的影响

结合数值模拟与实验研究了常规条件下Al-Bi偏晶合金难混溶区的凝固过程,分析了Bi含量对Al-Bi偏晶合金显微组织演变的影响。结果表明,成分为Al-10%Bi合金试样下部组织中富Bi液滴的平均直径和体积分数分别是成分为Al-5%Bi合金试样下部组织中富Bi液滴平均直径和体积分数的1.1倍和12.8倍,且Al-10%Bi合金试样下部聚集了大量的大尺寸富Bi液滴,其显微组织均匀性较差;成分为Al-5%Bi的合金试样中富Bi液滴的平均尺寸较小,显微组织较均匀;偏晶点成分的Al-3.4%Bi偏晶合金显微组织中无大尺寸富Bi液滴生成,显微组织均匀。分析表明,富Bi相含量的增加导致液滴受重力作用增强,加剧了难混溶区液-液相分离和分解的凝固行为,增大了富Bi液滴的尺寸和数量,液滴受到的Ostwald熟化和碰撞凝并的作用增强,促使大量的大尺寸富Bi液滴聚集在试样下部,加重了偏晶合金显微组织的宏观偏析。     

在磁场作用下Al-Pb合金连续凝固组织的形成过程

在恒定磁场作用下对Al-Pb合金进行连续凝固实验,研究了磁场对Al-Pb偏晶合金凝固特性的影响。结果表明,在恒定磁场作用下得到了弥散相均匀分布于基体的凝固试样。随着磁场强度的提高试样中富Pb相粒子的弥散度增加。恒定磁场能增加合金熔体的有效粘度,减小弥散相液滴相对于基体熔体的运动速度,抑制基体熔体对流,促进偏晶合金形成弥散型复合凝固组织。     

磁场作用下偏晶合金凝固组织演变的数值分析

建立偏晶合金难混溶区凝固过程的两相数学模型,模拟研究了有、无磁场Al-10%Bi过偏晶合金微观组织演变,研究了温度、速度、第二相体积分数等物性参数对凝固组织宏观偏析的影响。结果表明,在磁场作用下温度场为中心对称分布,更有利于第二相液滴的均匀分布;电磁力抵消了部分重力和Marangoni力,使无磁场时外环流的速度场变为有磁场时斜向下的速度场,且速度明显降低,从而减轻了强对流导致的重力偏析;在磁场的作用下,试样底部第二相的体积分数减小,凝固组织宏观偏析得到改善。     

有无重力作用下Al-Bi偏晶合金凝固过程的模拟研究

基于二元Al-Bi偏晶合金难混溶区间凝固过程的液-液相分解及分离机制,采用欧拉法求解该合金显微组织演化的数学模型。该模型考虑了实际凝固过程中导致第二相宏观偏析的诸多复杂因素,包括形核、扩散长大、Stokes运动、Marangoni运动及重力沉降的影响。模拟结果表明,无重力条件下Marangoni力驱动L2相小液滴向中心高温区迁移,而重力条件下却呈现轴对称环流运动形式;随着冷却过程的进行,L2相在重力作用下发生沉降聚积,铸件底部L2相体积分数和液滴直径增加显著;与重力条件相比,无重力条件更有利于晶核的形成。     

Al-Bi过偏晶合金凝固过程中富Bi相分布规律研究

在常规重力场条件下,通过极快速液淬冷却的实验方法,分析了Al-Bi过偏晶合金在难混溶区凝固过程中,不同凝固时刻和温度下富Bi相液滴的形核、扩散长大、Marangoni迁移、重力沉降和碰撞凝并等复杂的物理过程,考察了富Bi相液滴平均直径及最大直径等随时间变化的分布规律,揭示了偏晶合金宏观偏析的形成过程与影响因素的作用程度,为制备匀质偏晶合金提供理论参考和实验依据。     

Cu-Cr合金初生相对共晶生长的影响机制研究

制备了定向凝固Cu-1.0%Cr亚共晶自生复合材料,研究了初生α相生长对共晶生长的影响机制,探讨了亚共晶合金中共晶的生长规律.研究结果表明,Cu-1.0%Cr合金定向凝固时,在初生α相间生长的共晶受到初生相生长的影响,在热场不定向和生长空间受限的双重作用下,共晶无定向地杂乱生长.初生α相的生长引起枝晶间液相溶质分布的变化,随着凝固速度的增大,初生α枝晶间液相溶质的浓度分布趋于平缓,成分趋近于CE.Cu-1.0%Cr合金在快速凝固条件下,初生α相生长改变了共晶的生长环境,致使形成非平衡凝固组织--离异共晶.     

Pr对Sn-Cd包晶合金定向凝固组织的影响

为了解Pr对Sn-Cd包晶合金凝固组织的作用,在不同抽拉速率下对Sn-Cd-Pr合金进行定向凝固实验.采用光学显微镜、扫描电子显微镜(SEM)和能谱分析(EDS)研究了Pr对Sn-Cd包晶合金凝固组织的影响.研究发现:对于Sn-1.8%Cd过包晶合金,在2μm/s时0.5%的Pr增加了包晶相β的体积分数;在4μm/s时凝固组织明显细化.对于Sn-0.65%Cd亚包晶合金,在4μm/s时Pr的加入促进了平→胞界面的转变;在16μm/s时随着Pr含量的增大,凝固组织逐渐细化;在100μm/s时Pr含量的增大使凝固组织仅有微弱的细化.研究表明,Pr的加入可以改变组成相的体积分数、细化组织以及减小界面稳定性.     

综论偏晶合金的制备技术:外场下凝固、快速凝固及激光技术

当偏晶合金液过冷至液相分离温度(Tsep)以下时,进入亚稳态难混溶区间,由单一液相分离成两个液相:L1(主体液相,质量分数大于50%)与L2(次生液相,收缩成液滴)。微观组织演化呈现三个阶段:(1)相分离自发进行阶段;(2)主体相合金熔体进入结晶过程;(3)残余的次生相合金熔体进入凝固阶段。尤其是当次生相凝固后弥散分布于主体相基体内时,偏晶合金具有高强、高导以及高耐磨性能,其在航空航天和汽车等工业领域具有重要的应用前景,长期以来受到了研究者的广泛关注。偏晶合金组织结构特征有两种,即第二相弥散型和核/壳结构型。然而,常规凝固条件下,制备的偏晶合金极易形成严重偏析或分层组织,导致制备大块匀质偏晶合金变得困难。为了深入研究偏晶合金液相分离行为,以及微结构特征对偏晶合金性能的影响,研究者提出了许多制备偏晶合金的方法。早在1958年,液相分离现象就在Cu-Fe偏晶合金中被发现,当即引起学者们的广泛关注。近年来,为了制备组织均匀和性能优异的偏晶合金,开发了许多外场作用下的偏晶合金制备方法,旨在消除常规重力场下熔体对流造成的凝固组织偏析、位错、空洞等缺陷。例如,在微重力场条件下,对流作用减弱,可制备接近无偏析的凝固组织;在电磁场条件下,实现了对材料工艺过程的控制和材料组织与性能的改善;在直流磁场和电场交互作用下,熔体流动得到抑制,实现了电磁搅拌控制凝固;在交流磁场和电场交互作用下,实现了电磁搅拌和电磁悬浮,达到减小偏析和改善组织结构特征的目的;在超声场作用下,实现了材料无容器凝固。此外,快速凝固是一个典型的非平衡相变过程,可以消除合金的溶质偏析,获得常规凝固条件下无法获得的成分、相结构和显微组织,显著提高合金的强度、塑性、韧性、延展性和磁性等。为深入了解各类偏晶合金的制备方法,本文主要从外场下凝固、快速凝固、激光技术角度综述了偏晶合金的各种凝固制备工艺和研究方法。     

定向凝固超高强度NiTi基自生复合材料

在NiTi合金中添加少量Al、Nb、Hf强化元素, 采用定向凝固原位自生法制备了一种NiTi基自生复合材料, 并在950 ℃对定向凝固棒分别进行了12 h、50 h及100 h的均匀化热处理, 对热处理后的试样进行了室温抗拉强度测试。结果表明, 定向凝固组织为沿[001]方向生长的细小的棒状胞晶组织, β-Nb相和Ti₂Ni相增强体颗粒沿NiTi胞晶间的[001]方向排列。随热处理时间的延长, 胞晶尺寸逐渐粗化, 两增强相粒子分布更加弥散均匀。热处理后NiTi基自生复合材料的最大抗拉强度达到1972 MPa, 超过了4130和8640超高强度钢, 与4140和4340超高强度钢的强度基本相当, 达到或超过了多种国内外现役的超高强度钛合金。     

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．     

Studies on relative catalytic efficiencies of zinc and aluminium on nucleation of dispersed bismuth droplets

In Zn-Bi and Al-Bi systems, both of which belong to monotectic class, dispersion of second phase particles within the matrix have been produced through rapid solidification processing (RSP) route. In order to understand relative catalytic efficiencies of Zn and Al matrices in catalyzing nucleation of Bi particles entrapped in the respective metal matrices, thermal analysis in constant program mode was performed. Thermal analyses revealed that Bi undercools by 132° in Zn matrix and by 157° in Al matrix. Thermodynamic barrier to nucleation (ΔG*) for Bi has also been calculated, which is 39·8 kcal/mole and 47·085 kcal/mole, in Zn and Al matrices, respectively at the maximum recorded undercoolings.     

偏晶合金液—液相变过程中第二相液滴的碰撞粗化

建立偏晶合金液－液两相区内第二相液滴碰撞粗化的数学模型。并对Ｚｎ－Ｐｂ合金作数值模拟计算。结果表明，当第二相液滴平均半径〈１μｍ时，布朗凝并起主要作用。当第二相液滴平均半径较大时。重力凝并起主要作用。     

Coarsening kinetics of coherent γ′ precipitates in ternary Ni-based alloys: the Ni–Al–Si system

Coarsening of coherent precipitates γ' in alloys such as Ni-based alloys has been studied extensively not only for its practical significance in the design of engineering alloys but also in an effort to understand the phenomenon of coarsening. However, a complete understanding of the role of the multiple factors that can affect the coarsening kinetics in such systems is still lacking. Although some advances have been made through computer simulations, studying experimentally the influence of the volume fraction of the second phase and that of coherency strains on the kinetics of coarsening has been particularly challenging.This paper will highlight some of the issues that are relevant to the study of coarsening in multi-component alloy systems. Recent results obtained for the kinetics of coarsening of γ' precipitates in different alloys within the Ni-Al-Si system will be presented. Compositions of these alloys have been chosen so as to vary both the magnitude and the sign of coherency strains between the precipitate and the matrix. Some anomalies related to the composition dependence of the kinetics of coarsening will be highlighted. This paper will conclude with a discussion on the role of the volume fraction of the second phase and coherency strains in contributing to these anomalies and identify directions for future work.     

Phase field formulations for modeling the Ostwald ripening in two-phase systems

Phase field formulations have been constructed for modeling Ostwald ripening in two-phase systems. The microstructural evolution and the kinetics of Ostwald ripening were studied by numerically solving the time-dependent Ginzburg-Landau (TDGL) equations. The simulated microstructures are in a striking resemblance with experimental observations. The shape accommodation of second phase particles occurs as the volume fraction increases. It was observed that these two-phase systems reach the steady state or scaling state after a short transient time and the scaling functions are independent of time for all volume fractions of the second phase. The kinetics of Ostwald ripening in a two-phase mixture have been studied over a range of volume fractions of the coarsening phase. It was found that the coarsening kinetics of second phase particles follows the power growth law R_t^m − R₀^m = kt with M = 3, which is independent of the volume fraction of the coarsening phase. The kinetic coefficient k increases significantly as the volume fraction of the coarsening phase increases.     

The effect of an electric field on the solidification of off-eutectic tin-lead composite alloys

The influence of an electric field upon the directional solidification of hypoeutectic and hypereutectic Sn-Pb composite alloys was studied. Equations were derived to account for the effect of electrotransport of solute in the liquid upon the solid composite composition. Experimental results support the theoretical predictions. Mixing in the liquid is more strongly influenced by unstable solute gradients than by current induced convection. Electrotransport shows limited promise as a method for control of the volume fraction fibre phase in the preparation of aligned composites by directional solidification.     

微重力条件下偏晶合金的研究进展

介绍了重力对偏晶合金凝固过程的影响,综述了微重力环境下偏晶合金凝固研究中的新进展,总结了不同磁场模拟微重力环境下的实验研究成果,探讨了不同微重力条件制备偏晶合金的特点,指出了利用磁场制备偏晶合金的发展前景,最后提出了微重力条件下偏晶合金今后研究的方向和途径。     

The monotectic reaction in Cu-Nb alloys

The solidification of Cu-Nb alloys has been studied by a chill casting technique. Formation of small spherical particles, termed spheroids, was observed at compositions of 15 and 20 wt % Nb, and comparison to similar particles found in chill cast Cu-Pb alloys presents evidence that the spheroids are produced by a monotectic reaction. It is shown that the spheroids are induced by oxygen impurities in the alloys and that their formation is also promoted by high solidification rates. A mechanism for spheroid formation is postulated.