Al-Bi过偏晶合金显微组织演变的数值模拟

基于Al-Bi过偏晶合金凝固通过难混溶区阶段产生的液-液相分解及分离的运动行为,采用两相体积平均法,对质量、动量、能量、组分及液滴密度守恒方程进行数值模拟,计算中考虑了形核、扩散长大、Stokes运动及Marangoni运动等多种复杂物理现象的耦合作用,分析了两相运动速度、第二相尺寸分布、第二相体积分数分布以及液滴密度分布对过偏晶合金凝固的显微组织演变及宏观偏析的影响。结果表明,过偏晶合金凝固过程中显微组织演化在不同阶段的主要影响因素不同:凝固初始阶段主要以形核和扩散长大为主;凝固中期和后期第二相迁移运动行为将逐渐占主导作用。凝固过程中,铸件顶角位置首先获得过冷度驱动形核,并以较快的形核速率达到最大形核密度。随着凝固过程不断地进行,第二相小液滴受到的Marangoni力约为Stokes粘滞阻力的两倍,开始由铸件顶角和边缘低温区向中心高温区聚集。凝固时间为1 s时,铸件顶角和边缘第二相小液滴的生长直径和第二相体积分数大于铸件中心位置,而凝固时间为5和7 s时,第二相小液滴直径随铸件中心距离变化的曲线斜率随凝固过程的进行而逐渐变缓,长大速率逐渐变慢。     

单辊快速凝固法制备Co-Cu-Pb合金颗粒的结构与形成机制

分别以Co_(47.5)Cu_(47.5)Pb_5和Co_(42.5)Cu_(42.5)Pb_(15)三元偏晶合金作为母合金,采用单辊法急冷快速凝固制备Co-Cu-Pb三元难混溶合金颗粒,对颗粒的微观组织结构与尺寸进行观察与分析,并对不同结构颗粒的形成机制进行研究。结果表明:Co-Cu-Pb合金颗粒的直径为70~600μm,得到实心颗粒、空心颗粒及多层壳核结构3种不同结构的颗粒。Co-Cu-Pb合金颗粒发生包晶反应形成富Co(Cu)相的初生枝晶,富Pb相主要富集于枝晶间隙处。随辊面线速度从15 m/s增大到30 m/s,初生Co(Cu)相枝晶发生粗大枝晶→细小等轴晶的转变,合金颗粒的凝固组织显著细化,并且由于液态难混溶合金发生Marangoni运动,形成快速凝固多层壳核结构,最终获得均质化的Co-Cu-Pb合金凝固组织。     

镍基高温合金GH4151的偏析及相析出行为

摘要：利用电子探针(EPMA)、场发射扫描电镜（SEM）及差热分析（DTA）研究了GH4151合金的元素偏析行为、铸态组织特征以及析出相种类,并对合金凝固过程进行讨论。结果表明：GH4151合金凝固过程中,W元素偏聚于枝晶干,Mo、Nb、Ti元素偏聚于枝晶间,Co、Cr、Al元素几乎不发生偏析,Nb、Ti元素偏析较重。GH4151铸锭心部为粗大的等轴晶,主要析出相包括强化相γ′相、一次碳化物、η相、（γ+γ′）共晶相以及Laves相,其中枝晶间分布的η相、（γ+γ′）共晶相和Laves相为低温脆性相在凝固末期形成,扩大了合金的凝固区间,从而导致合金热裂敏感性增加。     

水平稳恒磁场对Al-Bi偏晶合金中富Bi相迁移和分布的影响

Al-Bi偏晶合金是潜在的新型汽车轴瓦材料,然而由于偏晶合金的凝固特点,凝固过程中的液-液分离反应极易造成第二相的宏观偏析,很难采用传统凝固方法制备。在水平稳恒磁场下进行Al-Bi偏晶合金的水冷铜模浇铸实验,使熔体内不同位置的温度梯度方向与磁场方向呈不同角度。研究当磁场与富Bi液滴Marangoni运动方向的夹角不同时,磁场对偏晶合金凝固组织影响效果的变化。研究结果表明当磁场方向与富Bi相液滴Marangoni运动方向平行时,磁场对宏观偏析的抑制效果最明显。对本实验条件下磁场在Al-Bi偏晶合金凝固过程中的作用机制进行了分析,Al-Bi偏晶合金富Bi液滴发生了由冷端向热端的Marangoni运动,磁场主要通过电磁拖曳力作用于液滴,降低液滴的迁移速度。当磁场与温度梯度方向垂直时,其对熔体传热的抑制减小了电磁拖曳力的作用效果,加重了富Bi相液滴Marangoni运动造成的偏析。     

锆对急冷凝固FGH96合金粉末显微组织和元素偏析的影响

采用金相显微镜(OM)、扫描电镜(SEM)及其附带的能谱仪,对不同Zr含量的急冷凝固FGH96合金粉末的显微组织、热学凝固参数和枝晶间合金元素偏析进行研究。结果表明:通过调整Zr含量,可以改变粉末颗粒内部树枝晶、胞状长大晶和微晶凝固组织的比例;粉末凝固组织形态主要取决于冷却速率T-、固液界面前沿温度梯度G和长大速度R;不同Zr含量的FGH96合金粉末颗粒中,Mo、Nb、Ti、Zr等元素富集于枝晶间,Co、Cr、W和Ni富集于枝晶轴,随Zr含量增加,Ti、Nb、Zr等元素的枝晶偏析程度减小。     

GH5188合金凝固过程动态原位观察研究

利用高温共聚焦激光扫描显微镜对GH5188合金在冷速为10～200℃/min条件下的凝固过程进行动态原位观察，并研究了冷速对GH5188合金凝固过程、凝固组织及析出相的影响。结果表明，GH5188合金的凝固过程为缓慢凝固-快速凝固-缓慢凝固过程，冷速越大，峰值凝固速度越大。冷速影响合金凝固温度，随着冷速的增加，凝固温度逐渐降低，冷速的增加使合金凝固后二次枝晶间距减小，析出相尺寸更均匀细小，有利于减少合金凝固过程的裂纹敏感性，获得了不同冷速下合金二次枝晶间距的预测公式。     

Fe-C合金凝固枝晶形貌及特性参数的相场模拟

 为了实现对合金凝固过程中枝晶形态的定量表征、揭示凝固前沿溶质分布与过冷度对微观偏析的影响,进而实现对凝固枝晶间液相渗透率的量化研究,采用相场模型探讨了Fe-0.5％C合金凝固过程中的显微组织和特征参数,并引入分形维数和无量纲周长定量分析了枝晶形貌、微观偏析和其糊状区的渗透性。结果表明,分形维数和无量纲周长可用于定量描述枝晶形态的复杂性。当过冷度从20增加到27 K时,分形维数从1.28增加到 1.791,无量纲周长从2.39增加到12.6;随着过冷度的增加,枝晶中心轴和固/液界面的溶质浓度均增加,并且枝晶尖端的扩散层厚度减小,即枝晶之间的偏析率增大。此外,利用分形维数和无量纲周长作为凝固过程中枝晶曲折因子实现了凝固过程枝晶间糊状区渗透率的量化计算。其中,与分形维数相比,利用无量纲周长作为曲折因子估测的渗透率与试验结果更为吻合,其在不同过冷度下数值为1.36×10-15~1.75×10-13 m2。     

稀土La在Al-Si共晶合金中的偏聚及其与合金细化变质的关系

本文通过定向凝固和液-固界面的微区成份分析及电镜组织观察,研究了La在Al-Si共晶合金中的分布规律和共晶两相生长特点与合金细化变质的关系。试验表明,La在液-固界面前沿明显富集,共晶生长形态发生变化,α-Al领先相树枝晶发展,共晶枝晶和α-Al枝晶均有颈缩和熔断现象。     

Pr基块体非晶的制备及特性研究

在铜模铸造条件下制备了直径5mm的P r61Cu19N i10A l10大块非晶合金,在普通DSC条件下观察了这种合金的玻璃转变温度。对制备的非晶合金进行晶化处理,测定了不同处理温度下的电阻率,观察了P r61Cu19N i10A l10块体合金的缓冷凝固组织。发现随晶化程度的增加,合金样品的电阻率下降,完全晶化后合金的电阻率比非晶态的电阻率低14%。P r61Cu19N i10A l10块体合金的缓冷凝固组织由大量细小规则的树枝状晶和少量共晶组织组成。     

影响钢锭中A,V形偏析的几种因素

通道偏析产生于凝固过程中的固－液两相区，并在最后的凝固组织中以Ａ形或Ｖ形分布，近十几年，通道偏析的研究已有了很大的进展，减少这种缺陷的主要手段是减少或消除糊状区中枝晶间的液体液动，因此，合金成分和凝固过程中的导热条件是影响钢锭中Ａ、Ｖ形偏状的决定因素，基于通道偏析的形成机理，提出一些可以防止Ａ、Ｖ形偏析的措施，以指导实际生产。     

气雾化Al-Pb系轴瓦合金

采用气雾化技术，制备了应用于工业化ＲＳＰＭ工艺的高质量ＡｌＰｂ系合金粉末。对雾化粉末显微结构的分析表明，第二相（铅相）在基体中分布均匀，其粒径大小取决于凝固过程的冷速；不同粒径的粉末第二相分布随冷速的增加而分布更均匀、细化。对雾化粉末中各元素分布的分析表明，硅、铜等元素在晶界上有富积现象。     

Alloy solidification in systems containing a liquid miscibility gap

Directional solidification methods have been used to examine the growth of fibrous, or tubular, composite samples in alloys close to the monotectic reaction: Liquid 1 Solid + Liquid 2. The binary systems Al-In, Cu-Pb and Cd-Ga have been examined and some use made of a transparent analogue, succinnonitrile-water. The solidification behavior appears to relate to the height of the miscibility gap in such systems and it is demonstrated how ternary additions which change this height (Sn to Al-In, Al to Cu-Pb) modify the microstructures dramatically. These structural changes are discussed with reference to the relative surface energies between two immiscible liquids and a third (solid) phase. The phase spacings and transitions in microstructure are discussed in terms of diffusion processes at the monotectic front. Formerly with University of Wisconsin-Milwaukee     

Solidification of Pb particles embedded in Al

Hypermonotectic alloys of Al-5 wt% Pb and Al-5 wt% Pb-0.5 wt% X where X = Mn, Cu, Zn, Fe and Si have been manufactured by chill-casting and melt-spinning. The resulting microstructures have been examined by a combination of optical microscopy, scanning and transmission electron microscopy, and electron probe microanalysis. The as-solidified hypermonotectic alloys exhibit a homogeneous bimodal distribution of faceted Pb particles embedded in a matrix of Al, with chill-cast Pb particle sizes of 1–2 μm and 5–50 μm, and melt-spun Pb particle sizes of 5–10 nm and 50–100 nm. The larger Pb particles are formed during cooling through the region of liquid immiscibility while the smaller Pb particles are formed during monotectic solidification of the Al matrix. The Pb particles exhibit a cube-cube orientation relationship with the Al matrix, and a truncated octahedral shape with {111} and {100} facets. The as-solidified Pb particle distributions are resistant to coarsening during post-solidification heat treatment. The equilibrium Pb particle shape and therefore the anisotropy of solid Al-solid Pb and solid Al-liquid Pb surface energies have been monitored by in situ heating in the transmission electron microscope over the temperature range between room temperature and 550°C. The anisotropy of solid Al-solid Pb surface energy is constant between room temperature and the Pb melting point, with the {100} surface energy 14% greater than the {111} surface energy, in good agreement with geometric near-neighbour bond energy calculations. The {100} facets disappear when the Pb particles melt, and the anisotropy of solid Al-liquid Pb surface energy decreases gradually with increasing temperature above the Pb melting point, until the Pb particles become spherical at about 550°C. The kinetics of Pb particle solidification have been examined by heating and cooling experiments in a differential scanning calorimeter. Pb particle solidification is nucleated catalytically by the Al matrix on the {111} facet surfaces, with an undercooling of 22K and a contact angle of 21°C. Ternary additions of Mn, Cu, Zn and Fe do not influence the Pb particle solidification behaviour, but Si is a potent catalyst and stimulates the Pb particles to solidify close to the equilibrium Pb melting point.     

In situ studies of precipitate formationin Al-Pb monotectic solidification by X-ray transmission microscopy

Al-1.5 wt pct Pb monotectic alloys were unidirectionally solidified. X-ray transmission microscope (XTM) observations, both during and after solidification, revealed various new morphological/compositional features in the melt and solid. In the melt, nonuniform lead-rich interfacial segregation layers and droplets were observed to form well ahead of the interface. In the solid, periodic striae formed at translation/solidification velocities as low as 6 × 10⁻⁶ m/s. The striae shape does not replicate that of the interface. The striae spacing decreases from 4 to 2 × 10⁻⁴ m with an increasing solidification rate between 6 and 16 × 10⁻⁶ m/s. High resolution postsolidification XTM examination reveals that these striae consist of Pb-rich particles of 2 to 3 × 10⁻⁶ m diameter. At translation/solidification velocities below 6 × 10⁻⁶ m/s, Pb incorporation into the solid occurs in the form of continuous fibers and strings of particles of about 5 × 10⁻⁶ m diameter. Bands, parallel to the interface, in which these fibers were aligned in the solidification direction, alternated with bands of poor fiber alignment. The width of these bands is comparable to the striae spacings obtained at the high solidification rates.     

Directional solidification of lead-copper immiscible alloys in a cyclic gravity environment

Hypermonotectic copper-lead alloys were directionally solidified at unit gravity on earth and also in the cyclic gravitational environment attainable during flight of NASA's KC-135 aircraft. In both cases macrosegregation developed that consisted of an initial lead-rich phase above which an aligned composite structure of apparent monotectic composition grew. Differences within these regions are examined, and the suitability of the KC-135 environment for directional solidification of monotectic alloys is discussed. This paper is based on a presentation made in the symposium “Experimental Methods for Microgravity Materials Science Research” presented at the 1988 TMS-AIME Annual Meeting in Phoenix, Arizona, January 25–229, 1988, under the auspices of the ASM/MSD Thermodynamic Data Committee and the Material Processing Committee.     

Variations of Microsegregation and Second Phase Fraction of Binary Mg-Al Alloys with Solidification Parameters

A systematic experimental investigation on microsegregation and second phase fraction of Mg-Al binary alloys (3, 6, and 9 wt pct Al) has been carried out over a wide range of cooling rates (0.05 to 700 K/s) by employing various casting techniques. In order to explain the experimental results, a solidification model that takes into account dendrite tip undercooling, eutectic undercooling, solute back diffusion, and secondary dendrite arm coarsening was also developed in dynamic linkage with an accurate thermodynamic database. From the experimental data and solidification model, it was found that the second phase fraction in the solidified microstructure is not determined only by cooling rate but varied independently with thermal gradient and solidification velocity. Lastly, the second phase fraction maps for Mg-Al alloys were calculated from the solidification model.     

The effect of solidification rate on microsegregation

An X-ray diffraction technique has been utilized to determine the second phase content and average composition of the primary phase in aluminum-copper and aluminum-silicon alloys solidified at a cooling rate in the range of 0.06 to l0⁵ K/s. For the Al-Cu samples, the normalized Θ phase content (ratio of the 6 content to the value predicted by the Scheil model) was found to be 0.71 at 0.1 K/s (solidification rate = 0.001 cm/s) and to increase with increasing cooling rate to 0.96 at about 180 K/s (1 cm/s). Beyond this cooling rate, it decreased with increasing the cooling rate to 0.44 at about 3.7 x lO4 K/s. The same trend was observed in the Al-Si samples, except that the normalized silicon content was much lower. Also, for both systems the normalized average composition of the primary phase was found to decrease progressively with increasing the solidification rate until it reached a minimum at 1 cm/s, beyond which it increased with higher solidification rates. The results are discussed with respect to the prevailing segregation models that include back-diffusion in the solid, dendrite tip undercooling, and the eutectic temperature depression. An equation which combines these effects at all cooling rates is given.