共查询到18条相似文献,搜索用时 125 毫秒
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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. 相似文献
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《材料研究学报》2016,(8)
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。 相似文献
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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。 相似文献
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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。 相似文献
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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. 相似文献
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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. 相似文献
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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。 相似文献
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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。 相似文献
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Sanjay Chaubey 《Bulletin of Materials Science》2007,30(2):81-87
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. 相似文献
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《Science and Technology of Advanced Materials》2000,1(1):51-62
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. 相似文献
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Danan Fan Long-Qing Chen S. P. Chen Peter W. Voorhees 《Computational Materials Science》1998,9(3-4):329-336
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 Rtm − R0m = 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. 相似文献
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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. 相似文献
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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. 相似文献