球状石墨的形核与孕育--球墨铸铁基础理论的最新发展(一)

石墨球的形成分两个阶段：形核与生长。虽然形核不影响最终的形状，但它是石墨球形成的重要过程。石墨的形核物质有石墨、硫化物、氧化物、碳化物、氮化物、金属间化合物及气体。各种物质的形核机制可以不同，但是都必须符合晶格匹配关系（失配度小于6%时，形核能力强），以及满足相互的界面能要求。由于球铁过冷倾向大，所以孕育是球铁生产的必须工序。鉴于硫氧化物是球墨形核的重要组分，特介绍一种新型高效孕育剂，其特点在于孕育剂中含有一定的硫氧非金属物质，可补充球化处理后铁液硫、氧的贫缺。     

风电球铁材料球墨形成的原理及工艺

阐述了风电球墨铸铁球墨形核、生长原理与孕育、球化处理的关系。石墨的形核分为均匀形核和异质形核,形核物质与石墨的失配度要小于12%是孕育处理和孕育剂选用的理论基础,孕育处理可以有效增加石墨核心数,孕育以邻近浇注为原则,且采用多次孕育法。球墨的生长必须有强烈的过冷以满足球墨缺陷生长的条件,介绍了提高铁液过冷度的主要方法——球化处理法,介绍了球化剂的选择和使用方法。     

低碳球铁中球状石墨形核及生长机制的探讨

研究表明，低碳球铁中的球状石墨内部结构为Ｃ轴呈放射状的多晶体，并呈现出明显的年轮状结构，球状石墨球能以“气泡”形核，也能以“球形夹杂物”形核。石墨球的核心应该是球形。     

对低碳球铁中石墨核心的分析和研究

通过对低碳球铁中球状石墨核心的分析和研究，探讨了核心形状及其物质构成对低碳球铁中石墨球化的影响规律。研究表明，核心的存在是否墨形核长大的基础，球状石墨形核的最佳核心应是某些非金属球形夹杂物或低沸点表面活性元素的气泡。结果还表明，气泡理论的确可以较好地解释石墨的球化现象，但这一理论至今尚存在不少问题，需要加以认真修改和完善。     

氩气流量对等离子体喷射法制备的金刚石膜形核的影响

采用100kW级直流电弧等离子喷射法进行金刚石膜沉积,阐述了氩气对维持电弧稳定的重要作用,讨论了氩气流量对衬底表面轰击的影响,氩气流量和不同电弧分区对形核期金刚石表面形貌和晶粒尺寸的影响,以及时间和不同预处理方式对金刚石形核密度的影响。结果表明:随着氩气流量的增加,氩气对衬底表面的轰击作用增强,金刚石膜表面形貌呈现从(111)到(100)的变化规律,金刚石晶粒尺寸减小,晶形变得不完整;弧边位置(100)取向更明显;使用金刚石微粒对衬底进行研磨预处理能显著提高金刚石的形核密度。     

铝合金中气孔的形核机理

综述了铝合金中气孔的形核机理。指出铝合金中气孔的形核主要是异质形核,形核位置主要在非润湿性的外生基底和／或液／固生长界面处,前者可以认为是凹槽机制起主要作用,而后者应该考虑非金属夹杂与生长界面的相互作用。只有被凝固界面推移的夹杂才能成为气孔形核的异质核心,而那些被吞没的夹杂则对气孔形成没有影响。     

关于铸铁孕育机理

为了解释铸铁凝固过程中的石墨形核机理,过去已经提出了几种形核理论.大多数理论都是基于这样一种假设,即石墨的形成是由于在凝固过程中非金属夹杂物作为异质形核核心,而微量元素如Ca、Ba和Sr在形核过程中发挥了重要的作用.     

中碳高硅高铝钢回火过程石墨析出动力学

通过光学显微镜、扫描电镜及石墨粒子生长动力学方程对中碳高硅高铝钢回火过程组织及石墨粒子的析出行为和形核长大机制进行了系统研究。结果表明，中碳高硅高铝钢在680℃回火2、5和8 h后的石墨粒子总密度分别2539、4791和6405个/mm²,对应的石墨粒子平均尺寸分别为1.30、1.80和1.90μm。在680℃回火过程中，石墨粒子主要在铁素体晶界形核，其临界形核尺寸约为0.6 nm,在铁素体中的长大行为遵循Ostwald熟化规律，熟化速率为29.38 nm·s^-1/3。     

化学气相沉积金刚石微球的生长机制研究

采用微波等离子体化学气相沉积法,在过饱和碳离子浓度条件下,在单晶硅衬底上制备了球形结构的多晶金刚石微球,通过控制沉积气压与温度的变化,研究了金刚石由石墨生长区向纳米晶的球形结构、再到具有良好结晶性的金刚石生长区的过渡过程。结果表明:沉积气压与温度的升高导致微球的粒径增大,微球由sp3、sp2键共存相转变为较纯的金刚石相;在一定的碳离子过饱和度和气压、温度范围内,微球的形成主要受二次形核过程的控制。气压和温度升高后,微球呈<110>取向生长,微球的形成主要受（111）面高密度孪晶和层错缺陷的控制,揭示了化学气相沉积金刚石不同生长区内二次形核机制与孪晶层错机制诱导的金刚石微球的生长过程。      

电磁场下近液相线高温合金熔体处理非枝晶组织的形成

采用电磁场近液相线熔体保温工艺对Ni-20Cr-18W(质量分数,%)高温合金进行处理,获得了平均晶粒尺寸d=127μm,凝固组织为球状晶粒的60 kg铸锭.借助于OM,SEM和EBSD等手段,针对所获得的电磁场对原子团簇、界面稳定化以及树枝晶向球状晶转变的研究结果,从形核热力学方面对近液相线处理后高温合金铸锭晶粒细化机制进行了研究.结果表明:该工艺能够细化凝固组织;其细化机制为原子团簇与球晶分别作为后续凝固过程中的形核核心,促进凝固过程中晶粒形核,细化组织.此外,在改善熔体宏观温度场均匀性的基础上,电磁场促进了球晶稳定化以及树枝晶向球状晶的转变,并提高其形核率.     

核心形状对石墨球化的影响

研究了低碳球铁中球状石墨的内部结构及核心形状，探讨了核心形状对石墨球化的影响规律。研究表明。核心的存在是石墨形核长大的基础，而核心形状对石墨的最终形状起着至关重要的影响；球形或接近球形的石墨核心长大后大多都能保持原来的球状，形状不规则的核心，则很难长大成为圆整的球状石墨。但是，铁水中如果只有气泡或球形核心存在，而没有足够的粘度、压力、过冷度和较高的表面能以及吸附等其它充分条件的配合，这些核心仍然很难长大成为球状石墨。也就是说，凡是有利于铁水中形成球形核心，并有利于这种核心在各个方向均匀生长的因素。都可看作是促进石墨球化的因素，反之亦然。研究还表明，表面活性元素的球化和反球化作用不是绝对的，在一定条件下它们可能会相互转化。传统理论中认定的球化和反球化元素，实际上并非在各种不同条件下都能促进或破坏球化。因此。有必要根据实际情况对球化元素与反球化元素进行重新界定。     

Tribological behaviors of Ag−graphite composites reinforced with spherical graphite

Two kinds of Ag−graphite composites reinforced with spherical graphite (SG) and conventional flake graphite (FG) were prepared by powder metallurgy technology. The effect of graphite morphology on the tribological behavior for the Ag−SG and Ag−FG under the dry sliding wear was investigated with a pin-on-disk tribometer at a load of 3.0 N in atmosphere condition. The results indicated that the minimum wear rate of 3.5×10⁻⁵ mm³/(N·m) for Ag−FG was achieved and it reduced by nearly an order of magnitude, reaching 1.6×10⁻⁶ mm³/(N·m) for the Ag−SG. The obviously different tribological behaviors between the Ag−SG and Ag−FG were closely related to the formation of cracks in the sub-surface. The stress concentration tended to generate at the edges of flake graphite during sliding process, which resulted in the cracks and severe delamination wear of Ag−FG. However, no cracks were found around the spherical graphite in Ag−SG. The spherical graphite can effectively inhibit the initiation and propagation of cracks, achieving high wear resistance.     

Complex（Mn, X）S compounds - major sites for graphite nucleation in grey cast iron

Despite the cubic system, the ability of sulphides to nucleate graphite can be enhanced by inoculating elements which transform them in complex compounds with a better lattice matching to graphite, a low coagulation capacity, good stability and adequate interracial energy. (Mn,X)S compounds, usually less than 5.0 μm in size, with an average 0.4-2.0 μm well defined core (nucleus), were found to be important sites for graphite nucleation in grey irons. A three-stage model for the nucleation of graphite in grey irons is proposed: (1) Very small micro-inclusions based on strong deoxidizing elements (Mn, Si, Al, Ti, Zr) are formed in the melt; (2) Nucleation of complex (Mn,X)S compounds at these previously formed micro-inclusions; (3) Graphite nucleates on the sides of the (Mn,X)S compounds with lower crystallographic misfit. Al appears to have a key role in this process, as Al contributes to the formation of oxides in the first stage and favors the presence of Sr and Ca in the sulphides, in the second stage. The 0.005-0.010% Al range was found to be beneficial for lower undercooling solidification, type-A graphite formation and carbides avoidance.     

铸铁中石墨的形核问题

ＳｉＯ２可以作为石墨的形核基底．纯Ｓｉ和Ｆｅ—Ｓｉ合金对铸铁都没有任何孕育效果，当含有少量Ｃａ、Ｓｒ、Ａｌ等元素时才会产生明显的孕育效果．孕育铸铁的共晶团的数目与石英棒晶化层的厚度有密切的关系，晶化层越厚，共晶团数目越多．用Ｃａ—Ｓｉ孕育时，石英玻璃棒的晶化速度相当于在空气中的１０００倍．当用含Ｃａ的Ｓｉ或Ｆｅ—Ｓｉ孕育剂孕育时，吸附于玻璃态ＳｉＯ２的表面，促使非晶态ＳｉＯ２发生晶化，晶化的ＳｉＯ２作为石墨的基底，产生明显的孕育作用．     

The effect of graphite shape on the tensile deformation behaviour in hot-swaged and hot-rolled ferritic spheroidal graphite cast iron

The shape of the graphite nodules in ferritic spheroidal graphite (SG) cast iron is changed by hot-swaging and hot-rolling, when symmetric ellipsoidal graphite and asymmetric ellipsoidal graphite can be obtained, respectively. For the symmetric ellipsoidal graphite, the graphite shape ratio tends to decrease with increasing reduction ratio. The increase of flow stress with increasing reduction ratio can be explained by the decrease in the stress concentration induced by the graphite with increasing reduction ratio. On the other hand, the graphite shape ratio is more complex for the asymmetric ellipsoidal graphite than the symmetric ellipsoidal graphite specimens. The graphite shape ratio tends to increase and decrease with increasing reduction ratio in two transverse directions. As a result, a balance exists in the specimen between increasing and decreasing stress concentration, showing that the tensile flow stress is independent of reduction ratio.

To understand the effect of the graphite shape ratio on the tensile flow stress during tensile deformation, specimens with different carbon contents and graphite shape ratios were used in this study. Furthermore, the triaxial parameter (k₁) has been defined such that the larger stress concentration effect will have the larger k₁ value. Based on the present experimental results, the graphite shape ratio tends to decrease with increasing reduction ratio or tensile strain, so that the stress concentration effect and triaxial parameter (K₁) tend to decrease with increasing reduction ratio and tensile strain. Therefore, the effect of the graphite nodules in ferritic SG cast iron on the tensile flow stress is not only reduction of the effective area fraction of the matrix but also development of an uneven triaxial stress field.     

Eutectic solidification mode of spheroidal graphite cast iron and graphitization

The shrinkage and chilling tendency of spheroidal graphite (abbreviated SG) cast iron is much greater than that of the flake graphite cast iron in spite of its higher amount of C and Si contents. Why? The main reason should be the difference in their graphitization during the eutectic solidification. In this paper, we discuss the difference in the solidification mechanism of both cast irons for solving these problems using unidirectional solidification and the cooling curves of the spheroidal graphite cast iron. The eutectic solidification rate of the SG cast iron is controlled by the diffusion of carbon through the austenite shell, and the final thickness is 1.4 times the radius of the SG, therefore, the reduction of the SG size, namely, the increase in the number, is the main solution of these problems.     

DISTRIBUTION OF NODULARIZING ELEMENTS IN R. E. TREATED SPHEROIDAL GRAPHITE CAST IRONS

用自射线照相、电子探针及离子探针等方法研究了铈球铁、轻稀土-镁球铁、钇基重稀土球铁、钇基重稀土-镁球铁中铈、钇、镁、钙等球化元素以及硫的分布。自射线照相所获得的乳胶干板各处的黑度分布近似地反映了石墨与基体中放射性物质的重量百分浓度。在金相显微镜和扫描电子显微镜观察中均发现,有的球墨有核心。电子探针的探测结果说明:在铈球铁中,有的球墨的核心为硫化铈;在用铈、钙稀土合金处理的球铁中,有的球墨的核心有硫化铈和硫化钙;在轻稀土-镁球铁中,有的球墨的核心有铈、镁、硅的化合物。 球化元素铈、镁、钇、钙等和反球化元素硫在球墨中的分布是多样的;这些元素的化合物可能构成某些球墨的结晶核心;有的球墨边缘或周围有铈、钇、镁、钙的富集;有的球墨中铈为均匀分布,钇为不规则分布;在相当多的球墨中,没有铈、钇等球化元素的富集。根据球化元素的多种分布形态,认为球墨的成因,也不限于一种。球化剂的主要作用可能是它能去除硫、氧等杂质,改变了馆体的表面张力和熔体-石墨间的界面张力,从而有利于球墨的形成。 对双层球墨的成因进行了初步分析。     

Synthesis and characterization of aminated SiO2/CoFe2O4 nanoparticles

A kind of newly aminated CoFe2O4 nanoparticles were synthesized by grafting process for biomedical applications, which were coated primarily with silicon dioxide（SiO2）. The characterizations of aminated SiO2/CoFe2O4（ASCN） and SiO2-coated CoFe2O4（SCN） nanoparticles were investigated using elemental analysis, thermogravimetric analysis（TGA）, differential thermal analysis（DTA）, infi＇ared spectroscopy（IR）, atomic force microscopy（AFM）, zeta-potential measurement and vibrating sample magneto-metry（VSM） The AFM micrograph shows that the ASCN nanoparticles are approximately spherical with an average diameter of 30 nm. Based on IR and TGA results, it is suggested that the surface of the SiO2-coated CoFe2O4 nanoparticles are graiied with amino compounds. The elemental analysis also shows the presence of 0.98 mmol/g of organic moieties immobilized on the surface ofASCN nanoparticles. Zeta-potential data ofASCN nanoparticles also reveal that amino compounds are bonded onto the surface of SiO2-coated CoFe2O4 nanoparticles by ether linkage. The magnetic parameters show that ASCN nanoparticles still have good magnetic property.