纯铁和高洁净钢的非均质形核

采用电解纯铁通过高温钼丝炉制备含有不同数量异质核心的试样,用差热分析法分别测定了纯铁和高洁净钢试样在相同冷却速度下的过冷度.结果发现,随着洁净度的提高,高洁净钢和纯铁凝固的过冷度增加,形核功降低,临界晶核半径变小,形核率增大,凝固组织中晶粒的数量增加,晶粒得到细化.在洁净度相同的情况下,高洁净钢形核凝固的过冷度比纯铁小.     

Relationship Between Inoculants and the Morphologies of MnS and Graphite in Gray Cast Iron

The influence of oxides and sulfides in gray cast iron on the growth morphologies of MnS and on the nucleation of graphite was experimentally investigated using scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDX) analysis with evidences that shows the possible nucleation sites for graphite nucleation. Thermodynamic studies have been done on the influence of varying sulfur concentrations on the nucleation of MnS in the melt and during solidification. The consumption of dissolved oxygen and sulfur in the melt during the cooling process was analyzed and we explored how this influenced the nucleation process of oxides and sulfides. A sequential nucleation concept of oxides and MnS is proposed in relation to the growth morphology of MnS and graphite with respect to the mechanical properties of cast iron. The nucleation of new oxides and sulfides was analyzed using thermodynamics and compared to our experimental results. Graphite nucleation on substrates other than MnS, such as MoS₂ oxides and (Mo,Cr)S, was experimentally analyzed along with the influence of the substrates on graphite nucleation and growth morphology.     

Correlation between Surface Structure and Iron Nucleation in the Wustite Reduction

The effects of the orientations of surface grains on the iron nucleation process during the wustite reduction at 1073 K were studied. The orientation of grains in polycrystalline wustite was measured by applying electron backscattering patterns technique. Comparing the numbers of the formed iron nuclei in the grains with different orientations, the reactivity of iron nuclei formation was established as a function of grain orientation. The highest number of iron nuclei was observed in the grain with the orientation of (649). The nucleation behaviour was discussed based on the BCF (Burton‐Cabrera‐Frank) model of crystal growth. The step density in the surface structure of the grains was found to be the dominant controlling factor for the iron nuclei formation during the wustite reduction process.     

Quantitative Relationship Between the Nucleation Undercooling of Liquid Iron and Its Liquid Structure: Investigated by In Situ Synchrotron Radiation

Metallurgical and Materials Transactions A - The nucleation behavior of liquid iron with various iron oxides was investigated through an in situ synchrotron radiation analysis and undercooling...     

The Influence of calcia and magnesia in wustite on the kinetics of metallization and iron whisker formation

Calcium oxide andJor magnesium oxide are introduced in different ways to high purity wustite powders, followed by metallization in mixtures of CO, CO₂, and N₂ at 750, 900, and 1100 °C. A new technique is developed to study the nucleation and growth of iron where wustite plates are coated with thin films of CaO of MgO prior to metallization. Calcia, in solid solution with wustite, influences the kinetics of metallization and the morphology of iron. It is reasonably confirmed that nucleation and outward growth of iron, to form iron whiskers, occur at sites higher in CaO concentration on the wustite surface. MgO, on the other hand, has almost no noticeable effect on the iron whisker formation.     

The transition from gray to white cast iron during solidification: Part I. Theoretical background

The present work is based on a heat balance during the solidification of cast iron. Accordingly, an analytical expression was derived to relate the chilling tendency (CT) of cast iron to nucleation and growth processes associated with the eutectic graphite and cementite constituents. A relationship is found between the CT and factors such as the physicochemical state of liquid, the distribution of nucleation sites, and the density of nucleation sites for eutectic cells. In particular, it is found that the CT can be related to the critical casting modulus (M _cr ), enabling determinations of minimum wall thickness for chilled castings or chill widths in wedge-shaped castings. Finally, the present work provides a rational for the effect of technological factors such as the chemistry of the melt, inoculation practice, holding temperature, and time on the resultant CT and chill of the cast iron.     

The mechanism of whisker growth in the reduction of wüstite

The growth of iron whiskers on wüstite is explained on the basis of C. Wagner’s mechanism for the reduction of nonstoichiometric oxides. The first metal nucleus develops into a whisker if the iron accumulation in the supersaturated wustite particle is nearly uniform, a condition which is favored by chemical control. A mathematical model is proposed in which the “filling ratio” of wüstite by iron prior to nucleation is expressed in terms of the reaction constant, the chemical diffusion coefficient of iron, the particle radius, the Fe/o ratio at equilibrium with the gas, and the critical Fe/o ratio for nucleation at the most favorable site. The morphology of iron in the early stages of reduction, which goes from a single cylindrical whisker, through sponge iron to a smooth layer, can be predicted by comparing at all times the map of the actual Fe/o ratio at the surface with the map of the local Fe/O ratio for nucleation. A satisfactory test of the theory is obtained through a survey of the experimental evidence available about the effect of several physico-chemical factors of reduction on the iron morphology,e.g. surface characteristics, particle size, composition of the reducing gas, solute cations in wüstite, temperature, and gas transport. Formerly with the Ecole Centrale des Arts et Manufactures, Chătenay-Malabry     

氧化钙在铜渣还原过程中的作用

 铜渣是火法炼铜过程中产生的固体废弃物，其中铁质量分数约为40%，中国每年排放的铜渣量高达1 400万t，因此，研究铜渣还原提铁对资源综合利用具有重要意义。结合铜渣的物相分布，介绍了氧化钙在铜渣还原过程中的促进和抑制两种作用机理。促进作用包括氧化钙可以显著降低铜渣还原体系的反应初始温度、破坏铜渣中铁橄榄石结构、促进渣相中氧化亚铁的释放、降低铁晶粒的成核势垒，并加速铁晶粒的成核。抑制作用为氧化钙与硅氧化物生成高熔点的钙硅酸盐，增加铜渣渣相的固相率，阻碍铁晶粒的聚集、长大。     

The effect of carbide and nitride additions on the heterogeneous nucleation behavior of liquid iron

A systematic study of carbide and nitride additions on the heterogeneous nucleation behavior of supercooled liquid iron was undertaken. It was found that titanium nitride and titanium carbide were very effective in promoting heterogeneous nucleation. These compounds were followed by silicon carbide, zirconium nitride, zirconium carbide, and tungsten carbide in decreasing order of effectiveness. The degree of potency of the nucleation catalysts is explained on the basis of the disregistry between the lattice parameters of the substrate and the nucleating phase. Through the inclusion of planar terms the Turnbull-Vonnegut “linear” disregistry equation was modified to more accurately describe the crystallographic relationship at the interface during heterogeneous nucleation.     

Thermodynamics of nucleation and supersaturation for the aluminum-deoxidation reaction in liquid iron

The nucleation process of alumina in aluminum-deoxidized liquid iron was investigated by computer simulation, in which the Gibbs free-energy change of the parent liquid iron, the dependence of interfacial free energy between liquid iron and α-alumina on oxygen content, and the dependence of the interfacial free energy on the curvature of a nucleus were considered. The calculated curve of the Gibbs free-energy change of the systems (ΔG), with respect to nuclear radius, has a maximum and a minimum. Nucleation occurs rapidly when the initial oxygen content is higher than the critical point of nucleation (C _O ^cr ), but the growth of nuclei stops just after ΔG reaches its minimum. At the minimum, the small alumina nuclei are suspended in liquid iron for an extended period of time. This suspension is one reason for the presence of excess oxygen in liquid iron above the α-alumina equilibrium level, which is characteristic in this system. The residual dissolved aluminum and oxygen at the ΔG minimum remain supersaturated in the liquid iron. At an initial oxygen content below C ₀ ^cr , no nucleation can occur, and the components in the liquid iron remain in the supersaturated state. This supersaturation is another reason for the phenomenon of excess oxygen in liquid iron.     

The rate of CO bubble nucleation at oxide metal interfaces within liquid iron alloys

Baker, Warner, and Jenkins found that levitated droplets of Fe-0.8 pet C alloys exploded when decarburized at 1660°C, whereas during the present investigation, the drops remained intact during decarburization at temperatures above 1850°C. Therefore, the object of this work was to determine whether heterogeneous nucleation of CO bubbles at an iron-iron oxide interface could occur at 1900°K but could not occur at 2200°K. An equation was developed to calculate the nucleation rate of CO bubbles at an iron-iron oxide interface in iron at 1900°K containing 0.8 pct C and in iron at 2200°K containing 0.1 pct C. The results of the calculation showed that an iron-iron oxide interface could not serve as a site for CO bubble nucleation. Therefore, a new mechanism is postulated in which cavities swept into the levitated droplet from the surface serve as nuclei for CO bubble formation instead of nuclei formed at the iron-iron oxide interface.     

Microstructural features produced by the reduction of wustite in H2/H2O gas mixtures

The systematic study of the reduction of pure wustites (FeO) between 600 and 1100°C in H₂/H₂O gas mixtures has revealed a number of important morphological changes. It has been shown that dense wustite can decompose to form a highly porous wustite before iron nucleation takes place. The product morphologies of iron formed on the wustite on reduction have been classified into three types, (a) porous iron, (b) porous wustite covered by dense iron, and (c) dense wustite covered by dense iron.     

Graphite Crystallization in Ductile Iron

The authors studied effect of cerium and lanthanum on crystallization parameters anti structure formationof cast iron.It was found that REM additives increased duration and temperature range of eutectic transforma-tion and prevented primary graphite precipitation.The change in crystallization pattern of iron is due to theREM effect on graphite nucleation and growth.On the basis of microprobe analysis of REM distribution and theDTA results,a mechanism of the REM effect on iron crystallization and graphite morphology was suggested.     

CO气氛下MgO对Fe2O3还原时金属铁析出微观行为的影响

利用高温光学体视显微镜和高温热台研究MgO对CO还原Fe2O3中铁晶须成核、生长的影响,并结合SEM、EDS、TG、红外光谱等,从微观角度揭示MgO对还原过程气固界面上金属铁的析出形态的影响机理。结果表明,掺入MgO能有效改变氧化铁还原后析出铁的形态,抑制了铁晶须成核和生长;当掺入质量分数超过2%时,还原后金属铁全部以层状晶析出,无铁晶须形成。     

MgFe_2O_4铁氧体改性羰基铁粒子制备及吸波性能

采用非均匀成核和化学沉淀相结合的方法制备了MgFe2O4铁氧体改性羰基铁(Carbonyl Iron,CI)复合粒子。用XRD和SEM对产物的晶体结构和微观形貌进行分析;研究了改性前后粒子在2~18GHz的电磁参数和吸波性能。结果表明:采用非均匀成核和化学沉淀相结合的方法可以制备MgFe2O4包覆CI复合粒子;改性粒子的复磁导率和介电常数实部较CI降低;介电常数虚部在频率为6~11.5GHz时大于CI,并在9 GHz左右有一个电损耗谱峰;改性粒子在-10 dB处频宽大于CI,最大吸收峰位置较CI向高频迁移;吸收层厚度为1.5 mm时,改性粒子的吸收峰值为-17.8241 dB,-10 dB的频宽为5.52 GHz,较改性前CI粒子,有效地改善了单一CI作为吸波剂的吸波性能。     

Nb合金化对高Cr铸铁轧辊组织的影响

微合金化是细化高Cr铸铁轧辊的凝固组织、提高轧辊的性能和使用寿命重要手段。研究了Nb微合金化对高Cr铸铁轧辊组织和硬度的影响,并采用Thermo-Calc软件分析了Nb对高Cr铸铁凝固组织的细化机制,结果表明,Nb合金化能够显著细化高Cr铸铁的轧辊的凝固组织,提高轧辊的硬度。Nb微合金化对高Cr铸铁的组织改善作用取决于MC型碳化物析出温度及其对奥氏体和共晶M₇C₃碳化物的形核的促进作用。当Nb质量分数为0.5%时,高Cr铸铁轧辊硬度最大;进一步提高Nb含量能显著细化高Cr铸铁轧辊组织,但使硬度降低。     

CO还原Fe2O3过程中金属铁析出的微观行为

 通过原位观察和同步热分析的方式,研究气固两相界面还原反应时金属铁析出形态、矿相结构演变过程,明确金属铁的析出形态与显微结构、黏结特性之间的关系。结果表明,CO气氛下Fe2O3还原时金属铁主要以纤维状生成的铁晶须为主,还原速率是决定这种形态差异的主要原因;在Fe2O3→Fe3O4→FeO中只会因晶格转变产生体积大小变化,且伴有裂纹和孔洞的发生,没有铁晶须生成;在FeO→Fe中出现铁晶须形核和生长,且铁晶须是100%的纯金属铁。要避免铁晶须生成,就要在FeO→Fe转变时抑制铁原子的定向迁移。     

Solidification structures in submicron spheres of iron-nickel: Experimental observations

Alloys of 0, 30, 40 and 50 at.% nickel in iron have been processed in vacuum by electrohydrodynamic atomization (EHD) to produce submicron droplets. The as-solidified spheres are studied to determine which of several solidification phases has appeared. Field-emission scanning transmission electron microscopy (STEM) is used to determine the microstructure, composition and crystal structure of the 10–150 run diameter spheres. It is believed that homogeneous nucleation must be the predominant nucleation mechanism in EHD droplets during free flight. The alternative crystallization phase, b.c.c., in the Fe-Ni alloy system, is found in 30 and 40 at.% Ni alloys but not in the 50 at.% Ni alloy. A new hexagonal crystal structure of Fe-50 at.% Ni is discovered. Furthermore, the smallest spheres of each alloy (<50 nm diameter) including pure iron are found to be amorphous. These findings are consistent with calculations (detailed in a second paper [Acta metall.36, 2537 (1988)], based on classical nucleation theory which predict the conditions under which alternative crystallization phases may appear.     

Influence of rare earth metals on the nucleation and solidification behavior of iron and 1045 steel

Two series of experiments have been conducted to determine the influence of rare earth additions on the nucleation and crystallization behavior of pure iron and 1045 steel. In the first series, additions of rare earth suicide or cerium dioxide powder to two-Kg 1045 steel ingots indicated that rare earth suicide can refine the as-cast structure of such ingots. However, if the holding time after rare earth silicide addition is over two minutes, the grain refinement decreases. With cerium dioxide additions, a relatively large columnar zone was obtained. In the second series, the effects of cerium metal or cerium dioxide powder additions on the degree of undercooling obtainable in pure iron and 1045 steel were examined by the lévitation melting method. Surface tension measurements of the levitated droplets were carried out at the same time to investigate the possible effects of surface tension variations on the nucleation and crystallization behavior of the metals. The experimental data show that rare earth inclusions can greatly reduce the degree of undercooling of iron and steel, and that a small amount of dissolved cerium can further reduce the degree of undercooling of levitated droplets. The structure and reaction products obtained with Fe-Ce levitated droplets were examined with both optical and scanning electron microscopy as well as X-ray diffraction analysis. The experimental results clearly indicated that cerium solute redistribution during solidification is the dominant factor in refining the as-cast structure. A nucleation and solidification model for the Fe-Ce levitated droplets has been developed, which can successfully explain the experimental results. on research leave from Beijing University of Iron and Steel Technology     

Influence of rare earth metals on the nucleation and solidification behavior of iron and 1045 steel

Two series of experiments have been conducted to determine the influence of rare earth additions on the nucleation and crystallization behavior of pure iron and 1045 steel. In the first series, additions of rare earth suicide or cerium dioxide powder to two-Kg 1045 steel ingots indicated that rare earth suicide can refine the as-cast structure of such ingots. However, if the holding time after rare earth silicide addition is over two minutes, the grain refinement decreases. With cerium dioxide additions, a relatively large columnar zone was obtained. In the second series, the effects of cerium metal or cerium dioxide powder additions on the degree of undercooling obtainable in pure iron and 1045 steel were examined by the lévitation melting method. Surface tension measurements of the levitated droplets were carried out at the same time to investigate the possible effects of surface tension variations on the nucleation and crystallization behavior of the metals. The experimental data show that rare earth inclusions can greatly reduce the degree of undercooling of iron and steel, and that a small amount of dissolved cerium can further reduce the degree of undercooling of levitated droplets. The structure and reaction products obtained with Fe-Ce levitated droplets were examined with both optical and scanning electron microscopy as well as X-ray diffraction analysis. The experimental results clearly indicated that cerium solute redistribution during solidification is the dominant factor in refining the as-cast structure. A nucleation and solidification model for the Fe-Ce levitated droplets has been developed, which can successfully explain the experimental results. on research leave from Beijing University of Iron and Steel Technology