石墨向稀土金属处理的铁水中的生长

为了阐明铸铁中球状和紧密蠕虫状石墨的形成机理,对衰退效应比镁小的稀土金属处理的铁水进行了两项试验:(1)用固着液滴技术测定了稀土金属处理的铁水和石墨晶体特定面之间的界面能;(2)将具有球状或片状石墨的Ni—C合金棒插入铁水中,然后观察从合金棒中生长出来的石墨向铁水中生长的形态。由实验得出如下结论:铁水和石墨基面间的界面能几乎与残余铈量无关,而铁水和石墨棱面之间的界面能则和用镁处理时情况相同,随着铈量的增加而增加。低铈量时,单晶的界面自由能比值低于多晶体,因而单晶(片状石墨)变得比较稳定。高铈量时,多晶体的这个比值低于单晶的,因而多晶体(球状石墨)变得较稳定。中铈量时,两种石墨的界面自由能比值几乎相等,形成紧密蠕虫状石墨。因此,无论石墨基质是球状的还是片状的,长入铁水中的石墨形态与铁水和特定面间的界面能有关。     

稀土变质铸铁石墨形态的转变

采用定向凝固实验方法考察了低凝固速度（下限为０．５ｍｍ／ｈ）下稀土变质铸铁石墨形态的转变。结果表明,随着稀土含量的增加,石墨形态呈现由Ａ型片状→Ａ′型片状→珊瑚状→蠕虫状→球状→开花状的一系列转变;石墨形态由片状到非片状的转变是由于生长方式改变所致,而这种改变取决于稀土含量,与凝固速度（或冷却速度）无关;石墨单体可以发生片状与非片状之间的连续转变,但在试样宏观的片状石墨区域与非片状石墨区域之间却存在着明显的分界线,说明石墨生长方式在试样宏观区域间发生了突变。     

稀土变质铸铁石墨形变的转变

采用定向凝固实验方法考察了低凝固速度（下限为０．５ｍｍ／ｈ）下稀土变质铸铁石墨形态的转变。结果表明,随着稀土含量的增加,石墨形态呈现由Ａ型片状→Ａ型片状→珊瑚状→蠕虫状→球状→开花状的一系列转变;石墨形态由片状到非片状的转变是由于生长方式改变所致,而这种改变取决于稀土含量,与凝固速度（或冷却速度）无关;石墨单体可以发生片状与非片状之间的连续转变,但在试样宏观的片状石墨区域与非片状石墨区域之间却存在     

球墨铸铁离异共晶凝固组织生长模拟

采用所开发的微观元胞自动机模型,耦合热力学模型计算获得的凝固路径,将浓度分布、温降与晶体生长相结合,模拟了亚共晶球墨铸铁(Fe-4wt.%C合金)离异共晶组织生长过程。结果表明,奥氏体与石墨生长相互促进,奥氏体晶粒快速包裹石墨,生长形态由枝晶状转为团簇状,石墨晶粒最终孤立分布于奥氏体基体中。等温下仅依靠溶质扩散的奥氏体与石墨生长速度略低于温降和溶质扩散共同作用下数值。二者生长速度随熔体初始过冷度增大均增快;随冷却速率增加均呈线性增长,增幅大致相同。本计算参数范围内,冷却速率对石墨生长速度的影响强度大于熔体初始过冷度。所预测的石墨平均半径与文献实验值相符。研究结果可为控制球墨铸铁凝固组织形貌及性能提供依据。     

球墨铸铁离异共晶凝固组织生长模拟

摘要：采用所开发的微观元胞自动机模型,耦合热力学模型计算获得的凝固路径,将浓度分布、温降与晶体生长相结合,模拟了亚共晶球墨铸铁(Fe-4wt%C合金)离异共晶组织生长过程。结果表明,奥氏体与石墨生长相互促进,奥氏体晶粒快速包裹石墨,生长形态由枝晶状转为团簇状,石墨晶粒最终孤立分布于奥氏体基体中。等温下仅依靠溶质扩散的奥氏体与石墨生长速度略低于温降和溶质扩散共同作用下数值。二者生长速度随熔体初始过冷度增大均增快;随冷却速率增加均呈线性增长,增幅大致相同。本计算参数范围内,冷却速率对石墨生长速度的影响强度大于熔体初始过冷度。所预测的石墨平均半径与文献实验值相符。研究结果可为控制球墨铸铁凝固组织形貌及性能提供依据。     

基于均匀形核的金属液滴凝固过程

研究了均匀形核的金属液滴凝固过程,应用渐近分析法求得金属液滴内晶核生长数学模型的渐近解,分析了表面张力、界面动力学参数、初始晶核尺寸和过冷度对晶核界面生长速度、晶核半径以及液滴凝固时间的影响.在一定的过冷条件下,表面张力和界面动力学参数显著减缓了晶核界面生长速度.在凝固开始的很短时间内晶核界面生长速度迅速上升,当速度上升到最大值后,随着晶核半径的增大,界面生长速度逐渐减慢,表面张力和界面动力学参数对晶核生长速度的作用也逐渐减小.过冷度越大,液滴凝固时间越短.经过在开始的瞬变凝固阶段之后,温度场从设定的初始分布迅速地调整为由过冷度、表面张力、界面动力学参数等所确定的特定温度分布.      

38MnVS5钢连铸坯中硫化物形貌及形成机制的研究

用扫描电镜观察分析了38MnVS5钢320mm×300姗连铸坯中硫化物的三维形貌,结果显示,从铸坯表面到心部硫化物类型由球状变成杆状,并且在偏析严重区域呈片状或片状-杆状共存.球状硫化物是由包晶反应生成的,杆状硫化物为稳态共晶反应的产物.当合金元素含量较高时,由于过冷导致硫化物生长速度较快,杆状硫化物向片状硫化物转变.     

铸铁碳硫专用标样的研制

国内生产的铸铁、球铁标样,均由冶炼、铸锭、切削加工的方法制备而成.受冶炼工艺所限,铸锭不可避免存在各元素成份的不均匀性,其中尤以碳、硫两元素最为显著.这是由于高碳铁水在样模内凝固过程中,具有选择结晶和结晶生长的方向性,促使碳以片状和球状石墨形态游离存在于铁基     

稀土变质铸铁中石墨—奥氏体共晶的生长方式

用Ｂｒｉｄｇｍ ａｎ 定向凝固技术制取了稀土变质共晶铸铁不同石墨形态时的液－固平界面试样。观察结果表明,稀土变质铸铁共晶凝固过程中,石墨相不论形态如何均为领先相;石墨与奥氏体共晶有共生生长和离异生长两种生长方式。在共生生长情况下,奥氏体紧贴石墨片两侧协同生长,形成锯齿状的液－固界面结构;在离异生长情况下,石墨相单独在液相中析出并充分长大,随后被奥氏体包围,液－固界面形态主要取决于奥氏体－液相界面的稳定性。分析表明,稀土在石墨－液相界面的富集是导致石墨－奥氏体共晶离异生长的主要原因。     

放电等离子烧结法制备高导热片状石墨/铝复合材料

以高导热片状石墨和铝粉为原料,通过放电等离子烧结法(SPS)制备高导热片状石墨/铝复合材料。使用金相显微镜(OM)、扫描电子显微镜(SEM)和X射线衍射仪(XRD)对高导热片状石墨/铝复合材料的显微结构和成分进行了表征,观察了复合材料的界面结合状况,分析了烧结温度和烧结压力对复合材料致密化的影响,研究了复合材料中石墨含量对复合材料热导率的影响。研究表明,片状石墨和铝界面结合良好,没有生成界面产物Al4C3。适当的提高烧结温度和烧结压力有利于促进复合材料的致密化,过高的烧结温度容易造成铝液的溢出。当烧结压力为40 MPa,烧结温度为580℃时,高导热片状石墨/铝复合材料的致密度能达到99.7%。当复合材料中石墨含量为60%时,高导热片状石墨/铝复合材料的面向热导率能达到440 W·m-1·K-1,很好地满足了现代社会对电子封装材料的散热要求。     

Relationship Between the Trace Elements and Graphite Growth Morphologies in Cast Iron

The graphite morphology transition was studied using various techniques and additives in ultra-pure binary and ternary alloys with hypo- and hypereutectic compositions. Some of the trace elements were observed to stabilize the flake growth morphology of graphite, while others did not. The distance between the graphite basal planes of spheroidal, flake, and undercooled fine graphite was measured and the lattice fringes were studied using high resolution transmission electron microscope, after preparing a thin lamella of graphite using focused ion beam. Latent heat measurement was performed using differential scanning calorimeter on the pure binary alloy with and without sulfur and oxygen additions. The substitution of various elements under study in a monolayer of graphene was analyzed by considering the binding energies of the elements with C and their bonding nature. Simulations were performed using a molecule editor program and visualizer (Avogadro software), which considers various types of interatomic forces to optimize a monolayer of graphene to a minimum energy. The effect of the type of cyclic C-ring structure and energy of the basal plane of graphite with a connection to the addition of trace elements individually in the monolayer of graphene was studied and simulated to understand the resulting bulk graphite growth morphology.     

A monte carlo simulation study of dissolution of graphite in iron-carbon melts

A Monte Carlo (MC) simulation study has been carried out on the dissolution of graphite in Fe-C melts in the temperature range 1300 °C to 1600 °C. Atoms in graphite and iron melt were arranged on a rigid graphitic hexagonal lattice and interactions were assumed to be pairwise and short ranged. This hexagonal model of iron melts has been validated using saturation solubility of C in iron melts. The aim of this study was to investigate the effect of the atomic nature of the interfacial region on graphite dissolution. Using canonical ensemble, simulations were carried out as a function of carbon content of the melt, temperature, interface orientation, and surface roughness. A contact between graphite and melt resulted in the formation of a broad interfacial region containing high concentrations of C and Fe atoms. During the initial stages of contact, strong C-C bonds in the basal plane hinder the dissociation of C atoms and affect the overall dissolution rate. As dissolution proceeds, interfacial effects become less important and dissolution is controlled by mass transfer in the melt. Interfacial effects do not play an important role across prismatic planes. The simulation results also show an excellent agreement with the basic trends in experimental results on graphite dissolution.     

Spangle formation in galvanized sheet steel coatings

Very large grains, termed “spangles,” are produced on galvanized sheet steel coatings when lead is added to the zinc bath. The spangles have been attributed to melt undercooling prior to solidification. The present results indicate this is not the case, undercooling being less than 1 °C. The spangle diameter is shown to be dependent on the alloy addition to the bath, large spangles being obtained with Bi and Sb as well as Pb. The spangle size is related to the surface tension of the alloying addition, the size decreasing as the melt vapor surface tension of the alloying element increases. It is proposed that spangles form dendritically from a nucleus in the melt. Alloy additions with low interfacial energies and very limited solid solubility are highly concentrated ahead of the dendrite tip. This decreases the tip radius and increases the dendrite velocity, producing large grains. The basal plane orientation of the samples varies between 17 and 80 deg with respect to the steel sheet surface, which is inconsistent with basal plane dendritic growth in Zn along (1010) directions. It is proposed that solute additions to the melt and growth in a thin liquid layer can modify the dendrite growth direction, accounting for the spangle orientation. On leave from Obafemi Awolowo University, lie Ife, Oyo State, Nigeria     

The role of interphase boundary adsorption in the formation of spheroidal graphite in cast iron

The interphase boundaries in gray and ductile cast iron were studied with a scanning Auger microprobe (SAM). Sulfur and oxygen were found to be adsorbed at the flake/ metal interfaces in the gray iron, while the nodule/metal and intercrystalline graphite interfaces in the ductile iron were free of foreign elements. The only magnesium detected in the magnesium modified ductile iron was combined with phosphorus and sulfur as a compound. A model is presented which proposes that Fe−C−Si eutectic alloys in the absence of surface active impurities (such as in vacuum casting of high purity materials) produce nodular graphite due to the inherent instability of the graphite/melt interface. The sulfur and oxygen always present in commercial alloys adsorb at the graphite/melt interface, effectively “stabilizing” the active sites on the graphite basal planes, and preventing spherulitic growth. The purpose of modifiers is to getter these impurities.     

Steady-state cellular solidification of Al-Cu Reinforced with alumina fibers

The steady-state directional solidification of aluminum-4.5 wt pct copper and aluminum-1.0 wt pct copper alloys reinforced with parallel,continuous, closely spaced alumina fibers is investigated under growth conditions that produce a plane front or cells in corresponding unreinforced alloys. Specimens were designed to have a central reinforced region surrounded by unreinforced metal of the composite matrix composition. Each was produced by pressure infiltration, subsequently remelted, directionally solidified, and quenched to reveal the liquid/solid metal interface. Both unreinforced and composite sections were characterized to determine solidification front morphology and degree of microsegregation. In the unreinforced portion of the samples, the transition from plane-front to cellular solidification was observed to correspond to a coefficient of diffusion of copper in liquid aluminum of 5 − 10⁻⁹ m² − s⁻¹, in agreement with published values. Cell lengths, analyzed using a finite-difference model of microsegregation, are in agreement with the Bower-Brody-Flemings (BBF) model for cell tip undercooling. In the composite portion of the samples, the alloys solidify free of lateral microsegregation for all solidification conditions investigated, in agreement with theory. The shape of the liquid/solid metal interface near the fibers indicates a much lower fiber/liquid metal interfacial energy than fiber/solid metal interfacial energy. In the composite, plane front solidification is therefore not observed even when plane front solidification obtains in the unreinforced alloy. It is shown that geometrical constraint imposed on deep cells by the fibers causes significant increases in cell tip undercoolings, in agreement with current analyses of deep cell solidification.     

Influence of melt carbon and sulfur on the wetting of solid graphite by Fe-C-S melts

The interfacial phenomena between carbonaceous materials such as graphite, coke, coal, and char and Fe-C-S melts are important due to the extensive use of these materials in iron processing furnaces. However, the understanding of the interfacial phenomena between these kinds of carbonaceous materials and molten iron alloys is far from complete. In this study, graphite was selected as the solid carbonaceous material because its atomic structure has been well established. The sessile drop method was adopted in this investigation to measure the contact angle between solid graphite and molten iron and to study the interfacial phenomena. The influence of carbon and sulfur content in Fe-C-S melts on the wettability of solid graphite has been investigated at 1600 °C. The melt carbon content was in the range of 0.13 to 2.24 wt pct, and the melt sulfur content was in the range of 0.05 to 0.37 wt pct. X-ray energy-dispersive spectrometer (EDS) analysis was conducted on an HITACHI S-4500 scanning electron microscope to detect composition distribution at the interfacial region. It was found that contact of solid graphite with Fe-C-S melts will result in a nonequilibrium reactive wetting. It involved carbon transfer from the solid to the liquid and iron transfer from the liquid to the solid. The Fe-C-S melts exhibited relatively poor wetting when the reactions were absent. The mass transfer between solid graphite and Fe-C-S melts was observed to strongly enhance the wetting phenomena. It is proposed that the decrease of system free energy corresponding to the mass transfer reactions strongly influences the formation of the interface region and results in the progressive spreading of the wetting line. The composition and thickness of the graphite/iron interfacial layer was dependent on the intensity of mass transfer across the interface. The resulting change in the interfacial energy γ _ls is a strong function of mass transfer, and it varies in accordance with time of contact. The influence of carbon content on the wetting phenomena could only be seen at in the initial stages, whereas the influence of sulfur on the wettability was found when the system approached equilibrium. Therefore, the interfacial tension in its equilibrium condition at the graphite/Fe-C-S melt interface was determined only by the extent of sulfur adsorption at this interface.     

The effect of undercooling on the cellular precipitation reaction in Cu-3 Pct Ti

The effect of undercooling on the morphology of the cellular precipitation reaction in Cu-3 Pct Ti is examined by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and serial sectioning experiments. The reaction front formed at small undercooling, which exhibits strong faceting of the precipitate growth interfaces, gradually changes with increasing undercooling to a smoothly curved reaction front with concave precipitate growth interfaces and convex grain boundary segments. This concealment of the faceted reaction front appears to be due to the rapid accumulation of growth ledges with increasing undercooling. This study also indicates that the cellular precipitation reaction, at small undercooling, is initiated by Widmanstätten precipitation. At larger undercoolings, a second mechanism is responsible for cellular genesis. Finally, contrary to accepted models of colony development, serial sectioning experiments show that nucleation of additional lamellae may occur at the faces of existing lamellae, from where they extend laterally to achieve the characteristic interlamellar spacing for that temperature.     

Formation of aluminum oxide scales in sulfur-containing high temperature environments

A model alloy consisting of Fe-18 wt pct Cr-6 wt pct Al was used to study the formation of α-aluminum oxide under various oxygen (1 to 10⁻²⁰ atm) and sulfur (10⁻¹³ to 10⁻⁶ atm) partial pressures at 900 °C. Acoustic emission results indicate that at constanti,e2051-01 (10⁻²⁰ atm) the oxide scale became much more resistant to isothermal cracking with increasing sulfur potentials. In addition, adherence of the oxide scale to the alloy was also enhanced and the oxidation rates increased with increasing sulfur potentials. Using inert palladium markers, the diffusion processes in the growing aluminum oxide scale were studied. In environments where oxygen was the only oxidant present, the results indicated that inward oxygen diffusion is the predominant scale growth mechanism. In contrast, oxides grown in sulfur containing environments revealed a drastic change of the marker position, indicating that the oxide growth occurred predominantly by outward aluminum ion diffusion. Oxide scale morphologies are shown using scanning and transmission electron microscopy. formerly a Graduate Student of Chemical Engineering and Materials Science Department of The University of Minnesota     

Study of Interfacial Tension between Molten Steel and Na_2O-Li_2O-SiO_ 2-B_2O_3 Slag

Ｔｈｅｒｅａｒｅｍａｎｙｃａｓｅｓｗｈｅｒｅｔｈｅｉｎｔｅｒｆａｃｅｂｅ ｔｗｅｅｎｍｏｌｔｅｎｍｅｔａｌａｎｄｓｌａｇｐｌａｙａｎｉｍｐｏｒｔａｎｔｒｏｌｅｉｎｔｈｅｓｔｅｅｌｍａｋｉｎｇ ｐｒｏｃｅｓｓｓｕｃｈａｓｔｈｅｆｏｒｍａｔｉｏｎ ,ａｇｇｒｅｇａｔｉｏｎ ,ｄｉｓｔｒｉｂｕｔｉｏｎａｎｄｓｉｚｅｏｆｎｏｎ ｍｅｔａｌｌｉｃｉｎ ｃｌｕｓｉｏｎｓｉｎｓｔｅｅｌ ,ｆｏｒｍａｔｉｏｎｏｆｍｅｔａｌ ｉｎ ｓｌａｇｅｍｕｌｓｉｏｎｉｎｔｈｅＢＯＦｐｒｏｃｅｓｓａｎｄｓｌａｇ ｍｅｔａｌｒｅａｃｔｉｏ…