Theory of eutectic growth under rapid solidification conditions

The Jackson-Hunt model of eutectic growth at small undercoolings is extended to large undercooling values which are commonly encountered under rapid solidification conditions. The parameters, λ²V and λΔT, are found to deviate from constant values at high velocities, and these deviations depend upon the nature of the metastable phase diagram below the eutectic temperature. A limiting velocity is predicted for the formation of a regular, coupled eutectic structure, and the reason for this limiting velocity is shown to be either the temperature dependent diffusion coefficient or the limit of undercooling.     

Modeling of crystal growth during rapid solidification

Models for computer simulation of solidification of alloys in two different geometries have been developed. These models allow us to study the roles of crystal nucleation and heat transfer and the effects of different modes of growth on the evolution of the microstructure during solidification. In particular, by applying the powerful and versatile tool comprised of the solute-drag model and the thermodynamic modeling of the alloy system, we are able to predict the formation of a microstructure consisting of alternating layers parallel to the growth front. This type of banded microstructure, experimentally observed in Al-Cu, Al-Fe, and Ag-Cu alloys, has not been satisfactorily explained before.     

Model for nonequilibrium partitioning during rapid solidification of binary concentrated solutions

During the past several years, a great deal of progress has been made in the use of both pulsed and continuous wave (CW) lasers in materials processing. Usually pulsed laser is used for annealing semiconductors and CW laser for alloying and cladding substrate materials. Inherent rapid cooling in such processes invariably produces novel materials due to nonequilibrium segregation of solute atoms during solidification. Such nonequilibrium phenomena have drawn a lot of interest in both theoretical and applied areas of solid-state physics and materials science. Dilute solution theory is adequate to study nonequilibrium solute (dopant) segregation during laser annealing of semiconductors because the concentration of solute atoms is very small compared to that of the solvent (host) atoms. Using kinetic model of nonequilibrium solidification, several studies have been already carried out under dilute solution approximation to derive an expression for nonequilibrium partition coefficient. However, concentrated solutions are frequently encountered in laser cladding and alloying processes. The nonequilibrium partitioning of solute in binary concentrated solutions is modeled in this study, and an expression for nonequilibrium partition coefficient with only one unknown parameter is obtained.     

Understanding texture domains and relations between pre-eutectic and eutectic phase during Al-Si alloy solidification

Phase morphology and crystallographic texture are important components of a solidification microstructure. Probability of nucleation of a nucleus is a strong function of local chemical & thermal conditions and its atomic orientation. Similarly, the growth kinetics for various crystal planes is dependent on its crystal orientation apart from local environmental conditions. The combined effects of nucleation & growth kinetics along with solidification conditions dictate the resulting microstructure of an alloy. In this study, various phases of a near eutectic Al-Si alloy are examined by serial sectioning and 3D reconstruction. A peculiar microstructure is observed in which primary silicon is present in the immediate proximity of primary aluminum. Observations through electron back scatter diffraction show that the microtexture of primary and eutectic Si is similar (with in a twin relation) and thus it is concluded that the formation of eutectic starts with eutectic Si forming on the primary Si.     

Nucleation of substitutional solid solutions during the solidification of binary liquid solutions

The thermodynamic parameters (entropy, enthalpy, interfacial surface energy) are analyzed for the substitutional solid solutions that solidify from liquid solutions of binary systems characterized by continuous solubility. Equations determining the concentrational dependences of these parameters, the critical sizes of nuclei, and their formation energies are obtained.     

Properties of titanium alloy VT6 fibers obtained by rapid solidification of the melt

We have studied the chemical, physicochemical, and technological properties of titanium alloy VT6 fibers in the initial state and after vacuum annealing for one hour at 500–1300°C. We have carried out fractographic, microstructural, and qualitative x-ray spectral microanalysis of the fibers. We have established that in the initial state, the fibers possess high values of the physicomechanical characteristics; the decrease in the properties of the fibers with a rise in temperature was determined by observing changes in the microscopic and macroscopic structure and the chemical composition. Vacuum annealing of VT6 fibers allows us to improve their compressibility with different combinations of hardness, strength, and conductivity characteristics.Institute of Problems in Materials Science, National Academy of Sciences of Ukraine, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 3–4, pp. 85–90, March–April, 1994.     

Containerless processing and rapid solidification of Nb-Si alloys in the niobium-rich eutectic range

Containerless processing and rapid solidification techniques were used to process Nb-Si alloys in the Nb-rich eutectic range. Electromagnetic ally levitated drops were melted and subsequently splat quenched from different temperatures. A variety of eutectic morphologies was obtained as a function of the degree of superheating or undercooling of the drops prior to splatting. Metallic glass was observed only in drops quenched from above the melting temperature. Micro-structures of splats deeply undercooled prior to quenching were very fine and uniform. These results are discussed in terms of classic nucleation theory concepts and the expected heat evolution at different regions of the splat during the rapid quenching process. The locations of the coupled-zone boundaries for the α-Nb + Nb₃Si eutectic are also suggested. Formerly Graduate Student, Vanderbilt University.     

Structure and mechanical properties of a eutectic high-temperature Nb-Si alloy grown by directional solidification

The structure and the short-term high-temperature strength of Ni-18.7 at % Si (Nb-Nb₃Si eutectic) alloys fabricated by vacuum electron-beam zone melting and induction melting in an argon atmosphere are studied. The structure of the samples prepared by vacuum electron-beam zone melting is characterized by the presence of primary Nb₅Si₃ intermetallic precipitates and the absence of its secondary precipitates. The structure of the samples prepared by induction melting in an argon atmosphere has two characteristic zones, namely, eutectic and eutectoid ones.     

急冷对Ti_(0.32)Cr_(0.345)V_(0.25)Fe_(0.03)Mn_(0.055)合金储氢性能的影响

为了研究急冷对储氢合金残余氢量的影响,利用真空电弧熔炼炉和铜模喷铸制备了Ti_(0.32)Cr_(0.345)V_(0.25)Fe_(0.03)Mn_(0.055)合金,采用XRD、PCT(压力-容量-温度)、TG/DTA等手段分析了急冷对储氢合金吸放氢性能的影响。结果表明,铸态合金和急冷合金均由BCC固溶体主相和Laves第二相组成;急冷对首次吸氢动力学行为影响较大,由铸态时的化学反应控制变为急冷时的新相晶核形成长大控制;急冷后,合金吸放氢平台压得到提高,且吸氢起始点左移,但吸放氢滞后性增大。TG/DTA曲线表明,急冷并没有改变合金的残余氢量,但氢化物放氢温度升高。     

Microstructures and superplastic behavior of eutectic Fe-C and Ni-Cr white cast irons produced by rapid solidification

Superplastic behavior of two commercial grade white cast irons, eutectic Fe-C and Ni-Cr white cast irons, was investigated at intermediate temperatures (650 to 750 °C). For this purpose, rapidly solidified powders of the cast irons were fully consolidated by compaction and rolling at about 650 °C. The volume fractions of cementite in the eutectic cast iron and in the Ni-Cr cast iron were 64 pct and 51 pct, respectively, and both cast irons consisted of fine equiaxed grains of cementite (1 to 2 μm) and ferrite (0.5 to 2 μm). The cast iron compacts exhibited high strain-rate sensitivity (strain-rate-sensitivity exponent of 0.35 to 0.46) and high tensile ductility (total elongation of 150 pct to 210 pct) at strain rates of 10^-4 to 10^-3 s^-1 and at 650 °C to 750 °C. Microstructure evaluations were made by TEM, SEM, and optical microscopy methods. The equiaxed grains in the as-compacted samples remained unchanged even after large tensile deformation. It is concluded that grain boundary sliding (e.g., along cementite grain boundaries in the case of the eutectic cast iron) is the principal mode of plastic deformation in both cast irons during superplastic testing conditions. Formerly with the Department of Materials Science and Engineering, Stanford University Formerly Visiting Scholar, Department of Materials Science and Engineering, Stanford University     

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。     

Growth temperatures and the limits of coupled growth in unidirectional solidification of Fe-C eutectic alloys

Growth temperature has been measured as a function of applied growth velocityV for grey (austenite-flake graphite) eutectic, austenite dendrites and white (austenite-cementite) eutectic in Fe-4.28 wt pct C at a temperature gradientG of ∼7 K/mm. Grey eutectic (0.4 to 65 μm/s) required an undercoolingΔTfor growth equal toK ₁ V ^1/2 withK ₁ as 3.4 ± 0.1 Ks^1/2/μm^1/2, giving values ofΔT nearly an order of magnitude larger than predicted for growth at the extremum, as shown previously by Toloui and Hellawell for the related Al-Si eutectic. Austenite dendrites growing together with the grey eutectic atV between 11 and 65 μ m/s exhibited a range of tip temperatures giving rise to average undercoolings of magnitude [GD/V +K ₂ V ⁿ ] withD as diffusion coefficient of carbon in the melt andK ₂ as 0.73 Ksⁿ μm⁻ⁿ withn as 0.46 predicted by Ivantsov’s theory for growth of an austenite needle. White eutectic displaced both grey eutectic and austenite dendrites atV of 100 μm/s. Except for the persistence of grey eutectic rather than white eutectic in the presence of austenite dendrites (11 <V < 65 μm/s), these observed growth transitions are consistent with the derived relationships governing growth temperatures on the basis of competitive growth,i.e. that the growth form with the highest growth temperature at a particularV should prevail. The results are then applied to derive the limiting conditions for growth of grey and white eutectics as a function of composition in Fe-C (i.e. their coupled zones). The significance of observed dependences onV of volume fraction and spacing of austenite dendrites is discussed.     

Al-Si合金快速等轴凝固界面响应函数及组织选择

在快速枝晶及共晶生长理论模型基础上,使用最高界面生长温度判据,建立了共晶合金快速等轴凝固界面响应函数(IRF)模型,分析了Al-Si合金系各种相及组织的竞争生长,绘制了非平衡组织选择图.计算结果与实验结果基本吻合,说明所建立的界面响应函数模型可以较好地预测Al-Si合金等轴凝固过程中的非平衡组织选择及形态演化.     

An integrated model for dendritic and planar interface growth and morphological transition in rapid solidification

Rapid solidification can be achieved by quenching a thin layer of molten metal on a cold substrate, such as in melt spinning and thermal spray deposition. An integrated model is developed to predict microstructure formation in rapidly solidified materials through melt substrate quenching. The model solves heat and mass diffusion equations together with a moving interface that may either be a real solid/liquid interface or an artificial dendrite tip/melt interface. For the latter case, a dendrite growth theory is introduced at the interface. The model can also predict the transition of solidification morphology, e.g., from dendritic to planar growth. Microstructure development of Al-Cu alloy splats quenched on a copper substrate is investigated using the model. Oscillatory planar solidification is predicted under a critical range of interfacial heat-transfer coefficient between the splat and the substrate. Such oscillatory planar solidification leads to a banded solute structure, which agrees with the linear stability analysis. Finally, a microstructure selection map is proposed for the melt quenching process based on the melt undercooling and thermal contact conditions between the splat and the substrate.     

The mechanisms of formation and prevention of channel segregation during alloy solidification

Conditions for the formation of macroscopic segregation channels have been examined in the ammonium chloride-water and lead-tin systems, using base chilled molds. Such channels develop when the rejected solute is less dense than the solvent and are therefore a result of density inversion, but slow (≺5 rpm) rates of mold rotation, about axes inclined to the vertical by 20 deg to 30 deg, throughout the time of solidification, effectively prevent the formation or propagation of these channels. Artificially created channels or those momentarily blocked fail to continue and are overgrown, but channels can be initiated by drawing liquid upward from close to the growth front in fine capillaries. Examination of these effects leads to the conclusion that channels originate at the growth front, rather than within the dendritic array, and that their formation is necessarily preceded by a liquid perturbation from the less dense boundary layer into the supernatant, quiescent bulk liquid. Intermittent ‘solute fingers’ are then fed by dendritic entrainment to produce stable convective plumes and concomitant channels. It is considered that the effects of mold precession are primarily caused by translation of bulk liquid across the dendritic growth front, shearing off convective perturbations from the boundary layer before they have time to develop. The nature of the liquid movements is discussed and shown to be a function of the mold dimensions. The inclination of the gravitational vector within the solid-liquid, dendritic array is considered to be of secondary importance to the formation or prevention of channels. This paper is based on a presentation made at the symposium “Fluid Flow at Solid-Liquid Interfaces” held at the fall meeting of the TMS-AIME in Philadelphia, PA on October 5, 1983 under the TMS-AIME Solidification Committee.     

Technique for the estimation of thermal resistance at solid metal interfaces formed during solidification and melting

The early stages of melting and/or dissolution of additions to liquid metals or slags involve the formation and melting back of a shell composed of solidified liquid. This constitutes a moving boundary problem which is further complicated by evidence of an interface thermal resistance. It is during this period that a thermal resistance exists at the addition-shell interface. In this paper, a technique is presented for the estimation of this resistance. Data, collected from experiments, were used as input to a model which solves the inverse heat conduction problem in terms of a resistance estimate. Validation procedures were developed which authenticate both the collected data from experiments as well as the model results. Results for a variety of metal combinations have shown a relationship between the estimated thermal resistance and the expansion of the added metal. Specifically, the ratio between the added and solidified liquid metal expansion coefficients indicates the relative magnitude of the thermal resistance.     

非调质钢中MnS的析出与长大

摘要：在立式电阻炉内熔化45MnVS钢,分析了水冷和空冷2种冷却方式下试样中MnS的形貌特征,并对试样中MnS的析出与长大进行了热力学和动力学计算。结果表明：水冷试样中MnS的平均尺寸为1.14μm,最大尺寸为7.80μm;空冷试样中MnS的平均尺寸为2.19μm,最大尺寸为8.78μm。钢液凝固过程中,MnS在固相分率达到0.69时析出;在相同固相分率下,水冷试样比空冷试样中MnS的尺寸小。固相分率小于0.97时,S元素为MnS长大的控制元素,固相分率大于0.97时,Mn元素为MnS长大的控制元素。计算所得水冷试样和空冷试样中MnS最终尺寸分别为5.27和9.54μm。     

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...