Formation of eutectic silicide coatings on molybdenum

General features of structural changes in silicide coatings on molybdenum during vacuum annealing in the temperature range between 1850 and 1920°C have been studied by electron microscopy, metallography, and x-ray phase analysis. The kinetics of eutectic MoSi₂-Mo₅Si₃ formation and structure are studied. It is found that fusion of the eutectic compound coating makes it possible to reduce defects substantially and thus improve surface layer quality.     

Heat storage in eutectic alloys

Classical thermodynamic equations based on the regular solution approximation yield enthalpic changes for eutectic transformation that agree roughly with values measured for several binary and ternary alloy systems by differential scanning calorimetry or differential thermal analysis. Restricting measurements to binary and ternary alloys of the relatively plentiful elements Al, Cu, Mg, Si and Zn, it has been verified that the best heat storage densities on a mass or volume basis are obtained with alloys rich in Si or Al, elements that have large heats of fusion. Several of these alloys have the highest heat-storage density reported for phase change materials that transform between 780 and 850 K. The Mg₂Si-Si eutectic, which has outstanding storage density at 1219 K, illustrates the utility of ordered intermetallic phases with large heat of formation that dissolve in the eutectic liquid to contribute to the entropy change.     

Microcalorimetric measurements in Fe-C and Fe-C-Si-Mn alloys

Microcalorimetric measurements of heat related to fusion and crystallization of the eutectic in Fe-C and Fe-C-Si-Mn alloys have been carried out. In the case of Fe-C alloys, the heat related to fusion and crystallization of the eutectic changed from 0 to 215 kJ/kg and from 0 to 207 kJ/kg, respectively. For Fe-C-Si-Mn alloys, the heat related to fusion and crystallization of the eutectic changed from 178 to 220 kJ/kg and from 169 kJ/kg to 216 kJ/kg, respectively. This was attributed to the change of the content of eutectic in the alloys.     

Mathematical modeling of microstructural development in hypoeutectic cast iron

A mathematical heat-transfer/microstructural model has been developed to predict the evolution of proeutectic austenite, white iron eutectic, and gray iron eutectic during solidification of hypoeutectic cast iron, based on the commercial finite-element code ABAQUS. Specialized routines which employ relationships describing nucleation and growth of equiaxed primary austenite, gray iron eutectic, and white iron eutectic have been formulated and incorporated into ABAQUS through user-specified subroutines. The relationships used in the model to describe microstructural evolution have been adapted from relationships describing equiaxed growth in the literature. The model has been validated/fine tuned against temperature data collected from a QuiK-Cup sample, which contained a thermocouple embedded approximately in the center of the casting. The phase distribution predicted with the model has been compared to the measured phase distribution inferred from the variation in hardness within the QuiK-Cup sample and from image analysis of photomicrographs of the polished and etched microstructure. Overall, the model results were found to agree well with the measured distribution of the microstructure.     

Microstructures of Al–Cu–Ag Ternary Eutectics Under Near-Isothermal Conditions

The three-phase eutectic microstructures in Al–Cu–Ag ternary alloys are complex and only directional solidification (DS) structures have been systematically studied in the literature. In order to better understand the large variation of eutectic structures in this system, three off-eutectic alloys in the vicinity of the nonvariant eutectic composition were solidified in a differential scanning calorimeter (DSC). The DSC was operated between 450 and 550 °C and at a heating and cooling rate of 1 °C/min to produce near-isothermal conditions. The intermetallic phase ζ-Ag₂Al is found to behave like pure metallic phase and forms primary dendrites. The ζ-Ag₂Al phase has a regular morphology in eutectics, while the θ-Al₂Cu exhibits big anisotropy of intermetallic nature and shows elongated morphology in the unconstraint grown eutectics. Three basic eutectic structures are categorized based on the relative phase arrangement: (1) semi-regular eutectic with elongated θ-Al₂Cu; (2) regular eutectic with rod-like ζ-Ag₂Al embedded in θ-Al₂Cu and (3) irregular eutectic with θ-Al₂Cu in ζ-Ag₂Al. They have nearly the same phase fraction and composition but distinct phase size distribution. The DSC eutectic structures are compared with reported DS eutectic structures.     

Creep behavior of an aligned Ag3Mg-AgMg eutectic

A eutectic alloy, Ag-32.2 at. pct Mg, has been directionally solidified at growth rates,R, ranging from 0.9 to 63.9 cm/h to produce an aligned lamellar structure. The alloy consists of AgMg, an ordered CsCl type phase, and a solid solution of approximate composition Ag-27 at. pct Mg. The investigation consisted of studying ordering effects in the Ag-27 pct Mg phase on creep behavior of the aligned eutectic at temperatures between 210 and 270°C. Ordering of Ag-27 pet Mg markedly reduced creep rate and increased rupture life of the eutectic alloy. An increase in R resulted in a finer interlamellar spacing and further reduced creep rate for both ordered and disordered material. An increase in the apparent activation energy for creep with order is consistent with an increase in the activation energy for vacancy motion in the Ag₃Mg phase. Creep tests on a single-phase Ag₃Mg alloy confirmed this conclusion. Both the ordered and disordered eutectic alloy exhibited a high sensitivity of steady-state creep rate, ε, to stress. The stress dependence was analyzed in terms of a proposed mechanism of creep in this alloy. Slip processes have been observed by optical microscopy, and fractographic techniques have been employed to explain the influence of long range order on rupture. Implications of this work to design of creep-resistant eutectic composites are discussed.     

Experimental Determination of the Austenite + Liquid Phase Boundaries of the Fe-C System

Differential thermal analysis has been employed to achieve an accurate determination of the solidliquid equilibria for γ-Fe at temperatures between 1147 °C and 1430 °C and compositions in the range 1.5 to 4.42 wt pct C. Several small but significant changes to the established phase diagram have been proposed. In particular, the liquidus in the vicinity of the eutectic has been raised by approximately 15 °C and the temperature of the graphite eutectic lowered to 1150.5 °C. Consequently, the eutectic compositions have been increased to 4.39 and 4.42 wt pct C for the graphite and cementite eutectics, respectively. The possibility of some modification to the γ-solidus has also been suggested, and a revised phase diagram is presented.     

Eutectic nucleation and growth in hypoeutectic Al-Si alloys at different strontium levels

The effects of different levels of strontium on nucleation and growth of the eutectic in a commercial hypoeutectic Al-Si foundry alloy have been investigated by optical microscopy and electron backscattering diffraction (EBSD) mapping by scanning electron microscopy (SEM). The microstructural evolution of each specimen during solidification was studied by a quenching technique at different temperatures and Sr contents. By comparing the orientation of the aluminum in the eutectic to that of the surrounding primary aluminum dendrites by EBSD, the eutectic formation mechanism could be determined. The results of these studies show that the eutectic nucleation mode, and subsequent growth mode, is strongly dependent on Sr level. Three distinctly different eutectic growth modes were found, in isolation or sometimes together, but different for each Sr content. At very low Sr contents, the eutectic nucleated and grew from the primary phase. Increasing the Sr level to between 70 and 110 ppm resulted in nucleation of independent eutectic grains with no relation to the primary dendrites. At a Sr level of 500 ppm, the eutectic again nucleated on and grew from the primary phase while a well-modified eutectic structure was still present. A slight dependency of eutectic growth radially from the mold wall opposite the thermal gradient was observed in all specimens in the early stages of eutectic solidification.     

Eutectic interface configurations during melting

Experimental studies are presented on the melting of a lamellar eutectic structure in a transparent model alloy system. Steady-state morphologies have been characterized for different dissolution rates. Both coupled and noncoupled melting of eutectic are observed. A diffusive model of coupled eutectic melting has been developed and compared with the experimental results. Because the eutectic spacing is fixed, the system is shown to select the shape so that the average position of the interface for the two phases remains isothermal. A comparison of the predicted shapes with the experimental data shows that the higher melting point phase adjusts its curvature to equate the average temperature of the two phases when the experimental conditions are in the coupled melting regime. The conditions for a coupled melting to be unstable are described for which one of the phases extends further into the liquid. When this extension is significant, a spherodization of the leading phase is observed.     

The role of iron in the formation of porosity in Al-Si-Cu-based casting alloys: Part III. A microstructural model

Iron has been shown to have a significant effect on the formation of porosity and shrinkage defects in Al-Si-Cu-based foundry alloys. This is not simply a direct consequence of the physical presence of the β-Al₅FeSi platelets in the microstructure, but is also due to the effect that these platelets have on the nucleation and growth of eutectic silicon. The alloy-dependent critical iron content determines when the β phase first solidifies and, hence, when it can participate in the silicon nucleation event. At critical iron contents, the β phase solidifies as the initial component of the ternary eutectic. However, at supercritical iron contents, the β phase is already well developed when ternary eutectic solidification begins, while, at subcritical iron contents, the β phase forms as a component of the ternary eutectic only after the binary Al-Si eutectic is well established. Each of these paths of microstructural evolution leads to different variations in microstructural permeability and, hence, interdendritic feedability and porosity formation. The actual porosity-forming response to these alloy-induced microstructural changes is influenced by the solidification conditions in the casting.     

Eutectic reaction and nonconstant material parameters in micro-macrosegregation modeling

Recently, the authors presented a general framework for the calculation of the local solidification path in micro-macrosegregation computations (Reference 1). For a volume element whose overall solute content is not necessarily constant, the problems of solute diffusion in the primary solid phase and of remelting were addressed. In handling the eutectic reaction, the mass fraction and the solute profile in the primary phase at the beginning of the eutectic reaction were assumed to be “frozen.” In the present study, this restriction has been relaxed, as the solute diffusion in the primary phase occurring during the eutectic reaction is taken into account as well as that taking place for temperatures between the liquidus and the eutectic. It is shown that there is a potential for dissolving the secondary phase, which is controlled by diffusion in the primary phase. Other extensions of the former modeling concept made in the present study are to allow different densities of the liquid and solid phases, nonlinear phase diagram characteristics, and a temperature and/or solute concentration-dependent solute diffusivity in the primary phase. Assuming that the variations of enthalpy and of average solute concentration are known from the solution of the macroscopic continuity equations, modeling examples focusing on the extensions of the former model have been carried out.     

Solidification of undercooled Sn-Bi and Pb-Sb alloys

In hypoeutectic and eutectic alloys of Sn-Bi and Pb-Sb, the first phase to solidify from the melt is the nonfaceted phase, tin and load, respectively. Since primary tin and lead are poor nucleants for the second phase of the eutectic, the faceted phase, substantial undercooling below the equilibrium eutectic temperature occurs before the eutectic is nucleated. Once nucleated the faceted phase does not grow preferentially to return the melt composition to the equilibrium eutectic composition. Instead growth of a complex regular structure takes place from a melt whose composition is displaced from the equilibrium eutectic composition towards the faceted component. Reheating experiments have confirmed that the complex regular structure is rich in the faceted component. This structure grows at a temperature depressed several degrees below the equilibrium eutectic temperature. An hypothesis is presented to explain the growth of a structure rich in the faceted component at a depressed temperature.     

简单共晶系相图的计算机计算

根据热力学原理和热力学数据，用计算机计算和绘制了一些简单共晶系相图，并用计算机打印相应的部分相图数据。     

Simulation of eutectic solidification structures of binary alloys: a multiparticle diffusion limited aggregation model

The (isothermal) eutectic solidification structures of binary alloys have been simulated using a two-dimensional monte carlo model. The model takes into account simultaneous diffusion of all liquid atoms, incorporates the energy contributions of like and unlike nearest and next-nearest neighbor bonds in the solid on solidification, and allows surface rearrangement at the solid-liquid interface. It has been demonstrated that results obtained from this model exhibit phenomena observed during eutectic solidification in practice, for example, the passage of the growth rate through a maximum upon decreasing temperature. Particular attention has been devoted to the effect of growth parameters, such as the next nearest neighbor interaction in the solid and the surface relaxtion length, on the morphology of the growing solid phase.     

La和Y对细化后A356铝合金显微组织的影响

利用金相显微镜和扫描电子显微镜等设备研究了稀土金属La和Y对经AlTiB和AlSr细化后的A356铝合金α-Al枝晶和共晶Si相显微组织的影响及作用机理。研究表明,金属La和Y加入A356铝合金后以AlSiLa或AlSiY的形式存在于α-Al相晶界中。La和Y不仅对A356铝合金共晶硅相具有明显的变质效果,而且对α-Al晶粒有细化作用。其中Y变质共晶硅和细化α-Al晶粒的效果均优于La。     

Microstructural Investigation and Phase Relationships of Fe-Al-Hf Alloys

The effect of Hf addition on microstructures, phase relationships, microhardness, and magnetic properties of Fe₅₀Al_50?n Hf _n alloys for n = 1, 3, 5, 7, and 9 at. pct has been investigated. At all investigated compositions, the ternary intermetallic HfFe₆Al₆ τ ₁ phase forms due to the limited solid solubility of Hf in FeAl phase and tends to develop a eutectic phase mixture with the Fe-Al-based phase. The Hf concentration of the eutectic composition is found to be 7 at. pct from the microstructural examinations and the eutectic phase transition temperature is determined as 1521 K (1248 °C) independent of Hf amount by differential scanning calorimetry measurements. Furthermore, the enthalpies and activation energies (based on Kissinger and Ozawa methods) of eutectic phase transitions are reported. The minimum activation energy is calculated for the fully eutectic composition. Moreover, variation of the microhardness of Fe-Al-based alloys as a function of the Hf content is investigated, and its dependence on the thermal history of the alloys is explained.     

Calculations of Stable and Metastable Equilibrium Diagrams of the Ag-Cu and Cd-Zn Systems

Gibbs energies have been modeled for the binary alloy systems Ag-Cu and Cd-Zn, simple eutectic systems of interest in the area of rapid solidification. Parameters of the thermodynamic functions are derived from phase diagram and thermochemical data. Metastable phase equilibria have then been calculated, as well as the chemical spinodals and the locus of compositions and temperatures where liquid and solid have equal Gibbs energies (T ₀).     

A Few Aspects on the Processing and Deformation Behavior of Advanced Eutectic Alloys

The present work is aimed to develop homogeneous ultrafine eutectic composites without any micrometer size second phase dispersions. Ti_70.5Fe_29.5 is chosen as a model eutectic system and ingots were prepared by arc melting upon addition of different amount of Sn. The effect of Sn addition to Ti_70.5Fe_29.5 on the interlamellar spacing, microstructure, and elastic properties has been studied. The load dependence microhardness or indentation size effect (ISE) in ultrafine lamellar eutectic has been investigated to understand the mechanism of ISE in terms of strain gradient on the evolution of geometrically necessary dislocations. Furthermore, high-resolution transmission microscopy of deformed composites has been performed in order to reveal the role of different eutectic phases during the transfer of slip across their interface.     

Modeling of irregular eutectic microstructures in solidification of Al-Si alloys

A modified cellular automaton (MCA) model was developed and applied to simulate the evolution of solidification microstructures of both eutectic and hypoeutectic Al-Si alloys. The present MCA model considers the equilibrium and metastable equilibrium solidification processes in a multiphase system. It accounts for the aspects including the nucleation of a new phase, the growth of primary α dendrites and two eutectic solid phases from a single liquid phase, as well as the coupling between the phase transformation and solute redistribution in liquid. The effects of alloy composition and eutectic undercooling on eutectic morphology and eutectic nucleation mode were investigated. The simulated results were compared with those obtained experimentally.