Quantification of microsegregation during rapid solidification of Al-Cu powders

A new technique is introduced to quantify microsegregation during rapid solidification. The quantification involves calculation of the average solute solubility in the primary phase during solidification of an Al-Cu binary alloy. The calculation is based on using volume percent eutectic and weight percent of second phase (in the eutectic), which were obtained experimentally. Neutron diffraction experiments and stereology calculation on scanning electron microscope images were done on impulse atomized Al-Cu alloys of three compositions (nominal), 5 wt pct Cu, 10 wt pct Cu, and 17 wt pct Cu, atomized under N₂ and He gas. Neutron diffraction experiments yielded weight percent CuAl₂ data and stereology yielded volume percent eutectic data. These two data were first used to determine the weight percent eutectic. Using the weight percent eutectic and weight percent CuAl₂ in mass and volume balance equations, the average solute solubility in the primary phase could be calculated. The experimental results of the amount of eutectic, tomography results from previous work, and results from the calculations suggest that the atomized droplets are in metastable state during the nucleation undercooling of the primary phase, and the effect of metastability propagates through to the eutectic formation stage. The metastable effect is more pronounced in alloys with higher solute composition.     

Rapid solidification of Al-Cu eutectic alloy by laser remelting

Rapid surface resolidification using a high powered CO₂-laser has been performed on eutectic Al-32.7 wt% Cu at speeds between 0.2 and 8 m/s. By means of longitudinal cuts through the centre of the laser trace, the local growth rate has been measured by observation of the orientation of the microstructure using transmission electron microscopy. The various microstructures as a function of growth rate, are:

• 1.(a) regular lamellar eutectic α-Al/θ-Al₂Cu structure for growth rates below 20 cm/s with interlamellar spacing as fine as 17 nm;
• 2.(b) a new wavy eutectic α-Al/θ'-Al₂Cu morphology for growth rates of between 20 and 50 cm/s;
• 3.(c) a banded structure formed by alternating supersaturated α-Al solid solution and the wavy eutectic for growth rates greater than 50 cm/s.

A recent analytical model for eutectic growth under rapid solidification condition is compared to the experimental results. Contrary to the classical λ²V_s = const. relationship, which predicts a continuous decrease in spacing as the growth rate increases, this new theoretical model clearly predicts a limit for the coupled eutectic growth which finds its analogy in single phase solidification in the limit of absolute stability.     

Microsegregation in cellular solidification

Microsegregation in a binary alloy solidified in the form of deep cells is predicted using a simplified finite difference model. The model accounts for solid state diffusion and for flow of liquid between cells driven by solidification shrinkage. Cell tip undercooling is predicted using the expression originally derived by Boweret al. Cells are assumed to be cylindrical, and solid state diffusion along the cell axis is ignored, simplifying considerably prediction of solid state diffusion and cell shape behind the tip, which are treated as a one-dimensional moving boundary problem. Experiments were conducted on binary Al-4.5 wt pct Cu, solidified in the cellular growth regime using a Bridgman furnace. Microsegregation in the samples was measured and is compared to predictions; good agreement is found, both for cell heights and microsegregation in the fully solidified material. It is found that intercellular fluid flow exerts a small, but discernable, influence on microsegregation and cell shape. Formerly Research Assistant, Department of Materials Science and Engineering, Massachusetts Institute of Technology     

Solute redistribution in cellular solidification

Mathematical solutions and experimental results are presented which describe solute redistribution during cellular or dendritic solidification at highG/R (thermal gradient divided by growth rate) of binary and ternary alloys. The solutions assume negligible constitutional supercooling in the vicinity of the growing cells or dendrites and negligible effects of curvature and interface kinetics. Data are in the form of measurements of tip temperature and composition, and, for the ternary system, temperature at which two-phase cells or dendrites start to form. In all cases agreement between experiment and theory is good.     

Measurements and modeling of the microstructural morphology during equiaxed solidification of Al-Cu alloys

The morphology predicted by an equiaxed growth model recently presented elsewhere has been compared with quantitative experimental morphology measurements on a range of Al-Cu alloys. In the experiments, the samples have been solidified with a uniform temperature and quenched from the mushy state at the instant when the eutectic temperature was reached. The copper content and the amount of grain-refiner additions have been varied, resulting in both “clover-leaf” and dendritic equiaxed morphologies. Morphology characterization on both the intragranular and extragranular length scales has been performed on the quenched samples. Average heat-extraction rates, grain densities, and alloy compositions from the experiments have been used as input to the equiaxed grain-growth model, and the resulting morphology predictions have been compared with the morphology measurements. For the morphologies observed in the present study, the equiaxed growth model predicts higher values of the internal solid fraction than observed experimentally. This has been indicated to be due to the failure of commonly made modeling assumptions during the later stages of the solidification.     

Heat flow parameters affecting dendrite spacings during unsteady-state solidification of Sn-Pb and Al-Cu alloys

Solidification thermal parameters and dendrite arm spacings have been measured in hypoeutectic Sn-Pb and Al-Cu alloys solidified under unsteady-state heat flow conditions. It was observed that both primary and secondary spacings decreased with increased solute content for Sn-Pb alloys. For Al-Cu alloys, the primary spacing was found to be independent of composition, and secondary spacings decrease as the solute content is increased. The predictive theoretical models for primary spacings existing in the literature did not generate the experimental observations concerning the Sn-Pb and Al-Cu alloys examined in the present study. The theoretical Bouchard-Kirkaldy’s (BK’s) equation relating secondary spacings with tip growth rate has generated adequately the experimental results for both metallic systems. The insertion of analytical expressions for tip growth rate and cooling rate into the predictive model, or into the resulting experimental equations in order to establish empirical formulas permitting primary and secondary dendritic spacings to be determined as functions of unsteady-state solidification parameters such as melt superheat, type of mold, and transient metal/mold heattransfer coefficient is proposed.     

Simulation of metal solidification using a cellular automaton

The solidification of a hypothetical liquid was studied by means of simulations conducted using a cellular automaton. Construction and operation of the automaton are described. The effects of varying the temperatures of the liquid and solid phases, the freezing point, the velocity of dendrite growth relative to thermal diffusion, the latent heat, and the probability of nucleation within the bulk of the liquid were examined. Simulated microstructures are shown, and an attempt to assess these quantitatively has been made using parameters such as the roughness of the solid-liquid interface, the depth of the «slushy» zone, and the average width of a columnar grain. The applications of cellular automata in general are also briefly reviewed. It is concluded that the model described is able to realistically simulate several aspects of solidification.     

Creep behavior of an AZ91 magnesium alloy reinforced with alumina fibers

Creep tests were conducted at elevated temperatures on an AZ91 alloy reinforced with 20 vol pct Al₂O₃ fibers. When the creep data are interpreted by incorporating a threshold stress into the analysis, it is shown that the true stress exponent, n, is ∼3 at the lower stress levels and increases to >3 at the higher stresses. The true activation energy for creep is close to the value anticipated for interdiffusion of aluminum in magnesium. This behavior is interpreted in terms of a viscous glide process with n =3 and a breakaway of the dislocations from their solute atom atmospheres at the higher stress levels. The threshold stresses in this composite appear to arise from an attractive interaction between mobile dislocations in the matrix alloy and Mg₁₇Al₁₂ precipitates. The experimental results reveal several important similarities between the creep behavior of this magnesium-based composite and the well-documented creep properties of aluminum-based composites.     

Correlation between unsteady-state solidification conditions, dendrite spacings, and mechanical properties of Al-Cu alloys

The wide range of operational conditions existing in foundry and casting processes generates as a direct consequence a diversity of solidification microstructures. Structural parameters such as grain size and interdendritic spacings are strongly influenced by the thermal behavior of the metal/mold system during solidification, imposing, as a consequence, a close correlation between this system and the resulting microstructure. Mechanical properties depend on the microstructural arrangement defined during solidification. Expressions correlating the mechanical behavior with microstructure parameters should be useful for future planning of solidification conditions in terms of a determined level of mechanical strength, which is intended to be attained. In the present work, analytical expressions have been developed describing thermal gradients and tip growth rate during one-dimensional unsteady-state solidification of alloys. Experimental results concerning the solidification of Al-4.5 wt pct Cu and Al-15 wt pct Cu alloys and dendritic growth models have permitted the establishment of general expressions correlating microstructure dendrite spacings with solidification processing variables. The correlation of these expressions with experimental equations relating mechanical properties and dendrite spacings provides an insight into the preprogramming of solidification in terms of casting mechanical properties.     

Rheological behavior of Al-Cu alloys during solidification constitutive modeling,experimental identification,and numerical study

The rheological behavior of a solidifying alloy is modeled by considering the deforming material as a viscoplastic porous medium saturated with liquid. Since the solid grains in the mush do not form a fully cohesive skeleton, an internal variable that represents the partial cohesion of this porous material is introduced. The model parameters are identified using shear and compressive stress states under isothermal conditions on an Al-Cu model alloy. The model is partially validated with non-isothermal conditions and we complete this study with tensile conditions. Such conditions, when applied on the mush, may lead to severe defects in many casting processes. The model has been implemented into a commercial finite-element code to simulate a tensile test. Comparison with experimental data shows that the model is able to reproduce the main features of a solidifying alloy under tension, although fracture is not directly addressed here. We show that two critical solid fractions must be introduced in the model to account for the rheology: the coherency solid fraction at which the mush acquires significant strength and the coalescence solid fraction at which solid grains start to form solid bridges.     

Analytical,numerical, and experimental analysis of inverse macrosegregation during upward unidirectional solidification of Al-Cu alloys

The present work focuses on the influence of alloy solute content, melt superheat, and metal/mold heat transfer on inverse segregation during upward solidification of Al-Cu alloys. The experimental segregation profiles of Al 4.5 wt pct Cu, 6.2 wt pct Cu, and 8.1 wt pct Cu alloys are compared with theoretical predictions furnished by analytical and numerical models, with transient h _i profiles being determined in each experiment. The analytical model is based on an analytical heat-transfer model coupled with the classical local solute redistribution equation proposed by Flemings and Nereo. The numerical model is that proposed by Voller, with some changes introduced to take into account different thermophysical properties for the liquid and solid phases, time variable metal/mold interface heat-transfer coefficient, and a variable space grid to assure the accuracy of results without raising the number of nodes. It was observed that the numerical predictions generally conform with the experimental segregation measurements and that the predicted analytical segregation, despite its simplicity, also compares favorably with the experimental scatter except for high melt superheat.     

Steady state and transient creep properties of an aluminum alloy reinforced with alumina fibers

Steady state and transient creep experiments have been conducted on a metal matrix composite consisting of an aluminum-5 wt% magnesium alloy matrix reinforced with 26 vol.% alumina fibers. The composite exhibits high steady state stress exponents which range from 12.2 at 200°C to 15.5 at 400°C. The apparent activation energy for creep over this temperature range is found to be 225 kJ/mol. After unloading, large anelastic strains are observed. The anelastic recovery process is found to vary non-linearly with stress and temperature and exhibits an activation energy of 151 kJ/mol. A finite element model of the composite microstructure has been developed by treating the alumina fibers as an interconnected network of elastic beam elements. This model accurately simulates the transient response of the material to changes in loading. The model suggests that the high stress exponents, high activation energy, and large recoverable strains observed for this material can be explained by a balance between load transfer to and damage of the fiber network.     

Preferred-pattern formation during the initial transient in cellular solidification

Critical information on the pending problem of preferred-pattern formation and wavelength selection in cellular directional solidification is currently expected from the study of initial transients. Therefore, experiments were carried out in lead—thallium alloys to analyse the dynamical process by which a periodic array of cells is formed by using two classical but different experimental procedures, one violent and one soft. The statistical analysis of pattern characteristics (average wavelength, level of disorder) shows that both procedures converge towards a common asymptotic state, which means that a preferred pattern does exist. Although different, the initial transients are qualitatively similar. It is found that in both cases, the solid—liquid interface is first dendritic and then restabilizes into a cellular array. Oscillations, in periodicity and in the sense of disorder, are observed, that are associated to the repetition of phases of cell divisions, probably by tip-splitting, separated by rearrangement periods.     

Al2O3短纤维增强Al-Cu合金的微观组织结构和机械性能研究

为了研究铜元素含量变化对复合材料界面反应、微观组织结构和机械性能的影响，利用挤压铸造法制备了体积分数均为40％的Al2O3纤维增强纯铝和Al—Cu合金（1％，3％和5％）复合材料。采用X射线、TEM、SEM和拉伸实验手段，观察和测试了4种复合材料的微观组织和机械性能。结果表明，Al2O3纤维表面含有非晶SiO2成分，在高应力下易于开裂。铜元素的加入对材料的析出产生和机械性能有重要影响。铜元素引入后在复合材料中纤维表面处偏聚和富集，促进了界面θ相析出，并随基体中Cu含量提高而增加。当铜含量增加到5％后，基体内部也出现明显的析出相。拉伸实验结果表明随着Cu含量的增加复合材料的抗拉强度增高，Al2O3f/Al-Cu与Al2O3f／纯Al相比，抗拉强度分别增加了102％，146％和171％。SEM断口观察表明：基体合金的断口基本上都呈宏观脆性断口，具有低的展延性和撕裂纹理；大量的纤维从复合材料基体中拔出，一些纤维被拉断，这些特点与界面结合物和多晶的Al2O3纤维结构密切相关。     

Microstructure of a pressure-cast Fe3AI intermetallic alloy composite reinforced with zirconia-toughened alumina fibers

A composite of Fe-28Al-2Cr-lTi (at. pct) reinforced with 20-@#@ μm diameter zirconia-toughened alumina fiber, PRD-166, was pressure cast and examined by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). A new phase, tentatively identified as Fe₂AlZr, with an fcc crystal structure and a lattice parameter of 1.18 nm was occasionally found at fiber/ matrix interfaces. It was proposed that the phase formed by the eutectic reaction L → Fe(Al) + Fe₂AlZr. The Zr in the compound became available as a result of the dissolution of ZrO₂ from the fiber into the molten alloy. The matrix contained a high density of dislocations resulting from a difference in the coefficients of thermal expansion between the matrix and fiber. It was proposed that dislocations which formed at high temperatures in either A2 or B2 states were incompatible with the low-temperature DO₃ state. Geometrically necessary antiphase boundaries have been proposed to provide compatibility between dislocations formed in either the A2 or B2 states and the DO₃ state.