The undercooling of aluminum

Metallurgical and Materials Transactions A - An important parameter affecting microstructure development during solidification is the amount of undercooling prior to nucleation. The undercooling...     

The undercooling of aluminum

An important parameter affecting microstructure development during solidification is the amount of undercooling prior to nucleation. The undercooling potential of aluminum has been assessed by thermal analysis measurements on powder dispersions of the liquid metal. A number of variables have been identified which influence the undercooling of powder Al samples including powder coating, powder size, melt cooling rate, and melt superheat. Surface analysis by Auger electron spectroscopy indicates that changing the medium in which the powders are produced is an effective method of altering the coating chemistry. Factorial design analysis has been employed to quantify the potential of processing variables to increase the undercooling level obtainable in aluminum. The factorial analysis indicates that control of the powder coating through changing the medium in which the powders are produced is most effective in decreasing the nucleation temperature. Additionally, the finest powders produced in the medium which induces the least catalytic coating, when cooled at high rates,T = 500 °C/s, from low superheats,T _s = 710 °C, are found to achieve the deepest undercooling, ΔT = 175 °C. These studies provide the basis for further increases in undercooling and for future investigations into the solidification reactions which produce both stable and metastable structures in aluminum alloys. Formerly Research Assistant in the Department of Metallurgical and Mineral Engineering, University of Wisconsin-Madison     

The pitsch-petch orientation relationship in ferrous pearlite at small undercooling

The factors behind the occurrence of the Pitsch-Petch pearlitic orientation relationship (OR) in ferrous systems are investigated using a combination of crystallographic information obtained through electron-backscatter diffraction (EBSD) and knowledge of the three-dimensional (3-D) morphology of pearlite from serial sectioning. Both hypereutectoid and hypoeutectoid ferrous systems of various compositions providing a variety of morphologies are examined to consider the relationship between pearlite nucleation sites and ferrite-cementite interlamellar ORs. It is shown that the Pitsch-Petch OR occurs when a colony is cementite nucleated and this conclusion is valid for both sides of the eutectoid composition.     

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.     

Nucleation,undercooling and homogeneous structures in rapidly solidified powders

The possibility is considered that sufficient undercooling of powder particles can be achieved to result in a microstructure consisting of homogeneous microcrystallites. A relation is developed for the maximum achievable undercooling for comparison with the undercooling required for such homogeneous powder. Sample calculations show that such undercooling can be attained for the case of mercury, where homogeneous nucleation data are available, and for iron and cobalt, but the possibility cannot be decided upon for other metals. From a practical viewpoint, the results show that effort to achieve even greater rates of cooling in powder production would be worthwhile. Also, they indicate that in addition to cooling rate, attention should be directed to decreasing impurities and other heterogeneities which would tend to cause heterogeneous nucleation and prevent the attainment of the maximum possible undercooling.     

The effect of carbon on wetting of aluminum oxide by aluminum

Vapor-deposited carbon films on sapphire enhance its wettability by high-purity aluminum. A reduction in the terminal contact angle of 30 deg, to a value of 31 deg, occurs at 1223 K under vacuum conditions. This effect is related to either the dissolution of carbon in aluminum or the formation of aluminum carbide. Etching of the sapphire by molten aluminum is greatly enhanced by the presence of carbon, as revealed by profilometry of substrates following removal of the aluminum sessile drops.     

The deformation of aluminum foams

The compressive flow behavior of Al, Al-7 pct Mg and 7075 Al alloy foams has been determined in structures whose void fraction varies from 0.80 to 0.95 of the total volume. In all cases, a greater than linear increase in flow strength with increase in density was exhibited, indicating that bending stresses within the foam structure are an important feature of the collapse mode. The flow strength did not follow proportionately changes in bulk flow strength in comparisons of either alloy or of heat-treatment conditions. Ancillary tensile and metallographic observations show that this lack of correlation arises because the different foams collapse by different modes with localized fracture becoming dominant in the higher strength 7075 alloy. The energy absorbing efficiency was found to be independent of foam density for all the materials. However, the efficiency was found to be a strong function of the alloy and heat treatment increasing from about 30 pct in Al, to 43 pct in Al-7 pct Mg and to 50 pct in the solution heat treated and aged 7075 alloy. The increase in efficiency occurs because of an increase in the propensity to fracture in the higher strength alloys which introduces the potential for a propagating constant-stress collapse process.     

The dislocation microstructure of aluminum

Aluminum of 99.999 pct purity was deformed in torsion at 644 K and an equivalent uniaxial strain rate of 5.04 × 10⁻⁴ s⁻¹ to various steady-state strains up to 16.33. The subgrain size and density of dislocations not associated with subgrain boundaries remained fixed throughout the wide steady-state strain range. The subgrain boundaries, however, underwent two important changes. At the onset of steady state (ε ~0.2) all of the subgrain boundaries had relatively small misorientation angles averaging about 0.5 deg. With increased strain, however, an increasing fraction of the subgrain facets were high-angle boundaries. At strains greater than about four nearly a third of the boundaries were high-angle. In specimens with both types of boundaries, the high-angle boundaries have misorientation angles (θ) greater than 10 deg, while θ for low-angle boundaries is nearly always less than 3 deg. Only rarely do subgrain boundaries have misorientation angles between 3 deg and 10 deg. In aluminum, the increased high-angle boundary area at larger strains originates from the extension of the initial boundaries through the mechanism, recently introduced by others, of “geometric dynamic recrystallization” in aluminum. The average misorientation across low-angle boundaries initially increases during steady state but eventually reaches a maximum value of about 1.2 deg at ε ≃ 1.2. Since the flow stress stays nearly constant, the dramatic changes in the character of the subgrain boundaries that are observed during steady state suggest that the details of the boundaries arenot an important consideration in the rate-controlling process for creep.     

The Deformation of aluminum foams

The compressive flow behavior of Al, Al−7 pct Mg and 7075 Al alloy foams has been determined in structures whose void fraction varies from 0.80 to 0.95 of the total volume. In all cases, a greater than linear increase in flow strength with increase in density was exhibited, indicating that bending stresses within the foam structure are an important feature of the collapse mode. The flow strength did not follow proportionately changes in bulk flow strength in comparisons of either alloy or of heat-treatment conditions. Ancillary tensile and metallographic observations show that this lack of correlation arises because the different foams collapse by different modes with localized fracture becoming dominant in the higher strength 7075 alloy. The energy absorbing efficiency was found to be independent of foam density for all the materials. However, the efficiency was found to be a strong function of the alloy and heat treatment increasing from about 30 pct in Al, to 43 pct in Al−7 pct Mg and to 50 pct in the solution heat treated and aged 7075 alloy. The increase in efficiency occurs because of an increase in the propensity to fracture in the higher strength alloys which introduces the potential for a propagating constant-stress collapse process.     

Large undercooling,rapid solidification,and nucleation mechanism during multistage atomisation

Abstract

The large undercooling and rapid quenching that can occur during multistage atomisation are studied both theoretically and experimentally. Results show that these two effects are closely related to and promoted by each other. The level of undercooling for droplets is dependent on alloy composition, powder particle size, and atomisation condition. The cooling rates of droplets depend heavily on their particle size. Both experiments and theoretical analysis on Sn-Pb alloys reveal that a certain function of undercooling has a linear relationship with the logarithm of the mean powder particle particle size. Based on the present results, a mechanism of nucleation preferred on surface oxide is proposed to give a quantitative interpretation of the experimental observations.     

The nucleation of aluminum grains in alloys of aluminum with titanium and boron

The microstructure of a ternary alloy, Al-5 wt pct Ti, 1 wt pct B, has been examined by optical and electron transmission microscopy, by selected area diffraction, and electron probe microscopy, by selected area diffraction, and electron probe microanalysis. Particles of Al₃Ti are found at the center of grains and there exist preferred epitaxial orientations between this compound and the surrounding aluminum. Particles containing titanium and boron occur at aluminum grain boundaries and have no preferred configurations with respect to the aluminum or to one another. It is concluded that the active heterogeneous nuclei are therefore Al₃Ti and that particles of TiB₂, AlB₂, or a ternary compound are not active in this alloy. Grain size measurements in binary Al-Ti alloys suggest that particles of a nucleating phase must be present at concentrations as low as 0.01 wt pct Ti, and it is suggested that these could be Al₃Ti if the existing binary phase diagram Al-Ti is in error.     

The effect of aluminum hydroxide dissolution on the bleeding of aluminum lake dyes

The effect of pH on the bleeding of FD&C yellow No. 5 aluminum lake and FD&C red No. 40 aluminum lake was investigated. The pH-bleeding profiles corresponded to the pH-solubility profile of aluminum hydroxide. The similarity of the bleeding profiles of both lake dyes and the pH-solubility profile of aluminum hydroxide indicates that pH related bleeding, other than that occurring by competition with anions, is a result of dissolution of the aluminum hydroxide substrate. This dissolution is related to the properties of the substrate rather than to the structure of adsorbed dye.     

The oscillatory spectra of aluminum nitride

Translated from Poroshkovaya Metallurgiya, No. 2(326), pp. 80–82, February, 1990.     

The solidification of inoculated aluminum ingots

The macrostructures of aluminum and its alloys solidified with inoculants are reexamined here by comparing structures produced through normal solidification with those produced by the external force fields of steady-state mold oscillation. The inoculation reaction of one specific commercial product shows how foreign additions used for nucleation interact with the thermal and fluid flow fields to modify structure. A special combination of rotation and oscillation within the selfsame ingot allows discrimination between the grain refinement produced by an inoculant and that produced by any usual alloying element.     

超声外场对AlTiC细化剂中TiC粒子过冷形核的影响

在7085铝合金凝固过程中添加质量分数为5%的Al-5Ti-0.2C细化剂,并施加超声外场,运用EPMA、SEM检测超声外场对铝合金溶质元素分布的影响,对比其铸锭凝固组织,探讨超声外场对AlTiC细化剂细化率的影响。研究结果表明:添加AlTiC细化剂的细化率为44.7%,再施加超声外场,细化率可再提高31.4%;EPMA元素面扫描结果表明,施加超声后Ti元素和C元素在晶界的聚集明显减少;SEM元素线扫描结果表明,施加超声外场使Zn、Mg、Cu主要溶质元素相对均匀地分布在铝熔体中;施加超声外场后,TiC粒子形核的成分过冷度增大并且过冷度局部差异减小,有利于形成均匀细小的等轴晶组织。