C. Triveño Rios

Fracture toughness of the eutectic alloy Al3Nb-Nb2Al

Abstract Presenting high fracture toughness is a decisive condition to any structural material, and when considering brittle alloys, the Vickers indentation method to determine fracture toughness is an interesting alternative. Like many other intermetallics, the Al 3 Nb–Nb 2 Al eutectic alloy shows high strength at high temperatures and low fracture toughness at room temperature. Al 3 Nb–Nb 2 Al samples, both in the as-solidified condition and in the directionally solidified condition, had their hardness and fracture toughness determined by the Vickers indentation method. Lower values of hardness were found when higher loads were used, and fracture toughness was found to be about 2.0 MPa m 1/2 . The as-solidified condition is harder and less tough, and when fracture occurs, cracks always develop by cleavage.

Rapidly Solidified Al-Si-Mg Alloy

Rapid solidification processes, RSP, are powerful tools to induce microstructural modifications, which may improve mechanical properties of metallic alloys. In this paper the influence of rapid solidification on the formation of the undesirable brittle intermetallic compounds promoted by Mg, Si and Fe in Al-9Si-0.7Mg (A359-type) alloy have been investigated by using of water-cooled wedge-copper mould for the rapid solidification process. The microstructures have been evaluated by using a combination of X-ray diffraction (XRD), optical (OM), scanning (SEM) and transmission electron microscopy (TEM), and by Vickers microhardness. By increasing the cooling rate the secondary dendritic arm spacing decreased and the formation of Mg2Si and Al8Si6Mg3Fe phases were suppressed. At the same time an increase in the hardness was observed. These microstructural and mechanical properties changes associated with the rapid solidification process might be attributed to the increased solid solution content of the alloying elements in the Al matrix. Introduction Aluminum-silicon alloys are widely used for automobile parts production due to their excellent castability and high strength-to-weight ratio. Magnesium is often added to increase the strength and hardenabilty of the Al-Si cast parts [1]. Iron and manganese are also present in these alloys, either introduced by the scrap or deliberately added to provide material special properties. On the other hand, due to the low solid solubility of Fe, Si and Mg in Al, during conventional casting precipitation of Al5FeSi, Al8Si6Mg3Fe, Mg2Si and other intermetallic phases with complex structures is observed. Rapid solidification processes, RSP, are powerful tools to induce microstructural modifications; besides the formation of homogeneous and refined microstructures [2], they can also increase the solid solubility limit of some elements in the matrix [3, 4] and therefore suppress the formation of some precipitates or to modify their morphology. The objective of the present study is to analyze the influence of rapid solidification process on the microstructure of an Al-9Si-0.7Mg (A359 type) alloy, focusing on the possibility of minimizing the formation of intermetallic compounds. Experimental Procedure Ingots of commercial Al-9Si-0.7Mg (A359-type) alloy, whose chemical composition is shown in Table 1, have been melted and cast into a water-cooled wedge-section copper mould, with a wedge angle of 5 o , 10 mm wide and 50 mm high. The melt, pouring systems and the mould were installed in a single melt-spinning device chamber. Processing was under argon atmosphere and the molten alloy at a temperature of 830°C was ejected with an overpressure of 200 mbar into of the copper mould. The samples naturally aged for more than a week have been characterized by using a combination of optical microscopy (OM), scanning (SEM) and transmission electron microscopy (TEM, Philips CM120), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and by Vickers microhardness measurements. Samples for TEM were thinned by ion milling using a dual guns system operating at 5kV with 800 μA per gun. The secondary dendrite arm spacing, DAS, was Journal of Metastable and Nanocrystalline Materials Online: 2004-07-07 ISSN: 2297-6620, Vols. 20-21, pp 594-598 doi:10.4028/www.scientific.net/JMNM.20-21.594

Microstructural Characterization of Spray Formed A380 Alloy

The microstructural features of spray formed deposits are a function of both the dynamic and the thermal behavior of the atomized droplets during its flight toward the substrate and of the thermal and solidification behavior of the deposited material. In this work, the microstructural evolution of Al-9Si-3Cu (A380-type) alloy during spray deposition process has been investigated, with special emphasis on the formation of intermetallic compounds, which may act as stress raisers with detrimental effects on the mechanical properties of the cast alloy. Argon has been used as gas atomizer and a cooper plate as substrate. The microstructure was evaluated by a combination of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray difraction (DRX). The formation of Al-Si irregular eutectic structure with Si flakes, normally observed in conventionally cast material, was suppressed in both overspray powder and deposit. The microstructure was formed by equiaxed grains of alfa-Al with Si as spheroidaltype particles in the intergranular region and inside the grains. The equilibrium Al15(MnFe)3Si2 and Al2Cu phases were hard to identify due to its metastable state. On the other hand, the amount of the detrimental Al5FeSi phase, usually found in conventionally cast material was reduced and it was observed as very fine platelets with lengths shorter than 15 microns.

Rapidly Solidified Al-6Si-3Cu Alloy

Rapid solidification processes, RSP, are powerful tools to induce microstructural modifications, which may improve mechanical properties of alloys. In this paper the influence of rapid solidification on the formation of the undesirable brittle intermetallic compounds promoted by Si and Fe in Al-6Si-3Cu (A319-type) alloy have been investigated. The alloy have been casted using both conventional method and water-cooled wedge-copper mould. The microstructures have been evaluated by using a combination of X-ray diffraction, optical, scanning and transmission electron microscopy, and by Vickers microhardness. By increasing the cooling rate the length of the intermetallic β-Al5FeSi phase decreased, accompanying the same tendency of the secondary dendritic arm spacing. These results are accompanied by an increasing in hardness. Moreover, the formation and growth of the Al2Cu phase have been suppressed. These microstructural and hardness changes with the rapid solidification might be attributed to the increased solid solution content of the elements in the Al matrix.

Microstructural Characterization of As-Quenched and Heat Treated Al-Si-Mg Melt-Spun Ribbons

Al-Si based alloys are the most important and widely used among the family of aluminum cast alloys. With small additions of magnesium these alloys become heat treatable, increasing the mechanical strength with toughness. In this work, the commercial A359 alloy was rapidly quenched by using melt spinning process, using rotating speeds of 22 and 56 m/s. The asmelt spun ribbons were characterized by a combination of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The microhardness of the ribbons was also measured. It was found that the quenched of the ribbon (wheel side) presents a featurless zone and a second zone composed by dendritic-cellular grains. Silicon particles, with nanometer size were found to preferentially precipitate along the grain boundaries. During heat treatment the over-saturated silicon precipitated with morphology spherical. The microstructures changes as well as the increasing of hardness with the increasing of cooling rate might be attributed to the supersaturated solid solutions of Si in Al matrix and to the refinement of the microstructure. Introduction Aluminum-Silicon alloys are widely used in the aluminium foundry due to their excellent castability and high strength-to-weight ratio. Magnesium is frequently added to these alloys to promote an increase of the strength and hardenabilty of the cast part [1]. On the other hand, due to the low solid solubility of Fe, Si and Mg in Al, during conventional casting, precipitates of Al5FeSi, Al8Si6Mg3Fe, Mg2Si and others with more complex structures are formed. However, a substantial increase of solid solubility of those elements in aluminum might be achieved by non-conventional techniques such as the rapid solidification, and consequently to suppress the formation of those precipitates. The objective of the present study is to analyze the influence of rapid solidification process on the microstructure in melt-spun of the Al-9Si-0.7Mg (A359 type) alloy. Experimental Procedure The chemical composition of the Al-9Si-0.7Mg (A359-type) as-cast ingots is shown in Table 1. Approximately 4 g of the molten alloy with 250°C above the liquidus temperature was ejected with an overpressure of 200 mbar onto a rotating Cu-Be wheel. For rotating speed of 56 m/s, the melt-spun ribbon was produced with 2.7 mm in width and 28-34 μm of thickness. On the other hand, for 22 m/s the ribbon was obtained with 3.4 mm in width and 45-55 μm of thickness. The samples have been characterized by using a combination of optic, MO, scanning, SEM, and transmission electron microscopy, TEM, energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and by Vickers microhardness testing. The samples were prepared by standard metallographic procedures. The XRD measurements were carried out in a Siemens D5005 using CuKα radiation. Samples for TEM (M120-Philips with EDS) were thinned by ion milling using dual gun system operating at 5kV with 800 μA per gun. The ribbons were heat treated at three temperatures: 200°C, 350°C and 450°C for 12 hours in air. After each heat treatment, measurements of Vickers hardness were performed with a load of 10gf. Journal of Metastable and Nanocrystalline Materials Online: 2004-08-01 ISSN: 2297-6620, Vol. 22, pp 103-108 doi:10.4028/www.scientific.net/JMNM.22.103

Crystallization of Amorphous Al85Ce5Ni10 Ribbon

In the present work the crystallization process of an aluminum-based amorphous metal have been investigated. Rapidly quenched Al85Ce5Ni10 ribbon has been produced by melt-spinning. The amorphous structure evolution during heating has been studied by a combination of X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Thermograms obtained in continuous heating regime reveal a glass transition, Tg, resulting in a supercooled liquid temperature range of ∼16°C. Multiple crystallization events were observed by isothermal annealing of the as-quenched melt-spun ribbon at temperatures below Tg; precipitation of a metastable phase in the amorphous matrix has been observed. Further heating at increasing temperatures resulted in complete crystallization with α-Al and intermetallic compounds. Kinetics analyses indicate that crystallization occurs though nucleation and three-dimensional growth.

Thermodynamic Analysis and Experimental Assessment of Al-Fe-Nd Alloys

The solidification behaviour of several Al-Fe-Nd alloys with compositions chosen in the Al-rich corner has been studie d to understand the phase stability of this glassformer system. From the amorphous solid, several Al-rich compositions can crystallise in two stages, resulting in a nanocomposite structure formed by primary Al phase surrounded by a residual amorphous matrix, a type of microstructure which can be responsible for excelent values of mechanical strength. Therefore, the knowledge of the phase stability is an important step in understanding the crystallisation behaviour which leads to the nanostructured alloys. In our work, we studied the phase evolution and stability in Al-rich compositions of the Al-Fe-Nd system by thermodynamic calculations and experimental assessment of equilibrium phases. The phases obtained after equilibrium thermal treatments have been compared with the phases which resulted from the crystallisation of the amorphous phase. The results have shown that although the final phases are the same for both equilibrium solidification and for crystallisation from the amorphous solid, the crystallisation paths are completely different.