Dendritic Arm Spacing Affecting Mechanical Properties and Wear Behavior of Al-Sn and Al-Si Alloys Directionally Solidified under Unsteady-State Conditions

Alloys of Al-Sn and Al-Si are widely used in tribological applications such as cylinder liners and journal bearings. Studies of the influence of the as-cast microstructures of these alloys on the final mechanical properties and wear resistance can be very useful for planning solidification conditions in order to permit a desired level of final properties to be achieved. The aim of the present study was to contribute to a better understanding about the relationship between the scale of the dendritic network and the corresponding mechanical properties and wear behavior. The Al-Sn (15 and 20 wt pct Sn) and Al-Si (3 and 5 wt pct Si) alloys were directionally solidified under unsteady-state heat flow conditions in water-cooled molds in order to permit samples with a wide range of dendritic spacings to be obtained. These samples were subjected to tensile and wear tests, and experimental quantitative expressions correlating the ultimate tensile strength (UTS), yield tensile strength, elongation, and wear volume to the primary dendritic arm spacing (DAS) have been determined. The wear resistance was shown to be significantly affected by the scale of primary dendrite arm spacing. For Al-Si alloys, the refinement of the dendritic array improved the wear resistance, while for the Al-Sn alloys, an opposite effect was observed, i.e., the increase in primary dendrite arm spacing improved the wear resistance. The effect of inverse segregation, which is observed for Al-Sn alloys, on the wear resistance is also discussed.

Crystallisation behaviours of Al-based metallic glasses: Compositional and topological aspects

The different types of thermal crystallisation behaviours observed during continuous heating of M-based metallic glasses have been successfully associated with the topological instability. criterion, which is simply calculated from the alloy composition and metallic radii of the alloying elements and aluminium. In the present work, we report on new results evidencing the correlation between the values of X and the crystallisation behaviours in Al-based alloys of the Al-Ni-Ce system and we compare the glass-forming abilities of alloys designed with compositions corresponding to the same topological instability condition. The results are discussed in terms of compositional and topological aspects emphasizing the relevance of the different types of clusters in the amorphous phase in defining the stability of the glass and the types of thermal crystallisation. (C) 2008 Elsevier B.V. All rights reserved.

Thermodynamic and topological instability approaches for forecasting glass-forming ability in the ternary Al-Ni-Y system

A thermodynamic approach to predict bulk glass-forming compositions in binary metallic systems was recently proposed. In this approach. the parameter gamma* = Delta H-amor/(Delta H-inter - Delta H-amor) indicates the glass-forming ability (GFA) from the standpoint of the driving force to form different competing phases, and Delta H-amor and Delta H-inter are the enthalpies for-lass and intermetallic formation, respectively. Good glass-forming compositions should have a large negative enthalpy for glass formation and a very small difference for intermetallic formation, thus making the glassy phase easily reachable even under low cooling rates. The gamma* parameter showed a good correlation with GFA experimental data in the Ni-Nb binary system. In this work, a simple extension of the gamma* parameter is applied in the ternary Al-Ni-Y system. The calculated gamma* isocontours in the ternary diagram are compared with experimental results of glass formation in that system. Despite sonic misfitting, the best glass formers are found quite close to the highest gamma* values, leading to the conclusion that this thermodynamic approach can lie extended to ternary systems, serving as a useful tool for the development of new glass-forming compositions. Finally the thermodynamic approach is compared with the topological instability criteria used to predict the thermal behavior of glassy Al alloys. (C) 2007 Elsevier B. V. All rights reserved.

Predicting glass-forming compositions in the Al-La and Al-La-Ni systems

In this work, a criterion considering the topological instability (lambda) and the differences in the electronegativity of the constituent elements (Delta e) was applied to the Al-La and Al-Ni-La systems in order to predict the best glass-forming compositions. The results were compared with literature data and with our own experimental data for the Al-La-Ni system. The alloy described in the literature as the best glass former in the Al-La system is located near the point with local maximum for the lambda.Delta e criterion. A good agreement was found between the predictions of the lambda.Delta e criterion and literature data in the Al-La-Ni system, with the region of the best glass-forming ability (GFA) and largest supercooled liquid region (Delta T(x)) coinciding with the best compositional region for amorphization indicated by the lambda.Delta e criterion. Four new glassy compositions were found in the Al-La-Ni system, with the best predicted composition presenting the best glass-forming ability observed so far for this system. Although the lambda.Delta e criterion needs further refinements for completely describe the glass-forming ability in the Al-La and Al-La-Ni systems, the results demonstrated that this criterion is a good tool to predict new glass-forming compositions. (C) 2010 Elsevier B. V. All rights reserved.

Porous alumina-spinel ceramics for high temperature applications

In order to reduce energy costs, high-temperature insulation porous refractory ceramics have been subjected to increasing demands. Among the techniques used to produce these materials (such as the addition of foaming agents and organic compounds), the pore generation via phase transformation presents key aspects, such as easy processing and the absence of toxic volatiles. In this study, this technique was applied to produce porous ceramics by decomposing an aluminum magnesium hydro-carbonate known as hydrotalcite (Mg(6)Al(2)(CO(3))(OH)(16)center dot 4H(2)O). It was found out that by using this complex compound, a large fraction of pores can be generated and kept at high temperatures (above 1300 degrees C) due to the in situ formation of spinel-like phases (MgAl(2)O(4)). (C) 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Internal Residual Stresses in Sintered and Commercial Low Expansion Li(2)O-Al(2)O(3)-SiO(2) Glass-Ceramics

We performed Synchrotron X-ray diffraction (XRD) analyses of internal residual stresses in monolithic samples of a newly developed Li(2)O-Al(2)O(3)-SiO(2) (LAS) glass-ceramic produced by sintering and in a commercial LAS glass-ceramic, CERAN (R), produced by the traditional crystal nucleation and growth treatments. The elastic constants were measured by instrumented indentation and a pulse-echo technique. The thermal expansion coefficient of virgilite was determined by high temperature XRD and dilatometry. The c-axis contracts with the increasing temperature whereas the a-axis does not vary significantly. Microcracking of the microstructure affects the thermal expansion coefficients measured by dilatometry and thermal expansion hysteresis is observed for the sintered glass-ceramic as well as for CERAN (R). The measured internal stress is quite low for both glass-ceramics and can be explained by theoretical modeling if the high volume fraction of the crystalline phase (virgilite) is considered. Using a modified Green model, the calculated critical (glass) island diameter for spontaneous cracking agreed with experimental observations. The experimental data collected also allowed the calculation of the critical crystal grain diameters for grain-boundary microcracking due to the anisotropy of thermal expansion of virgilite and for microcracking in the residual glass phase surrounding the virgilite particles. All these parameters are important for the successful microstructural design of sintered glass-ceramics.

Is Nitric Oxide a Mediator of the Effects of Low-Intensity Electrical Stimulation on Bone in Ovariectomized Rats?

Low-intensity electrical stimulation (LIES) may counteract the effects of ovariectomy (OVX) on nitric oxide synthase (NOS) expression, osteocyte viability, bone structure, and microarchitecture in rats (Lirani-Galvo et al., Calcif Tissue Int 84:502-509, 2009). The aim of the present study was to investigate if these effects of LIES could be mediated by NO. We analyzed the effects of NO blockage (by l-NAME) in the response to LIES on osteocyte viability, bone structure, and microarchitecture in OVX rats. Sixty rats (200-220 g) were divided into six groups: sham, sham-l-NAME (6 mg/kg/day), OVX, OVX-l-NAME, OVX-LIES, and OVX-LIES-l-NAME. After 12 weeks, rats were killed and tibiae collected for histomorphometric analysis and immunohistochemical detection of endothelial NOS (eNOS), inducible NOS (iNOS), and osteocyte apoptosis (caspase-3 and TUNEL). In the presence of l-NAME, LIES did not counteract the OVX-induced effects on bone volume and trabecular number (as on OVX-LIES). l-NAME blocked the stimulatory effects of LIES on iNOS and eNOS expression of OVX rats. Both l-NAME and LIES decreased osteocyte apoptosis. Our results showed that in OVX rats l-NAME partially blocks the effects of LIES on bone structure, turnover, and expression of iNOS and eNOS, suggesting that NO may be a mediator of some positive effects of LIES on bone.

Reduction of Chromium from Al(2)O(3)-CaO-SiO(2)-CrOx Slags by Carbon Dissolved in Liquid Iron

The goal of this work is to investigate the reduction of chromium from a quaternary slag by carbon dissolved in liquid steel. Laboratory scale experiments were conducted to study the reduction of chromium oxides in the slag by carbon dissolved in the melt. These experiments were made under different conditions of slag basicity and amount of added carbon. Thermodynamic calculations based on Double Sublattice model were applied using the commercial software Thermo-Calc, with the IRSID database. The results obtained showed good correlation with practical and calculated results, making it possible to predict equilibrium conditions of the system and to determine the activities of chromium oxides in the slag.

Microstructure and texture assessment of Al-Mn-Fe-Si (3003) aluminum alloy produced by continuous and semicontinuous casting processes

Aluminum sheets are currently produced by the direct-chill process (DC). The need for low-cost aluminum sheets is a challenge for the development of new materials produced by the twin roll caster (TRC) process. It is expected that sheets produced from these different casting procedures will differ in their microstructure. These differences in microstructure and in the crystallographic texture have great impact on sheet mechanical properties and formability. The present study investigated microstructure and evaluated texture of two strips of Al-Mn-Fe-Si (3003) aluminum alloy produced by TRC and by hot-rolling processes. It was possible to notice that the microstructure, morphology, and grain size of the TRC sample were more homogenous than those found in hot-rolled samples. Both strips, obtained by the two processes, showed strong texture gradient across the thickness.

Hardness and structure characterization of Ti(6)Al(4)V films produced by reactive magnetron sputtering on a conventional austenitic stainless steel

Ti(6)Al(4)V thin films were grown by magnetron sputtering on a conventional austenitic stainless steel. Five deposition conditions varying both the deposition chamber pressure and the plasma power were studied. Highly textured thin films were obtained, their crystallite size (C) 2008 Elsevier Ltd. All rights reserved.

Magnetic loss, permeability dispersion, and role of eddy currents in Mn-Zn sintered ferrites

Magnetic energy losses and permeability have been investigated in laboratory prepared and commercial Mn-Zn sintered ferrites from quasi-static conditions up to 10 MHz. The mechanisms leading to energy dissipation, either due to domain wall displacements or magnetization rotations, have been quantitatively assessed and their respective roles have been clarified. Domain wall processes dissipate energy by pure relaxation effects, while rotations display resonant absorption of energy over a broad range of frequencies. Their specific contributions to the permeability and its frequency dispersion are thus identified and separately evaluated. It is shown that eddy currents are always too weak to appreciably contribute to the losses over the whole investigated frequency range and that rotations are the dominant magnetization and loss producing mechanisms on approaching the MHz range, as predicted by the Landau-Lifshitz-Gilbert equation with distributed anisotropy fields. (C) 2008 Elsevier B.V. All rights reserved.

Electrochemical study of modified cerium-silane bi-layer on Al alloy 2024-T3

In this paper, the performance of bis-1, 2-(triethoxysilyl) ethane (BTSE) as a pre-treatment to protect the AA 2024-T3 against corrosion has been investigated by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization curves, and the scanning vibrating electrode technique (SVET). The microstructural and morphological characterizations were carried out via scanning electron microscopy and atomic force microscopy and the chemical composition evaluated using contact angle measurements and X-ray photoelectron spectroscopy (XPS). The electrochemical results showed that the additives improved the anticorrosion properties of the coating. The chemical characterization indicated that additives contribute to an increased degree of surface coverage, as well as to a more complete reticulation. The SVET results evidenced the self-healing abilities of Ce ions. (C) 2009 Elsevier Ltd. All rights reserved.

Electrochemical study of modified non-functional bis-silane layers on Al alloy 2024-T3

In the last few years great efforts have been made in order to find and to develop environmentally friendly substitutes for Cr6+ pre-treatments applied on aluminium alloys used in the aircraft industry. Among the potential substitutes, silane layers have attracted considerable interest from researchers and from the industry. The present work investigates the anti-corrosion behaviour of (bis-1, 2-(triethoxysilyl) ethane (BTSE)) silane layers modified with Ce ions and/or silica nanoparticles applied on Al alloy 2024-T3 substrates. The corrosion behaviour was investigated in 0.1 M NaCl solution via d.c. polarization and electrochemical impedance spectroscopy (EIS). Contact angle measurements and XPS were used to assess information on the chemistry of the silane pre-treated surfaces. The results have shown that the introduction of additives improves the corrosion protection properties of the silane layer. (c) 2008 Elsevier Ltd. All rights reserved.

Thermodynamic Assessment of the Aluminum Corner of the Al-Fe-Mn-Si System

A new assessment of the aluminum corner of the quaternary Al-Fe-Mn-Si system has been made that extends beyond the COST-507 database. This assessment makes use of a recent, improved description of the ternary Al-Fe-Si system. In the present work, modeling of the Al-rich corner of the quaternary Al-Fe-Mn-Si system has been carried out by introducing Fe solubility into the so-called alpha-AlMnSi and beta-AlMnSi phases of the Al-Mn-Si system. A critical review of the data available on the quaternary system is presented and used for the extension of the description of these ternary phases into the quaternary Al-Fe-Mn-Si.

Thermodynamic modelling of an Al(2)O(3)-MnO system using the ionic model

The thermodynamic assessment of an Al(2)O(3)-MnO pseudo-binary system has been carried out with the use of an ionic model. The use of the electro-neutrality principles in addition to the constitutive relations, between site fractions of the species on each sub-lattice, the thermodynamics descriptions of each solid phase has been determined to make possible the solubility description. Based on the thermodynamics descriptions of each phase in addition to thermo-chemical data obtained from the literature, the Gibbs energy functions were optimized for each phase of the Al(2)O(3)-MnO system with the support of PARROT(R) module from ThemoCalc(R) package. A thermodynamic database was obtained, in agreement with the thermo-chemical data extracted from the literature, to describe the Al(2)O(3)-MnO system including the solubility description of solid phases. (C) 2009 Elsevier Ltd. All rights reserved.