L. F. Malheiros

Thermodynamic assessment of the Li-Si system

The Li-Si binary system was thermodynamically assessed using experimental phase diagram and thermodynamic data. Due to inconsistencies in the experimental data, two optimizations were performed. In both cases, the obtained sets of parameters are different and are discussed.

Evaluation of the heat of solidification of cast irons from continuous cooling curves

Abstract This article describes an evaluation procedure which uses a thermal analysis technique based upon cooling curves, taken from the geometrical centre of casting sections, in order to evaluate the heat released during a transformation reaction, with special emphasis on the study of the solidification of cast irons. The mathematical procedure allows the evaluation of the heat of solidification, as well as the associated error, through the product of VρC p by the area between the first derivative of the cooling curve of the real section and the first derivative of the cooling curve of a fictitious neutral body, generated according to the Newtonian cooling principle, which assumes that there are no thermal gradients before the beginning of the reaction. The method has been applied to the study of the solidification of some nodular, grey and ledeburitic irons. Different types of test castings have been used: a step casting with a different section thickness, isolated sections with the same thickness as the different sections of the step casting and, finally, a Tectip cartridge. The results obtained enable the conclusion to be made that the neutral body has to be generated very carefully in order to avoid the influence of the sharp transition that occurs at the end of the solidification on the value calculated for the heat of solidification.

Isopiestic determination of the coefficients of activity of magnesium in AlCuMg liquid alloys

Abstract An isopiestic method has been employed for the determination of the activity coefficients of magnesium in AlMg and AlCuMg liquid alloys, in the 992–1138 K temperature range. The experiments have been developed in closed systems where the magnesium activities were imposed by the temperature of a magnesium reservoir. In all the experiments, the magnesium activity has been deduced from thermodynamic data already known. The quenched alloys have been observed by scanning electron microscopy and its chemical composition determined by energy-dispersive spectroscopy.

The Cu-Li-Mg system at room temperature

The Cu‐Li‐Mg system still remains very unknown. Mel’nik et al. (Russ. Metall. 3 (1976) 152‐156) studied it at 643 K. They considered the existence of a stoichiometric compound Cu8Li2Mg15 with an orthorhombic structure. In this work, the Cu‐Li‐Mg system was studied at room temperature by means of SEM (scanning electron microscopy) / EDS (energy dispersive spectroscopy) and X-ray diffraction. Results were compared with the ones from (Russ. Metall. 3 (1976) 152‐156) and with the assessments for the binaries Cu‐Li, Cu‐Mg and Li‐Mg present in the literature (COST-507, Thermochemical database for light metal alloys, in: I. Ansara (Ed.), Concerted Action on Materials Sciences, European Commission DGXII Publ., 1994, pp. 132‐134; A.A. Nayeb-Hashemi, J.B. Clark, Phase Diagrams of Binary Magnesium Alloys, ASM International, 1988, pp. 184‐194). # 2000 Elsevier Science B.V. All rights reserved.

HT-XRD in the study of Cu-Li-Mg

In a previous study on Cu-Li-Mg system, the authors of the present paper concluded that the ternary phase in that system corresponds to CuMg2-xLix (x ~ 0.11), with a hexagonal structure, space group P6222 (180), and lattice parameters a = b = 0.5260 nm, c = 1.3649 nm [1]. The structure was refined by the Rietveld method [2]. In order to characterize the thermal behaviour of the ternary compound and to assess the Cu-Li-Mg phase diagram [3], HT-XRD measurements were performed on samples whose compositions were close to the one corresponding to the ternary compound. SEM/EDS measurements of the phases’ compositions in equilibrium, as well as DSC/DTA heating curves, contributed to the identification of the transition temperatures and the phases present in equilibrium. It was concluded that the ternary phase decomposes at ~ 702 ± 2K. Introduction The Cu-Li-Mg system has not been, till now, the object of many studies although it is one of the ternaries of the Al-Cu-Li-Mg system which has been deeply studied, at least near the quasicrystalline T2 (Al6Li3Cu) phase, and which has many applications in the aeronautic industry. Mel’nik et al. [4] referred to the existence of a ternary phase in the Cu-Li-Mg system: Cu8Li2Mg15 with an orthorhombic structure (a = 0.524 nm, b = 0.899 nm, and c = 5.433 nm). Hämäläinen et al. [3] assessed the phase diagram of the system. Figure 1 presents two vertical sections for x(Li) = 0.04 and x(Li) = 0.07 from the Gibbs energy parameters obtained in [3]. In previous work, the present authors pointed out the existence of a phase with a stoichiometry close to that of Cu8Li2Mg15 [5] and, in recent work, the latter phase was defined as being CuMg2-xLix (x ~ 0.11) [1]. Taking into account that there were no experimental data about the thermal behaviour of the ternary phase, nor the high temperature equilibria, 300 European Powder Diffraction Conference, EPDIC 10 and that quenching seemed ineffective for such a narrow temperature range, some HT-XRD studies were developed. On the other hand, an important feature for a successful assessment is the crystallographic data; thus the models used to describe the phases should be supported by the phase’s crystal structure.

Experimental Phase Diagram of the Ternary Bi-Sn-Zn

Bi-Sn-Zn is one of the systems being used as substitute of the traditional lead solders. Therefore a deeper knowledge of its phase diagram is a priority. Due to the lack of data about the thermodynamic properties of terminal solid solutions as well as on their phase boundaries, only binary contributions were utilized and the ternary terms were ignored on the published assessments [1, 2]. Samples corresponding to several vertical sections were prepared and DTA, SEM/EDS and XRD at room and high temperatures were performed. It was concluded that ternary terms should be held in consideration in a future assessment of the system.

Potential use of continuous cooling curves for prediction of solidification structure of nodular iron casting sections

This article describes an evaluation procedure to forecast the structure resulting from a certain reaction which uses a thermal analysis technique based upon cooling curves, taken from the geometrical centre of casting sections, in order to evaluate the heat released during the transformation reaction. The method has been applied to the study of the solidification of some nodular, lamelar and ledeburitic melts; special emphasis has been given to the study of the solidification of nodular cast irons. The mathematical model, already described by some authors, allows the evaluation of the heat of solidification and the error associated. Three types of test castings have been used: a step casting, with decreasing section thickness, isolated sections with the same thickness as the different sections of the step casting and, finally, a TecTip cartridge.The results obtained let us suggest that the neutral body has to be generated very wisely in order to avoid the influence of the sharp transition, which occurs a...