Papers by Keyword: MgZn₂

Abstract: Cu and Ni impurities in Mg alloys are deleterious contaminants that reduce the corrosion resistance of the alloy. Mg₂Cu and Mg₂Ni precipitates can cause significant anodic dissolution of the Mg matrix, owing to their potential difference. Suppression of these phases can prevent the deterioration of corrosion resistance. The neutralization of these impurities through the formation of MgZn₂ intermetallic phases has been studied, because the atomic radii of Cu and Ni are similar to that of Zn. As a result, the MgZn₂ phase may precipitate during the rapid cooling that occurs during the solidification of the Mg-6 mass% Zn alloy, and introduce substitutional impurity atoms in the crystal lattice. Mg (Zn, Cu, Ni)₂ phase can be formed instead of Mg₂Cu and Mg₂Ni, in the presence of both of Zn and these impurities. In this study, the solubility of Cu and Ni into MgZn₂ phase when they contain simultaneously was investigated by preparing the intermetallic compounds and the crystal structure evaluation. The lattice parameter of the MgZn₂ phase containing both Cu and Ni impurity decreased to about 5.14 Å with increasing impurity concentration. Cu and Ni impurities are concentrated in the MgZn₂ phase while maintaining the crystal structure, when the impurity concentrations are within the composition range of Cu + 2Ni ≦ 20 at.%. When the concentrations of Cu and Ni exceeded this range, the crystal structure of the MgZn₂ phase transformed from hexagonal (hP12) to cubic (cF24).

Abstract: Aluminium is a light-weight material and possesses high corrosion resistance, so that it is widely used in manufacturing industries. The Al-Zn series have the highest strength compared to other aluminium alloys. To further increase the strength of Al-Zn alloys, Mg and Cu are added and age hardening treatment is applied. This research studied the precipitation process in Al-9Zn-4Mg (wt. %) alloys with Cu content of 0, 1, 3 and 5 wt.%. The alloys were produced through investment casting taking the shape of turbine impeller. The samples were solution treated at 460 °C for 2 hours and then aged at 130 °C. The characterization included hardness testing to observe response of age hardening, microstructural observation and Differential Scanning Calorimetry (DSC) testing. Microstructural observation was conducted by optical microscope and Scanning Electron Microscope (SEM) which was combined with Energy Dispersive Spectroscopy (EDS). The results showed that addition of Cu initially decreased the hardness during early ageing (2 hours) due to segregation of Cu-V complexes toward the grain boundaries which then decreased the hardness and enlarged the grain boundary phases. However, the peak hardness of the alloys was not affected by the increase in Cu content. due to high concentration of Zn and Mg. Exothermic reactions of formation of GP zones, η", η' and η (MgZn₂) were found during precipitation process while endothermic reaction were observed due to dissolution of the phases. Presence of MgZn₂ and Al₇Cu₂Fe second phases were observed in grain boundaries.

Abstract: The effect of rapid solidification on the microstructure of Al-Zn-Mg-Cu alloys with 8 wt.% Zn-2 wt.% Mg-2.3 wt.% Cu and 0.2 or 0.5 wt.% of Zr additions were investigated using X-ray diffraction measurements (XRD), scanning (SEM) and transmission electron microscopy (TEM) combined with energy dispersive X-ray (EDX) microanalysis. Rapidly solidified ribbons with thickness of 70-100 m were performed by melt spinning technique. The mould cast alloys as well as the melt spun ribbons revealed dendritic microstructure of (Al) solid solution and η Mg(Zn,Cu)2 phase in interdendritic areas. The refinement of the microstructure and reduction of the volume fraction of the η phase up to 1.7%, as compared to 4% in the mould cast alloys was observed in the ribbons. Copper dissolution up to about 20 wt % in the η phase causes a decrease of the lattice parameters. The Al3Zr primary precipitates were observed in the mould cast alloy containing 0.5 wt % of Zr while in the ribbons all zirconium dissolved in the aluminium solid solution.

Abstract: The in-situ investigation on morphology, chemical contents and crystal structure of non-equilibrium eutectics in semicontinuous casting ingot of Al-6.2Zn-2.3Mg-2.3Cu super-high strength aluminum alloy have been carried out by transmission electron microscopy equipped with energy dispersive X-ray spectroscopy (TEM-EDX). The results show that a large amount of lamellar non-equilibrium eutectics exists in semicontinuous casting ingot of this alloy; the non-matrix phase of this eutectics is a secondary phase which contains Al, Zn, Mg and Cu elements, with the atom ratio of approximately 1:1:1:1, and this non-matrix phase of non-equilibrium eutectics owns the same crystal structure as that of η phase (MgZn2).

Abstract: The influence of Zn content and Zn/Mg ratio on the equilibrium phase amounts of major ageing strengthening phase η’ (MgZn2) in lithium containing 7000 series aluminum alloys has been investigated by means of thermodynamic calculations. The results show that, comparing with Li-free 7000 series aluminum alloys, Zn/Mg ratio is no longer the determinant for the amount of η’ and only increasing Zn content that can increase the amount of η’. And the results were discussed with existing experiments.