Experimental investigation of the Ca-Mg-Zn system via Diffusion Couples and Key Experiments.
Masters thesis, Concordia University.
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Nine diffusion couples and 32 key samples have been used to study the phase diagram of the Ca-Mg-Zn system at 335ºC and crystal structures of ternary intermetallics. Four ternary compounds have been found in this system: Ca3MgxZn15-x (4.6≤x≤12 at 335ºC) (IM1); Ca14.5Mg15.8Zn69.7 (IM2); CaxMgyZnz (x = 8.2-9.1; y = 27.1-31.0; z = 60.8-64.7 at 335ºC) (IM3) and Ca1.5Mg55.3Zn43.2 (IM4). Phase relations and solubility limits have been determined for binary and ternary compounds using Scanning Electron Microscopy, Electron Probe Microanalysis and X-ray Diffraction techniques. Crystal structures of the IM1 and IM3 ternary compounds have been studied by means of XRD, Transmission Electron Microscopy and Electron Back Scattered Diffraction. The refinement of the XRD patterns for IM1 ternary compound has been carried out by Rietveld analysis. XRD data has shown that this solid solution crystallizes in hexagonal structure having P63/mmc (194) space group and Sc3Ni11Si4 prototype. The lattice parameters decrease linearly with decreasing Mg content obeying Vegard’s law. The fractional atomic occupancy of 6h, 4f, 2b and 12k sites of this compound are function of Mg concentration. Focused Ion Beam has been used to lift Transmission Electron Microscopy specimen of the ternary compound and the hexagonal structure has been confirmed by means of Selected Area Electron Diffraction data. Based on the atomic occupancy results and the crystallographic details, a three sublattice (Ca)(Zn)(Mg,Zn)4 model is proposed for this compound.
Three binary compounds are found to have extended solid solubility into the ternary system. CaZn11, CaZn13 and Mg2Ca are forming substitutional solid solutions where Mg substitutes Zn atoms in the first two compounds, and Zn substitutes both Ca and Mg atoms in Mg2Ca. Based on the current experimental results, the isothermal section of Ca-Mg-Zn phase diagram at 335ºC has been constructed. The morphologies of diffusion couples have been studied in the Ca-Mg-Zn system at 335ºC. Depending on the terminal compositions of the diffusion couples, the morphology of the two-phase regions in the diffusion zone has: ‘tooth-like’ morphology or matrix phase with isolated and/or dendritic shape precipitates morphology.
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Experimental investigation of the Ca-Mg-Zn system via Diffusion Couples and Key Experiments. (deposited 09 Jun 2011 14:37)
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