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Investigation of the Phase Relations in Noble Metal Alloys

Author: LongChaoHui
Tutor: JinZhanPeng; LiuHuaShan
School: Central South University
Course: Materials Science
Keywords: noble metal based alloy phase diagram thermodynamic calculation CALPHAD approach solidification simulation
CLC: TG146.3
Type: PhD thesis
Year: 2010
Downloads: 25
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Abstract

Because of the characteristic crystalline structure, noble metals have good electrical and thermal conductivity, good corrosion resistant, and good resistance to creep at high temperature. In order to improve the comprehensive properties of noble metal materials, such as strength, hardness, temperature below that at which it recrystallizes, stability at high temperature, and reduce the costs of materials, some alloying elements are necessary to add in noble metal matrix.To improve our understanding about the precipitating process and design alloy compositions, knowledge of phase diagrams and thermodynamic properties of the involved systems are necessary. In this paper, experimental investigation and thermodynamic calculation, including CALPHAD method and Miedema theory, were carried out on the phase relations of some noble metal-based systems as described in the following.(1) The isothermal section of Ag-Cu-Er (0-33.3 at.% Er) ternary system at 923 K was investigated by X-ray diffraction, scanning electron microscopy and electron-probe microanalysis. The experimental results showed that there were four three-phase regions, i.e. (Ag)+(Cu)+Cu9Er2, (Ag)+Ag51Er14+Cu9Er2, Ag2Er+Cu2Er+Cu9Er2 and Ag2Er+Ag51Er14+ Cu9Er2. The solubilities of the third element in binary compounds of Ag51Er14, Ag2Er, Cu9Er2 and Cu2Er are 17.96 at%,10.32 at%,53.08 at% and 22.08 at%, respectively. Ag and Cu substitute each other, while the composition of Er is tinily changed.Thermodynamic assessment of Ag-Er binary system was carried out. Then, based on the experimental results of the phase relations of Ag-Cu-Er system in present work and the reported thermodynamic parameters of Ag-Cu and Cu-Er system, Ag-Cu-Er ternary system was thermodynamic assessed. Reasonable agreement between the calculated results and experimental data or first-principles calculation results was obtained.Using the present thermodynamic database of Ag-Cu-Er ternary system, the solidification was calculated for one alloy in three-phase region. The calculated result was successfully applied to understand the experimental microstructures and phases obtained in present work for the alloy as cast state. The solidification path of the alloy was also calculated.(2) With X-ray diffraction, scanning electron microscopy and electron-probe microanalysis, a part of phase relations in Ag-Si-Zr ternary system at 1023 K were investigated. Five three-phase regions were measured, that is (Ag)+(Si)+Si2Zr, (Ag)+AgZr+SiZr2, SiZr+Si4Zr5+Ag4Si3Zr3, AgZr+AgZr2+SiZr2 and AgZr2+SiZr2+(Zr). A ternary intermetallic compounds, i.e. Ag4Si3Zr3 was discovered. Based on the phase relations obtained in the present work and the reported thermodynamic parameters of the involved binary systems, thermodynamic assessment of Ag-Si-Zr ternary system was carried out. Good agreement between the thermodynamic calculation and experiments was achieved.(3) Thermodynamic assessments of Ag-Dy and Dy-Sb binary systems were carried out based on critically evaluated experimental data. Combined with the optimized parameters in the literature for Ag-Sb system, Ag-Dy-Sb ternary system was optimized. The calculated results were good agreement with experimental data or first-principles calculation results.(4) Pt-Pb system was evaluated and optimized with CALPHAD method. The calculated phase relations were good agreement with experimental data, while the calculated thermodynamic properties agreed with the measured data and first-principles calculation results. Combined with the reported thermodynamic parameters of Ag-Pt and Ag-Pb systems, Ag-Pt-Pb ternary system was extrapolated. The liquid project and several isothermal sections were calculated.(5) According to the experimental thermodynamic properties and phase diagrams, Ru-Ge, Ru-Si and Ru-Sn binary systems were thermodynamic reassessed. Miedema theory was used to calculate the mixing enthalpies of liquid Ru-Ge alloys and the calculated results were used to optimize Ru-Ge binary system. Combined with the reported thermodynamic parameters of the other involved binary systems and the experimental ternary phase relations, Ru-Ge-Si, Ru-Si-Sn and Ru-Ge-Sn ternary systems were successfully optimized. The self-consistent and reasonable thermodynamic database for describing Ru-Ge-Si-Sn quaternary system was obtained.

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CLC: > Industrial Technology > Metallurgy and Metal Craft > Metallurgy and Heat Treatment > Metallic materials > Non - ferrous metals and their alloys > Precious metals, platinum group metals and their alloys
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