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Title

THE CHARACTERIZATION FOR B2 STRUCTURE AND L2\ STRUCTURE IN THE AG-MG AND AG-MG-IN SYSTEM

Creator

Kim, Do Hyung

Date

2013, 2013-05

Description

The concentration of point defects and the long range order for ordered B2 AgMg alloys, quenched from 973K, was investigated by statistical...
Show moreThe concentration of point defects and the long range order for ordered B2 AgMg alloys, quenched from 973K, was investigated by statistical thermodynamic modeling, powder X-Ray Diffractometery and diffraction simulation as a function of composition. The lattice parameter behavior on the Ag rich side are expectably having constitutional and thermal anti-site defects on both Ag and Mg sub-lattices, corresponding to the literature data. On the other hand, the Mg rich side has substantially thermal vacancy defects based on the lattice parameter data which shows lower, compared with previously reported data. Concentration of the equilibrium point defects at 973K was calculated from two thermodynamic models, where the Ag rich side was based on the constitutional and thermal anti-site defect formation and the Mg rich side was based on the hybrid defect formation consisting of vacancy, Mg and Ag anti-site defects The experimental long range order at 973K, determined from the integrated intensity ratio of (100) super-lattice reflection to (200) fundamental reflection, is in quite good agreement with the theoretical long range order at 973K based on the calculated integral intensities from the diffraction simulation with the equilibrium concentration of each point defect, obtained by two thermodynamic models. Furthermore, point defect hardening coefficients on both sides of stoichiometry were determined by the measurement of the Vickers hardness as a function of the equilibrium concentration of the main point defects deduced from two thermodynamic models. The hardening coefficient is G/16 for the Ag rich side with respect to Ag anti-site defects and G/3.1 for the Mg rich side with respect to vacancy defects. Also, two hardening coefficients are corresponding to the empirical correlation for the several binary B2 intermetallic compounds with anti-site defects (G/9 to G/85) xvi and vacancy defects (G/3 to G/4). This suggests that the elastic size effect on the Ag rich side is the primary hardening mechanism due to constitutional and thermal Ag anti-site defects while the Mg rich side is likely to have the elastic modulus effect due to constitutional and thermal vacancy defects. It is also indicated that the vacancy defect is more significant hardener than Ag anti-site defects for the ordered B2 AgMg intermetallic system. The partial liquidus projection in the Ag-Mg-In ternary system was established by the primary phase and liquidus temperature, using the Scanning Electron Microscopy, Energy Dispersive Spectroscopy and Differential Scanning Calorimetry. The results showed that the AgMg1-xInx phase of the Ag-Mg-In ternary system has a large primary solidification field up to 90 at.% of In, so that most ternary invariant reactions of the In rich field must be formed beyond 90 at.% of In. The liquid-solid schematic reactions in the Ag rich field were experimentally confirmed, but those of the In rich side have not been established. Furthermore, the ordering phase transition and melting temperature of the Heusler phase AgMg1-xInx alloys were investigated using Differential Scanning Calorimetry, Scanning Electron Microscopy, and powder X-Ray Diffractometery. The DSC results indicated that the melting temperature decreased with increasing the In composition, but a thermal peak for the ordering phase transition was not detected due to either a very small heat of transition or a second order transformation. The XRD results showed that the L21 structure of the Heusler phase was observed for the 15 at.% of In alloy and the degree of order of L21 structure continuously increased with the In composition, resulting from the (111) super-lattice intensity with respect to the In composition. The L21 structure ordering of the 15 at.% of In and 20 at.% of In system xvii was gradually decreased with increasing the annealing temperature, corresponding to decreasing the (111) super-lattice intensity and the long rang order parameters of the L21 structure. These XRD behaviors suggest that the L21/B2 ordering transformation phenomena is a second order transformation with respect to temperature.
PH.D in Material Science and Engineering, May 2013
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