Djordje Mirković

Solidification curves for commercial Mg alloys obtained from heat-transfer modeled DTA experiments

Abstract An improved method is presented for determination of solidification curves, i.e., solid fraction versus temperature, for commercial Mg alloys using heat-transfer modeled differential thermal analysis (DTA) curves. A better simulation of the measured DTA signal is attained through an independent measurement of the time constant as function of temperature for the applied equipment. This enables a better desmearing of the DTA signal. Challenging Mg alloys could be appropriately handled by redesigning the tantalum encapsulation. Due to high oxygen affinity and vapor pressure of the investigated magnesium alloys, this special adaptation of the DTA setup using sealed Ta capsules was indispensable for generation of reproducible and reliable data.

Integrated approach to thermodynamics, phase relations, liquid densities and solidification microstructures in the Al–Bi–Cu system

Abstract Al – Bi – Cu phase equilibria were studied using dedicated thermal analysis and microstructural investigation of slowly solidified samples. Experimentation in the most decisive Cu-rich region was enabled by developing a BN insert in combination with Ta-encapsulation for these challenging samples. A consistent thermodynamic model of the Al – Bi – Cu system is developed for the first time and applied to calculations of the entire phase diagram. The liquid miscibility gap of Al – Bi alloys extends dramatically upon addition of Cu. The density difference of the two liquids was determined in dedicated experimentation on buoyancy forces. The useful composition and temperature range for these experiments in the ternary L′ + L′′ region was provided by the thermodynamic calculations. The density data can be well interpreted on the basis of presently calculated coexisting liquid phase compositions. Finally, distinct ternary monotectic features on solidification microstructures of Al – Bi – Cu alloys are revealed and inter-related with the phase formation from thermodynamic calculations.

Tetragonal low-temperature structure of LiAl

Abstract The cubic face-centered structure of LiAl ( Fd 3 m , a=6.35971(3) A at 290 K ) transforms into a tetragonal body-centered structure (I41/amd, a=4.47831(1) A , c=6.34435(2) A at 12 K ). This first-order phase transition at about 93(2) K during heating is probably the reason for the so-called “ 100 K anomalies” in some physical properties like specific heat, electrical resistivity and nuclear-spin lattice relaxation. This transition seems to be correlated with the composition Li:Al of the alloy and the amount of Li vacancies.

Constitution of Magnesium Alloys

Multi-component magnesium alloys exhibit a complex constitution, requiring computational thermodynamics for a quantitative treatment that goes beyond “just” phase diagrams. The basis for this approach is a thermodynamic Mg alloy database, which is developed in an ongoing long-term project in our group since many years. Three distinctive features of this database are highlighted in this report: (i) Combination of own key experimental work with theoretical modeling to generate consistent data, (ii) Systematic quality control of the database using a variety of elaborated cross-checks, and (iii) Complete and entire composition range descriptions for all pertinent binary or ternary subsystems whenever possible. The latter point is decisive for the capability to use this tool for new alloys, far beyond the composition limits of conventional Mg alloys. It is demonstrated by correctly predicting the phase formation in aluminum-rich six component alloys. Also, new Mg-solder alloys can be tackled that are essentially Zn-rich.