Artem Kozlov

The Li–C phase equilibria

Abstract Experimental work using X-ray diffraction and differential scanning calorimetry was conducted on key samples in the Li–C binary system. Reproducible differential scanning calorimetry data with multiple heating cycles were produced only by samples sealed in arc welded Ta-capsules. Only one compound, α/βLi2C2, is found to be stable. A comprehensive Calphad-type assessment was performed and for the first time a consistent thermodynamic description, covering all thermodynamic and phase equilibrium data, is developed. Phase diagrams calculated from that validated database, including the gas phase, are presented. The phase LiC6, was also studied experimentally. It is metastable with respect to α/βLi2C2 + (C), but may be formed from Li + (C). Phase transitions of LiC6, claimed in the literature, are discussed.

Growth restriction factor in Al-Si-Mg-Cu alloys

The Growth Restriction Factor, Q, proved to be useful to analyse and control grain refinement during solidification of alloys [1-3]. It is known that in multicomponent alloys a simple summation of the Qi values of the individual constituents taken from the binary phase diagrams can lead to grossly wrong results and that the ternary or higher-level phase diagram needs to be evaluated. This work demonstrates that the actual evaluation of Q using the liquidus gradient and partition coefficients of the multicomponent phase diagram requires some precautions and may be cumbersome. More importantly, this approach entirely fails if an intermetallic phase turns out to be the primary solidifying phase even in tiny amount. A very simple and general solution of this problem is illustrated for Al-Si-Mg-Cu alloys.

The Mg–Ca–O system: Thermodynamic analysis of oxide data and melting/solidification of Mg alloys with added CaO

Abstract Some published experimental results on the thermodynamic data of pure solid CaO and MgO significantly contradict the widely used data in review books or tables, such as JANAF data. We scrutinized all the original experimental data forming the basis in review books and tables in order to establish a more realistic uncertainty range. The two Gibbs energy functions of CaO and MgO are combined with those of all other relevant phases to develop a thermodynamic database for the Mg–Ca–O system. The first systematic study of the calculated Mg–Ca–O phase diagram is presented. This phase diagram is validated by recently obtained dedicated experimental data using in-situ synchrotron radiation diffraction to study phase formation and reactions during melting and solidification of Mg and Mg–Ca alloys with CaO-additions below 1050 K. Intricacies in the solidification path of Mg-rich ternary alloys are revealed by the present thermodynamic calculations. Mg-rich alloys are categorized in four groups according to their participation in different invariant four-phase reactions. The development of non-equilibrium constitution of four phases, (Mg) + Mg2Ca + MgO + CaO, in the fully solidified sample is explained.

Thermodynamic description of reactions between Mg and CaO

CaO is considered as possible replacement for cover gases such as SF6 during melting and casting of Mg alloys. Such CaO additions to molten Mg increase the ignition resistance by forming a protective oxide layer. The actual reactions between liquid Mg and CaO are not well understood. An approach based on chemical reaction equations cannot capture the “CaO dissolution” process. This work presents the development of a consistent thermodynamic description of the ternary Mg-Ca-O alloy system. To that end a revision of the thermodynamic data of key oxides, CaO and MgO, has been performed based on original experimental work so far not considered in thermodynamic databases or tabulations. The formation of a liquid Mg-Ca-[O] alloy during the reaction is predicted from the thermodynamic calculations at melting temperatures; solidification simulations are also performed. These predictions from thermodynamic simulations are validated by experimental data using in situ synchrotron radiation diffraction.

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.

Thermodynamics of phase formation in Mg-La-Ce-Nd alloys

Experimentally validated thermodynamic descriptions have been developed for the ternary Mg-La-Ce, Mg-La-Nd, and Mg-Ce-Nd systems by selecting key alloys in both systems and analyzing the phase formation in both the as-cast and heat treated state by SEM/EDS and DSC. These results were combined to form the validated thermodynamic Calphad-type description for quaternary Mg-La-Ce-Nd alloys. It is shown that for these light rare earth elements (La, Ce, Nd) the intermetallic phases with Mg exhibit significant mutual solid solubility in the ternary systems, extending into the quaternary alloy system. This is reflected by considering the shared crystal structures in the thermodynamic modeling. Simulated solidification paths of three Mg-La-Ce-Nd alloys with different La:Ce:Nd ratios and a common total content of 5 wt.% rare earth (RE) metals are evaluated using computational thermodynamics. Unexpected and distinctly different solidification behavior of these three alloys is revealed. The sequence La→Ce→Nd in the periodic table is not at all reflected in a monotonous solidification behavior. The demonstrated individual impact of each of these elements forbids treating the RE additions as a mere wt.% sum of RE elements.

Phase formation in alloy-type anode materials in the quaternary system Li–Sn–Si–C

Abstract Investigations on the thermodynamics of alloy-type anode materials have been carried out for the quaternary Li–C–Si–Sn system. Phase equilibria and phase stabilities were characterized in the binary subsystems Li–C, Li–Si, Li–Sn. The Calphad method was first used to optimize or completely re-establish all binary subsystems containing Li. For reasons of consistency, the binary subsystem Si–C had to be revisited and its Calphad description was modified. The ternary phase diagrams were then tentatively calculated by extrapolation from the binary subsystems and confirmed by key experiments. No ternary compounds were found. In order to verify the applicability of the anode materials in real batteries, some of the materials were nanostructured by ball milling and spark plasma sintering, the corresponding nanostructures were characterized. Theoretical predictions that nanograined Li2C2 can also be used as cathode material were verified experimentally. The methodologies worked out in the present project (e. g. nanoscale structure transmission electron microscopy analysis, glow discharge optical emission spectroscopy) were also employed in other projects and led to publications concerning other materials such as Mg alloys, carbon nanofibers and an Mn-based antiperovskite.

Interlaboratory study of the heat capacity of LiNi1/3Mn1/3Co1/3O2 (NMC111) with layered structure

Abstract An interlaboratory study was performed to determine the heat capacity of an active material for lithium-ion batteries with layered structure and nominal composition LiNi1/3 · Mn1/3Co1/3O2 (NMC111). The commercial sample, which was characterized using powder X-ray diffraction and inductively coupled plasma–optical emission spectroscopy, is single phase (α-NaFeO2 crystal structure) with a composition of Li1.02Ni0.32Mn0.31Co0.30O2. Heat capacity measurements of the homogeneous sample were performed at five laboratories using different operators, methods, devices, temperature ranges, gas atmospheres and crucible materials. The experimental procedures from each laboratory are presented and the results of the individual laboratories are analyzed. Based on a comprehensive evaluation of the data from each laboratory, the heat capacity of the NMC111 sample from 315 K to 1 020 K is obtained with an expanded reproducibility uncertainty of less than 1.22 %.

Thermodynamics and Constitution of Mg-Zn-Ce alloys

Ternary Mg-Zn-Ce phase equilibria were investigated by key samples to determine the solubilities, primary crystallizing phases and invariant reactions. Alloys were prepared from pure elements and investigated by DTA/DSC and SEM/EDS. A consistent thermodynamic Calphad-type model of the ternary Mg-Zn-Ce system is developed. Phase diagram sections at constant 300°C, at constant 85 at.% Mg and the liquidus projection of the Mg-Zn-Ce ternary system were calculated and they compare reasonably well with all available experimental data, especially in the Mg-rich region. This work may, thus, form a reliable basis for the application of computational thermodynamics in Mg-Zn-Ce-X multicomponent alloys.

Evaluating the Growth Restriction Factor in Stable and Metastable Al-Si-Ti Alloy Solidification

Evaluation concepts for the Growth Restriction Factor, Q, in multicomponent alloys are discussed and illustrated for ternary Al-Si-Ti alloys involving precipitation of primary intermetallic phases.