Joachim Gröbner

Focused Development of Magnesium Alloys Using the Calphad Approach

In traditional alloy development, experimental investigations with many different alloy compositions are performed. The selection criteria for multicomponent alloying elements and their compositions become diffuse in a traditional approach. Computational thermochemistry as used in the Calphad approach can provide a clear guideline for such selections and helps to avoid large scale experiments with less promising alloys. Thus, it is a powerful tool to cut down on cost and time during development of Mg-alloys. An overview of the Calphad method is given. As an example of applications, recent developments of new creep resistant magnesium alloys that show about 100 times less creep than the best commercial alloys are reported. Also outlined are the methods used in our long-term project of construction of the necessary thermodynamic magnesium alloy database for several alloying elements, such as Al, Li, Si, Mn, Ca, Sc, Y, and Zr, and rare earth elements (Ce, Gd, Nd), using the Calphad method combined with key experiments.

Experimental investigation and thermodynamic calculation of ternary Al–Ca–Mg phase equilibria

The ternary Al-Ca-Mg phase equlibria were investigated using X-ray diffraction methods, metallography, scanning electron microscopy with energy- and wave-length-dispersive X-ray microanalysis, and differential thermal analysis. The phase diagram was determined in the complete composition range. Large ternary solubilities were found for the binary phases CaMg 2 , Al 2 Ca and Al 3 Ca 8 . Microstructures of samples with compositions near the ternary monovariant eutectic L → CaMg 2 + Al 2 Ca show remarkable rod-like crystals that are identified as intergrowth between CaMg 2 and Al 2 Ca. A consistent thermodynamic model was developed using the Calphad method incorporating all experimental data. It was used to calculate all pertinent phase equilibria of the Al-Ca-Mg system.

Selection of promising quaternary candidates from Mg–Mn–(Sc, Gd, Y, Zr) for development of creep-resistant magnesium alloys ☆

Abstract Recently developed Mg–Mn–Sc alloys show a considerable increase of creep resistance at elevated temperature. The endeavor to further improve the properties and to reduce cost of high-price Sc metal initiated a search for additional alloying elements. Gd, Y and Zr were considered for this purpose. The aim is to achieve a large quantity of suitable precipitations to improve mechanical properties using a minimum of expensive alloy element addition. The huge amount of possibilities of combining the elements Mg–Mn–(Sc, Gd, Y, Zr) and the time and cost effort of technological experiments require a preselecting of systems and alloy compositions. Thermodynamic phase diagram and phase amount calculations were performed to give hints for selecting promising candidates. A priority list of three quaternary systems is established: Mg–Mn–Gd–Sc, Mg–Mn–Sc–Y and Mg–Mn–Y–Zr, based on the classification of individual alloys. Most promising is MgMn1Gd5Sc0.8 (wt.%), but the alloys MgMn1Gd5Sc0.3 and MgMn1Y5Sc0.8 are also promising. The entire quaternary Mg–Mn–Y–Zr system is disqualified because of phase diagram features that are detrimental for the required microstructural engineering. The focused alloy development following this approach avoids a waste of time and effort.

Thermodynamic calculation of the binary systems M-Ga and investigation of ternary M-Ga-N phase equilibria (M=Ni, Co, Pd, Cr)

A thermodynamic modeling of the binary systems Ni-Ga, Co-Ga, Pd-Ga, and Cr-Ga has been performed using the CALPHAD method. This modeling is focused on a simplified description of the solid-state equilibria. From published data on binary nitrogen systems, isothermal sections of the ternary M-Ga-N systems have been calculated by extrapolation. Ternary samples were prepared from M and GaN powders in various ratios, pressed to pellets, and annealed between 500 and 700 °C for up to 162 h. Phase analyses were carried out by x-ray diffraction, and the results compared to the calculated ternary phase equilibria; the comparisons indicate that M+GaN are not in equilibrium in any of these systems. Experimental results show that the M+GaN reactions are sluggish. In the ternary Ni-, Co-, and Pd-Ga-N systems, no ternary compounds were observed. The solid reaction products are essentially the metal-rich binary intermetallics (Ni3Ga, CoGa, Pd2Ga). Thermodynamic calculations show that, at elevated (local) pressure, reaction slows down. Pressure buildup must have occurred inside the tightly pressed pellet, since only small amounts of nitrogen gas were found to have escaped from the pellet. In the Cr-Ga-N system, two ternary compounds are known to exist along the Cr-GaN section. Therefore, the reaction of GaN with Cr to form the ternary phases could occur without gas liberation. Even this reaction is sluggish and was not completed in the samples investigated.

Thermodynamic optimization of the systems Mn–Gd and Mn–Y using new experimental results

Abstract The literature concerning binary systems of manganese and rare earth metals is very scarce. Thermodynamic optimizations using the Calphad method are only available for the Mn–Y system [Flandorfer et al., Z. Metallkd. 88 (1997) 529–538]. In this work, thermodynamic optimizations for the binary systems Gd–Mn and Mn–Y are investigated. New experimental data for the enthalpies of formation of the binary Mn–Y phases were considered. The resulting enthalpies fit in the trend of stabilities within the series of RE–Mn systems described by Saccone et al. [Z. Metallkd. 84 (1993) 563–568].

Key Issues in a Thermodynamic Mg Alloy Database

Key issues in a large thermodynamic Mg alloy database (20 components, over 450 assessed phases) are consistency, coherency, and quality assurance. These issues and the basic concept are elaborated. The structurally supported modeling, especially of pertinent solid phases, requires proper consideration of multicomponent solid solutions of intermetallic phases, which are abundant in magnesium alloys. The second focus of this work is to give evidence on the database application by predicting phase formation during solidification or heat treatment in advanced multicomponent magnesium alloys from thermodynamic calculations.

Measurement and calculation of the viscosity of metals—a review of the current status and developing trends

Viscosity is an important rheological property of metals in casting because it controls the rate of transport of liquid metals, which may lead to casting defects such as hot tearing and porosity. The measurement methods and numerical models of the viscosity of liquid and semi-solid state metals that have been published to date are reviewed in this paper. Most experimental measurements have been performed with rotational and oscillatory viscometers, which offer advantages at low viscosities in particular. Besides these two traditional methods for measuring viscosities, a couple of studies also introduced the technique of isothermal compression for alloys in the semi-solid state, and even an optical basicity method for the viscosity of slags. As to numerical models, most published results show that the viscosity of liquid and semi-solid state metals can be described by the Arrhenius, Andrade, Kaptay or Budai–Bemkő–Kaptay equations. In addition, there are some alternative models, such as the power model and the isothermal stress–strain model.

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.

Application of computational thermochemistry to Al and Mg alloy processing with Sc additions

Abstract Thermochemical calculations have been used to discuss various aspects of AlMgSc alloy processing: alloying of scandium, alloy melting, solidification and heat treatment. The feasibility of replacing expensive scandium by cheap scandium oxide for alloying into liquid Al is supported by these quantitative data, whereas the alloying into liquid Mg is found to be unfeasible. Calculations of ternary AlMgSc phase equilibria are based on an experimentally supported thermodynamic data set. Liquidus surfaces of both the Al and Mg rich corners depict the narrow ranges for complete melting of alloys subject to reasonable temperature limits. The joint solid solubilty limits in both the (Al) and (Mg) matrix are given as a basis for heat treatments. The solidification of one typical Al and one Mg alloy are discussed based on the equilibrium calculations and also using the Scheil model.

Microstructural and thermodynamic investigations on friction stir welded Mg/Al-joints

Abstract With friction stir welding it is possible to join light metals in the solid state. However joints between dissimilar metals are a special challenge. At the Institute of Materials Science and Engineering of the University of Kaiserslautern the friction stir weldability of magnesium–aluminium-joints was investigated. For the realization of high strength joints between these materials the knowledge of the developing microstructure in the welding zone is of especially high interest. Therefore, light microscopic and scanning electron microscopic investigations were carried out and the results compared with phase diagrams and phase fractions for the bonding zone which were calculated at Institute of Metallurgy of the University of Clausthal.