Rainer Schmid-Fetzer

The PANDAT software package and its applications

Abstract PANDAT is a software package for multicomponent phase diagram calculation. Given a set of thermodynamic parameters for all phases in a system and a set of user constraints, PANDAT automatically calculates the stable phase diagram without requiring either prior knowledge of the diagram or special user skills. The features of PANDAT are discussed and some application examples presented. In addition to PANDAT, its calculation engine, PanEngine, is also discussed.

Phase diagram calculation: past, present and future

The past, present and future of phase diagram calculations for multicomponent alloys are reviewed and assessed. The pioneering studies of Van Laar and Meijering in the first half of the 20th century led to the use of phase equilibrium information as a supplement to single phase thermodynamic property data in these calculations. The phenomenological modeling or the Calphad approach is the primary focus of this review due primarily to its great success in calculating multicomponent phase diagrams for technological applications. In this approach, thermodynamic descriptions of multicomponent alloys are obtained by appropriate extrapolations of descriptions obtained for the lower order systems, viz., the constituent binaries and ternaries. Some shortcomings of the Calphad route to obtaining phase diagrams are pointed out. These include (a) the inability of first generation software to always automatically calculate the stable phase diagram of a system given a thermodynamic description and (b) the use of some inappropriate thermodynamic models, particularly those used for ordered phases. The availability of second generation software eliminates the first shortcoming and a physically more realistic model, the cluster/site approximation, has been formulated which is more suitable for describing the thermodynamics of ordered alloys. The results obtained to-date using the new software and the new model open up new avenues for calculating more reliable multicomponent phase diagrams for technological applications.

On the calculation of multicomponent stable phase diagrams

In a thermodynamic system, a stable phase diagram is uniquely determined by the thermodynamic parameters of each phase. A generalized phase equilibria calculation software package should enable the calculation of the phase diagram from the parameters automatically, without requiring either prior knowledge of the diagram or special user skills. Improvements to the existing software packages have been achieved in the new software package PANDAT. The approach used in this software is described briefly and some examples of its advantages are given.

Thermodynamic measurements and assessment of the Al–Sc system

Abstract A study of the binary Al-Sc phase diagram has been performed by means of thermodynamic calculations and experimental measurements. The enthalpy of formation of all intermetallic compounds has been determined and a cursory examination of the phase equilibria carried out, for compositions greater than 40 at% Sc. Two new invariant reactions have been identified in the Sc-rich part of the diagram: L ↔ (βSc)+Sc2Al at 1185°C and (βSc) ↔ Sc2Al+(αSc) at 970°C. A coherent set of Gibbs energy expressions for all the phases in the system has been generated by a least square optimisation procedure using all the experimental data available. The overall agreement is satisfactory but some uncertainties still persist, especially concerning the ScAl phase, owing to experimental difficulties.

Reassessment of the Al–Mn system and a thermodynamic description of the Al–Mg–Mn system

Abstract A thermodynamic optimization for the Al – Mn system is performed by considering reliable literature data and newly measured phase equilibria on the Al-rich side. Using X-ray diffraction, differential thermal analysis, and scanning electron microscopy with energy dispersive X-ray spectroscopy methods, the melting behavior of λ-Al4Mn was correctly elucidated, and two invariant reactions associated with λ-Al4Mn (L + μ-Al4Mn λ-Al4Mn at 721 ± 2 °C and L + λ-Al4Mn Al6Mn at 704 ± 2 °C) are observed. The model Al12Mn4(Al, Mn)10 previously used for Al8Mn5 was modified to be Al12Mn5(Al, Mn)9 based on crystal structure data. In addition, the high-temperature form of Al11Mn4 is included in the assessment. Employing fewer adjustable parameters than previous assessments, the present description of the Al – Mn system yields a better overall agreement with the experimental phase diagram and thermodynamic data. The obtained thermodynamic description for the Al – Mn system is then combined with those in the Al – Mg and Mg – Mn systems to form a basis for a ternary assessment. The thermodynamic parameters for ternary liquid and ternary compound Mn2Mg3Al18 (τ) are evaluated on the basis of critically assessed experimental data. The enthalpy of formation for τ resulting from CALPHAD (CALculation of PHAse Diagrams) approach agrees reasonably with that via first-principles methodology. Comparisons between the calculated and measured phase equilibria in the Al – Mg – Mn system show that the accurate experimental information is satisfactorily accounted for by the present description. A reaction scheme for the whole ternary system is presented for practical applications.

Stability of metal/GaAs-interfaces: a phase diagram survey

Calculated phase diagrams of ternary Ga-As-metal systems for the metals Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Re, Os, Rh, Ir, Ni, Pd, Pt, Cu, Ag and Au are presented. The predictive calculations are based on the following simplifications: Ternary phases and solid solubilities are disregarded and the Gibbs energy of formation of binary compounds is estimated by the enthalpy of formation and calculated from Miedema’s model. The predicted diagrams agree surprisingly well with experimental data and they may be useful for the many cases where data are lacking or fragmentary. The phase diagrams and the thermodynamic data are shown to be a powerful tool for the understanding of interface reactions of metallic contacts to GaAs and hence for the development of improved contact materials.

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.

Development of Mg-Sc-Mn alloys

Abstract Scandium is a potential alloying element for improving the high temperature properties (above 300°C) of magnesium alloys. Scandium ( T m =1541°C) increases the melting point of the magnesium solid solution, diffuses slowly in magnesium and has a lower density than other potential alloying additions (3 g cm −3 ). Based on thermodynamic equilibrium calculations the alloys MgSc6Mn1 and MgSc15Mn1 were prepared using the squeeze casting technique. The alloys showed a strong annealing response due to the formation of Mn 2 Sc precipitates. The low diffusivity of the alloying elements retards the overageing process. The existence of the Mn 2 Sc precipitates was confirmed by X-ray diffraction and energy dispersive X-ray microanalysis. The ultimate tensile strength of the new alloys was somewhat lower than the corresponding values for the high creep resistant conventional alloy WE43 (Mg–4.0wt.% Y–1.0wt.% Heavy Rare Earths–2.25wt.% Nd–0.5wt.% Zr) whereas the tensile yield strength was comparable to WE43 at higher Sc content. The alloys showed a low elongation to fracture due to a strongly localised plastic deformation. Creep tests of the material in the as cast condition resulted in secondary creep rates which were comparable to WE43 at higher stresses but significantly lower at lower stresses. A T5 heat treatment of the new alloys led to creep rates which were up to two orders of magnitude lower than for WE43. Hence it is possible to increase the operating temperature by 50°C for the new alloys.

Interface reactions between silicon carbide and metals (Ni, Cr, Pd, Zr)

Abstract The interface reactions between silicon carbide (SiC) and metals Cr, Zr, Ni and Pd have been studied in the temperature range of 700–1300°C by employing bulk diffusion couples. The reaction products have been identified by SEM/EDX and EPMA. It has been observed that interface reactions between SiC/Cr and SiC/Zr show layered structures and interface reactions between that of SiC/Ni and SiC/Pd show periodic bands in the reaction zone. The observed diffusion paths are related to the respective ternary isothermal phase diagrams. Based on the ternary solid state equilibria and diffusion paths, interpretations pertaining to the nature of reaction products and reaction morphology are discussed.

Design strategy for microalloyed ultra-ductile magnesium alloys

This article describes a design strategy deployed in developing ultra-ductile Mg alloys based on a microalloying concept, which aims to restrict grain growth considerably during alloy casting and forming. We discuss the efficiency of the design approach, and evaluate the resulting microstructural and mechanical properties. After processing, the so-designed alloys ZQCa3 (Mg–3Zn–0.5Ag–0.25Ca–0.15Mn, in wt.%) and ZKQCa3 (Mg–3Zn–0.5Zr–0.5Ag–0.25Ca–0.15Mn, in wt.%) reveal very fine grains (<10 µm), high ductility (elongation to fracture of up to 30%) at moderate strength or high strength (ultimate tensile strength of up to 350 MPa) at reasonable ductility. These properties are explained based on thermodynamic modelling, microstructure analysis including transmission electron microscopy studies, and microstructural and mechanical testing after annealing, and are compared to a related commercial alloy (ZK31).