Volume 100, Issue 2

The research areas of materials constitution and computational thermodynamics have traditionally been covered by numerous publications in the International Journal of Materials Research – Zeitschrift für Metallkunde. From the very beginning in the year 1909 experimental work and also the modelling of thermodynamic functions and phase diagram calculations were presented. Since the early 1970s results of Calphad-type modelling including thermodynamic database developments were also published by numerous groups. The use of computational thermodynamics for multi-component materials design (metal alloys, ceramics, composites) are an additional focus of recent works.

By amending the time and space evolution of interfaces to the CALPHAD Gibbsian phase descriptions, the phase-field method now makes realistic microstructure simulations possible. This combination allows incorporating more than one century of accumulated experimental information, consistently synthesized in the thermodynamic and kinetic CALPHAD databases, into multicomponent, multiphase, 3-dimensional microstructure evolution simulations. It represents a large step in materials design and a call for creative contributions from a growing interdisciplinary community. The approach is illustrated by steel and magnesium alloy microstructure simulations.

Traditionally, the determination of phase diagrams has followed the inductive paradigm, where experimental observations provide the phase boundaries in more or less detail and phenomenological and semi-phenomenological models are employed to interpolate between the experimental data points, and by extrapolation to predict the shape of the phase boundaries in experimentally inaccessible regions. Over the past fifteen years, a new methodology has been developing, the aim of which is the prediction, determination and validation of phase diagrams in chemical systems without any recourse to experimental information. The founding stone of this deductive approach is the global study of the energy landscape of the chemical system. In this review, we present an introduction to the concept of energy landscapes in the context of phase diagram calculations, an overview over the large variety of methods employed to study energy landscapes and compute free energies on the ab initio level, and a number of typical examples of first-principle predictions, determinations and validations of phase diagrams.

The development of new alloys has always been a prominent field in materials science. For modern materials, the conditions that need to be considered are manifold. The properties of a material depend firstly on its composition, but also on the processing during its production and on the initial microstructure. In view of the complexity of modern materials and the numerous possibilities to influence their properties it is necessary to use a variety of tools for their optimization. Theoretical and empirical approaches should be combined in order to achieve a goal in a reasonable amount of time. On the theoretical side there is the calculation of type and amount of phases considering thermodynamic equilibrium, or in more sophisticated cases the combination of thermodynamic and kinetic calculations. If the necessary data are not available, some entities can also be estimated. On the experimental side all of the available methods for producing and analyzing metal alloys should be applied wherever necessary. The goal is a more profound understanding of microstructural evolution so that it can be influenced specifically and alloy candidates are not excluded prematurely if the properties of an alloy are not satisfactory after a first unsuccessful attempt. Examples that are presented here are development steps for Pb-free solder alloys (Bi and Zn based alloys) and the interrelation of alloy and processing development for an Al – Fe – Si alloy.

Based on the critical review of the experimental data available in the literature, 7 decisive ternary alloys were prepared. New phase transition temperatures and phase equilibria at 800°C in the Fe – Mn – Ni system were measured. An effective method to accurately determine phase transition temperatures from differential thermal analysis curves with overlapped peaks and nonlevel baselines within a narrow temperature range was developed. A massive transformation was observed in one quenched Fe-rich alloy. In order to make the fcc_A1 phase always in the fully disordered state in the ternary system, the effect of the binary reciprocal parameter on the Gibbs energy of ternary ordered L1 2 phase was considered. It was found that the introduction of only one parameter could describe all reliable experimental data in the Fe – Mn – Ni system satisfactorily. The presently obtained parameters were successfully applied in two practical cases.

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.

In order to define the ability of magnetic elements M = Mn, Fe, Co to stabilise clathrate structures, alloys of the Ba – M – Ge system were investigated in the as-cast state and after annealing at 700°C and 800°C by means of X-ray powder diffraction, light optical and electron-probe microanalysis. Temperatures of phase transformations were derived from differential thermal analysis. Results are summarised in (i) isothermal sections at 700°C and 800°C, (ii) solidus and liquidus surfaces covering the region of existence for both clathrate phases in these systems. Invariant reactions during crystallisation are presented in form of Schultz – Scheil diagrams. In all three cases only limited solubility of the M element was found for clathrate IX (Ba 6 Ge 25 ) i. e. the Ge-framework in the crystal structure of Ba 6 M x Ge 25 – x dissolves 0.6 atom of Mn, and about 1 atom of Fe and Co per unit cell. The maximum solubility of iron in clathrate type I (Ba 8 Ge 46 – x ) was found to be less than 0.5 Fe atom per unit cell, and clathrates with Mn and Co contain up to 1.0 and 2.5 atoms in the unit cell, respectively. Whilst Fe does not decrease the formation temperature of the clathrate phase, Mn and Co decrease it from 770°C (for binary Ba 8 Ge 43 ) to 766°C and 749°C, respectively.

Amorphous Si – C – N domains, nanocrystalline SiC and nanocrystalline Si 3 N 4 identified in the structure of Si – C – N polymer derived ceramics (SiCN PDCs) have been proposed as metastable phases am-SICN, NASIC and NASIN. To study structural transformations of SiCN PDCs, the thermodynamic equilibria of these phases have been computed using previously derived Gibbs energies G(am-SICN), G(NASIC) and G(NASIN). The computational results are presented in the form of metastable phase diagrams. A new paradigm of these diagrams is explained as due to time dependence implemented into the Gibbs energies G(NASIC) and G(NASIN) and the application to interpretation of the crystallization course observed for SiCN PDCs is discussed.

The binary Ce – Si and Y – Si systems have been critically evaluated based upon available phase equilibrium and thermodynamic data, and optimized model parameters have been obtained giving the Gibbs energies of all phases as functions of temperature and composition. The liquid solution has been modeled with the Modified Quasichemical Model to account for the short-range ordering. The results have been combined with those of our previous optimizations of the Mg – Si, Mg – Ce and Mg – Y systems to predict the phase diagrams of the Mg – Ce – Si and Mg – Y – Si systems. The predictions have been compared with available data.

A CALPHAD based re-assessment of the thermodynamic description of the Ti – Al – Nb system was performed to take into account experimental evidence of the extension of the primary crystallization of the -phase to higher Al contents. The adjustable parameters of the analytic expressions of the Gibbs free energy for the Liquid, β-, β 0 -, δ-, σ-, γ-, α-, and 2 -phases were re-optimized using available experimental data from a critical assessment of the literature. The calculated primary crystallization of -phase is in better agreement with experiment although the extension of the single phase -field to higher Al compositions could not be calculated. Calculated isothermal sections are in good agreement with experiment in the temperature range 1923 K to 1273 K.

Thin films of zinc aluminate are prepared by reactive evaporation. The composition of these films is varied by changing the oxygen partial pressure and by post-deposition annealing. It is observed that there is wide variation in the band gap values (2.34 – 4.54 eV) depending on the oxygen partial pressure. Further, there is a strong dependence of band gap values on the post-deposition annealing conditions. One of the films exhibits spinel (AB 2 O 4 ) structure, whereas the rest are amorphous. A resistivity value of as low as 3.6 × 10 – 3 ohm-cm has been measured. The compositional studies, using mass spectrometry, reported in this paper point towards novel transparent conducting oxides.

Trivalent cerium actived yttrium aluminum garnet (YAG: Ce) phosphor was prepared using the Pechini method. Phase transition during calcination was investigated through Fourier transform infrared spectroscopy and X-ray diffraction. The results show that the powders directly transform into the pure YAG phase at 80°C. The dimension of phosphor calcined at 1200°C is micro-scale, which consists of nanometer scale primary particles. The effect of Ce doping content and calcination temperature on the luminescence properties is also discussed. The maximum emission is obtained for about 1 at.% Ce additions. The concentration quench effect is observed when Ce content increases beyond this limit. With the increment of calcination temperature, the emission intensity and lifetime increase.

The microstructure and mechanical property of a eutectic Ni-33Al-28Cr-5.94Mo-0.05Hf-0.01Ho (holmium) alloy fabricated by using directional solidification under two withdrawal rates (3 mm min – 1 and 10 mm min – 1 ) are presented. It is shown that a promising as-cast microstructure is formed under the higher withdrawal rate, which can be characterized by the refinement of eutectic cells as well as eutectic lamellae between the NiAl and Cr(Mo) phases within the cells. As a positive response to this, superior room temperature mechanical properties with yield strength of 1153 MPa and compressive strain of 37 % are obtained. Based on these investigations, the mechanism responsible is discussed.

Using the Langmuir equation and penetration theory, the evaporation process of Al during the electron beam cold hearth melting of Ti64 alloy under steady state conditions was calculated and the evaporation mechanism was analyzed. The calculation results show that it is necessary to consider both the evaporation step at the liquid/gas surface and the diffusion step in the molten pool during calculation. Comparison of the mass transfer coefficient of the evaporation step and the diffusion step shows that the Al evaporation process is a double controlled process. The Al concentration at the interface decreases from 260 kg m – 3 to 140 kg m – 3 , and the evaporation rate increases rapidly from 0.001 kg m – 2 s – 1 to 0.012 kg m – 2 s – 1 when the temperature increases from 1950 K to 2300 K. To control the evaporation loss of Al, surface temperature should be controlled during the electron beam melting of Ti64 alloy.

The 560°C isothermal section of the Zn – Fe – Ni – Si quaternary system with the Zn composition being fixed at 93 at.% has been determined using optical microscopy, scanning electronic microscopy coupled with energy dispersive X-ray spectroscopy, and X-ray diffractometry. It was found that FeSi is in equilibrium with almost all phases in the section, including the δ-Fe – Zn, FeSi 2 , T (Zn – Fe – Ni ternary compound), γ-Ni – Zn, NiSi 2 and liquid phases. The existence of the L + FeSi + NiSi 2 + γ-Ni – Zn four-phase equilibrium prevents other Ni-Si compounds from entering equilibrium with the liquid phase at 560°C. No true quaternary compound was found in the present study.