Aleš Kroupa

Phase diagram in the iron-rich corner of the Fe-Cr-Mo-V-C system below 1000 K

Relevant experimental phase equilibria were used with the THERMOCALC program to refine thermodynamic parameters for low-temperature region, in the steels with composition 0.4 to 2.5 wt.% Cr, 0 to 1 wt.% Mo, and 0 to 0.5 wt.% V. The refined parameters were used to calculate phase relationships, which were then compared with experimental results. Results of experiments indicate that M7C3 replaces M23C6 carbides in the temperature region betweeen 773 and 873 K. Similar behavior was also found for molybdenum-rich phases M6C and M2C. Some modifications of existing thermodynamic parameters in the low temperature region were necessary.

Theoretical assessment of the Ni-Sn system

A new theoretical assessment of the Ni–Sn system has been performed by use of the CALPHAD method. Recent experimental results were significantly different from older experimental data and, therefore, a new reassessment of older theoretical work was necessary. The theoretical models for some intermetallic phases were changed to make them consistent with other binary systems in the thermodynamic database developed in the scope of COST action MP0602. Very good agreement was reached both with new experimental phase equlibrium data and older thermodynamic data.Graphical abstract

Phase changes in superaustenitic steels after long-term annealing

A structural study was performed on the austenitic steels Avesta 254 SMO and Avesta 654 SMO after annealing at 700 C for 500, 3188 and 6170 h. Both Avesta steels initially show an unexpectedly large amount of the Laves phase, followed by a relative slow development of the Sigma phase with equilibrium apparently not yet reached after 3188 h. Thermodynamic calculations confirm that the driving forces for alternative precipitates are very similar thus making it easy to form metastable precipitates that only change very slowly to the equilibrium state. TTT calculations also comnfirm that the Laves phase precipitates earlier than the Sigma phase as the temperature is lowered.

Theoretical and experimental study of the γ and γ' equilibrium in Ni-based superalloys

Abstract Ni-based superalloys with aluminium belong to systems with order–disorder transition for the γ and γ′ structures. Because of the complexity of these alloys, quaternary systems can be taken as suitable and sufficiently representative model systems for a theoretical study of their phase equilibria. In recent years several assessments including various model descriptions of ordered phases and ordering based on the CALPHAD formalism have been made with the aim to describe phase diagrams of relevant binary, ternary and some quaternary systems. For our study, the Ni–Al–Cr–Co system was chosen and phase boundaries of γ and γ′ phase equilibrium were calculated based on the data for lower-order systems. The calculations were performed by means of the software ‘Thermo-Calc’. Values obtained were compared with experimentally determined phase information for five alloys with constant Ni content (70 at.%) after long-term annealing at 900°C. The experiments were realized by means of energy-dispercive microanalysis in scanning transmission electron microscopy. For better agreement of theory with experiment, an thermodynamic optimization was performed.

The thermodynamic database COST MP0602 for materials for high-temperature lead-free soldering

The current state of thermodynamic modelling in the field of high-temperature lead-free soldering is presented. A consistent thermodynamic database, containing 18 elements (Ag, Al, Au, Bi, Co, Cu, Ga, Ge, Mg, Ni, P, Pb, Pd, Sb, Sn, Ti and Zn) has been created. The thermodynamic data for the most of the important binary and selected ternary systems were checked and included into the database. The database was tested using major commercial software packages. Such reliable and sophisticated software coupled to reliable thermodynamic databases are necessary prerequisites for application of thermodynamics in advanced alloys design.

Application of Ab Initio Electronic Structure Calculations in Construction of Phase Diagrams of Metallic Systems with Complex Phases

Ab initio electronic structure theory has achieved considerable reliability concerning predictions of physical and chemical properties and phenomena. It provides understanding of matter at the atomic and electronic scale with an unprecedented level of details and accuracy. In the present contribution, the electronic structure theory and state-of-the-art ab initio calculation methods in solids are briefly reviewed and the application of the calculated total energy differences between various phases (lattice stabilities) is illustrated on construction of phase diagrams by the CALPHAD (CALculation of PHAse Diagrams) method in systems containing phases with complex structures, as e.g. Laves phases or sigma phase. Particular examples include description of the Laves phases in the Cr-Nb, Cr-Ta and Cr-Zr systems, sigma-phase in the Fe-Cr system and prediction of the phase composition of ternary Fe-Cr-Mo system and super-austenitic steels. It is shown that the utilization of ab initio results introduces a solid basis of the energetics of systems with complex phases, allows to avoid unreliable estimates and extrapolations of Gibbs energies and brings more physics into the CALPHAD method.

Experimental and thermodynamic studies of phase transformations in Cr–V low alloy steels

Abstract Phase transformations in three Cr–V low alloy steels at 773 and 853 K were studied experimentally in a transmission electron microscope (TEM). The theoretical calculations were carried out for ternary Fe–Cr–C and quaternary Fe–Cr–V–C systems corresponding to the investigated steels. The calculations of phase equilibria were performed using the thermodynamic database program thermo-calc . M3C in 1Cr and 1Cr–0.3V steels, M7C3 in 1Cr–0.3V steel, and MC in 1Cr–0.3V and 1Cr–0.5V steels were identified. The formation M3C precedes the formation of M7C3. The M7C3 carbide in all investigated steels and the MC carbide in steels containing vanadium were determined by thermo-calc as equilibrium phases. The comparison of experimental and computational results indicates that the investigated steels have not achieved equilibrium even after aging for 5000 h at 773 and 853 K.

Phase transformations and phase equilibria in Ti -25V -N system at 1200°C

Abstract The nature of phase transformations and phase equilibria in the Ti- V - N system with about 20 at.-%V and 11 -15 at.-%N has been studied at 1200°C. The results obtained for the titanium rich corner of the Ti - V- N phase diagram provide new information on phase coexistence and the composition of particular phases in this system. Thermodynamic calculations, based on the use of thermodynamic parameters from the three binary subsystems, together with newly assessed thermodynamic parameters published previously, were carried out. Reasonable agreement was obtained between the experiment and the calculations.

Experimental study and thermodynamic assessment of the Ni-Al-Cr-Mo system at 1173 K

The Ni-Al-Cr-Mo system was studied experimentally and theoretically at 1173 K. Reasonable agreement between theory and experiment was stated.

Lead-Free Soldering: Environmentally Friendly Electronics

Abstract The substitution of lead in the electronics industry is one of the key issues in the current drive toward ecological manufacturing. This chapter is aimed at giving an overview of requirements, current situation, problems, and possible development of lead-free soldering and other environmentally friendly joining technologies. It starts with the overview of existing EU regulations and directives. The current situation in the lead-free soldering field is mapped here and a review of the literature, including a patent search, reveals systems and technologies that are at the focus of research effort. New approaches that employ the use of theoretical modeling in the development of new materials is briefly discussed in this chapter. The final part shows the difficulties that the industry faces when implementing new materials into production. The task of this part is to summarize the likely impacts of the phasing out of lead-containing solders for industry, especially small- and medium-sized enterprises.