New descritpion of the Al-Cu-X (X=Si, Zn) phase diagram

Alloys based on AlCu in general, and especially on the AlCuSi/Zn systems are of high interest for many different applications in automotive and aerospace industry. The AlCuZn system is interesting for shape memory alloys above that. An important source of the information for the design of such new materials is the equilibrium phase diagram. Phase diagrams are usually investigated experimentally and then theoretically modelled by the so-called CALPHAD approach which is based on the sequential modelling of energies from the simplest (pure elements) to more and more complex systems (alloys with two and more elements). Detailed knowledge of the coexistence and stability of phases in stable equilibrium significantly rationalizes the materials development and is also important for the prediction of materials properties. This project is oriented towards the complete description of the phase equilibria in AlCuSi and AlCuZn. This will be done by a combination of a thorough experimental investigation and the subsequent CALPHAD modelling. The main emphasis in the design of experiments will be targeted on questions that have not been satisfactorily resolved yet in the scientific literature and on the divergent results from different publications. The focus will be the characterization of the composition ranges and crystal structures of the phases in the Cu-rich part of AlCuSi at high temperatures. The AlCuZn system will be studied with the main emphasis on the description of the gamma brass phase and ternary intermetallic phases. Existence of some stable phases, specific for the ternary systems only, are known from the scientific literature but the knowledge about the crystal structure and thermodynamic properties is very poor. These phases can significantly influence materials properties of designed alloys. Theoretical modelling will be based on own experimental data as well as on previously published experiments and results from quantum mechanical calculations. Theoretical modelling will be based on the commonly used SGTE unary database to preserve the consistency with thermodynamic description of similar systems. In addition, a second reassessment of our alloys based on a new generation of the thermodynamic data for the pure elements will be performed in order to make these assessments consistent with upcoming developments in the CALPHAD community.

Advanced duralumin alloys based on binary system Al-Cu are of high interest for many different applications in automotive and aerospace industry. Relevant equilibrium phase diagrams are an important source of the information for the design of such new materials. Detailed knowledge of the coexistence and stability of phases in stable equilibrium significantly rationalizes their development from the point of view of time and costs. This project was oriented towards the complete description of the phase equilibria in ternary alloys Al-Cu-Si and Al-Cu-Zn by a combination of experimental investigation and the subsequent theoretical modelling using semiempirical CALPHAD approach. The main emphasis of the experimental program was to target questions that have not yet been satisfactorily resolved in the scientific literature, or where divergent results exist in various sources. Key parts of the experimental phase diagrams were experimentally studied. It was found that the widely accepted Al-Cu experimental phase diagram still contains some uncertainties and inconsistencies. The Al-Cu phase diagram was studied with special focus to the most problematic central part of the phase diagram. The Al-Cu-Zn system was experimentally studied in whole concentration and temperature range. The two-phase field of crystallographically closely related _brass_AlCu and '_brass_CuZn phases was observed up to 400 C. At higher temperature the phase transition between these phases is proposed to be of second order and therefore no two phase field exists between them. A vertical section between the binary phases and ' and isothermal sections at 400C, 550 C, 700C and 820C were constructed. Experimental isothermal sections of Al-Cu-Si phase diagram at 600C and 800C were constructed, and the previously published isothermal section at 700C was corrected. In the scope of the theoretical part of the project, the thermodynamic reassessment of binary subsystems Al-Cu, Cu-Zn and Cu-Si was carried out using the CALPHAD approach based on new experimental results. Complex 4-sublattice model was used for the modelling of the phases _brass_AlCu and '_brass_CuZn. A new sublattice model of the ternary phase in system Al-Cu-Zn based on the observed crystal structure was proposed for the thermodynamic modelling. The heat of formation of ternary phase was calculated by ab-initio methods and implemented to the thermodynamic modelling, and a tentative theoretical description of Al-Cu-Si and Al-Cu-Zn systems were proposed. Novel thermodynamic assessment of binary systems Al-Zn, Al-Si and Si-Zn were based on 3rd generation of unaries using Einstein model for solid phase and two-state model for the liquid phase. The variation of the Einstein temperature of the solid solution and liquid phases as a function of composition was newly designed. This novel physically based model allows to predict phase diagrams in whole temperature range from 0K.