Erwin Povoden-Karadeniz

Influence of trace impurities on the in vitro and in vivo degradation of biodegradable Mg-5Zn-0.3Ca alloys

The hydrogen evolution method and animal experiments were deployed to investigate the effect of trace impurity elements on the degradation behavior of high-strength Mg alloys of type ZX50 (Mg-5Zn-0.3Ca). It is shown that trace impurity elements increase the degradation rate, predominantly in the initial period of the tests, and also increase the materials susceptibility to localized corrosion attack. These effects are explained on the basis of the corrosion potential of the intermetallic phases present in the alloys. The Zn-rich phases present in ZX50 are nobler than the Mg matrix, and thus act as cathodic sites. The impurity elements Fe and Mn in the alloy of conventional purity are incorporated in these Zn-rich intermetallic phases and therefore increase their cathodic efficiency. A design rule for circumventing the formation of noble intermetallic particles and thus avoiding galvanically accelerated dissolution of the Mg matrix is proposed.

Assessment of parameters for precipitation simulation of heat treatable aluminum alloys using differential scanning calorimetry

Abstract Differential scanning calorimetry (DSC) has been used extensively to study different solid state reactions. The signals measured in DSC are associated with the growth and dissolution of different precipitates during a specific heat cycle. The time-temperature dependence of heat cycles and the corresponding heat flow evolution measured in the sample by DSC provide valuable experimental information about the phase evolution and the precipitation kinetics in the material. The thermo-kinetic computer simulation was used to predict the DSC signals of samples taken from 6xxx and 2xxx alloys. In the model, the evolution of different metastable and stable phases and the role and influence of excess quenched-in vacancies in the early stage of precipitation were taken into account. Transmission electron microscopy (TEM) and high-resolution TEM were used to verify the existence of precipitates, their size and number density at specific points of the DSC curves.

Interlayer growth kinetics of a binary solid-solution based on the thermodynamic extremal principle: Application to the formation of spinel at periclase-corundum contacts

A thermodynamic model has been developed for interlayer growth in a binary system between two phases of fixed composition producing an intermediate solid-solution phase. Thereby long-range diffusion, interface migration and generation/annihilation of vacancies at the reaction interfaces have been considered as potentially rate limiting. The coupling among these processes governs overall growth rate, position of the Kirkendall plane and the compositions of the solid-solution phase at the reaction interfaces. Model calculations illustrating the relations between the corresponding kinetic parameters and system evolution are presented. In particular, the systematics of non-equilibrium element partitioning across moving reaction interfaces is addressed. It is found that the deviation from equilibrium element partitioning at a moving reaction interface is a more sensitive monitor for the departure from local equilibrium than the deviation from parabolic growth behavior. Finally, the model is applied to interlayer growth of magnesio-aluminate spinel.

Thermo-Kinetic Simulation of the Yield Strength Evolution of AA7075 during Natural Aging

The yield strength evolution in aluminum alloy 7075 is investigated during natural aging. The thermo-kinetic simulation, capable of predicting nucleation, growth, coarsening and dissolution of metastable and stable hardening precipitates in Al-Zn-Mg-Cu during natural aging, is outlined briefly. A recent strengthening model for shearing and bypassing of precipitates by dislocations is utilized to calculate the evolution of the macroscopic yield strength at room temperature. The simulation accounts for vacancy-solute binding energies calculated with the help of first principles simulations that influence the diffusivity of the system due to the presence of excess quenched-in vacancies. These results provide predictions about the amount of excess vacancies trapped by solid solution alloying elements and how the lifetime of vacancies changes due to attractive or repelling binding forces between vacancies and different solid atoms in the aluminum matrix. In our approach, we calculate the strength evolution after quenching due to interaction between dislocations and changes in the microstructure by precipitation of different kinds of secondary phases. The predicted evolution of yield strength is finally verified on experimental measurements.

Saturation of Deformation Twinning in Magnesium Alloys

The saturation of primary tensile twins in heavily textured Mg-alloy AZ31 is investigated, and their strain accommodation limit is evaluated. EBSD results suggest that the mean number of twins per grain saturate rapidly, followed by the stop of twin growth. Twinning saturation is included in a physical model of twin evolution.

Coupling of Computational Thermodynamics with Kinetic Models for Predictive Simulations of Materials Properties

We present successful examples of CALPHAD thermodynamics-based precipitation simulations for three important alloy groups: Single-crystal Ni-base superalloy, austenitic stainless steel and hardenable Al-alloy. Underlying physical models for special features, such as, energies of diffuse interfaces between coherent precipitates and matrix, precipitation of incoherent particles at grain boundaries, evolution of excess vacancies during quenching and continuous aging and their role for metastable precipitate nucleation, are discussed.

The Bustling Nature of Vacancies in Al Alloys

The evolution of quenched-in vacancies, diffusing through the crystal lattice in Al alloys, is simulated within a physical modeling framework. In the context of initial decomposition phenomena in Al-alloys interactions between vacancies different alloying elements are of particular importance, since these decide over the occurrence of trapping or repelling effects on vacancies. We use first-principles analysis to evaluate solute-vacancy interactions. These results lead to improved understanding of trapping and repelling behavior. Generation and annihilation of non-equilibrium vacancies at different sources and sinks as well as binding energies of solute-vacancy complexes are considered, thus allowing for the prediction of the excess vacancy evolution. Moreover, their influence on the diffusivities and their role in the formation and growth of early precipitates is investigated. The present parameter-free model is a valuable tool in interpretation of experimental observations. Simulation studies in multi-component multiphase Al-alloys are presented, which show the bustling activities of quenched-in vacancies during various time-temperature treatments.

Thermodynamics-Integrated Simulation of Precipitate Evolution in Al-Mg-Si-Alloys

Metastable precipitates govern the mechanical properties of hardenable Al-alloys. A computational precipitation simulation approach is presented that is based on a combination of compiled and assessed thermodynamic and diffusion data with predictive physical models. Predictive precipitation kinetics simulation delivers approximations of thermodynamic properties that would otherwise require time-consuming computational techniques based on density functional theory. Coupling of thermodynamics with thermo-kinetic simulation of hardenable Al-alloys Al-Mg-Si 6016 is presented.