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Lang P.,Materials Center Leoben Forschungsgesellschaft MbH | Lang P.,Vienna University of Technology | Povoden-Karadeniz E.,Christian Doppler Laboratory | Povoden-Karadeniz E.,Vienna University of Technology | And 4 more authors.
8th Pacific Rim International Congress on Advanced Materials and Processing 2013, PRICM 8 | Year: 2013

The evolution of quenched-in vacancies, diffusing through the crystal lattice in A1 alloys, is simulated within a physical modeling framework. In the context of initial decomposition phenomena in A1-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 A1-alloys are presented, which show the bustling activities of quenched-in vacancies during various time-temperature treatments.

Kozeschnik E.,Materials Center Leoben Forschungsgesellschaft MbH | Kozeschnik E.,Vienna University of Technology | Kozeschnik E.,Christian Doppler Laboratory | Svoboda J.,Materials Center Leoben Forschungsgesellschaft MbH | And 5 more authors.
Modelling and Simulation in Materials Science and Engineering | Year: 2010

In this paper, a model for growth and coarsening of precipitates at grain boundaries is developed. The concept takes into account that the evolution of grain boundary precipitates involves fast short-circuit diffusion along grain boundaries as well as slow bulk diffusion of atoms from the grain interior to the grain boundaries. The mathematical formalism is based on a mean-field approximation, utilizing the thermodynamic extremal principle. The model is applied to the precipitation of aluminum nitrides in microalloyed steel in austenite, where precipitation occurs predominately at the austenite grain boundaries. It is shown that the kinetics of precipitation predicted by the proposed model differs significantly from that calculated for randomly distributed precipitates with spherical diffusion fields. Good agreement of the numerical solution is found with experimental observations as well as theoretical treatment of precipitate coarsening. © 2010 IOP Publishing Ltd.

Ahmadi M.R.,Christian Doppler Laboratory | Ahmadi M.R.,Materials Center Leoben Forschungsgesellschaft mbH | Ahmadi M.R.,Vienna University of Technology | Whitmore L.,Christian Doppler Laboratory | And 6 more authors.
Advanced Materials Research | Year: 2014

In the present study, we describe a comprehensive and consistent physical model for the yield strength change in Allvac® 718Plus™ caused by precipitation strengthening. The model incorporates the effect of different shearing and non-shearing mechanisms with respect to atomic continuity between the lattices of precipitates and matrix. We demonstrate that coherency and antiphase boundary effects are the major strengthening mechanisms in this alloy. The final yield strength of Allvac® 718Plus™ during aging is investigated using the thermo-kinetic software MatCalc. The calculated final yield strength evolution is consistent with experimental results. © (2014) Trans Tech Publications, Switzerland.

Stechauner G.,Vienna University of Technology | Stechauner G.,Materials Center Leoben Forschungsgesellschaft mbH | Kozeschnik E.,Vienna University of Technology | Kozeschnik E.,Christian Doppler Laboratory
Advanced Materials Research | Year: 2014

Cu precipitation in steel has been investigated numerous times. Still, a consistent simulation of the nucleation, growth and coarsening kinetics of Cu precipitates is lacking. Major reason for this is the fact that Cu precipitation involves complex physical interactions and mechanisms, which go beyond the classical precipitation models based on evaporation and absorption of precipitate-forming monomers (atoms). In the present work, we attempt a comprehensive modeling approach, incorporating coalescence results from Monte Carlo simulation, prediction of the nucleus composition based on the minimum energy barrier concept, diffusion enhancement from quenched-in vacancies, dislocation pipe diffusion, as well as the transformation sequence of Cu-precipitates from bcc-9R-fcc. Our simulations of number density, radius and phase fraction coincide well with experimental values. The results are consistent over a large temperature range, which is demonstrated in a TTP-plot. © (2014) Trans Tech Publications, Switzerland.

Lang P.,Materials Center Leoben Forschungsgesellschaft mbH | Weisz T.,Christian Doppler Laboratory | Ahmadi M.R.,Christian Doppler Laboratory | Ahmadi M.R.,Vienna University of Technology | And 5 more authors.
Advanced Materials Research | Year: 2014

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. © (2014) Trans Tech Publications, Switzerland.

Werinos M.,University of Leoben | Antrekowitsch H.,University of Leoben | Kozeschnik E.,Materials Center Leoben Forschungsgesellschaft MbH | Kozeschnik E.,Vienna University of Technology | And 6 more authors.
Scripta Materialia | Year: 2016

This study investigates the effect of trace Sn additions on the artificial aging of an Al-Mg-Si alloy at unconventionally high temperatures (> 210 °C), where such additions generate both ultrafast aging kinetics and superior peak hardness. The study shows that the effect of Sn is comparable to the influence of natural pre-aging on high temperature artificial aging, and explains this by solute-vacancy interactions, proposing that during artificial aging Sn-vacancy pairs contribute to diffusion and/or Sn retards the annihilation of quenched-in vacancies. © 2015 Elsevier Ltd. All rights reserved.

Lang P.,Materials Center Leoben Forschungsgesellschaft mbH | Lang P.,Vienna University of Technology | Shan Y.V.,Materials Center Leoben Forschungsgesellschaft mbH | Shan Y.V.,Vienna University of Technology | And 2 more authors.
Materials Science Forum | Year: 2014

Vacancies are the simplest type of lattice defect. However, they play a major role in the kinetics of diffusional processes, such as solid-state precipitation, where mass transport is directly proportional to the concentration of vacancies. We present a physical modelling framework, where we simulate the evolution of excess vacancies on the example of Al-alloys during simplified time-temperature treatments. Interaction energies between solute atoms and vacancies are evaluated by first-principle analysis. Assuming that the escape of vacancies from existing traps is dependent on temperature and binding energies, we explore the life-time of non-equilibrium vacancies and the natural and artificial aging response of Al alloys. The predictions of the model are finally compared to experimental data. © (2014) Trans Tech Publications, Switzerland.

Ahmadi M.R.,Christian Doppler Laboratory | Ahmadi M.R.,Materials Center Leoben Forschungsgesellschaft mbH | Ahmadi M.R.,Vienna University of Technology | Sonderegger B.,University of Graz | And 6 more authors.
Materials Science and Engineering A | Year: 2014

In precipitation strengthening of non-spherical precipitates, the effective radius is depending on the relative orientation between particle and the direction of dislocation movement. Expressions for the equivalent precipitate radius applicable to particle shearing and non-shearing mechanism are derived for ellipsoidal particles taking into account their growth direction and the dislocation character (edge and screw). The results are compared to spherical particles and presented in the form of closed analytical correction factors. © 2013 Elsevier B.V.

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