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Stockholm, Sweden

Zhu Z.,University of Oxford | Hoglund L.,KTH Royal Institute of Technology | Larsson H.,Thermo Calc Software | Reed R.C.,University of Oxford
Acta Materialia | Year: 2015

The multicomponent composition space pertinent to the single crystal nickel-based superalloys is mapped and searched, using computational modelling. A resolution of 0.1 wt.% for the alloying elements is assumed, consistent with manufacturing practice. Databases are constructed of alloy compositions which are predicted to be of promising microstructural architecture: e.g. equal fractions of the γ and γ′ phases. These may be regarded as maps - one might term them genomes - of this class of structural alloy. By combining the databases with additional composition-dependent property models, it is demonstrated that compositions can be identified which - subject to the accuracy and limitations of the sub-models - are likely to prove optimal, e.g. on the basis of their creep resistance, density and cost. The methods circumvent the need for the traditional empirically-driven approaches to alloy design. © 2015 Acta Materialia Inc. Source

Xiong W.,KTH Royal Institute of Technology | Xiong W.,Central South University | Selleby M.,KTH Royal Institute of Technology | Chen Q.,Thermo Calc Software | And 2 more authors.
Critical Reviews in Solid State and Materials Sciences | Year: 2010

Phase equilibria and thermodynamic properties in the Fe-Cr system have been reviewed comprehensively based on experimental information and available computer simulations in different scales. The evaluated phase equilibria show significant differences from the currently accepted thermodynamic description by CALPHAD (calculation of phase diagram) approach. The thermodynamic properties of the Fe-Cr system, such as heat capacity, enthalpy, and activity, have been evaluated in reported experiments. The experiments on phase separation in the Fe-Cr system have also been critically reviewed with a focus on spinodal decomposition. The reported data are concentrated in the temperature range from 673 to 823 K. In addition, there is a transition region between spinodal decomposition and nucleation regimes within the composition limit from 24 to 36.3 at.% Cr and the temperature range between 700 and 830 K. In view of the importance of magnetism in the Fe-Cr system, some inadequacies of the currently used thermodynamic description are pointed out in addition to some problems with the current magnetic model. Remaining issues on the thermodynamics of the Fe-Cr system have been elaborated for future refinement of the thermodynamic description of the Fe-Cr system. According to the present review, the melting temperature of Cr is recommended to be about 2136 K, which is 44 K lower than the value adopted in the research community on thermodynamics, such as the Scientific Group Thermodata Europe. © 2010 Taylor and Francis Group, LLC. Source

Software related to the calculation of phase diagrams (CALPHAD) is being used to aid in the design of new alloys and optimize the welding processes to join them. By adopting the CALPHAD technique, all available thermochemical information both thermodynamic and phase equilibria data is used to fit model parameters used to describe the Gibbs energy of individual crystallographic phases. The Gibbs energy of each phase is described by an appropriate thermodynamic model dependent on its physical and chemical properties. Thermodynamic calculations can be made to predict properties such as liquidus and solidus temperatures and also the phases to form from the liquid during welding. DICTRA is a software package for accurate simulations of one-dimensional diffusion in multicomponent alloys. It can be used to investigate microsegregation effects during solidification taking into consideration cooling rates and time. These software tools can reduce the time and costs associated with trial-and-error experiments. Source

Larsson H.,Thermo Calc Software
Calphad: Computer Coupling of Phase Diagrams and Thermochemistry | Year: 2014

Two previously suggested simulation models, for multiphase simulations (Larsson and Engström, 2006; Larsson and Höglund, 2009) and diffusion controlled growth (Larsson and Reed, 2008), respectively, are unified to form a generalized model for 1D simulations of diffusion controlled growth under local equilibrium conditions where multiple phases are allowed on either side of an interface. © 2014 Elsevier Ltd. All rights reserved. Source

Xiong W.,KTH Royal Institute of Technology | Hedstrom P.,KTH Royal Institute of Technology | Selleby M.,KTH Royal Institute of Technology | Odqvist J.,KTH Royal Institute of Technology | And 2 more authors.
Calphad: Computer Coupling of Phase Diagrams and Thermochemistry | Year: 2011

A thermodynamic modeling of the FeCr system down to 0 K is performed on the basis of our recent comprehensive review of this binary system [W. Xiong, M. Selleby, Q. Chen, J. Odqvist, Y. Du, Evaluation of phase equilibria and thermochemical properties in the FeCr system, Crit. Rev. Solid State Mater. Sci. 35 (2010) 125152]. The model predicts a sign change for the magnetic ordering energy of mixing rather than the enthalpy of mixing in the bcc phase at 0 K. Designed key experiments are performed not only to check the validity of the present modeling but also to assist in understanding the mechanism for spinodal decomposition of the FeCr alloy. Heat capacities and Curie temperatures of several Fe-rich alloys are determined between 320 and 1093 K by employing differential scanning calorimetry. The measured heat capacities are found to be in remarkable agreement with the prediction based on the present modeling. Microstructural patterns and frequency distribution diagrams of Cr are studied in alloys containing 26.65, 31.95, and 37.76 at.% Cr by using atom probe tomography. The observed phase separation results correspond well with our model-predicted boundary for the spinodal decomposition. Interestingly, a horn on the Cr-rich spinodal boundary is predicted below 200 K for the first time. This work demonstrates a way to bridge the ab initio calculations and CALPHAD approach. © 2011 Elsevier Ltd. All rights reserved. Source

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