Thermodynamic properties of solid solutions from special quasirandom structures and CALPHAD modeling application to Al-Cu-Mg-Si and Hf-Si-O /

by 1975- Shin, Dongwon

This thesis focuses on calculating thermodynamic properties of solid solution phases from first-principles studies for the CALPHAD thermodynamic modeling. Since thermodynamic properties of solid solutions cannot be determined accurately through experimental measurements, various efforts have been made to estimate them from theoretical calculations. First-principles studies of Special Quasirandom Structures (SQS) deserve special attention among the available approaches. SQS’s are structural templates whose correlation functions are very close to those of completely random solid solutions, thus can be applied to any relevant system by switching the atomic numbers in first-principles calculations. Moreover, the effect of local relaxation can be considered by fully relaxing the structure. In this thesis, SQS’s for both substitutional and interstitial solid solutions are considered. For substitutional solid solutions, binary hcp SQS’s and ternary fcc SQS’s are generated. First-principles results of those SQS’s are compared with experimental data and/or thermodynamic modelings where available and verified that they are capable of reproducing thermodynamic properties of substitutional binary hcp and ternary fcc solid solutions, respectively. For interstitial solid solution, binary hcp and bcc SQS’s are generated by considering the mixing of vacancy and interstitial atoms while the atoms in the parental structures are considered as frozen. SQS’s for substitutional solid solutions are applied to the Al-Cu-Mg-Si system with previously developed binary fcc and bcc SQS’s to investigate the enthalpy of iii mixing for binary bcc, fcc, and hcp solid solutions and ternary fcc solid solutions. Binary hcp and bcc SQS’s for interstitial solid solutions are used to calculate enthalpy of mixing for ?-Hf (hcp) and ?-Hf (bcc) phases in the Hf-O system to be used in the thermodynamic modeling of the Hf-Si-O system. This thesis shows that first-principles studies of SQS’s can provide insight into the understanding of mixing behavior for solid solution phases and calculated thermodynamic properties, for example enthalpy of mixing, can be readily used in thermodynamic modeling to overcome scarce and uncertain experimental data. iv