Medium-entropy alloys (MEAs) are multicomponent systems characterized by high entropy of mixing, which ensures the formation of stable solid solutions. However, in some cases, phase separation due to thermodynamic factors is observed in MEA. The Al–Cu–Fe–Mn system is a striking example of an immiscible MEA, where it is necessary to take into account the interaction between the components to predict phase stability.
Thermodynamic description of the Al–Cu–Fe–Mn system requires the construction of a mathematical model that takes into account the Gibbs energies of the various phases. CALPHAD (Calculation of Phase Diagrams) approaches based on a database of thermodynamic parameters are commonly used. For each component and binary interaction, Margules or Redlich-Kister parameters are used to describe deviations from the ideal behavior of the solution.
To create high-strength, medium-entropy, two-phase alloys whose components are immiscible, a thermodynamic representation of the Al-Cu-Fe-Mn quaternary system was developed. A new thermodynamic evaluation of the Al-Cu-Mn ternary subsystem was carried out, which is extended to the entire concentration range. The thermodynamic parameters of the B2, AlCuFe_τ and φ phases in the Al-Cu-Fe ternary subsystem were also adjusted based on the previous description, which used different binary descriptions of the components. The correctness of the developed thermodynamic description for the Al-Cu-Fe-Mn quaternary system was confirmed experimentally through the design, production and study of the phase structures of Al-Cu-Fe-Mn medium-entropy alloys.
Particular attention is paid to the description of the Gibbs energy of solid solutions, intermetallic compounds and the liquid phase state. Experimental data on phase composition and phase transition temperatures are used to optimize the thermodynamic parameters. In the case of immiscible SES, it is important to accurately describe the liquation regions and metastable phases formed during cooling.
Predicting the phase stability of the Al–Cu–Fe–Mn alloy allows optimizing the composition and heat treatment modes to obtain the desired microstructures and properties. Detailed thermodynamic modeling allows identifying the influence of each component on phase stability and predicting the phase composition depending on temperature and element concentration. This is an important step in the development of new medium-entropy alloys with specified characteristics.
Author: Hyeon-Seok Do, Jongun Moon, Hyoung Seop Kim, Byeong-Joo Lee
Institute: Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, 37673, Republic of Korea