Thermodynamic phase predictions in High entropy alloys

Thermodynamic phase predictions in High Entropy Alloys

Guruvidyathri K

The high configurational entropy resulting from the sort of compositions in HEAs was expected to stabilize simple solid solutions in microstructure. But, exceptions to such behaviour were reported in several cases. Therefore, search for the rules governing the phase formation in HEAs has become a major academic interest. In that direction, we do phase prediction by computational materials thermodynamics, using the CALPHAD (CALculaion of PHAse Diagram) technique. The challenge in applying this technique for HEAs is in developing a Gibbs energy database that works for wider range of compositions. This technique uses extrapolation of thermochemical data of lower order systems (binary and ternary) to higher order systems (quarternary, quinary, sexinary, etc.).

For example, phase prediction for Co-Cr-Fe-Ni alloy was done using such Gibbs energy database in combination with Thermo-Calc software. Fig. 1 shows a isoplethal section in Co-Cr-Fe-Ni phase diagram. Fig. 2 shows the phase fraction plot for the equiatomic CoCrFeNi alloy. For validation of these predictions, alloys are prepared by vacuum arc melting. In order to achieve the equilibrium microstructure, long term heat treatments are being done at various temperatures.

Fig 1. Isoplethal section of Co-Cr-Fe-Ni system

Fig 1. Phase fraction plot for equiatomic CoCrFeNi alloy