A CLOSE EXAMINATION OF THE MICROSTRUCTURAL SHIFTS OCCURRING AS THE TEMPERATURE OF 2ND GENERATION HIGH ENTROPY ALLOYS IS INCREASED

High Entropy Alloys (HEAs) are a class of equimolar alloys, which exhibit a single, disordered solid solution rather than multiple intermetallic compounds. In these alloys the solid solution is preferred over the multi-intermetallide form because of the high entropy component of the solid solution, reducing its free energy to well below that of the multi-intermetallide form. Contrary to other alloys, the stability of the microstructure increases with temperature, as the free energy further decreases.

A thermodynamic model considering the overall Gibbs free energy of the alloy as the sum of the free energy values of each phase was formulated. It enabled to determine that some desired modifications of HEAs` compositions cause thermodynamic preference of the ordered multi-intermetallide form over the solid solution form at low temperatures.

In order to analyze the microstructural changes arising in CrNbTiVZr and CrMo_0.5NbTa_0.5TiZr alloys, thermal treatments at various elevated temperatures, followed by water-quenching were performed. SEM, EPMA and XRD characterization techniques were utilized to identify the various phases existing in each temperature.

The CrNbTiVZr alloy manifests two phases up to 1000°C: NbTi solid solution incorporating some Cr, V and Zr, and a combined Laves phase consisting of ZrCr₂ and ZrV₂ and some dissolved Nb and Ti. At 1300°C ZrV₂ undergoes decomposition followed by V dissolution in the original NbTi phase. The existence of NbTi solid solution and two laves phases of the remaining elements was predicted by the model to exist below 450°C. The fact that one solid solution of all five elements was not observed even at 1300°C, combined with the lack of complete phase separation below 1000°C attests to impeded atomic mobility in this alloy.