Effect of Al Content on Microstructure and Mechanical Properties of the fcc and bcc Phases of Al_xCoCrFeNi High Entropy Alloys

In the Al_xCoCrFeNi high entropy alloys (HEAs), adding Al promotes a microstructure shift from fcc to bcc or their mixtures, which increases the overall strength of the alloys. To understand the strengthening mechanism, it is crucial to study the microstructure and mechanical properties of each constitution phase with varying compositions in addition to the amounts of strengthening phases. However, systematic research on the strengthening effect of Al content in individual fcc and bcc phases is lacking. In the present work, seven Al_xCoCrFeNi HEAs with different Al contents were prepared by arc melting. According to XRD, SEM and TEM observations and thermodynamic calculations, the phase constitutions of the alloys change in a sequence of fcc → fcc + A2 (disordered bcc) + B2 (ordered bcc) → A2 + B2 as the Al content increases. The addition of Al to the fcc phase generates substantial lattice distortion, but the nanoindentation hardness of the fcc phase is stabilized at about 3.6 GPa, meaning no solid solution hardening effect present in the fcc phase. However, with increasing Al content, the hardness of the bcc-based alloys increases, which is mostly attributable to the increasing amount of the B2 phase.