Microstructure and hardening mechanisms of oxygen-doped (Fe3Co2Ni2Cr3)94-xAlxO6 multi-principal element alloys

A series of oxygen-doped (Fe3Co2Ni2Cr3)(94-x)AlxO6 (x = 3-7 at.%) multi-principal element alloys (MPEAs) was synthesized via mechanical alloying (MA) and subsequently consolidated by high-pressure and high-temperature sintering (HPHT), and the effect of Al contents on the microstructure and hardening mechanisms was investigated systematically. Detailed microstructural characterizations indicate that (Fe3Co2Ni2Cr3)(94-x)AlxO6 MPEAs are composed of nanocrystalline FCC matrix, and ultrafine grained Cr-rich oxides, as well as the particle size of the Cr-rich oxides decreases and the distribution is gradually uniform with increasing Al contents. The (Fe3Co2Ni2Cr3)(87)Al7O6 MPEA contains small nanocrystalline Al-rich oxides. Attributed to grain boundary strengthening and strain hardening, (Fe3Co2Ni2Cr3)(87)Al7O6 shows a high Vickers hardness of 6.98 +/- 0.16 GPa, higher than that of most previously reported FCC structured HEAs. Estimated from Tabor's equation, the Tabor's ratio attains a value of similar to 2.96, consistent with that of conventional polycrystalline materials. This work provides a novel pathway for hardening MPEAs and widens the design toolbox for other high-performance materials, given the typical Tabor ratio.