AlCo-rich AlCoNiFe and AlCoNiFeCr high entropy alloys: Synthesis and interaction pathway at high heating rates

Abstract

Novel AlCo-rich AlCoNiFe and AlCoNiFeCr high entropy alloys (HEAs) were prepared from the non-activated and mechanically activated (MA) quaternary Al40Co40Ni10Fe10at% and quinary Al35Co35Ni10Fe10Cr10at% mixtures. The wide temperature range of the formation of a single high entropy alloy of BCC structure in quaternary system and the formation of pure quinary AlCoNiFeCr alloy above 740 °C was predicted by thermodynamic modelling. The underlying interaction mechanism was proposed, and the effective activation energy values were deduced at high heating rates up to 2600 °C·min−1 by thermal analysis method through high-speed temperature scanner. The presence of a single-stage exothermic interaction in the non-activated and mechanically activated quaternary and quinary mixtures was attributed to HEA formation via either solid + liquid or solid + solid mechanism depending on the heating rate and mechanoactivation (MA) duration. The influence of heating rate and MA duration on the self-heating profile and reaction onset temperature was also deduced. Microstructure evolution and phase formation sequence were monitored by the examination of products quenched at characteristic temperatures at synthesis and processing of AlCo-rich AlCoNiFe and AlCoNiFeCr HEAs. The crystallite size (CS), lattice strain (LS) and lattice parameter (LP) of CoFeNiAl and CoFeNiAlCr HEAs were estimated according to MA duration. The thermal stability in argon and air flow conditions at heating rate of 40 °C·min−1 for up to 1200 °C was studied by thermogravimetric analyser complemented with X-ray diffraction analysis.