Development of ultrafine grained cobalt-free AlCrFe2Ni2 high entropy alloy with superior mechanical properties by thermo-mechanical processing

Abstract

The microstructure and properties of cobalt-free cost-effective AlCrFe2Ni2 high entropy alloy (HEA) in the as-cast condition and after thermo-mechanical processing by severe cold-rolling and annealing were investigated in the present work. The as-cast HEA showed a heterogeneous microstructure consisting of relatively coarse lamellar and much finer intertwined regions. The coarse regions consisted of eutectic mixture of FCC and ordered B2 (along with minor BCC) phases. The FCC phase was enriched in Fe and Cr, while the B2 phase was found enriched in Ni and Al but depleted in Cr. The BCC/B2 phase in the as-cast material showed phase separation due to spinodal decomposition to two different B2 phases in the fine regions. The overall FCC phase and BCC/B2 phase fractions were ∼60% and 40%, respectively. Despite the complex microstructure, the presence of a high fraction of the ductile FCC phase rendered remarkable workability, allowing heavy cold-rolling up to 90% reduction in thickness. Heavy deformation resulted in the development of intriguing microstructural features such as folding and bending of the lamellae, local shearing, and finally, deformation-induced nanocrystallization of the FCC phase. However, the B2 phase retained the ordered structure even after 90% cold-rolling. Annealing at 800 °C resulted in the formation of an ultrafine microduplex structure with significant resistance to grain growth even up to an annealing temperature of 1200 °C. A high yield (∼880 MPa) and tensile strength (̴ 1100 MPa) coupled with appreciable elongation (∼10%) could be achieved after annealing at 800 °C. The tensile properties obtained were superior to the other cobalt-free HEAs, which indicate the promising application of the cobalt-free cost-effective AlCrFe2Ni2 HEA as an advanced structural material.