Relation of phase fraction to superplastic behavior of multi-principal element alloy with a multi-phase structure

Abstract

An Al0.3CoCrNi medium-entropy alloy was annealed in two conditions to differentiate the phase fraction of nano-scaled B2 and sigma precipitates in FCC microstructure, processed by high-pressure torsion, and subjected to a superplasticity test. The Al0.3CoCrNi with different microstructure exhibited distinct superplastic performance, with maximum elongation of 1900% and 1735% from tensile deformation at 1073 K at the strain rate at 5 × 10−3 s−1 and 10−2 s−1, respectively. The microstructural analysis revealed that the superplasticity is dominantly governed by the grain boundary sliding mechanism with the accommodation of intragranular dislocations. The Al0.3CoCrNi alloy, after superplastic deformation, consists of the mixture of FCC, B2, and sigma phases with different phase fractions. The comparison of this multi-phase structured alloy at two conditions and the sister alloy, Al0.5CoCrFeMnNi high-entropy alloy, suggests that increment of B2 and sigma phase fraction is beneficial to promote high-strain rate superplasticity by limiting the grain growth.