Engineering heterogeneous microstructures in additively manufactured high entropy alloys for high strength and strain hardenability

Abstract

A novel hierarchically heterogenous microstructure has been achieved by taking advantage of residual stresses in an additively manufactured (AM) Al0.3Ti0.2Co0.7CrFeNi1.7 HEA. This hierarchical heterogeneous microstructure was achieved by a single-step annealing of the AM processed HEA, and is manifested not only via a bimodal distribution of grain sizes, but also via concurrent differences in the morphology and scale of ordered precipitates within the grains. Firstly, at the macroscale, bimodally distributed finer recrystallized and coarser non-recrystallized γ grains were observed. Secondly, at the nanoscale, precipitates of two different types of ordered precipitates, γ′ and σ formed within these grains. Furthermore, two different morphologies of the γ′ precipitates formed in the annealed conditions; equiaxed/near-spherical in the non-recrystallized γ grains resulting from continuous precipitation and rod-like in the recrystallized γ grains resulting from discontinuous precipitation. Uniaxial tensile tests revealed that such heterogeneous microstructures lead to YS values as high as ∼1150 MPa, coupled with very high strain hardening rates, and reasonable tensile ductility. This investigation demonstrates a novel approach for achieving complex hierarchical heterogeneous microstructures in HEAs via AM processing, leading to a competitive balance of mechanical properties.