Effect of annealing on microstructure and mechanical properties of AlCrFe2Ni2 medium entropy alloy fabricated by laser powder bed fusion additive manufacturing

Abstract

In this paper, a Co-free AlCrFe2Ni2 medium entropy alloy (MEA) has been fabricated by laser powder bed fusion additive manufacturing and treated by subsequent annealing at various temperatures ranging from 750 °C to 1050 °C to achieve good strength-ductility synergy. The effect of annealing on the microstructure, phase distribution and mechanical properties was systematically investigated by characterizing the as-built and the subsequently annealed alloys. The as-built MEA showed a microstructure of homogeneously distributed ordered B2 single-phase, and exhibited a great compressive yield strength of over 1850 MPa but relatively insufficient ductility of 13.8%. After annealing, the microstructure transformed to a heterogeneous structure with Ni–Al rich ordered B2 and Fe–Ni–Cr rich FCC phases, as well as a few Fe–Cr rich BCC phase. With the increase of annealing temperature to 1050 °C, the ductility of the annealed MEAs significantly improved while the compressive yield strength decreased from 1185.85 MPa (as-built) to 614.93 MPa. Despite the certainly compromised yield strength, a good strength-ductility synergy and superior strain hardening capacity of the additively manufactured MEA were achieved via subsequent heat treatment. Furthermore, the underlying mechanisms of the decreased yield strength under high annealing temperature and the superior strain hardening capacity of the annealed alloy were analyzed. The findings of this work show the feasibility of achieving the required strength-ductility synergy of additively manufactured HEAs/MEAs by tailoring microstructural heterogeneity using subsequent heat treatment.