Effects of Ti and Al doping methods on microstructure and properties of laser-directed energy deposited CrCoNi medium-entropy alloys

Abstract

CrCoNi medium-entropy alloys with superior mechanical properties were fabricated via laser-directed energy deposition (L-DED), incorporating Ti/Al elements through two doping strategies: elemental powders and pre-alloyed TiAl powder. Microstructure and mechanical properties were systematically characterized through OM, SEM, TEM and mechanical testing at 298 K and 77 K. TEM revealed two FCC-structured nano-precipitates: TiO-coated Al₂O₃ core–shell structures and spherical complex oxide twins of Ti₂Al₂O₇ and Ti₂AlO₄. A notable distinction lies in the predominant spherical morphology of precipitate particles within the (CrCoNi)₉₄Ti₃Al₃ samples. This arises from the pre-alloyed TiAl powder enabling more complete bonding of Ti, Al, and O during material processing, which facilitates sequential oxidation and encapsulation to form core–shell structures. In contrast, elemental Ti/Al doping results in fewer core–shell precipitates, leading to significantly reduced overall nanoprecipitate density, and mainly in the formation of spherical oxides. In addition, higher dislocation density exists in the (CrCoNi)₉₄(TiAl)₆ alloy. The (CrCoNi)₉₄(TiAl)₆ alloy achieved excellent mechanical properties at 77 K with an elongation of 45.0 % and a tensile strength of 1296.7 MPa, which was superior to the (CrCoNi)₉₄Ti₃Al₃ alloy. This enhancement stems from reduced Ti/Al burn-off via pre-alloying, which increased precipitation density and dislocation accumulation, thereby optimizing strain hardening and mechanical performance.