Enhancing strength-ductility synergy by introducing multilattice defects and heterogeneous structures in CoCrNi-based medium-entropy alloys prepared by powder plasma arc additive manufacturing

Abstract

In the work, cold rolling and annealing were applied to powder plasma arc additively manufactured (CoCrNi)₉₄Al₃Ti₃ medium-entropy alloy (MEA) to efficiently attain different types of lattice defects and heterogeneous structures, thereby enhancing the strength of the alloy. Tensile tests show that mechanical properties of the MEA were significantly enhanced after cold rolling and annealing treatments compared to the directly deposited alloys. The microstructure and mechanical properties of cold rolled samples (50 % thickness reduction) annealed at 1073–1273 K for 1 h are compared. It has been shown that the MEAs prepared by additive manufacturing accumulate a large amount of deformation energy within the grains during the cold rolling process. This lead to recrystallized grains first nucleating within the original columnar grains, and efficient recrystallization could be realized. At annealing temperatures ≤1173 K, the recrystallized grain size has not been coarsened, the coarse and fine grains formed a heterogeneous grain structure, leading to significant back stress strengthening. TEM observations at different alloys indicate that the formation and increase in the number of multiple lattice defects (SFs, DTs, and L–C locks) is the main reason for the high work-hardening capacity of the alloy. This investigation demonstrates that the combined approach provides a novel means to fabricate high strength and ductile CoCrNi-based MEAs.