Microstructural evolution and high strain rate deformation response of SLM-printed CoCrFeMnNi after annealing and deep-cryogenic treatment

Abstract

This study examines the microstructural evolution and high strain rate deformation response of Selective Laser Melted (SLM) CoCrFeMnNi high-entropy alloys (HEAs) after annealing and deep cryogenic treatment. Annealing treatment has traditionally improved the ductility of SLM materials. However, in this work, a significant improvement in strength was observed in the annealed SLM CoCrFeMnNi after high strain rate deformation testing. TEM/HRTEM investigations revealed the formation of refined oxides, generated from the processing chamber environment and constituent powder feedstock. These oxides were homogenously distributed within microstructure, reinforcing its structural integrity. Initial microstructural analysis of the as-printed samples showed Mn₂O₃ oxides sparsely distributed within a cellular dislocation structure and significant Mn segregation. Unique grain growth with a less prominent cellular dislocation structure was observed in the annealed specimen. Deep cryogenic treatment induced oriented cellular structures with higher dislocation density and rounded oxides that were 56 % smaller. High strain rate impact tests (up to 6500 s-1) demonstrated that the as-printed sample was sensitive to the strain rate and reached a yield strength of ∼920 MPa at 6000 s⁻¹ and a strain deformation of ∼55 % making it desirable for high strain rate applications. Remarkably, up to 22 % higher strength and ∼10 % greater ductility were achieved after annealing. The strengthening mechanisms in the samples and their contributions to the overall material strength were thoroughly analyzed. It was determined that a substantial portion of the strength in the annealed samples was due to the contributions of the precipitates within the alloy. The observed increase in strength was primarily attributed to the presence of two distinct nano-precipitates in the annealed specimens. However, there was no change in ductility after the deep cryogenic treatment but ∼10 % higher yield strength values at equivalent strain rates also attributed to the increased dislocation density.