Nano-dispersion strengthened and twinning-mediated CoCrNi medium entropy alloy with excellent strength and ductility prepared by laser powder bed fusion

Laser powder bed fusion (LPBF) was explored to produce CoCrNi medium entropy alloys (MEAs) from pre-alloyed powder with a relatively high oxygen concentration (870 ppm). Microstructure, strength, and toughness were investigated, revealing the effect of oxide dispersion strengthening (ODS). LPBF CoCrNi MEAs had a porosity of <0.10 % and superior mechanical properties compared to CoCrNi-based MEAs reported in the literature. Notably, the CoCrNi MEAs denoted as S1 and S2, exhibited an excellent combination of yield strength (>735 MPa), a UTS > 1 GPa, and a total elongation of similar to 50 %. High strength results from the high number density of nano chromium oxides (average particle size similar to 20 nm), which were in situ formed during LPBF, contributing to a calculated dispersion strengthening of similar to 220 MPa. The deformation mechanism was investigated by TEM characterization of the specimen after tensile testing, revealing a large number of nano-twins for S1, cellular structure and nano-twins formed for S3. Extremely low porosity and deformation twinning ensure steady plastic deformation in LPBF CoCrNi, enabling high ductility. The prospect of ODS via LPBF provides a pathway for the strengthening and toughening of medium to high entropy alloys (HEAs).