Engineering fine grains, dislocations and precipitates for enhancing the strength of TiB₂-modified CoCrFeMnNi high-entropy alloy using laser powder bed fusion

TiB₂ nano-particle reinforced CoCrFeMnNi composite has been additively manufactured by using laser powder bed fusion (LPBF) technique. In comparison with the matrix CoCrFeMnNi sample, the average grain size of the TiB₂ doped CoCrFeMnNi (CoCrFeMnNi + TiB₂) sample reduces from 26.27 μm to 7.51 μm, appearing a morphological transformation from columnar grains to fine equiaxed grains and fine dendritic grains. Correspondingly, the compressive yield strength (σ_y) sharply increases from 484.41 ± 57.81 MPa of CoCrFeMnNi HEA to 952.62 ± 38.15 MPa of CoCrFeMnNi + TiB₂, and the tensile yield strength increases from 480.27 ± 1.25 MPa to 834.21 ± 15.93 MPa, revealing a substantial enhancement of mechanical properties. The structural analysis unveils that the CoCrFeMnNi + TiB₂ sample formed a tetragonal σ phase beside the FCC matrix and TiB₂ phases compared with the single FCC phase of CoCrFeMnNi HEA. Especially, atomic resolution scanning transmission electron microscopy discloses that the TiB₂ nanoparticles epitaxially grow onto the FCC matrix phase with [01–10]_TiB2//[011]_FCC orientation relationship. Theoretical calculations indicate that the enhancement mechanism of the CoCrFeMnNi + TiB₂ sample should originate from a synergistic strengthening effect which is induced by grain refinement, TiB₂ particles, σ phases and dislocations. Our work should provide a new view to utilize the rapid solidification process of LPBF and ceramic nano-particle reinforcement for the enhanced mechanical properties of HEAs with great potential.