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 (σy) 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.