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By using a blend of pre-alloyed CoCrFeNi powder and elemental Mn powder, quasi-equiatomic CoCrFeMnNi high entropy alloy (HEA) has been in-situ alloyed via laser powder bed fusion (LPBF). Besides being homogeneously dissolved into the HEA matrix, Mn also forms oxide particles with oxygen originating from both powder feedstock and printing atmosphere, resulting in an in-situ alloyed, oxide-dispersion-strengthened (ODS) HEA. The tensile strength of the ODS HEA is significantly improved to a yield strength of 620 MPa and a tensile strength of 730 MPa. The tensile ductility of the ODS HEA is lower than the ones prepared by using fully pre-alloyed powders, but an outstanding compression ductility maintains for the ODS HEA. The deforming behaviour of the alloy has been revealed, and the contribution of Orowan strengthening has been quantified along with other mechanisms. The study provides an …