Phase transition and heterogeneous strengthening mechanism in CoCrFeNiMn high-entropy alloy fabricated by laser-engineered net shaping via annealing at intermediate-temperature

High-entropy alloys (HEAs) have attracted tremendous attention owing to their controllable mechanical properties, whereas additive manufacturing (AM) is an efficient and flexible processing route for novel materials design. However, a profound appraisal of the fundamental material physics behind the strengthening of AM-printed HEAs upon low/intermediate-temperature annealing is essential. In this work, CoCrFeNiMn HEAs have been prepared using laser-engineered net shaping (LENS) and subsequently annealed at different temperatures. The CoCrFeNiMn HEA annealed at intermediate-temperature (873 K) exhibits a strong strain hardening capability, resulting in ultimate strength of 725 MPa and plasticity of 22%. A ternary heterogeneous strengthening mechanism is proposed to explain this phenomenon, in which equiaxed grains, columnar grains, and sigma precipitates play different roles during tensile deformation. The resultant excellent strength and ductility can be ascribed to the heterostructure-induced mismatch. The equiaxed grains provide adequate grain boundaries (GBs), which induce dislocation plugging-up and entanglement; the columnar grains induce the onset and arrest of the dislocations for plastic deformation; and the sigma precipitates hinder the movement of slip dislocations. The results provide new insights into overcoming the strength-ductility trade-off of LENS-printed HEAs with complex geometries. (C) 2021 Published by Elsevier Ltd on behalf of Chinese Society for Metals