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

Abstract 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 σ 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 σ 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.