Formation mechanisms of two types of sigma phases and the effects on mechanical behavior in a quaternary metastable high-entropy alloy

Metastable non-equiatomic FeMnCoCr high-entropy alloys (HEAs) exhibit excellent strength-ductility synergy mainly due to the transformation-induced plasticity (TRIP) effect. However, at relatively high Cr content and certain temperatures, the brittle sigma (σ) phase is prone to be induced, which may deteriorate the mechanical properties. In this work, we reveal two types of σ phases formed in a quaternary Fe40Mn20Co20Cr20 (at.%) HEA with a metastable face-centered cubic (FCC) matrix and illustrate their effects on the deformation behavior of the material. The first type of Cr-rich σ phase (T-type) is formed by the solid-state transformation of the BCC-δ phase and thus they have similar sizes (∼20 μm). The second type of σ phase (P-type) with an average size of ∼500 nm is formed by precipitation with the segregation of Cr at grain boundaries and grain-interior dislocations during recrystallization. Upon deformation, the propagation of cracks in the P-type σ phase particles can be suppressed by martensitic transformation and 9R structure formation in the alloy matrix, leading to a crack buffering effect and contributing to a high ultimate tensile stress of ∼1.2 GPa at a total elongation of ∼15%. Yet, the crack buffering effect can hardly develop in the T-type σ phase with larger sizes and irregular shapes, which accelerates the fracture of the material. This work thus provides insights for controlling the formation of σ phases and the associated design strategies of strong and ductile complex alloys.