Abstract: The phase metastability and precipitation are now considered to be an important strategy in designing Fe-rich high entropy alloys (HEAs). In this study, the influence of silicon addition on the initial and strain-induced microstructure evolution and related mechanical property of Fe_52-xMn₂₇Cr₁₅Co₆Si_x (x=0, 0.3, 0.5, 1.0, 1.5, at.%) HEAs was systematically investigated by utilizing the in-depth microstructural characterization coupled with X-ray diffractometer (XRD), secondary electron microscopy (SEM), and transmission electron microscopy (TEM). The addition of Si to Fe_52-xMn₂₇Cr₁₅Co₆Si_x HEAs facilitates the triplex structure consisting of fcc-γ matrix, thermally-induced ε-martensite and sigma phase (σ). The lattice distortion energy by Si atoms is suggested to promote the formation of σ phase consisting of Cr, Si and Co and consequently influence the metastability of the matrix. In 0.3 at.% Si HEA, the strain-induced body-centered tetragonal (bct)-type α'-martensite were observed at the intersection of bi-directional strain-induced ε-martensite laths, enhancing the ultimate tensile strength to ∼851 MPa from ∼618.3 MPa with ductility increment (∼73.1% from ∼71%). In 0.3 at.% Si and 0.5 at.% Si alloys, the granular-type σ phase was observed both at grain boundaries and in grain interior, and the size of granular-type σ phase at grain boundary and intra-granular σ phase were found to be similar. The deformation mode altered from the transformation-induced plasticity (TRIP) to twinning-induced plasticity (TWIP) with an increase of Si content to 1.5 at.%, due to the enhanced fcc-γ stability induced by the compositional modulation driven by increased σ phase formation. The propagation of microcracks inside brittle σ phase could be suppressed by homogeneous slip through strain-induced martensite transformation (SIMT) in HEAs with low Si addition of 0.3at.% -0.5 at.%.

Key words: High entropy alloy (HEA), ε -martensite, σ phase, Metastability, Microcrack