Abstract: C-alloying is a well-established method for enhancing the strength of face-centered cubic (FCC) high-entropy alloys (HEAs). However, to minimize the detrimental effects of carbides on ductility, C concentration is normally limited below a critical threshold to prevent carbide precipitation in many HEAs, despite the strength-enhancing effect of carbides. In this study, we present a novel strategy that combines intragranular carbide distribution with transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects to simultaneously enhance both strength and ductility in FCC Cr₂₀Mn₂₀Fe₂₀Co_35-xNi₅Cx (x = 0.5, 1, and 2) HEAs. Increasing C concentration leads to reduced grain size, fewer stacking faults (SFs), and higher dislocation density in the initial microstructures of these HEAs after undergoing identical thermomechanical processing. Carbides are absent in the C0.5 HEA, while the C1 and C2 HEAs contain M₂₃C₆ carbides, predominantly located within the grain interiors. During plastic deformation, besides dislocation slip, deformation-induced phase transformation, or twinning are activated, with the C0.5 HEA exhibiting the TRIP effect, and the C1 and C2 HEAs displaying a combination of TRIP and TWIP effects. The yield strength (YS) and total elongation (TEL) of these three HEAs all increase with increasing C concentration. The enhancement in YS is primarily attributed to the reduced grain size and the increased presence of M₂₃C₆ carbides. The improvement in ductility is attributed to several factors arising from the increased C concentration: sustained high strain hardening rates at higher strain levels, improved strain distribution uniformity, alleviation of strain localization at grain boundaries, and promotion of crack arrest within the FCC matrix.

Key words: High-entropy alloys, M₂₃C₆ carbides, Transformation-induced plasticity (TRIP), Twinning-induced plasticity (TWIP)