Phase structure, microstructure, and mechanical properties of FeCoCrNi-based eutectic high-entropy alloys reinforced with MWCNTs and Gr

Abstract

This study primarily focuses on the phase structure, microstructure, and mechanical behaviour of Fe₂₅Co₂₅Cr₂₅Ni₂₅ equiatomic EHEAs upon adding 2 wt% of Multi-Walled Carbon Nanotubes (MWCNTs) and Graphene (Gr) as reinforcements. The alloying processes include Mechanical Alloying (MA) and Vacuum Arc Melting (VAM). The as-milled MA powder is irregularly shaped, with an average particle size of 23.5 μm. Samples subjected to MA followed by VAM exhibited a single-phase alloy composition, with a near-equal chemical distribution of major Face-Centered Cubic (FCC) and minor Body-Centered Cubic (BCC) crystal structures, as confirmed by X-ray diffraction (XRD) analysis. The Vickers microhardness values of the Fe₂₅Co₂₅Cr₂₅Ni₂₅ EHEAs samples were 123 ± 7 HV, while the additions of MWCNTs and Gr increased the hardness to 146 ± 6 HV and 155 ± 9 HV, respectively. To further enhance the strengthening behaviour, the EHEAs samples were heat-treated in a Nabertherm furnace at 1100 °C under an argon atmosphere, resulting in hardness values of 134 ± 6 HV, 164 ± 8 HV, and 171 ± 7 HV for the base alloy, MWCNTs addition, and Gr addition. Adding MWCNTs and Gr enhances the thermal stability of the as-milled powder, preventing secondary phase formation and improving the alloy stability of the equiatomic Fe₂₅Co₂₅Cr₂₅Ni₂₅ composition. Specifically, Fe₂₅Co₂₅Cr₂₅Ni₂₅ exhibited thermal stability up to 534 °C, while Fe₂₅Co₂₅Cr₂₅Ni₂₅+MWCNTs achieved 612 °C, and Fe₂₅Co₂₅Cr₂₅Ni₂₅+Gr demonstrated thermal stability up to 713 °C, with no mass loss or phase change observed, as revealed by thermogravimetric analysis (TGA). Furthermore, adding 2 wt% graphene resulted in superior hardness, residual compressive stress, and thermal stability compared to the MWCNTs addition.