Physical mechanisms behind the different doping strengthening effects of metalloid and non-metallic elements on high-entropy alloys: Taking boron and carbon as examples

In this paper, boron (B) and carbon (C) are chosen as the representatives of metalloid and non-metallic elements, respectively. They were doped into a typical high-entropy alloy (HEA) Co₃₅Cr₂₅Fe₂₀Ni₂₀, to study their different strengthening-toughening effects. Tensile results confirmed that the doping of both B and C atoms can obviously increase the yield and tensile strengths, and the strengthening effect of C is stronger than that of B. The apparent reasons for their different strengthening effects are as follows. The electron localization function (ELF) shows that C is to produce more pronounced covalent bonding features than B. B is more inclined to form covalent bonds with Fe atoms. Whereas C is more likely to form covalent bonds with Cr atoms, which explains the formation of Cr₂₃C₆ carbides. Although both B and C atoms exist as octahedral (OCT) interstitials in the HEA system, their standard errors (SD) of the overall transfer of Bader's charge demonstrate that the C-atom doping generates a greater atomic-level stress than the B atoms. That means, the C atoms doping causes more pronounced localized lattice distortion, producing a stronger strengthening effect than that of B-atom doping. Thus, apart from grain boundary strengthening, the strengthening mechanism of doping metalloid elements like B is mainly dislocation strengthening, but is chiefly lattice friction stress for non-metallic elements like C.