Carbon-driven coherent nanoprecipitates enable ultrahigh yield strength in a high-entropy alloy

A novel C-doped high entropy alloy (Al0.5CoFeNiC0.1) with nano-scale coherent precipitates was successfully designed and fabricated by arc-melting. The nano-scale phase has been studied using a complementary range of techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), atom probe tomography (APT) and density functional theory (DFT). The effect of nano-sized particles on the mechanical behavior of the alloy has been evaluated by tensile testing. The nano-sized precipitate was identified to be the off-stoichiometric k' phase, which has an L012-crystal structure, wherein Fe/Ni/Co and Al are ordered on simple cubic sublattices and C occupies the octahedral interstices surrounded by only transition metal atoms, together forming a perovskite-type structure. DFT simulation shows that C triggers the ordered arrangement of Al and Fe/Ni/Co, thus promoting the formation of k' phase. k' particles precipitate during cooling after solidification via a classical nucleation and growth mechanism. An addition of only 2.8 at.% C to as-cast Al0.5CoFeNi leads to a high volume fraction (-55%) of k0 precipitates with an average size of -35 nm in as-cast Al0.5CoFeNiC0.1, thus enhancing dramatically the yield strength by a factor of 3 (from 385 MPa to 1115 MPa) compared to its carbon free counterpart (Al0.5CoFeNi). Such an efficient strengthening effect on k' phase could be associated with its high antiphase boundary (APB) energy.