Carbon and nitrogen co-doping enhances phase stability and mechanical properties of a metastable high-entropy alloy

Abstract To probe the significant interstitial effects coupling with metastability design of high-entropy alloys (HEAs), we introduced both interstitial C and N into a representative metastable HEA. The mechanical behavior of the novel C-N co-doped interstitial HEA (iHEA) with nominal composition Fe48.5Mn30Co10Cr10C0.5N1.0 (at. %) in different grain sizes has been investigated via digital image correlation (DIC) assisted tensile testing. The microstructure characteristics prior to and after tensile deformation with different local strain levels were probed by various techniques. The C-N co-doped iHEA shows a face-centered cubic (FCC) matrix without hexagonal closed packed (HCP) martensite after annealing plus the subsequent quenching, different from the reference metastable FCC-HCP dual-phase HEA without interstitials. Upon tensile deformation, dislocation slip and twinning prevail, and the new iHEA does not show phase transformation which is the primary deformation mechanism in the reference interstitial-free metastable HEA. This further confirms the enhanced FCC phase stability due to C-N co-doping. The interstitial effect coupling with the multiple strengthening mechanisms including twinning induced plasticity (TWIP) and nano-particle strengthening due to nano-sized carbonitrides leads to significantly higher yield strength (753 MPa) and ultimate tensile strength (1038 MPa) at good ductility (48.0%) compared to that of the reference interstitial-free metastable HEA (∼250 MPa, ∼850 MPa and ∼70%, respectively) at a similar grain size of ∼4 μm. Accordingly, our work demonstrates the significant capability of interstitials including C and N for tuning the phase stability and enhancing mechanical properties of HEAs.