A novel medium entropy alloy based on iron-manganese-aluminum-nickel: influence of boron addition on phase formation, microstructure, and mechanical properties

A novel Medium Entropy Alloy (MEA) based on Fe-Mn-Al-Ni has been designed adopting High Entropy Alloys' (HEAs) phase formation rules, and the effects of minor boron addition on phase content and mechanical properties have been separately investigated. Boron-free Fe(52.71-x)Mn31.11Al5.09Ni11.08Bx (x = 0, 0.05, 0.2, 0.5, 0.7 wt%) MEA showed a single face-centered cubic (FCC) structure. XRD results indicated that with 0.05 and 0.2 wt% boron addition the system maintains its single-phase structure. Further boron addition led to the formation of metal boride intermetallics by the volume fractions of 4.6 and 6.1% of Fe2B intermetallic phase in as-cast and 2.7 and 5.4% in Deformed and Heat-treated (D&H) samples according to Rietveld analysis for 0.5 and 0.7 wt% boron doped alloys, respectively. Boron addition had a positive effect on grain size reduction of the system where just by 0.05 wt% boron addition the grain size has been almost halved compared to boron free as-cast sample. Moreover, it was observed that as the boron level increases, the hardness value increases. With in the all samples subjected to thermomechanical process, 0.7wt % boron doped alloy showed the best yield strength (increasing by ∼50%, from 151 MPa to 222.5 MPa) and ultimate tensile strength (increasing by ∼15%, from 476 MPa to 543 MPa) compared to undoped MEA at comparable ductility ( > 60%).