A new class of light-weight metastable high entropy alloy with high strength and large ductility

Abstract

The strength-ductility (or density, or thermal conductivity) trade-off is unavoidable in most conventional alloys due to the limited composition space. Herein, we developed a new class of light-weight metastable high entropy alloy (LW metastable HEA) with high specific strength and large ductility. Firstly, a property-targeted alloy design to develop the LW TRIP-assisted dual-phase HEA was performed based on CALPHAD calculation by constituting the alloy with a higher fraction of LW elements in 3d transition metal elements and having the extremely low stacking fault energy. Secondly, we systematically add Al (low density, large atomic size misfit, and Ferrite stabilizer) up to 5 at.% in the non-equiatomic Cr24Mn6Fe40Co24Ni6 HEA developed through the first step. As a result, we confirmed that the phase constitution is changed with increasing Al contents from (FCC+HCP) dual-phase to FCC single phase to (FCC+BCC (B2)) dual-phase. In particular, the FCC single-phase HEA exhibits (1) a relatively low density (∼10% reduction compared to Cantor alloy), (2) an abnormally large lattice distortion (3 times greater atomic size mismatch compared to Cantor alloy), which can be confirmed by extremely low thermal conductivity as well as large lattice parameter; (3) a large solid solution strengthening, which can be confirmed by a significant Hall-Petch coefficient (∼50% increase compared to Cantor alloy); (4) TRIP/TWIP complex deformation mechanisms (large ductility over ∼40%). Thus, it can be understood that the developed single- phase FCC HEAs can provide exceptional combinations of density, strength, and ductility in the trade-off relationship through the confusion effect by the microstructural complexity as well as the chemical complexity. Consequently, these results provide a guideline on how to design a new class of LW metastable HEA with promising properties as a new generation of structural material.