Ductility improvement of amorphous steels: Roles of shear modulus and electronic structure

Metal–metalloid composition effects on the mechanical properties of Fe–Cr–Mo–P–C–B amorphous steel alloys have been investigated. Compressive plastic strain, elastic moduli and microhardness were measured. The present amorphous steels were found to exhibit enhanced plastic strains up to ∼3.6% and fracture strengths up to ∼3.5 GPa. Moreover, the plastic strain increased quite rapidly with the decrease in shear modulus. The shear modulus values measured are appreciably lower than those reported for previous amorphous steel compositions that did not contain phosphorus; and the Poisson’s ratios obtained are in the range ∼0.33–0.34. The present findings indicate that the ductility of amorphous steels can be significantly improved by chemically tuning the elastic properties which are determined by the amorphous structure and chemical bonding. First-principles electronic structure calculations show that ductility can be improved by partially replacing elements that create ionic and covalent bonds with other elements that favor metallic cohesion.