Mechanical properties of iron-based bulk metallic glasses

Iron-based bulk metallic glasses (BMGs) are characterized by high fracture strengths and elastic moduli, with some exhibiting fracture strengths near 4 GPa, 2–3 times those of conventional high-strength steels. Among the Fe-based BMGs, the non-ferromagnetic ones, designated “non-ferromagnetic amorphous steel alloys” by two of the present authors [S.J. Poon et al.: Appl. Phys. Lett.83, 1131 (2003)], have glass-forming ability high enough to form single-phase glassy rods with diameters reaching 16 mm. Fe-based BMGs designed for structural applications must exhibit some plasticity under compression. However, the role of alloy composition on plastic and brittle failures in metallic glasses is largely unknown. In view of a recently observed correlation that exists between plasticity and Poisson’s ratio for BMGs, compositional effects on plasticity and elastic properties in amorphous steels were investigated. For the new amorphous steels, fracture strengths as high as 4.4 GPa and plastic strains reaching ∼0.8% were measured. Plastic failure instead of brittle failure was observed as the Poisson’s ratio approached 0.32 from below. Investigation of the relationship between the elastic moduli of the alloys and those of the alloying elements revealed that interatomic interactions in addition to the elastic moduli of the alloying elements must be considered in designing ductile Fe-based BMGs. The prospects for attaining high fracture toughness in Fe-based BMGs are discussed in this article.