Super-high strength of over 4000 MPa for Fe-based bulk glassy alloys in [(Fe1−xCox)0.75B0.2Si0.05]96Nb4 system

New Fe-based bulk glassy alloys with diameters up to 5 mm were formed in [(Fe1−xCox)0.75B0.2Si0.05]96Nb4 system by the copper mold casting method. The substitution of Co for Fe causes an increase in the glass-forming ability (GFA). As the Co content increases, the liquidus temperature (Tl) decreases, resulting in an increase of reduced glass transition temperature (Tg/Tl) from 0.566 to 0.587. The bulk glassy alloys exhibit super-high fracture strength of 3900–4250 MPa, Young's modulus (E) of 190–210 GPa, elastic strain (ee) of 0.02, plastic strain (ep) of 0.0025 and Vickers hardness (Hv) of 1150–1220. The σf/E and Hv/3E of these glassy alloys are 0.02, respectively, in agreement with the previous data for a number of bulk glassy alloys. The agreement indicates that these Fe-based bulk glassy alloys have an elastic–plastic deformation mode. The syntheses of high-strength Fe-based bulk glassy alloys with super-high fracture strength of over 4000 MPa and high glass-forming ability are encouraging for future development of Fe-based bulk glassy alloys as structural materials.