An alloy development strategy coupled with toughness assessments and ultrasonic measurements is implemented to design a series of iron-based glass-forming alloys that demonstrate improved glass-forming ability and toughness. The combination of good glass-forming ability and high toughness demonstrated by the present alloys is uncommon in Fe-based systems, and is attributed to the ability of these compositions to form stable glass configurations associated with low activation barriers for shear flow, which tend to promote plastic flow and give rise to a toughness higher than other known Fe-based bulk-glass-forming systems. © 2009 American Institute of Physics. ͓DOI: 10.1063/1.3184792͔The remarkably high strength, modulus, and hardness of iron-based glasses, combined with their low cost, prompted an effort over the past five years to design amorphous steel suitable for structural applications. The development effort yielded glasses with critical rod diameters as large as 12 mm 1,2 and strengths in excess of 4 GPa. 3 These low-cost ultrastrong materials however exhibit fracture toughness values as low as 3 MPa m 1/2 , 4 well below acceptable toughness limits for structural materials. The low toughness has been linked to their elastic constants, specifically their high shear modulus, 5 which for some compositions exceeds 80 GPa. 3 Recent efforts to toughen these alloys by altering their composition yielded glasses with lower shear moduli ͑below 70 GPa͒, which exhibit improved notch toughness ͑as high as 50 MPa m 1/2 ͒ but compromised glass-forming ability ͑criti-cal rod diameters less than 3 mm͒. 5,6 In this study, we implement an alloy development strategy coupled with toughness assessment and ultrasonic measurements to design glassy steel alloys with particularly low shear moduli ͑below 60 GPa͒ that demonstrate high toughness ͑notch toughness in excess of 50 MPa m 1/2 ͒ yet adequate glass-forming ability ͑critical rod diameters as large as 6 mm͒.The link between the high shear modulus and the low toughness of Fe-based glasses rests on the argument that a high shear modulus implies a high resistance to relax stress by shear flow. In turn, this promotes cavitation and early fracture and thus limits toughness. Using a Frenkel-like analysis to study cooperative shearing, Johnson and Samwer 7 arrived at a quantitative expression for the activation energy for shear flow, that is, the energy barrier to initiate plastic flow. Specifically, a relationship was proposed between the shear-flow barrier W and the shear modulus G for a frozen-in atomic configuration at the glass transition temperature T g , given by W͑T g ͒ ϰ G͑T g ͒v m ͑T g ͒, 7 where v m is the molar volume, which usually varies little within an alloy family. Aside from their high G, the brittle behavior of these glasses can also be predicted by their high T g , which for some compositions exceeds 600°C. 1,2 The glass transition temperature is also a measure of W͑T g ͒, since the requirement for the liquid viscosity at T g ͑10 12 Pa s͒ gives W͑T g ͒Ϸ37RT g ....

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