1. Keck Laboratory of Engineering Materials, California Institute of Technology, Pasadena, California 91125, USA

Correspondence to: Douglas C. Hofmann¹ Correspondence and requests for materials should be addressed to D.C.H. (Email: dch@caltech.edu).

The selection and design of modern high-performance structural engineering materials is driven by optimizing combinations of mechanical properties such as strength, ductility, toughness, elasticity and requirements for predictable and graceful (non-catastrophic) failure in service¹. Highly processable bulk metallic glasses (BMGs) are a new class of engineering materials and have attracted significant technological interest^{2, 3, 4, 5, 6}. Although many BMGs exhibit high strength and show substantial fracture toughness, they lack ductility and fail in an apparently brittle manner in unconstrained loading geometries⁷. For instance, some BMGs exhibit significant plastic deformation in compression or bending tests, but all exhibit negligible plasticity (<0.5% strain) in uniaxial tension. To overcome brittle failure in tension, BMG–matrix composites have been introduced^{8, 9}. The inhomogeneous microstructure with isolated dendrites in a BMG matrix stabilizes the glass against the catastrophic failure associated with unlimited extension of a shear band and results in enhanced global plasticity and more graceful failure. Tensile strengths of 1 GPa, tensile ductility of 2–3 per cent⁹, and an enhanced mode I fracture toughness of K _1C  40 MPa m^1/2 were reported^{8, 9}. Building on this approach, we have developed 'designed composites' by matching fundamental mechanical and microstructural length scales. Here, we report titanium–zirconium-based BMG composites with room-temperature tensile ductility exceeding 10 per cent, yield strengths of 1.2–1.5 GPa, K _1C up to 170 MPa m^1/2, and fracture energies for crack propagation as high as G _1C  340 kJ m^-2. The K _1C and G _1C values equal or surpass those achievable in the toughest titanium or steel alloys, placing BMG composites among the toughest known materials.

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