Aaron Wiest

Designing metallic glass matrix composites with high toughness and tensile ductility

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 service. Highly processable bulk metallic glasses (BMGs) are a new class of engineering materials and have attracted significant technological interest. Although many BMGs exhibit high strength and show substantial fracture toughness, they lack ductility and fail in an apparently brittle manner in unconstrained loading geometries. 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. 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 cent, and an enhanced mode I fracture toughness of K1C ≈ 40 MPa m1/2 were reported. 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, K1C up to ∼170 MPa m1/2, and fracture energies for crack propagation as high as G1C ≈ 340 kJ m-2. The K1C and G1C values equal or surpass those achievable in the toughest titanium or steel alloys, placing BMG composites among the toughest known materials.

Development of tough, low-density titanium-based bulk metallic glass matrix composites with tensile ductility

The mechanical properties of bulk metallic glasses (BMGs) and their composites have been under intense investigation for many years, owing to their unique combination of high strength and elastic limit. However, because of their highly localized deformation mechanism, BMGs are typically considered to be brittle materials and are not suitable for structural applications. Recently, highly-toughened BMG composites have been created in a Zr–Ti-based system with mechanical properties comparable with high-performance crystalline alloys. In this work, we present a series of low-density, Ti-based BMG composites with combinations of high strength, tensile ductility, and excellent fracture toughness.

Glassy steel optimized for glass-forming ability and toughness

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.

Thermal and elastic properties of Cu–Zr–Be bulk metallic glass forming alloys

The compositional dependence of thermal and elastic properties of Cu–Zr–Be ternary bulk metallic glass forming alloys was systematically studied. There exists a linear relationship between the glass transition temperature Tg and the total Zr concentration. G decreases linearly with increasing Zr concentration as well. The results also show that Tg, shear modulus G, and Poissons ratio nu are very sensitive to changes in compositions. Low Tg, low G, and relatively high nu can be achieved with high Zr and Ti concentration.