Douglas C. Hofmann

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.

A damage-tolerant glass

Owing to a lack of microstructure, glassy materials are inherently strong but brittle, and often demonstrate extreme sensitivity to flaws. Accordingly, their macroscopic failure is often not initiated by plastic yielding, and almost always terminated by brittle fracture. Unlike conventional brittle glasses, metallic glasses are generally capable of limited plastic yielding by shear-band sliding in the presence of a flaw, and thus exhibit toughness-strength relationships that lie between those of brittle ceramics and marginally tough metals. Here, a bulk glassy palladium alloy is introduced, demonstrating an unusual capacity for shielding an opening crack accommodated by an extensive shear-band sliding process, which promotes a fracture toughness comparable to those of the toughest materials known. This result demonstrates that the combination of toughness and strength (that is, damage tolerance) accessible to amorphous materials extends beyond the benchmark ranges established by the toughest and strongest materials known, thereby pushing the envelope of damage tolerance accessible to a structural metal.

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.

Shape Memory Bulk Metallic Glass Composites

Glass-forming and shape memory metals may provide a route to fabricating materials with enhanced mechanical properties. Bulk metallic glasses (BMGs) are being studied extensively as potential structural materials as they have a unique array of mechanical properties compared to traditional crystalline metals (1–4). Their amorphous microstructure and variable composition give BMGs ultrahigh-yield strengths, large elastic strain limits, high hardness, corrosion resistance, and the ability to be processed like a plastic. So far, however, BMGs have not found many structural applications because of their catastrophic failure under tension (tensile loading) and their typically low fracture toughness (resistance to cracking), both resulting from the same amorphous microstructure that differentiates them from crystalline metals. This shortcoming has been addressed in recent years with the development of BMG matrix composites (BMGMCs)—two-phase alloys consisting of soft, crystalline dendrites grown in situ in a glass-forming matrix (5–9). When designed and processed properly, BMGMCs retain the positive structural features exhibited by monolithic (single-phase) BMGs, but can also exhibit enhanced tensile ductility, fracture toughness, and fatigue endurance, which makes them desirable as engineering materials (5, 10, 11).

Beating Crystallization in Glass-Forming Metals by Millisecond Heating and Processing

Resistive heating can be used to rapidly heat a bulk metallic glass without inducing crystallization. The development of metal alloys that form glasses at modest cooling rates has stimulated broad scientific and technological interest. However, intervening crystallization of the liquid in even the most robust bulk metallic glass-formers is orders of magnitude faster than in many common polymers and silicate glass-forming liquids. Crystallization limits experimental studies of the undercooled liquid and hampers efforts to plastically process metallic glasses. We have developed a method to rapidly and uniformly heat a metallic glass at rates of 106 kelvin per second to temperatures spanning the undercooled liquid region. Liquid properties are subsequently measured on millisecond time scales at previously inaccessible temperatures under near-adiabatic conditions. Rapid thermoplastic forming of the undercooled liquid into complex net shapes is implemented under rheological conditions typically used in molding of plastics. By operating in the millisecond regime, we are able to “beat” the intervening crystallization and successfully process even marginal glass-forming alloys with very limited stability against crystallization that are not processable by conventional heating.

Developing Gradient Metal Alloys through Radial Deposition Additive Manufacturing

Interest in additive manufacturing (AM) has dramatically expanded in the last several years, owing to the paradigm shift that the process provides over conventional manufacturing. Although the vast majority of recent work in AM has focused on three-dimensional printing in polymers, AM techniques for fabricating metal alloys have been available for more than a decade. Here, laser deposition (LD) is used to fabricate multifunctional metal alloys that have a strategically graded composition to alter their mechanical and physical properties. Using the technique in combination with rotational deposition enables fabrication of compositional gradients radially from the center of a sample. A roadmap for developing gradient alloys is presented that uses multi-component phase diagrams as maps for composition selection so as to avoid unwanted phases. Practical applications for the new technology are demonstrated in low-coefficient of thermal expansion radially graded metal inserts for carbon-fiber spacecraft panels.

Solution to the problem of the poor cyclic fatigue resistance of bulk metallic glasses

The recent development of metallic glass-matrix composites represents a particular milestone in engineering materials for structural applications owing to their remarkable combination of strength and toughness. However, metallic glasses are highly susceptible to cyclic fatigue damage, and previous attempts to solve this problem have been largely disappointing. Here, we propose and demonstrate a microstructural design strategy to overcome this limitation by matching the microstructural length scales (of the second phase) to mechanical crack-length scales. Specifically, semisolid processing is used to optimize the volume fraction, morphology, and size of second-phase dendrites to confine any initial deformation (shear banding) to the glassy regions separating dendrite arms having length scales of ≈2 μm, i.e., to less than the critical crack size for failure. Confinement of the damage to such interdendritic regions results in enhancement of fatigue lifetimes and increases the fatigue limit by an order of magnitude, making these “designed” composites as resistant to fatigue damage as high-strength steels and aluminum alloys. These design strategies can be universally applied to any other metallic glass systems.

Fracture toughness and crack-resistance curve behavior in metallic glass-matrix composites

Nonlinear-elastic fracture mechanics methods are used to assess the fracture toughness of bulk metallic glass (BMG) composites; results are compared with similar measurements for other monolithic and composite BMG alloys. Mechanistically, plastic shielding gives rise to characteristic resistance-curve behavior where the fracture resistance increases with crack extension. Specifically, confinement of damage by second-phase dendrites is shown to result in enhancement of the toughness by nearly an order of magnitude relative to unreinforced glass.

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.

Towards an understanding of tensile deformation in Ti-based bulk metallic glass matrix composites with BCC dendrites

The microstructure and tension ductility of a series of Ti-based bulk metallic glass matrix composite (BMGMC) is investigated by changing content of the β stabilizing element vanadium while holding the volume fraction of dendritic phase constant. The ability to change only one variable in these novel composites has previously been difficult, leading to uninvestigated areas regarding how composition affects properties. It is shown that the tension ductility can range from near zero percent to over ten percent simply by changing the amount of vanadium in the dendritic phase. This approach may prove useful for the future development of these alloys, which have largely been developed experimentally using trial and error.