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 approximately 1 GPa, tensile ductility of approximately 2-3 per cent, and an enhanced mode I fracture toughness of K(1C) approximately 40 MPa m(1/2) 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, K(1C) up to approximately 170 MPa m(1/2), and fracture energies for crack propagation as high as G(1C) approximately 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.

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.

Many engineering applications, particularly in extreme environments, require components with properties that vary with location in the part. Functionally graded materials (FGMs), which possess gradients in properties such as hardness or density, are a potential solution to address these requirements. The laser-based additive manufacturing process of directed energy deposition (DED) can be used to fabricate metallic parts with a gradient in composition by adjusting the volume fraction of metallic powders delivered to the melt pool as a function of position. As this is a fusion process, secondary phases may develop in the gradient zone during solidification that can result in undesirable properties in the part. This work describes experimental and thermodynamic studies of a component built from 304L stainless steel © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ incrementally graded to Inconel 625. The microstructure, chemistry, phase composition, and microhardness as a function of position were characterized by microscopy, energy dispersive spectroscopy, X-ray diffraction, and microindentation. Particles of secondary phases were found in small amounts within cracks in the gradient zone. These were ascertained to consist of transition metal carbides by experimental results and thermodynamic calculations. The study provides a combined experimental and thermodynamic computational modeling approach toward the fabrication and evaluation of a functionally graded material made by DED additive manufacturing.

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, highlytoughened BMG composites have been created in a Zr-Ti-based system with mechanical properties comparable with highperformance 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.R ecent progress in ductile-phase-reinforced bulk metallic glass (BMG) composites has demonstrated that with proper design and microstructural control enhanced toughness and tensile ductility can be achieved in 2-phase alloys with a glassy matrix phase that exhibits large glass-forming ability (GFA) (1,2). This work has opened potential structural applications for BMG composites that are not possible in monolithic BMGs, because of shear localization and subsequent catastrophic failure during unconfined loading (2). Although the problems associated with unlimited extension of shear bands and the resulting catastrophic failure seen in monolithic BMGs can be mitigated by adding soft crystalline inclusions to the glass, current BMG matrix composites that exhibit this structure are based on the relatively dense element zirconium (1-4). Reducing the cost and density of current Zr-based composites is beneficial for the commercialization of these new alloys. Low-density components with high toughness and strength could be particularly useful in the aerospace and aeronautics industry as replacements for some crystalline titanium alloy hardware.With this motivation, the current work explores Ti-based (in both weight and atomic percentage) BMG matrix composites with mechanical properties and low density matching or surpassing those of common engineering titanium alloys. Herein, we report BMG composites composed of 42-62 weight percentage (wt%) titanium, with densities ranging from 4.97 to 5.15 g/cm 3 , and all exhibiting at least 5% tensile ductility. We vary the volume fraction of the glass phase from 20% to 70%, investigate aluminum additions to lower density, and report 1 alloy with Ͻ1.0 wt% of beryllium. The current work demonstrates that Ti-based BMG composites are competitive with conventional titanium alloys for some structural applications where high strength and toughness are a necessity.Ti-based, ductile-phase-reinforced matrix composites have been the subject of significant recent research (5)(6)(7)(8)(9)(10)(11)(12)(13)(14). In other systems, nano-crystalline composites have been reported that display significant compressive plasticity (15)(16)(17)(18)(19). These alloys are designed in a similar manner to BMG-matrix composites with the significant difference being that the continuous matrix material is comprised of a nanostruct...

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