Bulk metallic glasses (BMGs) are a class of metals that are rapidly cooled from liquid metal melts such that the crystallization process is bypassed and amorphous structure resumes. The absence of crystalline grains and crystalline defects (e.g., dislocations, grain boundaries) in BMGs leads to superior mechanical performance. However, the unusual rapid cooling rate required to form a glass limits the available geometries into which it can be shaped by traditional manufacturing approaches such as casting. This has significantly impeded the structural application of BMGs. We demonstrated the feasibility of a novel approach to fabricating BMGs with versatile geometries through a direct ink writing (DIW)-based additive-manufacturing method combined with thermal sintering. Our systematic mechanical characterization of these additively manufactured BMG architectures indicated that ultrahigh strength can be realized in these complex architectures, which represents a benchmark toward the development of unprecedented structural materials. Our approach provided novel insight into the additive manufacturing of high-strength metals.
Bulk metallic glasses (BMGs) are capable of achieving high strength (>2 GPa) and high elasticity (approximately 2 percent) due to their amorphous nature and the absence of defects (Figure 1) (Johnson 1999; Schroers 2013; Schuh et al. 2007; Telford, M., 2004). The significantly higher strength and elasticity of BMGs compared to crystalline metals and ceramics offers the potential to develop extraordinarily strong and elastic "spring-like" cellular architectures. However, from a practical viewpoint, cellular BMG architectures, either periodic or stochastic, are very difficult to fabricate. Attempts have been made to fabricate stochastic cellular BMG architectures by foaming, but they often suffer from uneven pore-size distribution and uncontrolled architectural topology, resulting in detrimental degradation in material performance (Schroers et al. 2003; Chen et al. 2014a). Periodic structures with controlled and purpose-designed architectures may be engineered for optimal mechanical performance (e.g., strength-to-weight ratio or stiffness-to-weight ratio). However, such architectures are historically difficult to manufacture, particularly in the case of three-dimensional (3D) shapes. Recently, 3D BMG cellular structures have been attempted by mechanical assembly (Liu et al. 2016) and laser selective melting (Pauly et al. 2013), where the former method is limited in versatility (Liu et al. 2016) and the latter approach typically induces residual stresses and microcracks (Pauly et al. 2013).
We have introduced a novel and highly versatile approach to precisely fabricating 3D BMG cellular structures with well controlled architectures that were previously unachievable by conventional BMG processing technologies. Our 3D-printing approach, direct ink writing (DIW), is amenable to formulating carefully controlled inks based on BMG-based particulate feedstocks, for construction of periodic 3D BMG structures in a layer-by-layer fashion. After printing, the structures are post-processed to form BMG via thermoplastic sintering. By integrating BMGs' inherent material properties, we have demonstrated the feasibility of fabricating mechanically robust 3D BMG architectures.
We investigated materials and manufacturing approaches, as well as mechanical characterization. The DIW additive-manufacturing approach is an emerging low-cost 3D-printing technique that enables printing of materials into well-controlled complex architectures (Lewis 2006). During the DIW process, structures are built layer by layer through filamentary deposition of particulate-based or particle-free inks (Lewis et al. 2006). A key component of this technique is the development of inks with controlled flow (or rheological) properties. By tuning the rheology, ink can be made to flow through microscale nozzles to form continuous filaments at low pressure and solidify quickly upon exiting the nozzle to retain their filamentary shape. In this fashion, complex 3D structures with fine self-supporting features may be patterned. The viscoelastic inks are often highly concentrated; in the case of particulate-based inks, they are loaded with microscale and/or nanometer-scale particles (Lewis et al. 2006).
We developed a suite of novel BMG-based inks composed of Fe-Si-B BMG microparticles plus liquid suspension media (or carrier fluid) and polymer binders. During the DIW process, the liquid carrier fluid evaporates and the BMG particles become bound by the polymer matrix to achieve shape retention and formation of 3D constructs composed of a BMG-polymer composite material (Figure 2).
We demonstrated that the feature size can be tuned from approximately 0.1 to 1 mm in diameter by changing the particle size, nozzle size, print speed, and applied pressure. We added a post-printing step to remove the polymer binder and sinter the BMG particles. While crystalline metals or ceramics often display sluggish diffusion kinetics for sintering, even at elevated temperatures, BMGs can be rapidly and efficiently consolidated by spark-plasma sintering within the supercooled liquid regime above the BMG glass-transition temperature. Importantly, spark-plasma sintering prevents deleterious oxide films from forming on the BMG microparticles. Instead, BMGs become viscous and readily flow when exposed to plasma, forming a dense material. We employed spark-plasma sintering because the unique thermoplastic-like softening behavior (i.e., Newtonian viscous flow) of BMGs originating from the amorphous material structure allows a broad inter-atomic contact over large macroscopic areas (Chen et al. 2014b; Guoqiang et al. 2007). Hence, coupling BMGs' intrinsic material behavior with DIW offers a fundamentally new route to fabricating complex 3D architectures composed of pure, dense BMG filaments.
To compare our new 3D BMG architectures to the state of the art in BMG materials, we performed mechanical characterization of 3D BMG architectures using standard uniaxial-compression methods. A representative compressive stress-strain curve of a 3D BMG architecture with 50 percent printed macroporosity shows an extremely high strength of approximately 1.4GPa (Figure 3a), which is orders of magnitude higher than conventional metal alloys or foams of the same specific density. This strength is comparable to the widely used Ti-6Al-4V alloys (Figure 3b), yet our printed BMD structure has a lower density and the Fe-Si-B BMG alloy used in this study is significantly lower cost than Ti-6Al-4V. By varying the spacing of the filaments within the lattices during DIW (and hence the macroporosity), we can readily increase or decrease the density to achieve the desired mechanical properties, which suggests highly useful and versatile control over the mechanical properties of the resultant BMG architectures.
Our innovative approach to additive manufacturing BMGs has opened innovative design space for Lawrence Livermore National Laboratory's applications, with a focus on achieving ultrahigh strength, yet lightweight, mechanical metamaterials. In addition, this effort continues and builds upon a tradition of advancing the Laboratory's materials and manufacturing capabilities in support of its energy and environmental-security missions where lightweight and high-strength engineering materials are needed for transportation and aerospace applications.
We have demonstrated the feasibility of a novel method to fabricate 3D BMG architectures with previously unobtainable mechanical properties by a DIW-based additive-manufacturing technique. Such 3D BMG architectures have been impossible to fabricate in the past due to limitations of conventional BMG manufacturing and processing methods. By coupling BMGs' extraordinary intrinsic material properties with the exquisite architectural control afforded by our 3D-printing process, a broad range of 3D BMG structures can be achieved with tailorable mechanical properties. This work has opened new design space for creating complex 3D metallic architectures with previously unattainable properties and performance.
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