Full length article3D metallic glass cellular structures
Graphical abstract
A scalable fabrication method for 3DBMG structures by thermoplastic forming and parallel joining is introduced. Experimental characterization and theoretical analysis of 3D BMGs' mechanical behaviors reveal excellent elasticity and elastic energy storability paired with very high energy absorption ability. The combination of BMG properties and the developed versatile fabrication method suggest the possibility to develop a wide range of BMG structures with excellent performance for specific applications.
Introduction
Microstructural architectures with material specific design have been proven powerful in nature and engineering applications. Natural materials, such as shells [1], [2], [3], tooth [4], [5], or bone [6], [7], exhibit complex hierarchical structures spanning from microscopic to macroscopic length scales [8] and show significantly improved mechanical properties (e.g. strength and toughness) compared to base materials [9], [10]. Applications of engineering materials with synthetic structures range broadly, from advanced aerospace structures [11], [12], structural foams [13], [14], [15], multifunctional micro-lattices [16], [17], [18], and topology-optimized architectures [16], [19], [20], [21], [22] to bio-inspired architectures [23], [24]. Taking cellular metals for example, the high specific strengths and large compressibility makes them powerful for energy-absorbing applications. In general, energy absorption of a cellular material scales directly with its strength and plasticity. Bulk metallic glasses (BMGs) are a class of amorphous metals that possess very high strength. They are often not plastic in bulk forms (>1 mm) but can be very plastic at small scales (<1 mm) [25], [26], [27]. In addition, the significantly higher elasticity of BMGs (∼2%) over crystal metals (∼0.8%) or ceramics (∼0.2%) offers a high possibility to develop super-elastic “spring-like” cellular structures. The length scale effect has been widely utilized in BMG stochastic foams [28], [29], [30], [31], [32], [33], [34]. It has also inspired development of materials with architectural features scaled to the BMGs' critical crack length. By introducing isolated dendrites, spaced by tens of microns in a metallic glass matrix, toughness and ductility of the BMG composites are drastically increased [35], [36]. Through bio-inspired hierarchical arrangements of different structural elements at length scales spanning from nanometers to micrometers, Ti–N nanolattices with very high strength (up to 1.75 GPa) [37] and alumina nanolattices exhibiting very high elasticity (fully recoverable with strain up to 50%) [38] were fabricated by using a template-assisted electroless plating method [39].
Microstructural architectures are either periodic or stochastic. Stochastic architectures contain random distributions of heterogeneities. In the case of stochastic foams, which have been widely explored for BMGs [28], [29], [30], [31], [32], [33], [34], gas is incoorporated during processing and the resulting porous BMGs tend to contain uneven bubble (pore) size distributions throughout the resulting architectures. Although foams can generally be manufactured at low costs, the randomness of the underlying architectures leads to a significant reduction in material performance [40], [41]. Periodic architectures, on the other hand, feature ordered heterogeneities that may be engineered to optimally tailor material performance. Classic examples include microtruss lattice [17], [19], [40], [41] and topology-optimized architectures [24], [25]. The drawback of these architectures is that they are generally more difficult to manufacture than stochastic materials, particularly in the case of three-dimensionally architected BMGs, which have thus far not been accomplished successfully. Using the artificial microstructure strategy based on replicating silicon molds with BMGs, 2D cellular BMGs have been fabricated and characterized [36], [42], [43], [44]. The precision and versatility of this method has enabled the study of fundamental aspects of cellular BMGs [36], [42], [43], [44]. However this method is not extendable to the fabrication of 3D structures as microfabrication is a planar process. Recently, 3D printing [45] has offered a novel approach to designing and manufacturing three-dimensional complex parts from polymers, metals, and ceramics. Examples have included stretch-dominated ultra-stiff microlattices [46], and cellular architectures with negative stiffness [47] and negative Poisson's ratio [48]. The application of 3D printing to fabricate BMG cellular structures was recently achieved by using selective laser melting [49]. However, reducing microvoids during the melting of BMG powders and preventing crack formation during solidification are major challenges to this approach.
Here, we introduce a fabrication method that allows us to precisely fabricate BMG structures with desirable architectures and scalable size in 3D. Our method is based on thermoplastic forming (TPF)-based patterning of BMG sheets combined with a parallel joining technique. To demonstrate this capability, we have fabricated honeycomb-like BMG architectures covering a wide range of relative densities. These structures exhibit very high elasticity of up to 40% loading strain, high elastic energy storability, and high energy absorption.
Section snippets
BMG sheets through thermoplastic rolling
The proposed procedure for fabricating 3D BMG cellular architectures consists of four primary steps (Fig. 1 ): (1) formation of BMG sheets, (2) TPF-based patterning and perforation of BMG sheets, (3) parallel joining, and (4) demoulding. The starting material used in our fabrication experiments is cast Zr35Ti30Cu8.25Be26.75 rods, which is a highly reactive, low-cost, and readily accessible metallic glass. These rods have a diameter of 10 mm and length of 10 cm, and a glass transition
Experimental procedure
The fabricated 3D cellular architectures made from Zr35Ti30Cu8.25Be26.75 were quasi-statically compressed using an Instron universal testing machine with flat and parallel heads. All samples were loaded with an initial strain rate of 1.0 × 10−3 s−1. For cyclic loading, the unloading rate was one order of magnitude larger than the loading rate.
Experimental results
A typical stress–strain curve associated with the corresponding microstructural evolution (selected from an in situ movie, see Supplementary section) is
Finite element analysis (FEA) of honeycomb-like BMG architectures
In order to examine the failure mechanism of the BMG honeycomb-like architectures, finite element analysis (FEA) was performed on an idealized BMG honeycomb model. The BMG base material was assumed to exhibit elastic perfectly plastic constitutive behavior [60], with mechanical parameters for Zr35Ti30Cu8.25Be26.75 taken as Young's modulus E = 86.9 GPa, Poisson's ratio v = 0.37, and yield strength σs = 1.43 GPa, respectively [61]. The analyzed structures consisted of four horizontal and three
Summary
In summary, we have fabricated 3D BMG cellular structures. The developed fabrication method is based on thermoplastic forming and parallel joining. The versatility and practicability of our method allows fabrication of scalable and highly controllable 3D metallic glass structures (Table 1). Besides the previously observed drastically enhanced macroscopic plasticity, which is a requisite for energy adsorption, super-elasticity of 3D BMG honeycomb-like structures has been observed. For some
Acknowledgments
This work was primarily supported by DARPA through the MCMA program. Facilities use was supported by CRISP through the National Science Foundation under MRSEC DMR-1119826.
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