Three-dimensional architected materials and structures: Design, fabrication, and mechanical behavior

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Introduction The development of new engineering materials closely mirrored the ages of human development, with the particular periods of history labeled by the materials used in those eras. For example, the tools and weapons of prehistory, 300,000 or more years ago, were bone and stone; that period is referred to as the Stone Age. The discovery of ways to reduce ferrous oxides to make iron, a material with greater stiffness, strength, and hardness than any other then available, occurred around 1450 BC. However, two millennia passed before the blast furnace was developed in 1500 AD, enabling the widespread use of cast iron in the Iron Age. The development of new materials significantly accelerated the industrial revolution, and with the addition of polymers to the suite of materials available to engineers, enabled developments in multiple technologies. One way to examine the availability of engineering materials to possess a given set of properties is via material property charts or so-called “Ashby charts.”1 Material property charts display materials on axes based typically on two of their properties. Materials have many properties of course—mechanical, thermal, electrical, optical, and many more—so the number

of such pair-wise combinations is large. Each chart can thus be thought of as a slice through “material property space”—a multidimensional space with material properties as its axes. Figure 1 shows one such multidimensional chart displaying the relation between strength, modulus, and density of engineering materials. There are many obvious incentives for seeking materials with greater strength: more durable and effective tools; faster, more economical transport; and larger, more daring structures. In recent times, it has been high strength at low weight that is frequently sought, with transport and aerospace as the direct drivers. All such material property charts contain regions that are densely populated with materials, and other parts that are not populated—in the so-called “white spaces.” Some regions are inaccessible for fundamental reasons related to the size of atoms and the nature of the forces that bind them together. Other parts are empty even though, in principle, they could be filled. One approach to filling holes in material property space is by manipulating chemistry, developing new metal alloys, new polymer formulations, and new compositions of glass and ceramic that extend the populated areas of the property charts. A second is by manipulating microstructure,

Julia R. Greer, California Institute of Technology, USA; [email protected] Vikram S. Deshpande, Department of Engineering, University of Cambridge, UK; [email protected] doi:10.1557/mrs.2019.232

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