Showing papers by "Marc A. Meyers published in 2021"

Amorphization in extreme deformation of the CrMnFeCoNi high-entropy alloy

Abstract: Ever-harsher service conditions in the future will call for materials with increasing ability to undergo deformation without sustaining damage while retaining high strength. Prime candidates for these conditions are certain high-entropy alloys (HEAs), which have extraordinary work-hardening ability and toughness. By subjecting the equiatomic CrMnFeCoNi HEA to severe plastic deformation through swaging followed by either quasi-static compression or dynamic deformation in shear, we observe a dense structure comprising stacking faults, twins, transformation from the face-centered cubic to the hexagonal close-packed structure, and, of particular note, amorphization. The coordinated propagation of stacking faults and twins along {111} planes generates high-deformation regions, which can reorganize into hexagonal packets; when the defect density in these regions reaches a critical level, they generate islands of amorphous material. These regions can have outstanding mechanical properties, which provide additional strengthening and/or toughening mechanisms to enhance the capability of these alloys to withstand extreme loading conditions.

Hydration-induced reversible deformation of biological materials

Abstract: The influx and efflux of water in biological structures actuates reversible deformation and recovery processes that are crucial for mechanical functions in plants and animals. These processes utilize various mechanochemical mechanisms: swelling directed by the arrangement of cellulosic microfibrils in a bilayer construct, which generates different deformation patterns; lignification gradients; hierarchical foam-like inner structures, some of which also include swelling by hygroscopic cellulose inner cell layer; turgor pressure, which is activated by osmosis and acts at the cellular level, generating reversible motions. In this Review, we present representatives of each of these four mechanisms: pine cones, wheat awns, the twisted opening of Bauhinia pods and the seed of the stork’s bill; the resurrection plant; ice plant seed capsules and carrotwood seed pod; the wilting and redressing of plant stems. Natural polymeric materials produced by animals can also exhibit hydration-driven shape and strength recovery: bird feathers and hair are prime examples. Spider silk — a non-keratinous biopolymer — also exhibits humidity-driven reversible deformation. After describing these animal-based mechanisms, we outline bioinspired applications to actuate multifunctional and biocompatible smart materials, and indicate future directions of research with potential for new bioinspired designs. The influx and efflux of water in biological structures leads to reversible deformation, which has important functions in plants (for example, in seed protection and dispersal) and animals (for example, in the recovery of the strength and shape of feathers, and for reversible changes in silk and hair). Here the authors review the main hydration-induced deformation mechanisms and highlight applications inspired by these processes.

Additive manufacturing of structural ceramics: a historical perspective

Abstract: Additive manufacturing (AM) has created a new era of digital manufacturing, where engineering practices, computer-aided design platforms, and part sourcing pipelines are dramatically changing. AM techniques are capable of producing plastic, metal, and ceramic components for both prototyping and end-use purposes. In this review, the fabrication of dense, structural advanced ceramic components using the seven families of additive manufacturing is discussed through a historical perspective. Initial studies on additive manufacturing of ceramic materials were reported just a few years after those of metal and plastic materials. However, industrial application of ceramic additive manufacturing is more than a decade behind metallic and plastic materials. Many of the challenges of ceramic AM can be traced back to the intrinsic difficulties of processing structural ceramic materials, including high processing temperatures, defect-sensitive mechanical properties, and poor machining characteristics. To mature the field of ceramic AM, future research and development should focus on expanding material selection, improving printing and post-processing control, realizing single-step processing, and unique capabilities such as multi-material and hybrid processing.

Multi-material additive manufacturing of functionally graded carbide ceramics via active, in-line mixing

Abstract: Advanced ceramics are required in many applications including armor, engine components, and wear parts for abrasive, corrosive, and high temperature environments. Heterogeneous structuring in these materials has the potential to unlock extrinsic mechanisms that improve damage tolerance, of vital importance for structural functionality. However, traditional ceramic powder processing and forming techniques limit the design space to simple geometries with chemically homogenous microstructures. Thus, additive manufacturing by direct ink writing (DIW) was applied to fabricate multi-phase carbide specimens with tailored composition and mesostructure. A custom DIW system was developed to allow simultaneous extrusion and mixing of multiple inks, comprised of ceramic particulate suspensions, through a single nozzle. Boron carbide (B4C) and silicon carbide (SiC) were chosen for this study due to their excellent mechanical properties. Aqueous B4C and SiC inks were loaded to 47.5 vol% ceramic content and showed yield-pseudoplastic behavior. The carbide inks were characterized and modified to exhibit similar rheological behavior (yield stress and viscosity), and were used to produce B4C–SiC parts with either discrete or continuous composition variation. Specimens were hot pressed at 35 MPa and 1950 °C, yielding near full density with hardness (Knoop) values of 20–23 GPa. Tailored heterogeneity, achieved via active in-line mixing, is revealed through microstructural characterization. Cracking observed in the specimen with discretely varied composition is the result of thermally-induced residual stress, and is elucidated through analytical calculations.

Digital healthcare technologies: Modern tools to transform prosthetic care.

Abstract: Digital healthcare technologies are transforming the face of prosthetic care. Millions of amputees around the world do not currently have access to any form of prosthetic healthcare. However, digital technologies provide a promising solution. Digital healthcare technologies have the potential to augment the range and efficiency of prosthetists so they can reach more patients. These technologies will enable affordable prostheses to be built on a scale larger than currently possible with today’s clinical practices. In this paper, we explore the social aspects of amputation as a global issue, describe current practices for designing and manufacturing prosthetic sockets, and examine shifting trends towards virtual care models. Importantly, we assess the technologies used in these virtual health workflows to understand their critical needs. Large technological gaps need to be overcome in order to enable the mass production and distribution of prostheses digitally. However, recent advances in computational methods and CAD/CAM technologies are bridging this gap faster than ever before. We foresee that these technologies will return mobility and economic opportunity to amputees on a global scale in the near future.

Micro-mechanical response of ultrafine grain and nanocrystalline tantalum

Abstract: In order to investigate the effect of grain boundaries on the mechanical response in the micrometer and submicrometer levels, complementary experiments and molecular dynamics simulations were conducted on a model bcc metal, tantalum. Microscale pillar experiments (diameters of 1 and 2 μm) with a grain size of ~100–200 nm revealed a mechanical response characterized by a yield stress of ~1500 MPa. The hardening of the structure is reflected in the increase in the flow stress to 1700 MPa at a strain of ~0.35. Molecular dynamics simulations were conducted for nanocrystalline tantalum with grain sizes in the range of 20–50 nm and pillar diameters in the same range. The yield stress was approximately 6000 MPa for all specimens and the maximum of the stress–strain curves occurred at a strain of 0.07. Beyond that strain, the material softened because of its inability to store dislocations. The experimental results did not show a significant size dependence of yield stress on pillar diameter (equal to 1 and 2 um), which is attributed to the high ratio between pillar diameter and grain size (~10–20). This behavior is quite different from that in monocrystalline specimens where dislocation ‘starvation’ leads to a significant size dependence of strength. The ultrafine grains exhibit clear ‘pancaking’ upon being plastically deformed, with an increase in dislocation density. The plastic deformation is much more localized for the single crystals than for the nanocrystalline specimens, an observation made in both modeling and experiments. In the molecular dynamics simulations, the ratio of pillar diameter (20–50 nm) to grain size was in the range 0.2–2, and a much greater dependence of yield stress to pillar diameter was observed. A critical result from this work is the demonstration that the important parameter in establishing the overall deformation is the ratio between the grain size and pillar diameter; it governs the deformation mode, as well as surface sources and sinks, which are only important when the grain size is of the same order as the pillar diameter.

The role of pre-existing defects in shock-generated ejecta in copper

Abstract: The interaction of shock waves with non-planar free surfaces can cause atoms to eject from the surface, leading to the formation of ejecta. These non-planarities in the free surface can occur due to machining of the free surface or can be induced in the shock wave itself due to the presence of heterogeneities in the material. Both cases lead to the formation of ejecta. While the effect of machining on ejecta has been well studied, the latter has not been a topic of significant investigations. In this work, molecular dynamics simulations are used to systematically investigate the effect of size and concentration of He bubbles in Cu with planar free surfaces on ejecta production. It is shown that the presence of defects leads to the formation of non-planarity in the shock wave itself producing ejecta as the front reaches the flat free surface. The cluster size and velocity of ejected particles greatly exceeds that of pure Cu; the radius, density, and nature of the helium-filled voids alter the mass, velocity, and size distribution of the ejected matter.

Bite force mechanics and allometry of piranha (Serrasalmidae)

Abstract: The bite force of the piranha (Serrasalmidae) has drawn considerable attention due to its ability to effectively capture and masticate prey. Herein, we analyze theoretical anterior bite forces using a lever approach and compare them to in-vivo maximum bite forces. We provide a mechanics analysis that explains the scaling allometry of the bite force ( F o u t p u t ) with the length of the fish (l), F o u t p u t α l 2 .

Contributions to Dynamic Behaviour of Materials Professor John Edwin Field, FRS 1936–2020

Abstract: Professor John Edwin Field passed away on October 21st, 2020 at the age of 84. Professor Field was widely regarded as a leader in high-strain rate physics and explosives. During his career in the Physics and Chemistry of Solids (PCS) Group of the Cavendish Laboratory at Cambridge University, John made major contributions into our understanding of friction and erosion, brittle fracture, explosives, impact and high strain-rate effects in solids, impact in liquids, and shock physics. The contributions made by the PCS group are recognized globally and the impact of John’s work is a lasting addition to our knowledge of the dynamic effects in materials. John graduated 84 Ph.D. students and collaborated broadly in the field. Many who knew him attribute their success to the excellent grounding in research and teaching they received from John Field.

The rhythms of nature inspiring art and science

Abstract: The conquest of fire has been an essential component of the civilizing process. Milazzo and Buehler from MIT spearhead a novel direction of research by seeking inspiration from fire through deep learning, paving a new avenue for the design and fabrication of novel materials.