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2024
Controlling splat boundary network evolution towards the development of strong and ductile cold sprayed refractory metals: The role of powder characteristics
Mahsa Amiri, Kliah N. Soto Leytan, Diran Apelian, Daniel R. Mumm, Lorenzo Valdevit, Materials Science & Engineering A 902 (2024) 146559
Abstract
Feedstock powder characteristics such as composition (specific alloying elements and concentrations and impurity levels), microstructure, thickness/composition of surface oxide layers, and particle size distribution play a crucial role in determining the overall mechanical properties of cold sprayed deposits. Herein, we report on two deposits consolidated via cold spray processing from differently-sourced batches of nominally identical elemental refractory powders under identical spraying conditions, which exhibit bending strength and ductility values that differ by more than a factor of two – and with the weakest sample displaying negligible effective ductility. Through chemical, microstructural and micromechanical characterization of both the feedstock powders and cold sprayed deposits, we consider the possible influences of feedstock characteristics on the mechanical performance of cold spray consolidated deposits. It is shown that while differences in interstitial oxygen and hydrogen content may result in differences in the intrinsic yield characteristics of the feedstock material, both feedstocks maintain the ductile behavior required to induce good metallurgical bonding upon impact of optimally sized powder particles. We conclude that the deposits formed from the two feedstock powders are indistinguishable and exhibit high ductility when characterized locally within the relatively undeformed bulk of a single particle or splat. However, the two sprayed deposits show low ductility or brittle behavior when loaded in tension across intersplat boundary domains comprising material that has undergone extensive deformation. In addition, one feedstock incorporates a broader particle size distribution, with a long tail of larger-than-optimal particles. These larger particles are accelerated below the critical impact velocity, resulting in observed imperfect metallurgical bonding at splat interfaces and higher porosity at such intersplat boundaries. This result, combined with the overall differences in the morphology of the intersplat boundary network between the two deposits, explains the difference in macroscopic mechanical behavior. We conclude that accurate control of the powder feedstock particle size distribution is essential for optimizing the mechanical integrity of cold sprayed refractory elements and alloys.
Microstructural Control of a Multi-Phase PH Steel Printed with Laser Powder Bed Fusion
Brandon Fields, Mahsa Amiri, Jungyun Lim, Julia T. Pürstl, Matthew R. Begley, Diran Apelian, and Lorenzo Valdevit, Adv. Mater. Technol. 2024, 9, 2301037
Abstract
The established approach to materials design for additive manufacturing (AM) consists of attempting to reproduce the uniform structures and properties of conventionally processed materials. While this certainly helped facilitate material certification and the rapid introduction of AM technologies in several industries, the opportunity to exploit unique features of specific AM processes to generate spatially varying microstructures–and hence novel materials, remains largely untapped. This work presents a method for manufacturing materials through laser powder bed fusion (LPBF), in which control over the spatial variation in phase composition and mechanical properties is achieved. This technique is demonstrated using 17-4 precipitation-hardened stainless steel (17-4PH), by controlling spatial modulation of energy densities during printing. This results in local control of ferrite/martensite volume fractions, allowing the fabrication of metal/metal architected composites with hard/brittle regions interspersed with soft/tough regions. Local variations of ˜20% in tensile strength and ˜150% in elongation are achieved, with a spatial resolution of ˜100 microns. The approach is general and robust, fully compatible with commercially available LPBF equipment, and applicable to virtually any multi-phase alloy system. This work shifts the paradigm from attempting to print components with uniform properties to manufacturing alloys with controlled spatial property gradients.
Investigation of an additively manufactured modified aluminum 7068 alloy: Processing, microstructure, and mechanical properties
Brandon Fields, Mahsa Amiri, Benjamin E. MacDonald, Julia T. Pürstl, Chen Dai, Xiaochun Li, Diran Apelian, Lorenzo Valdevit, Materials Science & Engineering A 891 (2024) 145901
Abstract
Many additively manufactured alloys exhibit higher strengths than compositionally identical alloys processed via conventional processing routes. However, this enhancement is not consistently observed in 7xxx series aluminum alloys. These alloys present two complications when printed via Laser Powder Bed Fusion (LPBF): significant evaporation of strengthening elements from the melt pool and hot cracking during solidification. To address these issues, we introduce two modifications to the feedstock powder: (i) we increase the concentration of alloying constituents to counteract evaporation during printing, and (ii) we disperse TiC nanoparticles within the feedstock powder to promote heterogeneous nucleation and limit grain growth, thus avoiding hot cracking and improving strength. Relationships between the evaporation of alloying elements and laser energy density are quantified experimentally using inductively-coupled-plasma mass-spectrometry and are well captured by simple analytical models. The microstructures in as-printed and heat-treated conditions are characterized using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Printing parameters have been optimized to attain minimum porosity, resulting in tensile strengths up to 650 MPa, which are in good agreement with predictions from classic models of strengthening mechanisms.
Towards a self-healing aluminum metal matrix composite: Design, fabrication, and demonstration
David Svetlizky, Baolong Zheng, Xin Wang, Sen Jiang, Lorenzo Valdevit, Julie M. Schoenung, Enrique J. Lavernia, Noam Eliaz, Applied Materials Today 37 (2024) 102148
Abstract
This paper presents a novel approach to designing and synthesizing a self-healing aluminum-based metal matrix composite (MMC) at the macro-scale. The composite comprises an Al 5083 matrix embedded with low melting point particles (LMPPs) that act as healing agents. A two-step electroless micro-encapsulation process is developed to create LMMPs with a diffusion and thermal barrier designed to protect the Zn-8Al core with a Co-P shell. The MMC is fabricated using spark plasma sintering. Following controlled total fracture under tension, external compressive force is applied during heat treatment to heal the fracture effectively. The evolution of phases and interfaces is characterized using electron microscopy, and transient liquid phase bonding (TLPB) is identified as the fracture-healing mechanism, facilitated in areas with sufficiently high Zn concentration to fill the crack. The design can be expanded to incorporate other matrix and LMMP materials, mechanical crack volume reduction by integrating shape memory alloy (SMA) reinforcement during MMC synthesis, and processing of the self-healing MMC using Directed Energy Deposition additive manufacturing.
A multi-scale process for mechanical characterization of ceramic materials produced by Direct Ink Writing
Raphael Thiraux, Alexander D. Dupuy, Kate M. Ainger, Lorenzo Valdevit, Open Ceramics 17 (2024) 100544
Abstract
Mechanical characterization of ceramics is challenging, as the statistical nature of their strength requires numerous specimens to extract reliable distribution parameters. Here, we first propose a high-throughput testing procedure that allows extraction of statistical information on the strength of ceramic materials. We process large numbers of bending specimens from low volumes of material via Direct Ink Writing (DIW), and rapidly characterize them to extract Weibull parameters for bending strength. After investigating five ceramics and downselecting two formulations, we develop a multi-scale procedure to explore the impact of printing-induced defects on the strength distribution of DIW-processed ceramics. Finally, we demonstrate that a judicious choice of the printing strategy produces porous architected structures which can significantly exceed the strength of fully dense DIW-produced monolithic materials. While the results are presented on DIW-processed alumina-based ceramics, the approach is versatile and can provide rapid statistical data on the strength of many ceramic materials.
Neural network for predicting Peierls barrier spectrum and its influence on dislocation motion
Xinyi Wang, Lorenzo Valdevit, Penghui Cao, Acta Materialia 267 (2024) 119696
Abstract
The Peierls barrier represents the inherent lattice resistance to dislocation glide, controlling dislocation movement and dictating the resulting mechanical properties. The rise of multi-principal element alloys, with their vast compositional space and local chemical fluctuations, introduces notable challenges in efficiently computing the Peierls barriers. Here, we propose a neural network model that captures the chemistry and structure of screw dislocations. By incorporating two crucial inputs, the local atomic type and displacement, our model enables accurate and efficient prediction of the Peierls barriers in multi-component space. Using this neural network, we construct a barrier diagram across the entire ternary space of refractory Nb-Mo-Ta alloys, from which the compositions with high or low barriers can be quickly identified. We discover that the NbMo binary alloy can have a higher barrier than NbMoTa ternary system, suggesting chemical complexity may not be the predominant factor governing dislocation barrier. Subsequently, three screened compositions with different barriers are selected for studying their effects on dislocation motion. Atomistic simulations reveal that a higher mean barrier slows down dislocation mobility, while a broader distribution of barriers facilitates kink pair nucleation, altering the rate-limiting process from kink pair nucleation to kink glide and cross kinking. This study presents a general neural network model that enables rapid screening composition with the desired barrier spectrum and will impact the exploration and discovery of alloys with improved dislocation-controlled properties.
The defect sensitivity of brittle truss-based metamaterials
2023
In situ observation of melt pool evolution in ultrasonic vibration‑assisted directed energy deposition
Salma A. El‑Azab, Cheng Zhang, Sen Jiang, Aleksandra L. Vyatskikh, Lorenzo Valdevit, Enrique J. Lavernia, Julie M. Schoenung, Scientific Reports (2023) 13:17705
Abstract
The presence of defects, such as pores, in materials processed using additive manufacturing represents a challenge during the manufacturing of many engineering components. Recently, ultrasonic vibration-assisted (UV-A) directed energy deposition (DED) has been shown to reduce porosity, promote grain refinement, and enhance mechanical performance in metal components. Whereas it is evident that the formation of such microstructural features is affected by the melt pool behavior, the specific mechanisms by which ultrasonic vibration (UV) influences the melt pool remain elusive. In the present investigation, UV was applied in situ to DED of 316L stainless steel single tracks and bulk parts. For the first time, high-speed video imaging and thermal imaging were implemented in situ to quantitatively correlate the application of UV to melt pool evolution in DED. Extensive imaging data were coupled with in-depth microstructural characterization to develop a statistically robust dataset describing the observed phenomena. Our findings show that UV increases the melt pool peak temperature and dimensions, while improving the wettability of injected particles with the melt pool surface and reducing particle residence time. Near the substrate, we observe that UV results in a 92% decrease in porosity, and a 54% decrease in dendritic arm spacing. The effect of UV on the melt pool is caused by the combined mechanisms of acoustic cavitation, ultrasound absorption, and acoustic streaming. Through in situ imaging we demonstrate quantitatively that these phenomena, acting simultaneously, effectively diminish with increasing build height and size due to acoustic attenuation, consequently decreasing the positive effect of implementing UV-A DED. Thus, this research provides valuable insight into the value of in situ imaging, as well as the effects of UV on DED melt pool dynamics, the stochastic interactions between the melt pool and incoming powder particles, and the limitations of build geometry on the UV-A DED technique.
Residual stress mitigation in directed energy deposition
Aleksandra L. Vyatskikh, Xin Wang, James Haley, Baolong Zheng, Lorenzo Valdevit, Enrique J. Lavernia, Julie M. Schoenung, Materials Science & Engineering A 871 (2023) 144845
Abstract
Directed Energy Deposition (DED), a class of additive manufacturing techniques, has seen rapid growth over the last decade for potential applications in aerospace, medical devices, and energy systems. Despite notable progress in the research and development of AM, control and mitigation of residual stress during DED remains a challenge. In this work, we propose a novel approach that can be used for the mitigation of residual stresses in additively manufactured components. Specifically, we propose to mitigate the residual stress state of as-deposited components using alloy design, engineering of solid-state transformations, and the introduction of both hard and soft metallic phases. We demonstrate this strategy with a model system consisting of pure Fe and Fe–Cu. Experimental results indicate that residual stresses can be successfully manipulated by adjusting the alloy composition as a soft metallic phase can accommodate plastic deformation. Moreover, our findings suggest that the solid-state transformations experienced by the Fe and Fe-rich phases contribute to the observed differences in magnitude and location of residual stresses. This study is the first to suggest using residual stress as an engineering criterion in the design of alloys for metal additive manufacturing.
Influence of co-deposition strategy on the mechanical behavior of additively manufactured functionally integrated materials
Benjamin E. MacDonald , Baolong Zheng , Brandon Fields, Xin Wang , Sen Jiang , Penghui Cao , Lorenzo Valdevit , Enrique J. Lavernia , Julie M. Schoenung, Additive Manufacturing 61 (2023) 103328
Abstract
The co-deposition of multiple powder feedstocks during metal additive manufacturing (AM) can be used to fabricate materials with spatially dependent properties, which can be engineered to contain different function- alities (i.e., functionally integrated materials, FIMs). Although the transition region that forms between dissimilar materials has been studied in detail, the influence of co-deposition on the resultant spatial phase distribution and associated mechanical behavior has heretofore not been reported. In this study, FIM samples transitioning from stainless steel (SS) 316 L to Haynes 282 Ni-based superalloy were deposited via directed energy deposition (DED). The FIM samples were compared to baseline, homogeneous single-alloy deposited samples using digital image correlation during tensile testing, together with microscopy, energy-dispersive X-ray spectroscopy, elec- tron backscattered diffraction, and thermodynamic modeling, to assess the performance of different co- deposition strategies. Each FIM sample exhibited a compositionally and microstructurally unique transition re- gion from SS 316 L to Haynes 282, which was found to have implications on the strain localization across the transition region during uniaxial tensile loading. Finer step sizes in co-deposition were found to minimize strain localization by avoiding sharp compositional interfaces in the transition region.
Design, material, function, and fabrication of metamaterials
Amir A. Zadpoor, Mohammad J. Mirzaali, Lorenzo Valdevit, and Jonathan B. Hopkins, APL Mater. 11, 020401 (2023)
Abstract
Metamaterials are engineered materials with unusual, unique properties and advanced functionalities that are a direct consequence of their microarchitecture. While initial properties and functionalities were limited to optics and electromagnetism, many novel categories of metamaterials that have applications in many different areas of research and practice, including acoustic, mechanics, biomaterials, and thermal engineering, have appeared in the last decade. This editorial serves as a prelude to the special issue with the same title that presents a number of selected studies in these directions. In particular, we review some of the most important developments in the design and fabrication of metamaterials with an emphasis on the more recent categories. We also suggest some directions for future research.
Deformation behavior of cell walls in an additively manufactured hybrid metallic foam
2022
Nanoarchitected metal/ceramic interpenetrating phase composites
Architected metals and ceramics with nanoscale cellular designs, e.g., nanolattices, are currently subject of extensive investigation. By harnessing extreme material size effects, nanolattices demonstrated classically inaccessible properties at low density, with exceptional potential for superior lightweight materials. This study expands the concept of nanoarchitecture to dense metal/ceramic composites, presenting co-continuous architectures of three- dimensional printed pyrolytic carbon shell reinforcements and electrodeposited nickel matrices. We demonstrate ductile compressive deformability with elongated ultrahigh strength plateaus, resulting in an extremely high combination of compressive strength and strain energy absorption. Simultaneously, property-to-weight ratios outperform those of lightweight nanolattices. Superior to cellular nanoarchitectures, interpenetrating nanocom- posites may combine multiple size-dependent characteristics, whether mechanical or functional, which are radically antagonistic in existing materials. This provides a pathway toward previously unobtainable multifunctionality, extending far beyond lightweight structure applications.
Minimal Surface-Based Materials for Topological Elastic Wave Guiding
Abstract
Materials based on minimal surface geometries have shown superior strength and stiffness at low densities, which makes them promising continuous-based material platforms for a variety of engineering applications. In this work, it is demonstrated how these mechanical properties can be complemented by dynamic functionalities resulting from robust topological guiding of elastic waves at interfaces that are incorporated into the consid- ered material platforms. Starting from the definition of Schwarz P minimal surface, geometric parametrizations are introduced that break spatial sym- metry by forming 1D dimerized and 2D hexagonal minimal surface-based materials. Breaking of spatial symmetries produces topologically non-trivial interfaces that support the localization of vibrational modes and the robust propagation of elastic waves along pre-defined paths. These dynamic prop- erties are predicted through numerical simulations and are illustrated by performing vibration and wave propagation experiments on additively manu- factured samples. The introduction of symmetry-breaking topological inter- faces through parametrizations that modify the geometry of periodic minimal surfaces suggests a new strategy to supplement the load-bearing properties of this class of materials with novel dynamic functionalities.
Damage tolerance in additively manufactured ceramic architected materials
Abstract
Technical ceramics exhibit exceptional high-temperature properties, but unfortunately their extreme crack sensitivity and high melting point make it challenging to manufacture geometrically complex structures with sufficient strength and toughness. Emerging additive manufacturing technologies enable the fabrication of large- scale complex-shape artifacts with architected internal topology; when such topology can be arranged at the microscale, the defect population can be controlled, thus improving the strength of the material. Here, ceramic micro-architected materials are fabricated using direct ink writing (DIW) of an alumina nanoparticle-loaded ink, followed by sintering. After characterizing the rheology of the ink and extracting optimal processing parameters, the microstructure of the sintered structures is investigated to assess composition, density, grain size and defect population. Mechanical experiments reveal that woodpile architected materials with relative densities of 0.38–0.73 exhibit higher strength and damage tolerance than fully dense ceramics printed under identical conditions, an intriguing feature that can be attributed to topological toughening.
Alleviating expansion-induced mechanical degradation in lithium-ion battery silicon anodes via morphological design
Abstract
The mechanics of films undergoing volume expansion on curved substrates plays a key role in a variety of technologies including biomedical implants, thermal and environmental barrier coatings, and electrochemical energy storage systems. Silicon anodes for lithium-ion batteries are an especially challenging case because they can undergo volume variations up to 300% that results in cracking, delamination, and thus significant loss in performance. In this study, we use finite element analysis to model the volume expansion during lithiation for silicon coated on spinodal, inverse opal, gyroid, and Schwartz primitive nickel backbones and compare the distributions of maximum principal stress, strain energy density, and von Mises stress, which we use as indicators for propensity for cracking, delamination, and yielding, in order to explore the effect of backbone morphology on mechanical degradation during expansion. We show that, when compared to the inverse opal, the spinodal morphology reduces and uniformly distributes the maximum principal stress and strain energy density in the silicon layer, and delays the onset of expansion-induced yielding at all silicon layer thicknesses, which we ascribe to the unique interfacial curvature distribution of spinodal structures. This work highlights the importance of morphology on coatings undergoing volume variations and unveils the particular promise of spinodally derived materials for the design of next generation lithium-ion battery electrodes.
2021
Thickness-Dependent Microstructure in Additively Manufactured Stainless Steel
Abstract
Widespread industrial adoption of metal additive manufacturing (AM) requires an in-depth understanding of microstructural evolution during AM. In this study, the effect of process parameters and feature thickness on the microstructures of 316L stainless steel components fabricated by laser powder bed fusion (LPBF) was examined. A standard benchmark geometry developed by the National Institute of Standards and Technology, which contained walls of 0.5, 2.5 and 5.0 mm in thickness, was used. Optical microscopy, finite element analysis, scanning electron microscopy and electron backscatter diffraction revealed dramatic microstructural differences in features of different thickness within the same component. The feature thickness influenced the cooling rate, which in turn impacted the melt pool size, solidification microstructure, grain morphology and density of geometrically necessary dislocations. The relationship between feature size and grain morphology was dependent on the energy input used during LPBF. Such behavior suggested that local manipulation of LPBF process parameters can be employed to achieve microstructural homogeneity within the as-printed stainless steel components.
Tensegrity Metamaterials: Toward Failure-Resistant Engineering Systems through Delocalized Deformation
Abstract
Failure of materials and structures is inherently linked to localized mechanisms, from shear banding in metals, to crack propagation in ceramics and collapse of space-trusses after buckling of individual struts. In lightweight structures, localized deformation causes catastrophic failure, limiting their application to small strain regimes. To ensure robustness under real-world nonlinear loading scenarios, overdesigned linear-elastic constructions are adopted. Here, the concept of delocalized deformation as a pathway to failure-resistant structures and materials is introduced. Space-tileable tensegrity metamaterials achieving delocalized deformation via the discontinuity of their compression members are presented. Unprecedented failure resistance is shown, with up to 25-fold enhancement in deformability and orders of magnitude increased energy absorption capability without failure over same-strength state-of-the-art lattice architectures. This study provides important groundwork for design of superior engineering systems, from reusable impact protection systems to adaptive load-bearing structures.
A press release on this article is available here.
A link to a video emphasizing the main concept of this work can be found here.
Nanoscale investigation of two-photon polymerized microstructures with tip-enhanced Raman spectroscopy
Abstract
We demonstrate the use of tip-enhanced Raman spectroscopy (TERS) on polymeric microstructures fabricated by two-photon polymerization direct laser writing (TPP-DLW). Compared to the signal intensity obtained in confocal Raman microscopy, a linear enhancement of almost two times is measured when using TERS. Because the probing volume is much smaller in TERS than in confocal Raman microscopy, the effective signal enhancement is estimated to be ca. 10^4. We obtain chemical maps of TPP microstructures using TERS with relatively short acquisition times and with high spatial resolution as defined by the metallic tip apex radius of curvature. We take advantage of this high resolution to study the homogeneity of the polymer network in TPP microstructures printed in an acrylic-based resin. We find that the polymer degree of conversion varies by about 30% within a distance of only 100 nm. The combination of high resolution topographical and chemical data delivered by TERS provides an effective analytical tool for studying TPP-DLW materials in a non-destructive way.
Mechanically Compliant Thermal Interfaces Using Biporous Copper-Polydimethylsiloxane Interpenetrating Phase Composite
Abstract
Thermal interface materials are essential for thermal management in electronics packaging by providing a low resistance thermal pathway between heat sources and heat sinks. Nanostructured materials can be potential candidates for the next-generation interface materials by coupling their high thermal conductivity and mechanical compliance, suppressing failure even after large numbers of thermal cycles. This work investigates the thermal and mechanical characteristics of a new type of thermal interface materials, consisting of metal/elastomer interpenetrating phase composites. The 3D, highly porous copper scaffolds are fabricated via a fast and simple in situ bubble-templated electrodeposition process without the presence of solid templates; subsequently, the void fraction of the composite is filled by elastomer infiltration. The presence of elastomer matrix demonstrates limited impact on the thermal conductivity of the composite while it contributes substantially to the mechanical properties, providing the structural flexibility required. Thermal resistance values of 1.2–4.0 cm^2 K W^-1 are measured upon multiple thermal cycles, confirming the mechanical stability of the composite, without showing any noticeable degradation.
Mechanical performance of 3D printed interpenetrating phase composites with spinodal topologies
Abstract
The mechanical response of interpenetrating phase composites (IPCs) with stochastic spinodal topologies is investigated experimentally and numerically. Model polymeric systems are fabricated by Polyjet multi‐material printing, with the reinforcing phase taking the topology of a spinodal shell, and the remaining volume filled by a softer matrix. We show that spinodal shell IPCs have comparable compressive strength and stiffness to IPCs with two well‐established periodic reinforcements, the Schwarz P triply periodic minimal surface (TPMS) and the octet truss‐lattice, while exhibiting far less catastrophic failure and greater damage resistance, particularly at high volume fraction of reinforcing phase. The combination of high stiffness and strength and a long flat plateau after yielding makes spinodal shell IPCs a promising candidate for energy absorption and impact protection applications, where the lack of material softening upon large compressive strains can prevent sudden collapse. Importantly, in contrast with all IPCs with periodic reinforcements, spinodal shell IPCs are amenable to scalable manufacturing via self‐assembly techniques.
Architected implant designs for long bones: Advantages of minimal surface-based topologies
Abstract
Large bone fractures often require porous implants for complete healing. In this work, we numerically investigate the suitability of three topologically very different architected materials for long bone implants: the octet truss-based lattice, the Schwartz P minimal surface-based lattice and the spinodal stochastic surface-based lattice. Each implant topology (reinforcement) and its surrounding tissue (soft matrix) are modeled as a composite system via finite element analysis. Performance metrics are defined based on the Young’s modulus, the peak stress under service conditions, the interfacial surface area per unit volume and the relative bone growth rate (estimated based on the strain transferred to the soft matrix). We show that surface-based topologies are less prone to fatigue failure and may promote supe- rior bone growth than conventional truss-based designs. Spinodal surface-based architected materials have the best performance, and can be fabricated via self-assembly approaches followed by material con- version, potentially allowing scalable fabrication of implants with unit cell sizes at the micro-scale, thus dramatically amplifying surface area per unit volume and bone growth efficiency.
2020
Thermal post-curing as an efficient strategy to eliminate process parameter sensitivity in the mechanical properties of two-photon polymerized materials
Abstract
Two-photon polymerization direct laser writing (TPP-DLW) is one of the most versatile technologies to additively manufacture complex parts with nanoscale resolution. However, the wide range of mechanical properties that results from the chosen combination of multiple process parameters imposes an obstacle to its widespread use. Here we introduce a thermal post-curing route as an effective and simple method to increase the mechanical properties of acrylate-based TPP-DLW-derived parts by 20-250% and to largely eliminate the characteristic coupling of processing parameters, material properties and part functionality. We identify the underlying mechanism of the property enhancement as a self-initiated thermal curing reaction, which robustly facilitates the high property reproducibility that is essential for any application of TPP-DLW.
Surface oxide and hydroxide effects on aluminum microparticle impact bonding
Oxides, hydroxides, and other surface films act as impediments to metallurgical bonding during cold spray impact adhesion, raising the critical adhesion velocity and reducing the quality of the deposited coating. Using a single-particle impact imaging approach we study how altering the passivating surface oxides with exposures to various levels of heat and humidity affect the cold spray critical adhesion veloc- ity in the case of aluminum. We analyze the thickness, composition and microstructure of the passivation layers with transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Fourier- transform infrared spectroscopy (FTIR), and correlate our observations with a direct measurement of the critical adhesion velocity for each surface treatment. We conclude that exposures to temperatures as high as 300 °C for up to 240 min in dry air, or to room-temperature with humidity levels as high as 50% for 4 days, have negligible effect on the surface oxide layers, and by extension do not affect the critical adhesion velocity. In contrast, ambient-temperature exposure to 95% relative humidity levels for 4 days increases the critical adhesion velocity by more than 125 m/s, approximately a 14% percent increase. We observe that this distinct change in critical adhesion velocity is correlated with unique changes in the passivating layer thickness, thickness uniformity, crystallinity and composition resulting from exposure to high humidity. These results speak to particle surface treatments to improve the cold spray process.
Plate-nanolattices at the theoretical limit of stiffness and strength
Minisurf – A minimal surface generator for finite element modeling and additive manufacturing
Enhanced adhesion in two-photon polymerization direct laser writing
A versatile numerical approach for calculating the fracture toughness and R-curves of cellular materials
2019
Ultrahigh Energy Absorption Multifunctional Spinodal Nanoarchitectures
Nanolattices are promoted as next-generation multifunctional high-performance materials, but their mechanical response is limited to extreme strength yet brittleness, or extreme deformability but low strength and stiffness. Ideal impact protection systems require high-stress plateaus over long deformation ranges to maximize energy absorption. Here glassy carbon nanospinodals, i.e., nanoarchitectures with spinodal shell topology, combining ultrahigh energy absorption and exceptional strength and stiffness at low weight. Noncatastrophic deformation up to 80% strain, and energy absorption up to one order of magnitude higher than for other nano-, micro-, macro-architectures and solids, and state-of-the-art impact protection structures are shown. At the same time, the strength and stiffness are on par with the most advanced yet brittle nanolattices, demonstrating true multifunctionality. Finite element simulations show that optimized shell thickness-to-curvature-radius ratios suppress catastrophic failure by impeding propagation of dangerously oriented cracks. In contrast to most micro- and nano-architected materials, spinodal architectures may be easily manufacturable on an industrial scale, and may become the next generation of superior cellular materials for structural applications.
This article was reviewed in Science by Senior Editor Marc Lavine.
Thermal transport in hollow metallic microlattices
The mechanical response of cellular materials with spinodal topologies
Scalable synthesis of gyroid-inspired freestanding three-dimensional graphene architectures
Programmable Mechanical Properties of Two-Photon Polymerized Materials: From Nanowires to Bulk
Negative-Stiffness Inclusions as a Platform for Real-Time Tunable Phononic Metamaterials
Multiscale modeling and optimization of the mechanics of hierarchical metamaterialsoes Here
We present a survey of modeling techniques used to describe and predict architected cellular metamaterials, and to optimize their topology and geometry toward tailoring their mechanical properties such as stiffness, strength, fracture toughness, and energy absorption. Architectures of interest include truss-, plate-, and shell-based networks with and without periodicity, whose effective mechanical behavior is simulated by tools such as classical finite elements, further scale-bridging techniques such as homogenization and concurrent scale-coupling, and effective continuum descriptions of the underlying discrete networks. In addition to summarizing advances in applying the latter techniques to improve the properties of metamaterials and featuring prominent examples of structure–property relations achieved this way, we also present recently introduced techniques to improve the optimization process toward a full exploitation of the available design space, accounting for both linear and nonlinear material behavior.
Magnetoelastic Metamaterials for Energy Dissipation and Wave Filtering
A novel magnetoelastic mechanical metamaterial consisting of a hyperelastic 2D lattice incorporating permanent magnets is presented and characterized. When properly designed and fabricated, the metamaterial possesses two stable equilibrium configurations (henceforth referred to as hexagonal/hourglass and kagome), both stretching dominated (and hence stiff ). The two configurations have significantly different elastic properties and wave propagation characteristics, as shown numerically and experimentally. By switching between the two configurations via uniaxial loading cycles, the material displays hysteresis, thus dissipating substantial amounts of energy; in contrast with purely mechanical bistable structures (e.g., arches, hinged beams and buckled beams), the proposed magnetoelastic metamaterial does not require multiple unit cells in series or stiff boundary conditions to exhibit energy dissipation, thus enabling the implementation of compact stiff dampers. The presence of a bandgap in the kagome configuration (but not in the hexagonal/hourglass configuration) is attributed to an internal resonance mechanism and provides the foundation for the development of compact dynamic filters for mechanical signals.
Additive Manufacturing of Ductile, Ultrastrong Polymer-Derived Nanoceramics
Ceramics would be ideal engineering materials if their brittleness and scattered fracture strength could be overcome. While ductility and extraordinary strength have been reported at the nanoscale, they both rapidly disappear when samples reach micrometer dimensions; furthermore, manufacturing is limited to elaborate approaches, which are purely scientific in nature. Here, we present a robust route to additively manufacture ductile, ultrastrong silicon oxycarbide (SiOC) via two-photon polymerization direct laser writing (TPP-DLW) of a preceramic resin and subsequent pyrolysis. We 3D-print micrometer-size pillars and architected materials with feature sizes down to ∼200 nm and characterize them under uniaxial compression. Independent of size, SiOC micropillars consistently deform plastically with strains up to 25% and strengths >7 GPa, across the entire range of examined diameters (1–20 μm). Our findings demonstrate straightforward fabrication of ductile, ultrastrong ceramics at previously unprecedented scales, potentially enabling manufacturing of engineering systems up to tens of millimeters in size.
2018
The effect of manufacturing defects on compressive strength of ultralight hollow microlattices: A data-driven study
Hybrid Hollow Microlattices with Unique Combinations of Stiffness and Damping
LinkDamping of selectively bonded 3D woven lattice materials
ARCHITECTED MATERIALS: SYNTHESIS, CHARACTERIZATION, MODELING, AND OPTIMAL DESIGN
Architected materials are multi-phase and/or cellular materials in which the topological distribution of the phases is carefully controlled and optimized for specific functions or properties. Nearly two decades of research has resulted in the identification of a number of topologically simple, easy to fabricate, well established structures (including honeycombs and truss lattices), which have been optimized for specific stiffness and strength, impact and blast protection, sound absorption, wave dispersion, active cooling and combinations thereof.
Over the past few years, dramatic advances in processing techniques, including polymer-based templating (e.g., stereolithography, photopolymer waveguide prototyping, two-photon polymerization) and direct single- or multi-material formation (e.g., direct laser sintering, deformed metal lattices, 3D weaving and knitting), have enabled fabrication of new architected materials with complex geometry and remarkably precise control over the geometric arrangement of solid phases and voids from the nanometer to the centimeter scale.
The ordered, topologically complex nature of these materials and the degree of precision with which their features can now be defined suggests the development of new multi-physics and multi-scale modeling tools that can enable optimal designs. The result is efficient multi-scale cellular materials with unprecedented ranges of density, stiffness, strength, energy absorption, permeability, chemical reactivity, wave/matter interaction and other multifunctional properties, which promise dramatic advances across important technology areas such as lightweight structures, functional coatings, bio-scaffolds, catalyst supports, photonic/phononic systems and other applications.
Some of the most exciting recent developments in this field are the exploration of size effects in the development of nano-architected materials with superior combinations of properties, the investigation of geometrically complex unit cell architectures that enable non-linear effective mechanical response from linear-elastic materials, novel manufacturing approaches with increased resolution and scalability, and improved design optimization tools. Here we briefly review some recent progress in these areas, and conclude with some thoughts about opportunities for future development. The collection of articles in this focus issue is a wonderful exposure to some of the latest original work in this field.
2017
Topology optimization of multiphase architected materials for energy dissipation
In this article, we study the computational design of multiphase architected materials comprising a stiff phase, a dissipative phase, and void space, with enhanced vibration damping characteristics under wave propagation. We develop a topology optimization framework that maximizes a figure of merit comprising of effective stiffness, density and effective damping. We also propose novel material interpolation strategies to avoid the blending of different phases at any given point in the design domain. This is achieved by carefully defining different penalization schemes for different components of the merit function. The effective stiffness of the periodic multiphase material is calculated using homogenization theory and the Bloch–Floquet theorem is used to obtain its damping capacity, allowing for the investigation of the effect of wave directionality, material microarchitecture and intrinsic material properties on the wave attenuation characteristics. It is shown that the proposed topology optimization framework allows for systematic tailoring of microstructure of the multiphase materials for wide ranges of frequencies and densities and results in the identification of optimized multiphase cellular designs with void space that are superior to fully dense topologies.
Some Graphical Interpretations of Melan's Theorem for Shakedown Design
Optimal design of a cellular material encompassing negative stiffness elements for unique combinations of stiffness and elastic hysteresis
Nanolattices: An Emerging Class of Mechanical Metamaterials
Elastic Architected Materials with Extreme Damping Capacity
2016
Multistable Shape-Reconfigurable Architected Materials
Multistable shape-reconfigurable architected materials encompassing living hinges and enabling combinations of high strength, high volumetric change, and complex shape-morphing patterns are introduced. Analytical and numerical investigations, validated by experiments, are performed to characterize the mechanical behavior of the proposed materials. The proposed architected materials can be constructed from virtually any base material, at any length scale and dimensionality.
This article was featured in a Nature Research Highlight, Nature 535 (07/2016) 32
A Tri-Leaflet Nitinol Mesh Scaffold for Engineering Heart Valves
2015
Topology optimization of lightweight periodic lattices under simultaneous compressive and shear stiffness constraints
Push-to-pull tensile testing of ultra-strong nanoscale ceramic-polymer composites made by additive manufacturing
Novel insights from 3D models: the pivotal role of physical symmetry in epithelial organization
Macroscopic strain controlled ion current in an elastomeric microchannel
Incorporating Fabrication Cost into Topology Optimization of Discrete Structures and Lattices
2014
Glass-Blown Pyrex Resonator with Compensating Ti Coating for Reduction of TCF
Fabrication and Deformation of Metallic Glass Micro-Lattices
Energy Dissipation Mechanisms in Hollow Metallic Microlattices
Accurate Stiffness Measurement of Ultralight Hollow Metallic Microlattices by Laser Vibrometry
2013
The Effects of Tine Coupling and Geometrical Imperfections on the Response of DETF Resonators
Microlattices as Architected Thin Films: Analysis of Mechanical Properties and High Strain Elastic Recovery
Emergence of film thickness and grain size dependent elastic properties in nanocrystalline thin films
Compressive Strength of Hollow Microlattices: Experimental Characterization, Modeling and Optimal Design
2012
Ultra-high dynamic range resonant MEMS load cells for micromechanical test frames
Characterization of nickel-based microlattice materials with structural hierarchy from the nanometer to the millimeter scale
2011
Ultralight Metallic Microlattices
Protocol for the Optimal Design of Multifunctional Structures: From Hypersonics to Micro-Architected Materials
Mechanical Characterizations of Cast Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate)/Polyvinyl Alcohol thin films
The polymer Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate), hereafter referred to as PEDOT:PSS, has electrical properties superior to those of most conducting polymers, but it is too brittle to be employed in many applications. Blending PEDOT:PSS with other polymers is a promising route to reach a good trade-off between electrical and mechanical properties. This paper describes the mechanical characterization of PEDOT:PSS/PVA (Polyvinyl Alcohol) blends. The PEDOT:PSS/PVA films used in this study are produced by casting, and uniaxial tensile tests are performed to characterize the Young’s modulus, fracture strain, tensile strength, and plastic deformation behavior of the blends as a function of the weight fraction of the components. For pure PVA, the Young’s modulus, fracture strain and tensile strength are found to be, respectively, 41.3 MPa, 111% and 41.3 MPa. The strength exhibits a nearly perfect bimodal behavior, suddenly increasing by a factor 2 at a PEDOT:PSS content of 30%. Importantly, the ductility remains extremely high (∼94%, only 20% lower than pure PVA) up to PEDOT:PSS fractions of ∼50%. The Young’s modulus monotonically increases with PEDOT:PSS content, reaching 1.63 GPa at 50%. SEM imaging and XRD analysis allows correlation of these evolutions to substantial morphological changes in the PEDOT:PSS/PVA microstructure. When combined with a previously published electrical characterization study, the current work suggests that a PEDOT:PSS/PVA polymer blend with 30–40 wt% of PEDOT:PSS provides the best trade-off of conductivity and ductility. For non free-standing films, higher PEDOT:PSS fractions (70%) might be preferable.
Implications of Shakedown for Design of Actively-Cooled Thermostructural Panels
Concentration independent modulation of local micromechanics in a fibrin clot
Catastrophic vs. gradual collapse of thin-walled nanocrystalline Ni cylinders as building blocks of micro-lattice structures
Recent breakthroughs in manufacturing enable efficient fabrication of hierarchically architected microlattices, with dimensional control spanning seven orders of magnitude in length scale. These materials have the potential to exploit desirable nanoscale-size effects in a macroscopic structure, as long as their mechanical behavior at each appropriate scale nano, micro, and macro levels is properly understood. In this letter, we report the nanomechanical response of individual microlattice members. We show that hollow nanocrystalline Ni cylinders differing only in wall thicknesses, 500 and 150 nm, exhibit strikingly different collapse modes: the 500 nm sample collapses in a brittle manner, via a single strain burst, while the 150 nm sample shows a gradual collapse, via a series of small and discrete strain bursts. Further, compressive strength in 150 nm sample is 99.2% lower than predicted by shell buckling theory, likely due to localized buckling and fracture events observed during in situ compression experiments. We attribute this difference to the size-induced transition in deformation behavior, unique to nanoscale, and discuss it in the framework of “size effects” in crystalline strength.
2010
Pressure Induced Amorphization in Silicon Caused by the Impact of Electrosprayed Nanodroplets
MEMS resonant load cells for micro-mechanical test frames: Feasibility study and optimal design
Influence of Configuration on Materials Selection for Actively-Cooled Combustors
2009
Materials Property Profiles for Actively Cooled Panels: An Illustration for Scramjet Applications
Feasibility of metallic structural heat pipes as sharp leading edges for hypersonic vehicles
2008
Organic substrates for flip chip design: a thermo-mechanical model that accounts for heterogeneity and anisotropy
A materials selection protocol for lightweight actively cooled panels
2007
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2006
Structural performance of near-optimal sandwich panels with corrugated cores
Optimal active cooling performance of metallic sandwich panels with prismatic cores
2005
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