Showing papers in "Journal of Materials Research in 2018"

Development and exploration of refractory high entropy alloys—A review

Abstract: Open literature publications, in the period from 2010 to the end of January 2018, on refractory high entropy alloys (RHEAs) and refractory complex concentrated alloys (RCCAs) are reviewed. While RHEAs, by original definition, are alloys consisting of five or more principal elements with the concentration of each of these elements between 5 and 35 at.%, RCCAs can contain three or more principal elements and the element concentration can be greater than 35%. The 151 reported RHEAs/RCCAs are analyzed based on their composition, processing methods, microstructures, and phases. Mechanical properties, strengthening and deformation mechanisms, oxidation, and corrosion behavior, as well as tribology, of RHEA/RCCAs are summarized. Unique properties of some of these alloys make them promising candidates for high temperature applications beyond Ni-based superalloys and/or conventional refractory alloys. Methods of development and exploration, future directions of research and development, and potential applications of RHEAs are discussed.

High-entropy functional materials

Abstract: While most papers on high-entropy alloys (HEAs) focus on the microstructure and mechanical properties for structural materials applications, there has been growing interest in developing high-entropy functional materials The objective of this paper is to provide a brief, timely review on select functional properties of HEAs, including soft magnetic, magnetocaloric, physical, thermoelectric, superconducting, and hydrogen storage Comparisons of functional properties between HEAs and conventional low- and medium-entropy materials are provided, and examples are illustrated using computational modeling and tuning the composition of existing functional materials through substitutional or interstitial mixing Extending the concept of high configurational entropy to a wide range of materials such as intermetallics, ceramics, and semiconductors through the isostructural design approach is discussed Perspectives are offered in designing future high-performance functional materials utilizing the high-entropy concepts and high-throughput predictive computational modeling

Model interatomic potentials and lattice strain in a high-entropy alloy

Abstract: A set of embedded atom method model interatomic potentials is presented to represent a high-entropy alloy with five components. The set is developed to resemble but not model precisely face-centered cubic (fcc) near-equiatomic mixtures of Fe–Ni–Cr–Co–Cu. The individual components have atomic sizes deviating up to 3%. With the heats of mixing of all binary equiatomic random fcc mixtures being less than 0.7 kJ/mol and the corresponding value for the quinary being −0.0002 kJ/mol, the potentials predict the random equiatomic fcc quinary mixture to be stable with respect to phase separation or ordering and with respect to bcc and hcp random mixtures. The details of lattice distortion, strain, and stress states in this phase are reported. The standard deviation in the individual nearest neighbor bond lengths was found to be in the range of 2%. Most importantly, individual atoms in the alloy were found to be under atomic strains up to 0.5%, corresponding to individual atomic stresses up to several GPa.

Thermal reduction of graphene oxide: How temperature influences purity

Abstract: Among various methods used for the reduction of graphene oxide (GO) into a purer form of graphene, the thermal reduction method provides a simpler, safer, and economic alternative, compared to other techniques. Thermal reduction of GO causes significant weight loss and volume expansion of the material. Current work investigates the onset temperature where reduction in terms of exfoliation takes place, which is determined to be 325 °C at standard atmospheric pressure. Reduction temperature plays the most crucial role as it controls the quality of reduced graphene oxide in terms of weight percentage of carbon and lattice defect. The study leads to achieving highest content with a minimum defect in the graphene lattice at the optimum temperature, which is found to be 350 °C at standard atmospheric pressure. The thermal reduction process has been analyzed with the help of Fourier transform infrared spectroscopy, thermogravimetric analysis, and thermal degradation kinetics. From thermal degradation kinetics of GO, the rate of reaction has been found to be independent of concentration and is a sole function of temperature.

Breakthrough applications of high-entropy materials

Abstract: The concept of high-entropy alloys has been extended to ceramics, polymers, and composites. “High-entropy materials (HEMs)” are named to cover all these materials. Recently, HEMs has become a new emerging field through the collective efforts of many researchers. Basically, high mixing entropy can enhance the formation of solution-type phases for alloys, ceramics, and composites at high temperatures, and in general leads to simpler microstructure. Large degrees of freedom in composition design as well as process design have been found to provide a wide range of microstructure and properties for applications. There are many opportunities for HEMs to overcome the bottlenecks of conventional materials. In this article, several possible breakthrough applications are pointed out and emphasized for turbine blades, thermal spray bond coatings, high-temperature molds and dies, sintered carbides for cutting tools, hard coatings for cutting tools, hardfacings, and radiation-damage resistant materials. In addition, more possible breakthrough examples are briefly described.

Orbital-free density functional theory for materials research

Abstract: Orbital-free density functional theory (OFDFT) is both grounded in quantum physics and suitable for direct simulation of thousands of atoms. This article describes the application of OFDFT for materials research over roughly the past two decades, highlighting computational studies that would have been impractical (or impossible) to perform with other techniques. In particular, we review the growing body of simulations of solids and liquids that have been conducted with planewave-pseudopotential (or related) techniques. We also provide an updated account of the fundamentals of OFDFT, emphasizing aspects—such as nonlocal density functionals for computing the kinetic energy of noninteracting electrons—that enabled much of the application work. The article concludes with a discussion of the OFDFT frontier, which contains brief descriptions of other topics at the forefront of OFDFT research.

Systematic characterization of 3D-printed PCL/β-TCP scaffolds for biomedical devices and bone tissue engineering: influence of composition and porosity.

Abstract: This work aims at providing guidance through systematic experimental characterization for the design of 3D-printed scaffolds for potential orthopedic applications, focusing on fused deposition modeling with a composite of clinically available polycaprolactone (PCL) and β-tricalcium phosphate (β-TCP). First, we studied the effect of the chemical composition (0–60% β-TCP/PCL) on the scaffold’s properties. We showed that surface roughness and contact angle were, respectively, proportional and inversely proportional to the amount of β-TCP and that degradation rate increased with the amount of ceramic. Biologically, the addition of β-TCP enhanced proliferation and osteogenic differentiation of C3H10. Second, we systematically investigated the effect of the composition and the porosity on the 3D-printed scaffold mechanical properties. Both an increasing amount of β-TCP and a decreasing porosity augmented the apparent Young’s modulus of the 3D-printed scaffolds. Third, as a proof of concept, a novel multimaterial biomimetic implant was designed and fabricated for potential disc replacement.

Modeling the structure and thermodynamics of high-entropy alloys

Abstract: High-entropy and multiprincipal element alloys present exciting opportunities and challenges for computational modeling of their structure and phase stability. Recent interest has catalyzed rapid development of techniques and equally rapid growth of new results. This review surveys the essential concepts of thermodynamics and total energy calculation, and the bridge between them provided by statistical mechanics. Specifically, we review the electronic density functional theory of alloy total energy as applied to supercells and special quasirandom structures. We contrast these with the coherent potential approximation and semi-empirical approximations. Statistical mechanical approaches include cluster expansions, hybrid Monte Carlo/molecular dynamics simulations, and extraction of entropy from correlation functions. We also compare first-principles approaches with Calculation of Phase Diagrams (CALPHAD) and highlight the need to augment experimental databases with first-principles derived data. Numerous example applications are given highlighting recent progress utilizing the concepts and methods that are introduced.

Combinatorial metallurgical synthesis and processing of high-entropy alloys

Abstract: High-entropy alloys (HEAs) with multiple principal elements open up a practically infinite space for designing novel materials Probing this huge material universe requires the use of combinatorial and high-throughput synthesis and processing methods Here, we present and discuss four different combinatorial experimental methods that have been used to accelerate the development of novel HEAs, namely, rapid alloy prototyping, diffusion-multiples, laser additive manufacturing, and combinatorial co-deposition of thin-film materials libraries While the first three approaches are bulk methods which allow for downstream processing and microstructure adaptation, the latter technique is a thin-film method capable of efficiently synthesizing wider ranges of composition and using high-throughput measurement techniques to characterize their structure and properties Additional coupling of these high-throughput experimental methodologies with theoretical guidance regarding specific target features such as phase (meta)stability allows for effective screening of novel HEAs with beneficial property profiles

L12-strengthened high-entropy alloys for advanced structural applications

Abstract: Advanced alloys with both high strength and ductility are highly desirable for a wide range of engineering applications. Conventional alloy design strategies based on the single-principle element are approaching their limits in further optimization of their performances. Precipitation-hardened high-entropy alloys (HEAs), especially those strengthened by coherent L12-nanoparticles, have received considerable interest in recent years, enabling a new space for the development of advanced structural materials with superior mechanical properties. In this review, we highlight recent important advances of the newly developed L12-strengthened HEAs, including the aspects of computation-aided alloy design, unique properties, atomic-level characterization, phase evolution, and stability. In particular, we focus our attention on elucidating fundamental scientific issues involving the alloying effects, precipitation behaviors, mechanical performances, and the corresponding deformation mechanisms, all of which provide a comprehensive metallurgical understanding and guidance for the design of this new class of HEAs. Finally, future research directions and prospects are also critically assessed.

The effect of architecture on the mechanical properties of cellular structures based on the IWP minimal surface

Abstract: Architected materials are materials engineered to utilize their topological aspects to enhance the related physical and mechanical properties. With the witnessed progressive advancements in fabrication techniques, obstacles and challenges experienced in manufacturing geometrically complex architected materials are mitigated. Different strut-based architected lattice structures have been investigated for their topology-property relationship. However, the focus on lattice design has recently shifted toward structures with mathematically defined architectures. In this work, we investigate the architecture-property relationship associated with the possible configurations of employing the mathematically attained Schoen’s I-WP (IWP) minimal surface to create lattice structures. Results of mechanical testing showed that sheet-based IWP lattice structures exhibit a stretching-dominated behavior with the highest structural efficiency as compared to other forms of strut-based and skeletal-based lattice structures. This study presents experimental and computational evidence of the robustness and suitability of sheet-based IWP structures for different engineering applications, where strong and lightweight materials with exceptional energy absorption capabilities are required.

Lattice distortions in high-entropy alloys

Abstract: One of the founding concepts of the high-entropy alloy (HEA) field was that the lattice structures of multicomponent solid solution phases are highly distorted. The displacement of the constituent atoms, away from their ideal locations (local lattice strain), has been widely cited as the reason for a number of the observed physical and mechanical properties. However, very little data directly characterizing these lattice distortions exist and, thus, the validity of this hypothesis remains an open question. Here, the concept is reviewed by considering the underlying principles of the lattice distortions, the suitability of different assessment methods, and the direct experimental data currently available. It is found that, at present, there is no clear evidence that the lattice distortions in HEAs are significantly greater than those of conventional alloys. However, so few alloys have been appropriately characterized that this conclusion cannot be considered overarching and further research is required.

Wear behavior of Al0.6CoCrFeNi high-entropy alloys: Effect of environments

Abstract: Environment can impact the wear behavior of metals and alloys substantially. The tribological properties of Al0.6CoCrFeNi high-entropy alloys (HEAs) were investigated in ambient air, deionized water, simulated acid rain, and simulated seawater conditions at frequencies of 2–5 Hz. The as-cast alloy was composed of simple face-centered cubic and body-centered cubic phases. The wear rate of the as-cast HEA in the ambient air condition was significantly higher than that in the liquid environment. The wear resistance in seawater was superior to that in ambient air, deionized water, and acid rain. Both the friction coefficient and wear rate in seawater were the lowest due to the formation of oxidation film, lubrication, and corrosion action in solution. The dominant wear mechanism in the ambient air condition and deionized water was abrasive wear, delamination wear, and oxidative wear. By contrast, the wear mechanism in acid rain and seawater was mainly corrosion wear, adhesive wear, abrasive wear, and oxidative wear.

Phase stability as a function of temperature in a refractory high-entropy alloy

Abstract: Refractory high-entropy alloys (RHEAs) have recently attracted much attention, primarily due to their mechanical properties at elevated temperatures. However, the equilibrium phase-stability of these alloy systems is not well established. The present investigation focuses on the phase stability of Al0.5NbTa0.8Ti1.5V0.2Zr RHEA at temperatures ranging from 600 to 1200 °C. The detailed phase characterization involves coupling of scanning electron microscopy, transmission electron microscopy, and atom probe tomography. The stable phases present at these temperatures are (i) 1200 °C—body-centered cubic (BCC) matrix with nano-B2 precipitates; (ii) 1000 °C and 800 °C—a BCC matrix phase with Al–Zr rich hexagonal closed packed intermetallic precipitates and, (iii) 600 °C—a BCC + B2 microstructure, comprising a continuous BCC matrix with discrete B2 precipitates. These results highlight the substantial changes in phase stability as a function of temperature in RHEAs, and high-entropy alloys in general, and also the importance of accounting for these changes especially while designing alloys for high temperature applications.

Metastability in high-entropy alloys: A review

Abstract: Classical alloy design strategies often aim to benefit from metastability Examples are numerous: metastable transformation- and twinning-induced plasticity steels, cobalt or titanium based alloys, age hardenable aluminum alloys, and severe plastic deformed nanostructured copper In each of these cases, superior engineering property combinations are achieved by exploring limits of stability For the case of high-entropy alloys (HEAs), on the other hand, majority of present research efforts focus on exploring compositions that would yield stable single-phase structures HEA metastability and its effects on microstructure and property development constitute only a relatively small fraction of ongoing work To help motivate and guide a corresponding shift in HEA research efforts, here in this paper, we provide an overview of the research activities on metastability in HEAs To this end, we categorize the past research on the topic into two groups based on their focus, namely, compositional and structural stability, and discuss the most relevant and exciting findings

Adsorption and photocatalytic properties of NiO nanoparticles synthesized via a thermal decomposition process

Abstract: NiO nanoparticles (NPs) were synthesized at different annealing temperatures via a thermal decomposition process and characterized using X-ray diffraction, scanning electron microscopy, and UV-vis spectroscopy. The NiO NPs prepared at higher annealing temperature (400 °C) were shown excellent adsorption and photocatalytic activity toward textile dyes reactive black 5 (RB-5) and methylene blue (MB). About 87.2% of RB-5 in 60 min and 70.2% of MB in 5 h was removed using NiO NPs synthesized at 400 °C. The photocatalytic degradation of MB was found to increase with an increase in the annealing temperature of the catalyst. Moreover, the kinetic study revealed that the adsorption and photocatalytic activity of NiO NPs followed the second and first-order kinetics, respectively. The enhanced performance of NiO NPs toward dye removal might be related to its optical and structural properties.

A superfine eutectic microstructure and the mechanical properties of CoCrFeNiMox high-entropy alloys

Abstract: A series of CoCrFeNiMox (x = 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2) high-entropy alloys were designed to develop a eutectic high-entropy alloy system and to acquire a superfine eutectic structure. The results show that for the CoCrFeNiMox alloys, with the increase of Mo content from 0.2 to 1.2, the microstructures shift from a typical dendrite structure to a hypoeutectic microstructure (x = 0.6), and then to a fully eutectic microstructure (x = 0.8) with a lamellar spacing only 110 nm, and finally culminate in the hypereutectic structure (x = 1.0, x = 1.2). The XRD results show that CoCrFeNiMox alloys have a single FCC phase when x is 0.2 or 0.4. When Mo content is over 0.6, it begins to separate Cr9Mo21Ni20 intermetallic compounds. The hardness of the CoCrFeNiMox alloys is increasing significantly from 172.8 to 763.7 HV with the increase of Mo content. Meanwhile, the fracture strength increased but the ductility decreases. Among these alloys, the CoCrFeNiMo0.6 alloy shows excellent integrated mechanical properties of compressive fracture strength and strain, which are 2051 Mpa and 23%, respectively.

Additive Manufacturing and size-dependent mechanical properties of three-dimensional microarchitected, high-temperature ceramic metamaterials

Abstract: 3D microarchitected metamaterials exhibit unique, desirable properties influenced by their small length scales and architected layout, unachievable by their solid counterparts and random cellular configurations. However, few of them can be used in high-temperature applications, which could benefit significantly from their ultra-lightweight, ultrastiff properties. Existing high-temperature ceramic materials are often heavy and difficult to process into complex, microscale features. Inspired by this limitation, we fabricated polymer-derived ceramic metamaterials with controlled solid strut size varying from 10-µm scale to a few millimeters with relative densities ranging from as low as 1 to 22%. We found that these high-temperature architected ceramics of identical 3D topologies exhibit size-dependent strength influenced by both strut diameter and strut length. Weibull theory is utilized to map this dependency with varying single strut volumes. These observations demonstrate the structural benefits of increasing feature resolution in additive manufacturing of ceramic materials. Through capitalizing upon the reduction of unit strut volumes within the architecture, high-temperature ceramics could achieve high specific strength with only fraction of the weight of their solid counterparts.

Structure–property relationships for 3D-printed PEEK intervertebral lumbar cages produced using fused filament fabrication

Abstract: Recent advances in the additive manufacturing technology now enable fused filament fabrication of polyetheretherketone (PEEK). A standardized lumbar fusion cage design was 3D printed with different speeds of the printhead nozzle to investigate whether 3D-printed PEEK cages exhibit sufficient material properties for lumbar fusion applications. It was observed that the compressive and shear strength of the 3D-printed cages were 63–71% of the machined cages, whereas the torsion strength was 92%. The printing speed is an important printing parameter for 3D-printed PEEK, which resulted in up to 20% porosity at the highest speed of 3000 mm/min, leading to reduced cage strength. Printing speeds below 1500 mm/min can be chosen as the optimal printing speed for this printer to reduce the printing time while maintaining strength. The crystallinity of printed PEEK did not differ significantly from the as-machined PEEK cages from extruded rods, indicating that the processing provides similar microstructure.

Enhanced photocatalytic activity of direct Z-scheme Bi2O3/g-C3N4 composites via facile one-step fabrication

Abstract: Coupling oxidation type semiconductors is a feasible strategy to improve the photocatalytic activity of reduction type g-C3N4 photocatalysts. In this work, Bi2O3 was used as an oxidation type semiconductor to construct direct Z-scheme Bi2O3/g-C3N4 photocatalysts by a one-step calcination method. The obtained Bi2O3/g-C3N4 composites exhibited excellent photocatalytic activity and stability toward methylene blue degradation under visible light irradiation. The composite with 1% weight content of Bi2O3 to g-C3N4 exhibited the highest photocatalytic activity with an apparent rate constant of 0.063 min−1, which was 3.0 and 3.7 times higher than that of pure Bi2O3 and g-C3N4, respectively. The enhanced photocatalytic activity of the Bi2O3/g-C3N4 composite was mainly attributed to the improved charge separation efficiency and stronger redox ability. This work gave a new insight in developing g-C3N4-based Z-scheme heterojunction photocatalysts with enhanced photocatalytic activity.

Black titania/graphene oxide nanocomposite films with excellent photothermal property for solar steam generation

Abstract: Solar steam generation is an efficient and green technology for desalination and drinking water purification, however, impeded by high cost, low efficiency, and complicated process. Black titania is expected to exhibit excellent solar steam performance due to its outstanding light absorption properties, chemical stability, low cost, and innocuity. Herein, we design a high absorbing and efficient solar steam generation system based on a black titania/graphene oxide nanocomposite film affixed to airlaid paper wrapped over the surface of expandable polyethylene foam; the system possesses several important criteria required for the ideal solar steam generator: wide-spectrum absorption, adequate water supply, reduced heat loss for localized water heating, and porous structure for steam flow. Remarkably, we realized a solar thermal conversion efficiency of 69.1% under illumination of 1 kW/m2 without solar concentration, and the device delivered remarkable cycle stability.

Additive manufacturing-enabled shape transformations via FFF 4D printing

Abstract: Fused-filament-fabrication (FFF) is a commonly used and commercially successful additive-manufacturing method for thermoplastics. Depending on the FFF process parameters, the internal-strains along print direction, thermal-gradient across layers, and anisotropy introduced during layer-by-layer build-up can significantly affect the macroscopic properties, dimensional stability, and structural performance of the final part. Conversely, these factors can be optimized to result in unique, controllable thermally actuated shape-transformations. This work aims at quantifying and understanding the underlying mechanisms that drive the thermally actuated shape-transformation in three commonly used thermoplastics fabricated by the FFF method namely, poly-lactic-acid (PLA), high-impact-polystyrene (HIPS), and acrylonitrile-butadiene-styrene (ABS). Initially, the release of internal-strains is analyzed for unidirectionally printed samples experimentally and computationally, employing a thermoviscoelastic-viscoplastic constitutive model. Subsequently, two basic initial (as-printed) configurations, namely, a beam and a circular-disc are chosen to study the 1D to 2D and 2D to 3D shape-transformations, respectively. The effect of process parameters such as the printing speed, print path, and infill density on the shape transformation behavior is investigated systematically. Finally, the results are applied to demonstrate shape-transformations for application in morphing-structures and/or as an alternative, simplified process in fabricating curved-components.

Molecular sieve membranes for N2/CH4 separation

Abstract: Natural gas consumption has grown from 5.0 trillion cubic feet (TCF) in 1949 to 27.0 TCF in 2014 and is expected to be ∼31.6 TCF in 2040. This large demand requires an effective technology to purify natural gas. Nitrogen is a significant impurity in natural gas and has to be removed since it decreases the natural gas energy content. The benchmark technology to remove nitrogen from natural gas is cryogenic distillation, which is costly and energy intensive. Membrane technology could play a key role in making this separation less energy intensive and therefore economically feasible. Molecular sieve membranes are ideal candidates to remove natural gas impurities because of their exceptional size-exclusion properties, high thermal and chemical resistance. In this review, the state of the art of molecular sieve membranes for N2/CH4 separation, separation mechanisms involved, and future directions of these emerging membranes for natural gas purification are critically discussed.

Nanoindentation of high-purity vapor deposited lithium films: A mechanistic rationalization of diffusion-mediated flow

Abstract: Nanoindentation experiments performed in 5 and 18 μm thick vapor deposited polycrystalline lithium films at 31 °C reveal the mean pressure lithium can support is strongly dependent on length scale and strain rate. At the smallest length scales (indentation depths of 40 nm), the mean pressure lithium can support increases from ∼23 to 175 MPa as the indentation strain rate increases from 0.195 to 1.364 s−1. Furthermore, these pressures are ∼46–350 times higher than the nominal yield strength of bulk polycrystalline lithium. The length scale and strain rate dependent hardness is rationalized using slightly modified forms of the Nabarro–Herring and Harper–Dorn creep mechanisms. Load-displacement curves suggest a stress and length-scale dependent transition from diffusion to dislocation-mediated flow. Collectively, these experimental observations shed significant new light on the mechanical behavior of lithium at the length scale of defects existing at the lithium/solid electrolyte interface.

Lead-free piezoelectric materials and composites for high power density energy harvesting

Abstract: In the emerging era of Internet of Things (IoT), power sources for wireless sensor nodes in conjunction with efficient and secure wireless data transfer are required. Energy harvesting technologies are promising solution toward meeting the requirements for sustainable power sources for the IoT. In this review, we focus on approaches for harvesting stray vibrations and magnetic field due to their abundance in the environment. Piezoelectric materials and piezoelectric–magnetostrictive [magnetoelectric (ME)] composites can be used to harvest vibration and magnetic field, respectively. Currently, such harvesters use modified lead zirconate titanate (or lead-based) piezoelectric materials and ME composites. However, environmental concerns and government regulations require the development of a suitable lead-free replacement for lead-based piezoelectric materials. In the past decade, several lead-free piezoelectric compositions have been developed and demonstrated with promising piezoelectric response. This paper reviews the significant results reported on lead-free piezoelectric materials with respect to high-density energy harvesting, covering novel processing techniques for improving the piezoelectric response and temperature stability. The review of the state-of-the-art studies on vibration and magnetic field harvesting is provided and the results are used to discuss various strategies for designing high-performance energy harvesting devices.

Tracer diffusion in single crystalline CoCrFeNi and CoCrFeMnNi high entropy alloys

Abstract: High entropy alloys are multicomponent alloys, which consist of five or more elements in equiatomic or nearly equiatomic concentrations. These materials are hypothesized to show significantly decreased self-diffusivities. For the first time, diffusion of all constituent elements in equiatomic CoCrFeNi and CoCrFeMnNi single crystals and additionally solute diffusion of Mn in the quaternary alloy is investigated using the radiotracer technique, thereby the tracer diffusion coefficients of 57Co, 51Cr, 59Fe, 54Mn, and 63Ni are determined at a temperature of 1373 K. The components are characterized by significantly different diffusion rates, with Mn being the fastest element and Ni and Co being the slowest ones. Furthermore, solute diffusion of Cu in the CoCrFeNi single crystal is investigated in the temperature range of 973–1173 K using the 64Cu isotope. In the quaternary alloy, Cu is found to be a fast diffuser at the moderate temperatures below 1273 K and its diffusion rate follows the Arrhenius law with an activation enthalpy of about 149 kJ/mol.

3D printing of poly(ε-caprolactone)/poly(D,L-lactide-co-glycolide)/hydroxyapatite composite constructs for bone tissue engineering

Abstract: Three-dimensional (3D) printing technology is a promising method for bone tissue engineering applications. For enhanced bone regeneration, it is important to have printable ink materials with appealing properties such as construct interconnectivity, mechanical strength, controlled degradation rates, and the presence of bioactive materials. In this respect, we develop a composite ink composed of polycaprolactone (PCL), poly(D,L-lactide-co-glycolide) (PLGA), and hydroxyapatite particles (HAps) and 3D print it into porous constructs. In vitro study revealed that composite constructs had higher mechanical properties, surface roughness, quicker degradation profile, and cellular behaviors compared to PCL counterparts. Furthermore, in vivo results showed that 3D-printed composite constructs had a positive influence on bone regeneration due to the presence of newly formed mineralized bone tissue and blood vessel formation. Therefore, 3D printable ink made of PCL/PLGA/HAp can be a highly useful material for 3D printing of bone tissue constructs.

Design and mechanical properties of elastically isotropic trusses

Abstract: The present article addresses design of stiff, elastically isotropic trusses and their mechanical properties. Isotropic trusses are created by combining two or more elementary cubic trusses in appropriate proportions and with their respective nodes lying on a common space lattice. Two isotropic binary compound trusses and many isotropic ternary trusses are identified, all with Young’s moduli equal to the maximal possible value for isotropic strut-based structures. In finite-sized trusses, strain elevations are obtained in struts near the external free boundaries: a consequence of reduced nodal connectivity and thus reduced constraint on strut deformation and rotation. Although the boundary effects persist over distances of only about two unit cell lengths and have minimal effect on elastic properties, their manifestations in failure are more nuanced, especially when failure occurs by modes other than buckling (yielding or fracture). Exhaustive analyses are performed to glean insights into the mechanics of failure of such trusses.

Microstructure evolution and thermal properties of an additively manufactured, solution treatable AlSi10Mg part

Abstract: Because of rapid solidification involved in the laser or e-beam based additive manufacturing (AM) process, solution treatable metallic parts made by these methods usually possess a unique nonequilibrium microstructure which changes significantly during subsequent thermal treatment. Such evolution alters the size, morphology, length scale, and distribution of microstructural features and has a substantial impact on thermal properties and possibly on electrical properties as well. This study focuses on effects of microstructural evolution on thermal properties of an additively manufactured AlSi10Mg part. The changes of thermal properties such as thermal expansion, heat capacity, thermal diffusivity, and thermal conductivity as a function of thermal treatment are reported. The results show that the formation of supersaturated primary α aluminum and unique cellular structure imparted by fast solidification in the AM process are the major cause for the low thermal diffusivity and low thermal conductivity observed in this solution treatable, as-built part. A correlation between microstructural evolution and changes in thermal properties is established. Advantages and tailoring of the thermal properties of additively built parts are discussed. Implications of these results are important for other additively manufactured components based on popular solution treatable alloys.

Enhanced strength and ductility of a tungsten-doped CoCrNi medium-entropy alloy

Abstract: Developing metallic materials with a good combination of strength and ductility has been an unending pursuit of materials scientists. The emergence of high/medium-entropy alloys (HEA/MEA) provided a novel strategy to achieve this. Here, we further strengthened a strong-and-ductile MEA using a traditional solid solution strengthening theory. The selection of solute elements was assisted by mechanical property and microstructure predictive models. Extensive microstructural characterizations and mechanical tests were performed to verify the models and to understand the mechanical behavior and deformation mechanisms of the designated CoCrNi–3W alloy. Our results show good experiment-model agreement. The incorporation of 3 at.% W into the ternary CoCrNi matrix increased its intrinsic strength by ∼20%. External strengthening through microstructural refinement led to a yield strength nearly double that of the parent alloy, CoCrNi. The increase in strength is obtained with still good ductility when tested down to 77 K. Nanoscale twin boundaries are observed in the post-fracture microstructure under 77 K. The combination of strength and ductility after W additions deviate from the traditional strength-ductility-trade-off contour.