Kelvin Xie

Stereolithography has been used to create ceramic parts with complex geometry that is difficult to achieve with conventional fabrication techniques. This study used stereolithography to print silica honeycomb structures with a commercial Formlabs Form2 printer. The printed samples were sintered at different temperatures, and the print shape was retained up to 1300°C, but significant distortion from partial melting occurred at 1400°C. Higher sintering temperatures lead to more shrinkage, but it is non‐uniform among directions, with the open cell plane shrinking more than the dense plane of the sample. As expected, the density of samples also increases with the sintering temperature. At higher sintering temperatures, there is an increase in cristobalite and a decrease in quartz, tridymite, and amorphous silica. Regarding mechanical properties, the out‐of‐plane compressive strength is approximately one order of magnitude higher than the in‐plane compressive strength. When compressed along the out‐of‐plane direction, the samples sintered at lower temperatures surprisingly exhibit higher strength, which is explained by the micro‐cracking mechanism. As expected, the samples sintered at higher temperatures display higher strength when compressed along the in‐plane direction.

Deformation at high strain rates often results in high stresses on many engineering materials, potentially leading to catastrophic failure without proper design. High-strain-rate mechanical testing is thus needed to improve the design of future structural materials for a wide range of applications. Although several high-strain-rate mechanical testing techniques have been developed to provide a fundamental understanding of material responses and microstructural evolution under high-strain-rate deformation conditions, these tests are often very time consuming and costly. In this work, we utilize a high-strain-rate nanoindentation testing technique and system in combination with transmission electron microscopy to reveal the deformation mechanisms and dislocation substructures that evolve in pure metals from low (10 ^–2 s ^–1 ) to very high indentation strain rates (10 ⁴ s ^–1 ), using face-centered cubic aluminum and body-centered cubic molybdenum as model materials. The results help to establish the conditions under which micro- and macro-scale tests can be compared with validity and also provide a promising pathway that could lead to accelerated high-strain-rate testing at substantially reduced costs.

Precession electron diffraction (PED) is a powerful technique for revealing the crystallographic orientation of samples at the nanoscale. However, the quality of orientation indexing is strongly influenced by the quality of diffraction patterns. In this study, we have developed a novel algorithm called Auto-CLAHE (automatic contrast-limited adaptive histogram equalization), which automatically enhances low-intensity diffraction pattern signals using contrast-limited adaptive histogram equalization (CLAHE). The degree of enhancement is dynamically adjusted based on the overall intensity of the diffraction pattern, with greater enhancement applied to patterns with fewer spots (i.e., away from zone axes) and little or no enhancement applied to patterns with many spots (i.e., at a zone axis). By improving the visibility of low-intensity diffraction spots, Auto-CLAHE significantly improves the template matching between experimentally acquired and simulated diffraction patterns, leading to orientation maps with dramatically higher quality and lower noise. We anticipate that Auto-CLAHE provides an efficient and practical solution for preprocessing PED data, enabling higher-quality crystal orientation mapping to be routinely obtained.

During martensitic phase transformations in thin films, substrates impact hysteresis by introducing an additional interface, which can inhibit martensite/austenite interface motion. In order to reduce hysteresis, we examine 2.9–14.5 μm thick Ni–Mn–Sn films, which in some cases have been delaminated from the substrates before or after annealing. We compare thermal hysteresis and defect densities at the interface. Delaminating films prior to annealing decreases hysteresis, whereas delaminating films after annealing does not significantly impact hysteresis. Substrate effects are attributed to the thermal expansion mismatch between the film and substrate, resulting in the formation of dislocations at the interface and, consequentially, an increase in frictional resistance to martensite/austenite interface motion.

In this work, we developed three methods to map crystallographic variants of samples at the nanoscale by analyzing precession electron diffraction data using a high-temperature shape memory alloy and a VO2 thin film on sapphire as the model systems. The three methods are (I) a user-selecting-reference pattern approach, (II) an algorithm-selecting-reference-pattern approach, and (III) a k-means approach. In the first two approaches, Euclidean distance, Cosine, and Structural Similarity (SSIM) algorithms were assessed for the diffraction pattern similarity quantification. We demonstrated that the Euclidean distance and SSIM methods outperform the Cosine algorithm. We further revealed that the random noise in the diffraction data can dramatically affect similarity quantification. Denoising processes could improve the crystallographic mapping quality. With the three methods mentioned above, we were able to map the crystallographic variants in different materials systems, thus enabling fast variant number quantification and clear variant distribution visualization. The advantages and disadvantages of each approach are also discussed. We expect these methods to benefit researchers who work on martensitic materials, in which the variant information is critical to understand their properties and functionalities.

Printing defects are known to degrade the performance of additively manufactured (AM) alloys. Thus, a thorough understanding of their formation mechanisms and effects on the mechanical properties of AM materials is critically needed. Here, we take CoCrFeNi high-entropy alloy as a model material and print this alloy by laser powder bed fusion over a wide range of printing conditions. We reveal the processing windows for the formation of different printing defects including lack of fusion (LOF), keyhole, and solidification cracking. LOF and keyholes can be well correlated with insufficient and excessive laser energy density inputs, respectively. Of particular interest, we observe that solidification cracks only emerge at the medium laser energy density region, where the porosity is minimal yet the grain size and misorientation are relatively large. Such observation is rationalized within the framework of Rappaz–Drezet–Gremaud solidification theory. Among the above printing defects, solidification cracks in AM CoCrFeNi result in less degradation of mechanical properties compared with LOF and keyholes due to their different defect densities and resultant capabilities of coalescence. Our work provides fundamental insight into understanding the physical origins underlying the formation of printing defects and their impacts on the mechanical properties of AM metals and alloys.

Over the last two decades, many studies have contributed to improving our understanding of the brittle failure mechanisms of boron carbide and provided a road map for inhibiting the underlying mechanisms and improving the mechanical response of boron carbide. This paper provides a review of the design and processing approaches utilized to address the amorphization and transgranular fracture of boron carbide, which are mainly based on what we have found through 9 years of work in the field of boron carbides as armor ceramics.

Compositional analysis of boron carbide on nanometer length scales to examine or interpret atomic mechanisms, for example, solid‐state amorphization or grain‐boundary segregation, is challenging. This work reviews advancements in high‐resolution microanalysis to characterize multiple generations of boron carbide. First, ζ‐factor microanalysis will be introduced as a powerful (scanning) transmission electron microscopy ((S)TEM) analytical framework to accurately characterize boron carbide. Three case studies involving the application of ζ‐factor microanalysis will then be presented: (1) accurate stoichiometry determination of B‐doped boron carbide using ζ‐factor microanalysis and electron energy loss spectroscopy, (2) normalized quantification of silicon grain‐boundary segregation in Si‐doped boron carbide, and (3) calibration of a scanning electron microscope X‐ray energy‐dispersive spectroscopy (XEDS) system to measure compositional homogeneity differences of B/Si‐doped arc‐melted boron carbides in the as‐melted and annealed conditions. Overall, the improvement and application of advanced analytical tools have helped better understand processing–microstructure–property relationships and successfully manufacture high‐performance ceramics.

Boron carbide is an excellent armor material due to its light weight and ultrahigh hardness. However, high‐rate mechanical behavior can be degraded by stress‐induced amorphization. In this paper, we review the progressive advances in the understanding of amorphization in three successive generations of boron carbide: stoichiometric (undoped), B‐rich, and B/Si codoped boron carbides. For each generation of boron carbide, the crystal structure and microstructure are first discussed. Then, we outline the experimental observations of amorphization made by Raman spectroscopy and transmission electron microscopy. The susceptibility of amorphization in each generation of boron carbide will be compared and the fundamental mechanisms that explain the reduction in amorphization for B‐rich and B/Si codoped boron carbides elucidated. Comments on future research directions to further broaden and deepen the understanding of stress‐induced amorphization of boron carbide are also provided.

Twin boundary migration transitions from atomic shear to atomic shuffle at high stresses and strain rates.

Downscaling material structure into ever-decreasing levels for modern technological advancements demands small-scale mechanical characterizations of materials, such as nanoindentation. Thus, a comprehensive understanding of indentation size effect (ISE) has a far-reaching impact, but such understanding remains incomplete at the submicron scale despite a decades-long quest since the 1980s. Here, we explored the origins of this effect experimentally by linking dislocation behavior evolution with nanoindentation mechanical data at various depths. Importantly, the dislocation behavior is found to be strongly depth dependent and gives rise to subgrain formation progressively. Aided by subgrain boundaries, the ISE mechanism transitions from dislocation source starvation to dislocation interaction as indentation depth increases. The critical mechanism transition elucidates the distinctive ISE behavior at the submicron scale.

In this work, B₄C, B₄C + 5 at.% Al, B₄C + B, and B₄C + B + 5 at.% Al were arc melted, and the resultant solid products were characterized. Results from x‐ray diffraction and scanning electron microscopy showed that adding Al alone in B₄C did not result in Al doping; adding Al and B in B₄C led to Al doping. Al‐doping also changed the surface energy of boron carbide in the liquid state, thus altered the wettability. Transmission electron microscopy revealed that stacking faults are more likely to form in the Al‐doped sample, especially in the regions where the Al concentration is high.

Galvanostatic electrodeposition from Grignard reagents in symmetric Mg–Mg cells is used to map Mg morphologies from fractal aggregates of 2D nanoplatelets to highly anisotropic dendrites with singular growth fronts and entangled nanowire mats.

Si-doped boron carbide could be a promising material for the next-generation body armor.

Small amount of TiB₂(<5 wt%) was added into B₄C through a novel method that combines the use of sputter deposition and hot pressing. Sputter deposition provided more uniform dispersion of TiB₂grains with smaller grain sizes as compared to the conventional particulate mixing. Small amount TiB₂addition demonstrated to be an effective way for improving the fracture behavior and toughness of B₄C while not sacrificing its outstanding lightweight property to a large extent: 2.3 wt% TiB₂addition brought 15% improvement in indentation fracture toughness while resulting in less than 2% increase in density. The improvement can be attributed to the combination of crack impeding by TiB₂grains and crack deflection at the B₄C–TiB₂interfaces. TiB₂also played as grain growth inhibitor resulting in a slight increase (2%) in Vickers hardness. Another intention of employing sputter deposition was to modify the grain boundary of B₄C; however, neither formation of Ti‐containing phase nor Ti segregation has been observed at grain boundaries likely due to the poor wettability of B₄C.

The lifespan of most organic electronics, and many types of food, depend on the oxygen and moisture barrier of the packaging materials. Despite exhibiting excellent oxygen barrier and being relatively easy to produce, polyelectrolyte multilayer nanocoatings (PEM) show limited moisture barrier. SiO_x thin films can exhibit good moisture barrier, but rigidity that leads to cracking is a challenge for flexible packaging. In this work, layer‐by‐layer assembled PEM coatings are paired with inorganic SiO_x to form a three‐layer sandwich‐type moisture barrier. The barrier property of this organic/inorganic/organic structure is an order of magnitude better than any individual component or other combination thereof. This behavior is attributed to the underlying PEM layer smoothing the substrate surface and the top PEM layer eliminating defects in the underlying SiO_x. This study provides a new twist in the development of flexible films with exceptional moisture barrier.

Submicrometer boron carbide powders were synthesized using rapid carbothermal reduction (RCR) method. Synthesized boron carbide powders had smaller particle size, lower free carbon, and high density of twins compared to commercial samples. Powders were sintered using spark plasma sintering at different temperatures and dwell times to compare sintering behavior. Synthesized boron carbide powders reached >99% TD at lower temperature and shorter dwell times compared to commercial powders. Improved microhardness observed in the densified RCR samples was likely caused by the combination of higher purity, better stoichiometry control, finer grain size, and a higher density of twin boundaries.

The extraordinary hardness of boron compounds is related to their internal structure, which is comprised of 12-atom icosahedra arranged in crystalline lattices. In these hierarchical materials, the icosahedra are easy to image with EM, but individual atoms are not. Here, we show that laser-assisted atom probe tomography can be used to deduce the atomic structure and relative interatomic bond strengths of atoms in boron carbide. To our surprise, the icosahedra disintegrated during the field evaporation process. Statistical analyses of event multiplicity and stoichiometry in the atom probe dataset substantiate that the icosahedra are less tightly bound than their interconnecting chains. Comparisons with quantum mechanics simulations further suggest that this instability plays a role in the amorphization of boron carbide.

Detailed microstructural characterization was carried out on a commercial‐grade hot‐pressed boron carbide armor plate. The boron carbide grains have close to B₄C stoichiometry, and most of them have no planar defects. The most prominent second phase is intergranular graphite inclusions that are surrounded by multiple boron carbide grains. Submicrometer intragranular and intergranular BN and AlN precipitates were also observed. In addition, fine dispersions of AlN nanoprecipitates were observed in some but not all grains. No intergranular films were found. These microstructural characteristics are compared with the lab‐consolidated boron carbide and their effects on the mechanical properties of boron carbide are addressed.

Uniform densification of relatively thick (~7 mm) consolidated boron carbide plates at relatively low temperatures (e.g. 1800°C) and low facture toughness are two of the primary challenges for further development of boron carbide applications. This work reports that these two challenges can be overcome simultaneously by adding 5 wt% alumina as a sintering aid. Nearly fully dense (97%), fine grained boron carbide (B₄C) samples were produced using spark plasma sintering at 1700°C and above in the B₄C‐5 wt% Al₂O₃ system. The alumina and boron carbide matrix reacted to form an Al₅O₆BO₃ (a mullite‐like phase) during sintering. The Al₅O₆BO₃ phase facilitated uniform densification via liquid phase sintering. This secondary phase is dispersed throughout the intergranular pores, providing obstacles for crack propagation and resulting in tougher boron carbide ceramics.

Atom probe is a powerful technique for studying the composition of nano-precipitates, but their morphology within the reconstructed data is distorted due to the so-called local magnification effect. A new technique has been developed to mitigate this limitation by characterizing the distribution of the surrounding matrix atoms, rather than those contained within the nano-precipitates themselves. A comprehensive chemical analysis enables further information on size and chemistry to be obtained. The method enables new insight into the morphology and chemistry of niobium carbonitride nano-precipitates within ferrite for a series of Nb-microalloyed ultra-thin cast strip steels. The results are supported by complementary high-resolution transmission electron microscopy.

Here we review research in which Vickers hardness tests, optical microscopy, electron backscatter diffraction, and atom probe tomography were used to understand the strengthening effects that can be found with Nb in CASTRIP® steels during thermo-mechanical processing and ageing. Nb addition favours the grain refinement of ferrite by inhibiting the austenite recrystallization when hot rolled and provides a strong cluster-hardening effect during ageing.

Microsphere systems with the ideal properties for bone regeneration need to be bioactive, and at the same time possess the capacity for controlled protein/drug‐delivery; however, the current crop of microsphere system fails to fulfill these properties. The aim of this study was to develop a novel protein‐delivery system of bioactive mesoporous glass (MBG) microspheres by a biomimetic method through controlling the density of apatite on the surface of microspheres, for potential bone tissue regeneration. MBG microspheres were prepared by using the method of alginate cross‐linking with Ca²⁺ ions. The cellular bioactivity of MBG microspheres was evaluated by investigating the proliferation and attachment of bone marrow stromal cell (BMSC). The loading efficiency (LE) and release kinetics of bovine serum albumin (BSA) on MBG microspheres were investigated after coprecipitating with biomimetic apatite in simulated body fluids (SBF). The results showed that MBG microspheres supported BMSC attachment and the Si‐containing ionic products from MBG microspheres stimulated BMSCs proliferation. The density of apatite on MBG microspheres increased with the length of soaking time in SBF. BSA‐LE of MBG was significantly enhanced by coprecipitating with apatite. Furthermore, the LE and release kinetics of BSA could be controlled by controlling the density of apatite formed on MBG microspheres. Our results suggest that MBG microspheres are a promising protein‐delivery system as a filling material for bone defect healing and regeneration. © 2010 Wiley Periodicals, Inc. J Biomed Mater Res Part A, 2010.

This study investigates the effect of N diffusion on a Nb-microalloyed steel made by twin roll casting at 525o C in a KNO3 salt bath. Nitriding up to 4 h increases the yield strength of the steel by ~50% with only a small drop in ductility, while 6 hours of nitriding causes brittle fracture. The improved mechanical performance after 4 hours of nitriding is thought to be a combined effect of solid solution strengthening of N diffusion and dispersion strengthening from extremely fine Nb-rich precipitates. Coarse features along grain boundaries consistently observed in steel nitrided for 6 hours are considered to be responsible for brittle fracture in samples nitrided for longer.