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Synthesizing bulk nanocrystalline materials is challenging since grain growth should be suppressed whereas densification promoted. Here, we demonstrate a novel route to synthesize superhard bulk nanocrystalline cubic boron nitride (cBN), which combines the use of emulsion detonation and high-pressure high-temperature transformation-assisted consolidation. The emulsion detonation process activates BN to possess unique structure and chemistry, i.e. wurtzitic BN nanograins in hexagonal BN matrix with enhanced structural disordering and oxygen impurity, a combination that enhances the nucleation rate of cBN and its densification leading to the formation of bulk nanocrystalline cBN at reduced conditions. The cBN, synthesized at 7.5 GPa and 1800 °C, displayed Vickers hardness values of 50−62 GPa for 5−20 N loads. The findings in the study suggest a feasible solution to synthesize bulk nanocrystalline cBN in a more scalable way, while also providing design insights on how to refine grain growth while enhancing densification to synthesize bulk nanocrystalline materials.

Graphitic carbon materials are commonly used for storing Li ions owing to their outstanding electrochemical stability and electrical conductivity, and their 3D porous structures are promising for achieving high capacity anodes by depositing Li metal beyond lithiation. However, lithiophobicity and high conductivity of the graphitic surface engender dendrite formation on the outer surface of the electrode rather than inserting Li metal into the pores. Here, we grafted mossy MnO2 uniformly on the entire surface of carbon nanotubes (CNTs), concurrently providing lithiophilic and dendrite-less surfaces. Our MnO2-decorated CNTs can deliver an outstanding performance parameter, which considers both areal capacity and lifetime, over 10 000 mA h2 cm−2, which is the highest to the best of our knowledge, due to a super-long lifetime over 1800 hours for repeated Li plating/stripping at a high areal capacity of 6 mA h cm−2. The striking improvement can be attributed to low overpotential due to superior lithiophilicity and electrolyte wetting characteristics of MnO2, large surface areas of the mossy structures (low local current density), distributed Li insertion into MnO2/CNTs for suppressing dendrite formation, and porous CNT frameworks with high conductivity according to our electrochemical impedance spectroscopy and density functional theory calculation results. We anticipate that our results will give rise to subsequent research about mossy structure coatings on porous structures with various metal oxides and Li attracting groups for further improving the energy density of Li batteries.

The corrosion susceptibility of recrystallized and un-recrystallized grains in equal channel angular pressed (ECAPed) Mg–9Al–1Zn (AZ91) alloys immersed in chloride containing media was investigated through immersion testing and an electrochemical microcell technique coupled with high resolution techniques such as scanning Kelvin probe force microscopy (SKPFM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD). During ECAP, dynamic recrystallization (DRX) and strain-induced dynamic precipitation (SIDP) simultaneously occurred, resulting in a bimodal grain structure of original elongated coarse grains and newly formed equiaxed fine grains with a large volume fraction of β-Mg17Al12 precipitates. Corrosion preferentially initiates and propagates in the DRXed grains, owing to the greater microchemistry difference between the β-Mg17Al12 precipitates formed at the DRXed grain boundaries and the adjacent α-Mg matrix, which induces a strong microgalvanic coupling between these phases. Additionally, the weaker basal texture of the DRXed grains also makes these grains more susceptible to electrochemical reactions than the highly textured un-DRXed grains. The influence of dynamic recrystallization and dynamic precipitation was also studied in ECAPed alloys with different levels of deformation strain through corrosion and electrochemical techniques. Increasing the strain level led to a more uniform corrosion with a shallow penetration depth, lower corrosion rate values, and higher protective ability of the oxide film. Furthermore, higher levels of strain resulted in greater hardness values of the ECAPed alloys. The superior corrosion resistance and strength of the ECAPed alloys with increasing strain level was attributed to the combination of smaller DRXed grain size, higher DRX ratio, and higher volume fraction of uniformly distributed fine β-Mg17Al12 precipitates.

We investigated the indentation response of Ta/Ti nanocomposite thin films with widely differing morphologies of the constituent phases, ranging from particulate to bicontinuously interpenetrating, but comparable characteristic dimensions of microstructure features. We find no influence of microstructure morphology on stiffness or hardness. However, we see a systematic dependence of indentation pileup height on microstructure morphology, with the largest pileups observed in composites with connected, Ta-rich networks and lowest for highly connected, Ti-rich networks. We attribute this dependence to an influence of microstructure morphology on strain hardening rates and propose mechanisms to explain it.

Inconel alloy 718 is a high-strength and corrosion resistant alloy that is commonly used as a beamline vacuum window. The accumulation of irradiation-induced damage substantially decreases the window’s service lifetime, and replacing it engenders significant beamline downtime. With this application in mind, herein we examine whether post-irradiation annealing can alleviate irradiation-induced damage of Inconel alloy 718. Inconel alloy 718 was received in a solution annealed state. We then irradiated samples using two different modalities (1.5 MeV H+ and 5 MeV Ni2+) at three representative temperatures for beamline windows (room temperature, 100 °C, and 200 °C), followed by annealing at temperatures viable for in-situ annealing processes (no anneal, 300 °C, and 500 °C). Using nanoindentation, we determined that irradiation-induced hardening occurs but is largely mitigated by post-irradiation annealing. Overall, our results suggest that in-situ annealing of radiation damage in Inconel alloy 718 vacuum windows appears feasible, which could potentially decrease beam downtime and maintenance costs.

Shear banding is an important deformation and failure mechanism in metallic systems, especially at high-rate straining. Dynamic recrystallization was often reported to account for the refined microstructure of shear bands but rarely confirmed using direct quantitative measurement. Here, we employ quantitative precession electron diffraction analysis to uncover shear band microstructure in pure titanium. The results reveal that the microstructure is dominated by early stages of grain subdivision process. Dynamic recrystallization is not as prevalent as perceived conventionally. Our results offer key insights into understanding shear banding and highlight the need for quantitative analyses of shear band microstructure. A critical quantitative assessment of shear band microstructures is made using precession beam diffraction imaging. Specifically, grain subdivision, as opposed to dynamic recrystallization, drives the shear band microstructure evolution.

The size, density, and distribution of helium (He) bubbles in a Ti-Ta single-phase alloy and a Ti/Ta dual-phase nanocomposite have been investigated using transmission electron microscopy. The Ti/T...

In this work, we statistically studied the activation and suppression of 〈c + a〉 dislocations in a textured Mg–3Al–1Zn alloy after compression. Our results revealed that the activation and suppression of 〈c + a〉 dislocations in polycrystals are primarily governed by the overall texture, regardless of the crystallographic orientation of each grain. This observation is explained by the requirement of five independent slip systems, reduced anisotropy, and twin-induced strain localization. These direct observations of 〈c + a〉 dislocation activities offer a key insight into understanding the deformation mechanism of polycrystalline Mg alloys.

dislocations in pure Mg have been reported to dissociate on basal planes, resulting in a sessile configuration that limits the ductility and formability of Mg. In this study, careful tilting experiments and weak-beam dark-field electron microscopy observations elucidated that dislocations in the commercial alloy AZ31 remain compact without apparent dissociation. The stabilization of the dislocation core structure with Al and Zn alloying may explain the improved strain to failure in the AZ31 alloy as compared to pure Mg samples.

The orientation dependence of tensile deformation in Fe40Mn40Co10Cr10 high entropy alloy (HEA) was investigated in [111], [001] and [123] oriented single crystals. Transmission electron microscopy investigations revealed three major mechanisms controlling the deformation stages, depending on the orientation: (i) deformation twinning, (ii) planar slip and (iii) dislocation wall/network formation. While twinning and planar slip were strongly orientation dependent, dislocation walls were observed in all orientations. Twinning was the dominant deformation mode in [111] crystals, while only multi-slip was observed in [001]. Both twins and planar slip were activated in [123] crystals. [111] crystals exhibited the highest strain hardening coefficients and ultimate tensile strength due to the strong twin-twin and twin-slip interactions where twin boundaries reduce the mean free path of dislocations, leading to dynamic Hall–Petch hardening. The decent ductility levels (∼45%) were attained in [111] due to nanoscale internal twins and tertiary twin system forming at the later stages of deformation and suppressing necking. In contrast, no twins or stacking faults were observed in [001] crystals, which is consistent with the Copley–Kear effect. [123] crystals had outstanding tensile ductility (∼65%), due to the activation of planar slip and twinning. Overall, in this off-stoichiometric HEA, we have determined the stacking faculty energy and critical resolved shear stresses for both twinning and slip, and demonstrated the formation of high dislocation density walls and wavy slip in [001], while the hardening stages of [123] and [111] are primarily governed by planar slip and twinning, which can be rationalized by the Copley–Kear effect.

The origin of the indentation size effect has been extensively researched over the last three decades, following the establishment of nanoindentation as a broadly used small-scale mechanical testing technique that enables hardness measurements at submicrometer scales. However, a mechanistic understanding of the indentation size effect based on direct experimental observations at the dislocation level remains limited due to difficulties in observing and quantifying the dislocation structures that form underneath indents using conventional microscopy techniques. Here, we employ precession electron beam diffraction microscopy to “look beneath the surface,” revealing the dislocation characteristics (e.g., distribution and total length) as a function of indentation depth for a single crystal of nickel. At smaller depths, individual dislocation lines can be resolved, and the dislocation distribution is quite diffuse. The indentation size effect deviates from the Nix–Gao model and is controlled by dislocation source starvation, as the dislocations are very mobile and glide away from the indented zone, leaving behind a relatively low dislocation density in the plastically deformed volume. At larger depths, dislocations become highly entangled and self-arrange to form subgrain boundaries. In this depth range, the Nix–Gao model provides a rational description because the entanglements and subgrain boundaries effectively confine dislocation movement to a small hemispherical volume beneath the contact impression, leading to dislocation interaction hardening. The work highlights the critical role of dislocation structural development in the small-scale mechanistic transition in indentation size effect and its importance in understanding the plastic deformation of materials at the submicron scale.

Here, an experimental study utilizing in-situ scanning electron microscopy (SEM) micro-compression testing and post-mortem transmission electron microscopy (TEM) imaging is presented to quantify the effect of temperature on the transition in deformation modes in twin-oriented Mg single crystals. Single crystal micropillars were fabricated using FIB milling, then tested by in-situ SEM micro-compression from 20 °C to 225 °C. It is observed that plasticity in the deformed Mg microcrystals at temperatures at and below 100 °C is governed by { 10 1 ¯ 2 } extension twinning. However, an anomalous increase of the flow stresses is observed at 100 °C, which is likely due to paucity of dislocation sources that are required to promote twin boundary migration. At 150 °C and above, extension twinning is suppressed and a continuous plastic flow and strain softening induced by prismatic dislocation mediated plasticity is observed. By comparing the current results with those from bulk scale studies for other hexagonal-closed-pack single crystals (e.g. titanium (Ti) and zirconium (Zr)), a general trend for the effect of temperature on the transition in deformation modes in HCP materials is proposed.

B4C-SiC composites were fabricated via the preceramic polymer (PCP) route combined with pressure-assisted sintering. Fully dense bodies were achieved by controlling surface oxide on B4C powder and pyrolysis conditions for PCP coated powder. We elucidate i) the microstructure and phase developments observed in the process of fabricating dense B4C-PCP derived SiC composites and ii) the mechanical properties and crack deflection behavior of dense bodies. The incorporation of PCP derived SiC to B4C decreases hardness due to the lower hardness value of SiC compared to B4C and the residual carbon accompanied by SiC formation. Instead, the PCP derived SiC improved indentation fracture toughness. The main toughening mechanism supposed is a combination of crack impeding by SiC grains and crack deflection within SiC grains, likely due to the presence of subgrains or layered microstructure in the PCP derived SiC grains.

Hexagonal boron nitride (h-BN) is a well-known ceramic that has wide application areas ranging from electronics to metallurgy. However, highly ordered h-BN is conventionally synthesized at high temperatures above 1800 °C. In this work, we investigated the formation of BN from boric acid (H3BO3)-ammonium chloride (NH4Cl) mixture in the sodium chloride (NaCl)-potassium chloride (KCl) eutectic salt. We report the synthesis of highly ordered and nanostructured h-BN at 1000 °C using molten salt synthesis. The effect of starting composition, synthesis temperature, and dwell time on BN formation and its structural ordering were systematically investigated. It is concluded that the molten salt plays important roles in the formation of BN and its structural ordering, which is achieved by i) decomposing the boron (B)-nitrogen (N) bearing reactants that lead to the formation of BN layers, and ii) increasing the mobility of BN layers formed. Furthermore, we propose a possible reaction mechanism that governs the BN formation from the reactant mixture in molten salts and explain the observations based on thermodynamic and kinetic considerations.

We have investigated the effect of synthesis conditions of BCNO compounds on BN formation and its structural ordering. BCNO compounds were synthesized from boric acid (H3BO3)-melamine (C3H6N6) mixtures with various molar ratios (H3BO3:C3H6N6 = 1:1 to 6:1) at 400 °C. The BCNO compounds were then subjected to post heating between 1400 °C and 1800 °C under argon flow. The final products were crystalline BN with hexagonal symmetry exhibiting varying degrees of structural ordering. As the H3BO3 to C3H6N6 ratio in the starting composition is increased, BN formation temperature decreases and the structural ordering of BN increases. Post heating at higher temperature also promotes BN formation and structural ordering. The findings indicate that synthesis and structural ordering of BN can be tailored by controlling starting composition and post heating temperature. These observations may provide a guideline to synthesize high or low-density polymorphs of BN at reduced pressure, temperature, or both.

Emulsion detonation synthesis (EDS) is a recently developed process to synthesize nano-sized ceramics based on detonation of two water-in-oil emulsions. The process produces high pressure and temperature along with fast cooling, thus providing ideal environment for metastable phase formation. Here we applied the process for the first time on hexagonal boron nitride (h-BN). Characterization studies demonstrated the formation of metastable explosion BN phase (e-BN) with grain sizes of 10–20 nm embedded in h-BN matrix. These findings support the potential use of EDS a novel and promising pathway for the synthesis of e-BN and other metastable BN phases.

Advanced metallic alloys are attractive in microelectromechanical systems (MEMS) applications that require high density, electrical and thermal conductivity, strength, and dimensional stability. Here we report the mechanical behavior of direct current (DC) magnetron sputter deposited Nickel (Ni)-Molybdenum (Mo)-Tungsten (W) films annealed at various temperatures. The films deposit as single-phase nanotwinned solid solutions and possess ultra-high tensile strengths of approximately 3 GPa, but negligible ductility. Subsequent heat treatments resulted in grain growth and nucleation of Mo-rich precipitates. While films annealed at 600 °C or 800 °C for 1 h still showed brittle behavior, films annealed at 1,000 °C for 1 h were found to exhibit strength greater than 1.2 GPa and near 10% tensile ductility. In addition to the excellent mechanical properties, alloy films further exhibit remarkably improved dimensional stability – a lower coefficient of thermal expansion and greater microstructural stability. An excellent balance between mechanical properties and dimensional stability make sputter deposited Ni-Mo-W alloys promising structural materials for MEMS applications.

Fully dense boron carbide discs were achieved by spark plasma sintering boron carbide powders with 10 wt% silicon. The silicon did not diffuse into boron carbide grains to produce a solid solution of Si-doped boron carbide; instead the silicon reacted with impurities in the starting powder to form β-SiC and borosilicate glass. The resultant new phases facilitated densification of the multiphase ceramic through liquid phase-assisted sintering. The resultant material exhibits improved hardness (34.3 GPa Vikers hardness under 1 kg load) with toughness comparable to both Si-free and commercially available boron carbide.

The starting alloy microstructure can be tailored to achieve varying degrees of grain refinement and enhance mechanical properties through severe plastic shear deformation during solid-phase processing. Crystal plasticity-based grain misorientation modeling, coupled with systematic pin-on-disk tribometry-based subsurface shear deformation experiments on as-cast Al-xSi alloys (x = 0, 1, 4 at%), was conducted. The post-deformation microstructural analysis, through a combined computational and experimental approach, conclusively shows that the initial volume fraction of the hard Si phase enhances the evolution of local lattice misorientation, leading to efficient grain refinement during severe plastic shear deformation. The shear-deformation–induced nanostructure resulted in more than double the nanoindentation hardness in the processed alloy.

A newly developed Mg–2Zn-0.3Ca-0.2Ce-0.1Mn (in weight %, ZXEM2000) wrought alloy exhibits an exceptionally high tensile ductility (29% ± 1.4% tensile elongation) along the rolling direction at room temperature. In this work, we investigated the fundamental plastic deformation mechanisms that can contribute to the high ductility of ZXEM2000. Scanning electron microscopy and transmission electron microscopy were carried out at selected strain levels to characterize the microstructural evolution. Compared with the deformed microstructures from the hot-rolled pure Mg and Mg–3Al–1Zn alloy at similar conditions, our results revealed that the high ductility of ZXEM2000 could be attributed to four key factors: weaker texture, fine grain size, reduced twinning, and increased cross-slip frequency of dislocation. These direct observations offer a key insight into improving ductility of magnesium alloys for future alloy design and development.

Boron carbide has a wide range of solubility, but the effects of stoichiometry on its microstructure and mechanical response are not well understood. In this study, detailed microstructural characterization was carried out on three hot-pressed B-rich boron carbide samples. Lattice parameter measurements from XRD identified the compositions to be B 4.2 C, B 5.6 C and B 7.6 C. Local substitutional disorder was observed by Raman spectroscopy, particularly for more B-rich samples. Electron energy loss spectroscopy observations suggest that excess boron preferentially substitutes for carbon atoms in the B 11 C icosahedra; after which additional boron modifies the CBC chains. Moreover, the boron content has salient effects on boron carbide densification and microstructure. Improved densification was observed in the more B-rich samples (B 5.6 C and B 7.6 C), and there is a transition from few or no intragranular planar defects (B 4.2 C), to numerous stacking faults (B 5.6 C), to copious twins (B 7.6 C). Nanoindentation experiments revealed that the highest value for B 4.2 C is statistically larger than that for B 5.6 C or B 7.6 C, suggesting that the hardness of boron carbide is reduced by boron substitution.

Magnesium holds considerable promise as a lightweight structural material, but structural applications of Mg alloys are generally limited to cast components because of its limited formability. Mg exhibits high hardening and low strain to failure, the attendant mechanisms of which remain elusive. In this work, we performed detailed TEM investigations on the dislocation structures in the c-axis compressed Mg single crystals. Systematic tilting experiments in TEM revealed the majority of 〈c + +a〉 dislocations to be dissociated and basal bound, suggesting that glissile pyramidal 〈c + +a〉 dislocations may transition and dissociate into a sessile configuration. Other 〈c + +a〉 dislocations were observed to decompose into individual 〈a〉 and 〈c〉 dislocations. Since 〈a〉 and 〈c〉 dislocations cannot accommodate c-axis compression, this process also manifests a glissile-to-sessile transition. Moreover, numerous sessile nanoscale dislocation loops were observed to form during plastic deformation, which can obstruct the movement of the mobile 〈c + +a〉 dislocations. These three mechanisms all impede the motion of glissile 〈c + +a〉 dislocations. Comparing these experimental with simulations that have been published in the literature is used to glean insight on the characteristics of 〈c + +a〉 dislocations and to explain the high hardening and low strain to failure that are widely observed in Mg.

It is well-established that Mg-3Al-1Zn (AZ31) alloy exhibits much better tensile ductility than pure Mg. However, the underlying mechanisms for such difference still remain relatively unexplored at the dislocation level. In this work, we deformed hot-rolled highly-textured pure Mg and AZ31 samples with similar initial microstructures (i.e., grain size and texture) under tension along the rolling direction. Apparent shear banding was only observed in the pure Mg samples from the early stage of the deformation. Shear bands could act as easy paths for crack propagation, leading to low ductility in pure Mg. Apparent shear banding was not noted in deformed AZ31 samples. We then investigated the deformation mechanisms at the dislocation level using transmission electron microscopy. Systematic tilting experiments and statistical analyses of multiple grains at different strain levels revealed a significant disparity of non-basal dislocation activities between pure Mg and AZ31. For pure Mg, dislocations were activated since the early stage of plastic deformation. For AZ31, dislocations were mostly absent at all strain levels, even in the strain-to-failure samples. Non-basal dislocations, including prismatic and pyramidal dislocations, were observed. The promotion of the non-basal dislocation activities and the suppression of dislocations in AZ31 are expected to offer more sustainable hardening, which could elucidate the absence of apparent shear banding and much-improved ductility in AZ31 compared to pure Mg.

We use precession electron diffraction to investigate the crystallographic character of copper (Cu)-tungsten (W) nanocomposites fabricated via physical vapor co-deposition at 400 °C. We observe sub-micron-scale regions, where apparently disconnected Cu and W grains have near-identical crystallographic orientations. This persistence of grain orientations suggests Cu and W grains within these regions are interconnected in 3-D when they first form and may be considered as intercalated, sub-micron-scale “super-grains”. Indeed, atom probe tomography provides direct evidence of 3-D interconnectivity of W domains. Our findings shed light on the structure and self-organization mechanisms of nanocomposites formed by spontaneous phase separation of co-deposited metals.

Precession electron diffraction is used to characterize the interaction between { 10 1 ¯ 2 } tensile twins and basal plate-like precipitates in a post-deformed, precipitate-dispersed Mg-9Al micropillar. We observed a heterogeneous distribution of precipitates in the micropillar sample, which enabled the study of the different stages involved in twin-precipitate interactions. We show that twin nucleation was promoted, taking place on the surface, as well as from the interior of the micropillar. Twin tip propagation and twin growth were hindered by the precipitates. Twin tips either were arrested by precipitates and new twins formed on the other side of the precipitate, or continued to grow around the precipitate without the re-nucleation events. During twin thickening, the precipitates did not significantly alter the shear-dominant twin boundary migration mechanism, as evidenced by the relatively flat twin boundaries around the partially embedded precipitates. However, the twin boundary migration was retarded by the precipitates, especially in regions confined by closely-spaced precipitates.

Sputter deposited nickel-molybdenum-tungsten (Ni-Mo-W) thin films possess a beneficial suite of properties that stem from the extremely fine growth twins that form during the deposition process. Previously these materials were only characterized in tension, however, in this study in situ micropillar compression and post-mortem microstructural analysis of nanotwinned Ni84Mo11W5 micropillars were employed to measure the compressive response and elucidate the attendant deformation mechanisms. The pillars exhibit Hookean behavior to compressive strengths of 3-3.5 GPa and the onset of non-linear plastic flow was manifest by discrete strain bursts and highly localized shear bands. Plastic deformation was concentrated at the top of the pillar, while the bulk of the micropillar was nominally unaffected. Post-mortem investigations indicate that at sufficiently high stresses shear banding is triggered, resulting in intense and highly localized plastic deformation that led to the formation of twin-free nanocrystalline grains within highly deformed shear bands. By contrast, the regions adjacent to the shear bands were unaffected. The absence of detwinning and dislocation glide mechanisms was unexpected and in direct contrast to what has been observed in nanotwinned Cu-Al. Post-mortem observations of the Ni-Mo-W micropillars suggest that the ultrafine twins create a unique form of dislocation starvation and source-limited plasticity. The ultrahigh compressive strength is governed by the triggering of shear bands rather than the activation of dislocation glide. The specialized nature of plasticity in nanotwinned Ni-Mo-W is clear, even though the precise trigger for shear band formation in nanotwinned Ni-Mo-W remains to be identified.

It has been documented that the corrosion behavior of AZ91 (Mg–9Al–1Zn, wt.%) alloy is strongly influenced by the morphology, distribution, and volume fraction of secondary phases, however, the proposed corrosion mechanisms are questioned due to new experimental findings. The present study focused on providing a more detailed understanding of the corrosion mechanism of a peak-aged AZ91 alloy exposed to a chloride environment by using scanning Kelvin probe force microscopy, immersion testing, optical microscopy, scanning and transmission electron microscopy techniques and electrochemical microcell technique. It was found that corrosion was simultaneously initiated in the interior of α-Mg grains and in the local α-Mg phase within the α + β lamellar precipitate; furthermore, the lamellar precipitate/α-Mg matrix boundary acted as a barrier for corrosion propagation. The proposed corrosion initiation mechanism is opposed to previous investigations reporting the development of micro-galvanic coupling between the β-Mg17Al12 phase and the adjacent α-Mg matrix, which cannot be supported by our experimental findings. The corrosion propagation in the peak-aged alloy was dominated by the preferential anodic dissolution at the interior of the α-Mg matrix due to its faster corrosion reaction kinetics compared to the micro-galvanic corrosion process between the β-phase and the local α-Mg phase occurring inside the lamellar precipitate.

The well-documented formation of amorphous bands in boron carbide (B4C) under contact loading has been identified in the literature as one of the possible mechanisms for its catastrophic failure. To mitigate amorphization, Si-doping was suggested by an earlier computational work, which was further substantiated by an experimental study. However, there have been discrepancies between theoretical and experimental studies, about Si replacing atom/s in B12 icosahedra or the C-B-C chain. Dense single phase Si-doped boron carbide was produced through a conventional scalable route. A powder mixture of SiB6, B4C, and amorphous boron was reactively sintered, yielding a dense single phase Si-doped boron carbide material. A combined analysis of Rietveld refinement on XRD pattern coupled with electron density difference Fourier maps and DFT simulations were performed in order to investigate the location of Si atoms in the boron carbide lattice. Si atoms occupy an interstitial position, between the icosahedra and the chain. These Si atoms are bonded to the chain end C atoms, which result in a kinked chain. Additionally, these Si atoms are also bonded to the neighboring equatorial B atom of the icosahedra, which is already bonded to the C atom of the chain, forming a bridge like structure. Owing to this bonding, Si is anticipated to stabilize the icosahedra through electron donation, which is expected to help in mitigating stress-induced amorphization. Possible supercell structures are suggested along with the most plausible structure for Si-doped boron carbide.

We demonstrate the growth of dendritic magnesium deposits with fractal morphologies exhibiting shear moduli in excess of values for polymeric separators upon the galvanostatic electrodeposition of metallic Mg from Grignard reagents in symmetric Mg–Mg cells. Dendritic growth is understood on the basis of the competing influences of reaction rate, electrolyte transport rate, and self-diffusion barrier.

Sputter deposited nanotwinned metal alloy films that possess exceptional properties attractive for next generation MEMS devices., Silicon-based microelectromechanical systems (MEMS) sensors have become ubiquitous in consumer-based products, but realization of an interconnected network of MEMS devices that allows components to be remotely monitored and controlled, a concept often described as the “Internet of Things,” will require a suite of MEMS materials and properties that are not currently available. We report on the synthesis of metallic nickel-molybdenum-tungsten films with direct current sputter deposition, which results in fully dense crystallographically textured films that are filled with nanotwins. These films exhibit linear elastic mechanical behavior and tensile strengths exceeding 3 GPa, which is unprecedented for materials that are compatible with wafer-level device fabrication processes. The ultrahigh strength is attributed to a combination of solid solution strengthening and the presence of dense nanotwins. These films also have excellent thermal and mechanical stability, high density, and electrical properties that are attractive for next-generation metal MEMS applications.

Our Letter reported high-resolution transmission electron microscopy on commercial quality boron showing that ∼2=3 of the grains exhibit smooth microstructure, leading to an x-ray diffraction pattern of well-known beta boron [1]. The other 1=3 grains exhibit a uniform zigzag pattern that extends across the entire grain and exhibits a very regular twinlike symmetry on every other lattice plane. This second phase gives diffraction patterns that are different from beta.

The theoretical strength of a material is the minimum stress to deform or fracture the perfect single crystal material that has no defects. This theoretical strength is considered as an upper bound on the attainable strength for a real crystal. In contradiction to this expectation, we use quantum mechanics (QM) simulations to show that for the boron carbide (B4C) hard ceramic, this theoretical shear strength can be exceeded by 11% by imposing nano-scale twins. We also predict from QM that the indentation strength of nano-twinned B4C is 12% higher than that of the perfect crystal. Further we validate this effect experimentally, showing that nano-twinned samples are harder by 2.3% than the twin-free counterpart of B4C. The origin of this strengthening mechanism is suppression of twin boundary (TB) slip within the nano-twins due to the directional nature of covalent bonds at the TB.

Elemental boron exhibits many polymorphs in nature based mostly on an icosahedral shell motif, involving stabilization of 13 strong multicenter intraicosahedral bonds. It is commonly accepted that the most thermodynamic stable structure of elemental boron at atmospheric pressure is the β rhombohedral boron (β−B). Surprisingly, using high-resolution transmission electron microscopy, we found that pure boron powder contains grains of two different types, the previously identified β−B containing a number of randomly spaced twins and what appears to be a fully transformed twinlike structure. This fully transformed structure, denoted here as τ−B, is based on the Cmcm orthorhombic space group. Quantum mechanics predicts that the newly identified τ−B structure is 13.8 meV/B more stable than β−B. The τ−B structure allows 6% more charge transfer from B_(57) units to nearby B_(12) units, making the net charge 6% closer to the ideal expected from Wade’s rules. Thus, we predict the τ−B structure to be the ground state structure for elemental boron at atmospheric pressure.