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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.

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.

The utilization of metallic anodes holds promise for unlocking high gravimetric and volumetric energy densities and is pivotal to the adoption of ‘beyond Li’ battery chemistries. Much of the promise of magnesium batteries stems from claims regarding their lower predilection for dendrite growth. Whilst considerable effort has been invested in the design of novel electrolytes and cathodes, detailed studies of Mg plating are scarce. Using galvanostatic electrodeposition of metallic Mg from Grignard reagents in symmetric Mg–Mg cells, we establish a phase map characterized by disparate morphologies spanning the range from fractal aggregates of 2D nanoplatelets to highly anisotropic dendrites with singular growth fronts and nanowires entangled in the form of mats. The effects of electrolyte concentration, applied current density, and coordinating ligands have been explored. The study demonstrates a complex range of electrodeposited morphologies including canonical dendrites with shear moduli conducive to penetration through typical polymeric separators. We further demonstrate a strategy for mitigating Mg dendrite formation based on the addition of molecular Lewis bases that promote nanowire growth through selective surface coordination.

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.

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.

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.

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.

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.

TEM observations have provided insight into the processing and microstructural evolution of a commercial hot-pressed boron carbide. Fine dispersions of nano-scale AlN precipitates and individual submicron AlN precipitates were observed in a modest fraction of the grains. The nano-precipitates were found to be coherent with a well-defined crystallographic relationship to the matrix. The chemistry, size and distribution of both types of precipitates and the coherency of the nano-precipitates indicate that both intragranular homogeneous and heterogeneous precipitation occurred during cooling.

The mechanical behavior of extruded pure magnesium was studied experimentally under high strain rate ( 10 3 s - 1 ) compression loading in the extrusion direction. Electron back scattered diffraction was used to examine the changes in the texture and transmission electron microscopy was used to investigate the dislocation structures in the material. Extensive grain reorientation due to extension twinning is observed. Dislocation activity is observed inside the parent region ( 〈 a 〉 slip) as well as the twinned region ( 〈 c + a 〉 slip). The high degree of strain hardening observed is postulated to be due to the texture hardening associated with extension twinning coupled with significant increase in the dislocation density with strain. In compression along the extrusion direction, extension twinning and dislocation activity are both needed to accommodate plastic deformation.

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.

Cluster-strengthened Nb-microalloyed strip cast steels are of interest as clustering during aging leads to an enhancement in strength without compromising ductility, resulting in desirable mechanical properties. However, the precise strengthening mechanism is not well understood. Using in situ heating transmission electron microscopy, clustering was found to impede the movement of dislocations during aging. The attractive combination of ductility and strength was attributed to the effects of recovery and the restricted movement of dislocations through clustering.

High strength, low Young's modulus and good biocompatibility are desirable but difficult to simultaneously achieve in metallic implant materials for load bearing applications, and these impose significant challenges in material design. Here we report that a nano-grained β-Ti alloy prepared by high-pressure torsion exhibits remarkable mechanical and biological properties. The hardness and modulus of the nano-grained Ti alloy were respectively 23% higher and 34% lower than those of its coarse-grained counterpart. Fibroblast cell attachment and proliferation were enhanced, demonstrating good in vitro biocompatibility of the nano-grained Ti alloy, consistent with demonstrated increased nano-roughness on the nano-grained Ti alloy. Results suggest that the nano-grained β-Ti alloy may have significant application as an implant material in dental and orthopedic applications.

This study investigated the microstructure and mechanical properties of Nb-microalloyed ultra-thin cast strip (UCS) steels produced by the CASTRIP ® process aged at temperatures below and above the Ac 1 temperature (525–800 °C). Aging provided a rapid strength increase without loss in ductility. This is significant because, ordinarily, one of these properties would increase at the expense of the other. Atom probe tomography (APT) and transmission electron microscopy (TEM) revealed that the strengthening was associated with the formation of Nb-rich solute atom clusters and nanoscale precipitates within the microstructure, which retard the movement of dislocations during deformation. The changes in strength and hardness were compared to the microstructural evolution across a range of different aging temperatures and explained in terms of the nucleation kinetics. Cluster-finding algorithms were used to analyse atom probe data, providing quantitative information about the dispersion of these clusters and precipitates. Optimum strengthening was observed where the clusters particles had a radius of 1–2 nm and a volume fraction of 0.027±0.004%.

The effects of two types of pre-existing defects, dislocations and clusters, on the strength and deformation behavior of body-centered cubic Fe nanopillars with a diameter of ∼150 nm were investigated using in situ nanocompression in a transmission electron microscope. The plastic deformation of nanopillars containing high initial dislocation densities was observed to be relatively continuous, proceeding via a series of small- and intermediate-scale strain bursts that were associated with the movement/escape of dislocations and the formation of slip bands. Mechanical annealing was observed in nanopillars with high dislocation densities. When the dislocation density was reduced by in situ heating, the nanopillars were much stronger and the plastic deformation behavior transformed to a more abrupt and explosive mode. The introduction of a dispersion of solute atom clusters into nanopillars caused further strengthening as a higher stress level is required for dislocations to pass the clusters. The strengthening effect of cluster dispersion in nanopillars is comparable to that observed in the bulk steel. These phenomena are universal for Fe nanopillars with different crystallographic orientations.

Cells can respond to mechanical forces and actively interact with mechanical stimulations in vitro. Understanding the effect of mechanical loading on cell morphology signifies a critical biomechanics issue in tissue engineering. In this study, human dermal fibroblasts (GM3384) underwent cyclic strain. This was done by culturing a monolayer of the cells onto a transparent membrane and applying a cyclic stress using a computer controlled bioreactor. The cells were mechanically stimulated at around 7% strain with 1 cycle per minute for 2 days. Finite element analysis (FEA) was then employed to characterize the strain field across the substrate membrane in the bioreactor. The results showed that strain distribution were non-uniform in the substrate membrane. The mapping of cell morphology with the strain field revealed that the cells exposed to the equibiaxial strain exhibited the classical spindle morphology while the cells subjected to uniaxial strain changed to a polygonal morphology. It is concluded that the nature of the strain has significant impact on the final cell morphology.

We demonstrate that a Nb-microalloyed ultra-thin cast strip steel can be strengthened substantially without compromising ductility by performing a simple heat treatment at 700 °C for 4 min. The strengthening was attributed to a fine dispersion of Nb-rich solute atom clusters. These clusters had an average size of ∼60 atoms at peak hardness and resembled Guinier–Preston zones in Al–Cu alloys. The application of the Ashby–Orowan equation indicates that these clusters are potent strengthening agents when compared to conventional Nb(C,N) precipitation strengthening.

In situ compression transmission electron microscopy was used to investigate the deformation behavior of alpha-Fe nanoblades. The results showed an inhomogeneous deformation process, composed of a series of short displacement bursts and single large displacement bursts. The short displacement bursts were associated with the collective motion of dislocations within the specimen, whilst the large displacement bursts occurred through dislocation-assisted slip band formation nucleated from the surface of the nanoblades.