Small Active Volume Research Articles

Response of silicon solar cells to neutrons in post-detonation monitoring

In this study, we evaluated the feasibility of using solar photovoltaic (PV) panels for the detection of prompt neutrons from a nuclear denotation. Monte Carlo (MC) simulations were performed to evaluate the response of silicon (Si) solar cells (SCs) to neutrons and support the experiments. The response of Si SCs to fission spectrum neutrons was measured experimentally using the fast neutron beam of a research reactor. Steady-state neutron measurements indicated a clear response of the Si SCs, which correlated with changes in the neutron intensity. Time-modulated neutron measurements performed using a neutron chopper demonstrated a clear transient response of Si SCs to neutrons, which was observed in the signal power spectral density plots containing peaks corresponding to frequencies of neutron pulses produced by the chopper. The resulting signal was found to be small, which was attributed to the extremely small active neutron interaction volume inside the SCs. In a real detonation scenario, the transient neutron flux is expected to be higher, and the effective neutron interaction volume of large area solar PV panels is much larger, which could significantly boost the signal. The study, overall, demonstrated the potential of large area solar PV panels or a solar farm for prompt detection and identification of a nuclear detonation.

Chemical complexity-changed deformation behavior of NiTi-based B2 low- to high-entropy intermetallic compounds: Atomistic simulations

Owing to the high chemical complexity and severe lattice distortion but still long-range ordered structure, the deformation behavior and defect activities of high-entropy intermetallic compounds are worth investigations. Hence in this study, the mechanical properties of NiTi-based low- to high-entropy intermetallic compounds were measured from different orientations at different temperatures simply by using nanoindentations, and compared with the structural evolution and defect activities suggested by molecular dynamics simulations. The deformation of the low-entropy intermetallic compound proceeded with regular martensitic transformation, followed by the heterogeneous nucleation of dislocations at the martensite/austenite interface and the subsequent long-distance gliding accompanied with sharp stress drops. With increasing temperature, a more intense shearing events and marked strength reduction were observed. In comparison, with the introduction of constitutional complexity, the martensitic transition was suppressed. Instead, the early homogeneous nucleation at a relatively small activation volume (acquired from indenting displacement bursts) and small-range activities of abundant defects mediated the smooth stress-strain response and uniform plastic deformation of the high-entropy intermetallic compound. At an elevated temperature, a similar deformation mechanism and the same level of strength were retained in the chemically complex alloy.

Thermally activated nature of synchro-Shockley dislocations in Laves phases

Synchro-Shockley dislocations, as zonal dislocation, are the major carrier of plasticity in Laves phases at high temperatures. The motion of synchro-Shockley dislocations is composed of localized transition events, such as kink-pair nucleation and propagation, which possess small activation volumes, presumably leading to sensitive temperature and strain rate dependence on the Peierls stress. However, the thermally activated nature of synchro-Shockley dislocation motion is not fully understood so far. In this study, the transition mechanisms of the motion of synchro-Shockley dislocations at different shear and normal strain levels are studied. The transition processes of dislocation motion can be divided into shear-sensitive and -insensitive events. The external shear strain lowers the energy barriers of shear-sensitive events. Thermal assistance is indispensable in activating shear-insensitive events, implying that the motion of synchro-Shockley dislocations is prohibited at low temperatures.

Temperature-Dependent Mechanical Properties of a Metal–Organic Framework: Creep Behavior of a Zeolitic Imidazolate Framework-8 Single Crystal

Zeolite Imidazole Framework-8 (ZIF-8) with a robust structure and high thermal stability is a strong candidate to act as the catalyst matrix for various chemical applications, especially for those at higher temperatures, like hydrogenation. In this study, the time-dependent plasticity of a ZIF-8 single crystal was explored by a dynamic indentation technique to explore its mechanical stability at higher temperatures. The thermal dynamic parameters for the creep behaviors, like activation volume and activation energy, were determined, and possible mechanisms for the creep of ZIF-8 were then discussed. A small activation volume implies the localization of the thermo-activated events, while high activation energy, high stress exponent n, and weak dependence of the creep rate on temperature all favor pore collapse over volumetric diffusion as the creep mechanism.

Enhanced mechanical properties of a carbon and nitrogen co-doped interstitial high-entropy alloy via tuning ultrafine-grained microstructures

C-N co-doped interstitial high entropy alloy (iHEA) was reported to have high strength and ductility. However, iHEA with fully recrystallized ultrafine grains (UFGs) and underlying thermally activated processes associated with dislocation slip, twinning, and solute drag have not been reported yet. In this work, a C-N co-doped iHEA with nominal composition Fe48.5Mn30Co10Cr10C0.5N1.0 (at.%) was prepared, and the microstructures were tuned by cold-rolling and annealing treatments to improve mechanical properties. Upon cold-rolling with a strain of 1.74, the main microstructures in the iHEA are composed of nano-grains, nano-twins, HCP laminates, and high density of dislocations, leading to ultrahigh hardness of 466.7 HV and tensile strength of 1730 MPa at the expense of ductility (2.44%). Both the nanostructures and the high hardness of the iHEA can be maintained up to an annealing temperature of 600 °C (462.5 HV). After annealing at 650 °C for 1 h, the UFG microstructures are obtained in the iHEA, containing recrystallized grains with an average grain size of 0.91 µm and nanoprecipitates with an average diameter of 90.8 nm. The combined strengthening and hardening effects of UFGs, nanoprecipitates, twinning, and solutes contribute to high strain hardening (n = 0.81), gigapascal yield strength (984 MPa), and good ductility (20%). The C-N co-doping leads to a strong drag effect on dislocation slip, resulting in a nano-scale mean free path of dislocation slip λ¯ (1.44 nm) and much small apparent activation volume V* (15.8 b3) of the UFG iHEA.

Anisotropic copper diffusion in olivine: Implications for Cu preservation in olivine and olivine-hosted melt inclusions

The rate of copper diffusion in olivine governs the preservation of copper in olivine and olivine-hosted melt inclusions, but Cu diffusivity in olivine has not been well determined especially regarding its anisotropy. In this study, Cu diffusion in single crystal olivine (Fo91Fa09) cuboid, prepared to comply with the three principle crystallographic axes, was investigated at 1273–1673 K and 1–2.5 GPa in a piston cylinder apparatus, using a diffusion-in approach with Cu or Cu2O as the source. Copper concentration profiles were analyzed by LA-ICP-MS scanning on the recovered samples along multiple directions. We found Cu diffusion in olivine to be appreciably anisotropic, with Cu diffusivity along the c axis being greater than that along the a axis by nearly an order of magnitude (Dc ≈ 3Db ≈ 7.5 Da). The activation energy for Cu diffusion is ∼210 kJ/mol, with little dependence on crystallographic orientation. Cu diffusivity at 2.5 GPa is only slightly lower than that at 1 GPa, yielding a small activation volume of 2.3 cm3/mol. While not being as rapid as the migration of H and interstitial Li, Cu diffusion, especially along the c axis, is significantly faster than the major elements of olivine (Fe-Mg, Si and O). Diffusion modeling indicates that the timescale for re-equilibration of Cu between olivine-hosted melt inclusion and external melt is relatively short (<60 yr at 1000 °C and <3000 yr at 800 °C) compared to the typical lifetime of magma chambers. Cu concentrations in olivine-hosted melt inclusions may therefore have been significantly modified from the pristine values. For Cu profiles preserved in olivine phenocryst, our determined Cu diffusivity can be used to constrain its residence time in magma chamber, and the diffusion anisotropy can provide a cross-check.

Static and Time-Dependent Plasticity of Monolithic Metal–Organic Frameworks

Mechanical properties of monolithic metal–organic frameworks (MOFs), especially static and time-dependent plasticity, are essential to the mechanical stability of their device applications. In this study, the hardness and elastic moduli of monoZIF-8 and monoUiO-66 MOFs were compared and analyzed in terms of formation enthalpy, topological structure, and crystal defects formed during sol–gel synthesis. For the first time, the creep behaviors of monoUiO-66 and monoZIF-8 were investigated using nanoindentation. The stress exponent n values for creep were calculated to be about 9 and 40 for monoUiO-66 and monoZIF-8, respectively, implying grain boundary sliding (GBS, n = 2) between nanocrystals is unlikely to occur. The suspected absence of GBS was also supported by the identical creep behavior of monoZIF-8 and a ZIF-8 single crystal. Small activation volumes V* determined for monoUiO-66 (0.721–0.839 nm3) and monoZIF-8 (1.29–2.54 nm3) correspond to only 2–3 intrinsic pores in the MOF frameworks, indicating that pore collapse is probably the predominant creep mechanism.

Functional Neuroplasticity and Motor Skill Change Following Gross Motor Interventions for Children With Diplegic Cerebral Palsy

Background Gross motor intervention designs for children with diplegic cerebral palsy (DCP) require an improved understanding of the children’s potential for neuroplasticity. Objective To identify relations between functional neuroplasticity and motor skill changes following gross motor interventions for children with DCP. Methods There were 17 participants with DCP (ages 8-16 years; 6 females; Gross Motor Function Classification System Level I [n = 9] and II [n = 8]). Each completed a 6-week gross motor intervention program that was directed toward achievement of individualized motor/physical activity goals. Outcomes were assessed pre/post and 4 to 6 months post-intervention (follow-up). An active ankle dorsiflexion task was completed during functional magnetic resonance imaging. The ratio of motor cortical activation volume in each hemisphere was calculated using a laterality index. The Challenge was the primary gross motor skill measure. Change over time and relations among outcomes were evaluated. Results Challenge scores improved post-intervention (4.57% points [SD 4.45], P = .004) and were maintained at follow-up (0.75% [SD 6.57], P = 1.000). The laterality index for dominant ankle dorsiflexion increased (P = .033), while non-dominant change was variable (P = .534). Contralateral activation (laterality index ≥+0.75) was most common for both ankles. Challenge improvements correlated with increased ipsilateral activity (negative laterality index) during non-dominant dorsiflexion (r = −.56, P = .045). Smaller activation volume during non-dominant dorsiflexion predicted continued gross motor gains at follow-up (R2 = .30, P = .040). Conclusions Motor cortical activation during non-dominant ankle dorsiflexion is a modest indicator of the potential for gross motor skill change. Further investigation of patterns of neuroplastic change will improve our understanding of effects. ClinicalTrials.gov Registry: NCT02584491 and NCT02754128

Superior ductility Mg-Mn extrusion alloys at room temperature obtained by controlling Mn content

The effect of Mn content (0.4, 0.8, 1.2 wt. %) on microstructure and mechanical properties of as-extruded dilute binary Mg-Mn alloys was studied. With the increase of Mn content from 0.4 wt. % to 1.2 wt. %, the volume fraction of dynamically recrystallized (DRXed) grains decreases from 89% to 40%, while the DRXed grain size decreases from 3.2 μm to 1.9 μm. The segregation of Mn atoms and nano α-Mn dynamical precipitates are observed at the grain boundaries of the extruded Mg-0.8Mn alloys and the amount of dynamically precipitates is increased with increasing Mn content. Tensile tests show that the Mn content significantly affects mechanical properties at room temperature (RT). The Mg-0.8 wt. % Mn alloy shows the excellent ductility of 81 % at room temperature in a strain rate of 1 · 10−3 s−1, the ductility decreases if the Mn content is lower or higher than 0.8 wt. %. The yield strength (YS) and ultimate tensile strength (UTS) increase with increasing Mn content. The large value of strain rate sensitivity exponent (m-value) and small activation volume (V*) of the as-extruded Mg-0.8Mn alloy suggest that grain boundary sliding (GBS) plays an important role in tensile plastic deformation. High volume fraction of the un-DRXed region leads to the reduced contribution of GBS to plastic deformation, decreases ductility at RT. The Atomic force microscopy (AFM) analysis indicates that the contribution of GBS to the total plastic deformation is 2.7 %, 55.1 % and 12.8 % for Mg-0.4Mn, Mg-0.8Mn and Mg-1.2Mn alloys, respectively.

Unveiling the grain boundary-related effects on the incipient plasticity and dislocation behavior in nanocrystalline CrCoNi medium-entropy alloy

• The incipient plasticity and dislocation behavior in nanocrystalline (NC) CrCoNi alloy were systematically investigated and compared with its coarse-grained (CG) counterpart and NC Ni. • Multiple pop-ins appear in the P-h curves of NC and CG CrCoNi alloys but disappear in the curves of NC Ni. • The incipient plasticity is mediated by dislocation nucleation in NC CrCoNi alloy but by GBs in NC Ni. • Chemically complex GBs in NC CrCoNi alloy facilitate dislocation nucleation but impede dislocation propagation. • Spatial restrictions from GBs in NC CrCoNi alloy induce size-dependent pop-in behaviors. The incipient plasticity and dislocation behavior in a nanocrystalline (NC) CrCoNi medium-entropy alloy were systematically investigated in terms of pop-in events during instrumental nano-indentation tests. Quantitative statistical analysis and molecular dynamic simulations were performed to reveal the effects of grain boundaries (GBs) on initial stages of plastic deformation . Multiple pop-in events appeared during loading on the NC CrCoNi. The first pop-in that represents the initial yielding was identified to be controlled by dislocation nucleation, which is in sharp contrast to the continuous elastic-plastic transition mediated by GB mechanisms in NC pure metals. This can be attributed to the sluggish kinetics of the chemically complex GBs (CCGBs) in the NC CrCoNi that hinders diffusive GB activities but facilitates dislocation nucleation. Subsequent pop-ins were also found to be closely related to the extra dragging effects imposed by the CCGBs on dislocation propagation in the NC alloy . Moreover, the extremely small grain sizes and the consequent high-volume fraction of GBs in the NC alloy severely restrict the lengths of dislocation source and the radii of dislocation loop, giving rise to a higher critical stress, smaller activation volume and lower pop-in width as compared with its coarse-grained counterpart. These results provide new insights into the onset of nano-plasticity in concentrated multi-principal element alloys.

Plastic flow behaviors of high-strength dual-phase Ni-SiOC nanocomposites

High-strength materials with good plastic flow stability are highly desirable for structural applications. Refining grain size can effectively enhance flow strength but often cause plastic flow instability. Here, we demonstrated that nanosized amorphous ceramics SiOC impart a simultaneous enhancement of strength and plastic flow stability to Ni-based nanocomposites. The co-sputtered Ni-SiOC nanocomposites exhibit core (crystalline Ni)-shell (amorphous SiOC) nanostructures and develop nanograined Ni composite containing amorphous ceramic SiOC nanoparticles during annealing up to 800 °C. Corresponding to the developed microstructures, in-situ scanning electron microscope (SEM) micropillar compression tests at deformation temperatures range from room temperature (RT) to 400°C reveal that the core-shell nanostructure shows high strength of 2.5 GPa at RT and 1.6 GPa at 400 °C with compressive strain up to 40%, while the nanograined Ni composite containing amorphous ceramic nanoparticles exhibits high strength of 3.0 GPa at RT and 2.0 GPa at 400 °C with compressive strain up to 50%. Most intriguingly, both nanocomposites do not exhibit obvious strain hardening/softening behavior during compression up to 50%. The high strain rate sensitivity (0.02 at RT to 0.05 at 400 °C) and the small activation volume (8b3 at RT to 10b3 at 400 °C) that were measured based on strain rate jump tests suggest dislocation slips as dominant deformation mechanisms in Ni grains, which is further evidenced by microscopy. Superb plastic flow stability is attributed to microstructure-promoted plastic co-deformation between amorphous ceramics and Ni grains as confirmed by microstructure characterizations. Moreover, amorphous ceramic SiOC inhibit grain coarsening of Ni, preventing deformation induced softening.

Cross-gain modulation-based photonic reservoir computing using low-power-consumption membrane SOA on Si.

We demonstrate photonic reservoir computing (RC) utilizing cross-gain modulation (XGM) in a membrane semiconductor optical amplifier (SOA) on a Si platform. The membrane SOA's features of small active volume and strong optical confinement enable low-power nonlinear operation of the reservoir, with 101-mW-scale power consumption and 102-µW-scale optical input power. The power consumption is about an order of magnitude lower than that of conventional SOAs that exhibit saturable nonlinearity. The XGM-based reservoir is configured by injecting a delayed feedback signal into the SOA from a direction opposite to the input signal. This configuration provides robust operation of the feedback circuit because of the phase insensitivity and the elimination of loop oscillation risk. The RC performance is evaluated via the information processing capacity (IPC) and a nonlinear benchmark task. It is revealed that the XGM-based reservoir performs strong nonlinear transformation of input time-series signals. The series of results consistently show that the membrane SOA performs RC-applicable nonlinear operations through XGM at a low power scale.

Uniting tensile ductility with ultrahigh strength via composition undulation

Metals with nanocrystalline grains have ultrahigh strengths approaching two gigapascals. However, such extreme grain-boundary strengthening results in the loss of almost all tensile ductility, even when the metal has a face-centred-cubic structure-the most ductile of all crystal structures1-3. Here we demonstrate that nanocrystalline nickel-cobalt solid solutions, although still a face-centred-cubic single phase, show tensile strengths of about 2.3 gigapascals with a respectable ductility of about 16 per cent elongation to failure. This unusualcombination of tensile strength and ductility is achieved by compositional undulation in a highly concentrated solidsolution. The undulation renders the stacking fault energy and the lattice strains spatially varying over length scales in the range of one to ten nanometres, such that the motion of dislocationsis thussignificantly affected. The motion of dislocations becomes sluggish, promoting their interaction, interlocking and accumulation, despite the severely limited space inside the nanocrystalline grains. As a result, the flow stress is increased, and the dislocation storageis promoted at the same time, which increases the strain hardening and hence the ductility. Meanwhile, the segment detrapping along the dislocation line entails a small activation volume and hence anincreased strain-rate sensitivity, which also stabilizes the tensile flow. As such, an undulating landscape resisting dislocation propagation provides a strengthening mechanism that preserves tensile ductilityat high flow stresses.

Diagnostic of Fusion Neutrons on EAST Tokamak Using 4H-SiC Detector

Due to significant radiation hardness, thermal properties, low sensitivity to gamma rays, and fast response, the 4H-SiC detector exhibits promising properties in measuring the neutron production in the high-power fusion devices. To demonstrate the potential value of the 4H-SiC detectors as a neutron flux monitor and a neutron spectrometer on experimentally advanced superconducting tokamak (EAST) during the 2021 EAST deuterium–deuterium plasma experimental campaigns, the 4H-SiC detector covered with a 25- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{m}$ </tex-math></inline-formula> -thick <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> LiF film 95% enriched in <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> Li and the 4H-SiC detector covered with a 1.5-mm-thick polyethylene layer were installed on a viewport located at the mid-plane of port F outside the EAST vacuum vessel and operated continuously during the whole experimental campaign with the goal to measure both the total and the time-dependent neutron emission from the plasma. The results are compared with those obtained with detectors routinely working at EAST ( <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">235</sup> U Fission Chambers). Despite their small active volumes, the 4H-SiC detectors were able to measure the total and the time-dependent neutron emission with good reliability and stability. The results of this work not only demonstrated the satisfactory performance of the 4H-SiC detector in the harsh environment of a mixed neutron–gamma high-radiation field and strong electromagnetic field, but also its applicability as a neutron spectrometer and a neutron flux monitor for fusion deuterium–plasma experiments.

Anomalous high strain rate compressive behavior of additively manufactured copper micropillars

Microscale dynamic testing is vital to the understanding of material behavior at application relevant strain rates. However, despite two decades of intense micromechanics research, the testing of microscale metals has been largely limited to quasi-static strain rates. Here we report the dynamic compression testing of pristine 3D printed copper micropillars at strain rates from ∼0.001 s−1 to ∼500 s−1. It was identified that microcrystalline copper micropillars deform in a single-shear like manner exhibiting a weak strain rate dependence at all strain rates. Ultrafine grained (UFG) copper micropillars, however, deform homogenously via barreling and show strong rate-dependence and small activation volumes at strain rates up to ∼0.1 s−1, suggesting dislocation nucleation as the deformation mechanism. At higher strain rates, yield stress saturates remarkably, resulting in a decrease of strain rate sensitivity by two orders of magnitude and a four-fold increase in activation volume, implying a transition in deformation mechanism to collective dislocation nucleation.

Dosimetric accuracy of a cross-calibration coefficient for plane-parallel ionization chamber obtained in low-energy electron beams using various cylindrical dosimeters

Abstract Introduction: The accuracy of the cross-calibration procedure depends on ionization chamber type, both used as reference one and under consideration. Also, the beam energy and phantom medium could influence the precision of cross calibration coefficient, resulting in a systematic error in dose estimation and thus could influence the linac beam output checking. This will result in a systematic mismatch between dose calculated in treatment planning system and delivered to the patient. Material and methods: The usage of FC65-G, CC13 and CC01 thimble reference chambers as well as 6, 9, and 15 MeV electron beams has been analyzed. A plane-parallel PPC05 chamber was calibrated since scarce literature data are available for this dosimeter type. The influence of measurement medium and an effective point of measurement (EPOM) on obtained results are also presented. Results: Dose reconstruction precision of ~0.1% for PPC05 chamber could be obtained when cross-calibration is based on a thimble CC13 chamber. Nd,w,Qcross obtained in beam ≥ 9MeV gives 0.1 – 0.5% precision of dose reconstruction. Without beam quality correction, 15 MeV Nd,w,Qcross is 10% lower than Co-60 Nd,w,0. Various EPOM shifts resulted in up to 0.6% discrepancies in Nd,w,Qcross values. Conclusions: Ionization chamber with small active volume and tissue-equivalent materials supplies more accurate cross-calibration coefficients in the range of 6 – 15 MeV electron beams. In the case of 6 and 9 MeV beams, the exact position of an effective point of measurement is of minor importance. In-water cross-calibration coefficient can be used in a solid medium without loss of dose accuracy.

Atomistic interpretation of extra temperature and strain-rate sensitivity of heterogeneous dislocation nucleation in a multi-principal-element alloy

Incipient plasticity of crystals is usually initiated from versatile pre-existing crystalline defects of different geometries and length scales that formed during materials fabrication and processing, which is closely associated with the mechanical properties. Here we study the heterogeneous dislocation nucleation behaviors from an existing nanoscale void embedded in a prototypical multi-principal-element (MPE) NiCoCr alloy via atomistical exploration of possible minimum energy pathways and analyzes of the dislocation kinetics by a continuum-level mechanistic model, in comparison with the same plastic mechanism in an elemental face-centered cubic copper. It is found that the rough nucleation pathway of dislocation brings about extra thermal softening and strain-rate sensitivity for the critical nucleation stress of dislocation in the MPE alloy, which reproduces the experimental observations. The extra temperature softening is revealed to be associated with the high total entropy of the MPE alloy, in which the configurational entropy plays a dominating role rather than the vibrational counterpart. The extra strain rate sensitivity is caused by the relatively small activation volume in MPE alloy. Our strategy offers physical understanding of the experimental phenomena with atomistic details, which also emphasizes the critical role of configurational disorder in the dislocation accommodated plasticity in the generic complex concentrated alloys.

Low-voltage surface-normal electroabsorption modulators.

Surface-normal electroabsorption modulators (SNEAMs) are appealing for short-reach communication systems because of their outstanding properties, such as ultrawide bandwidth and polarization-insensitive response; however, due to their small active volumes, large voltage swings are typically required to obtain the best performance. Here we propose and demonstrate a novel, to the best of our knowledge, design that dramatically reduces the voltage needed by SNEAMs and significantly increases their extinction ratio. By shrinking the multiple quantum well stack of SNEAMs to the minimum and by optimizing their reflectivity with dielectric coatings of suitable refractive index and thickness, we obtain modulators that require drive voltages of only 1-2Vpp. We show that these novel devices largely outperform conventional SNEAMs.

Strength, plasticity, thermal stability and strain rate sensitivity of nanograined nickel with amorphous ceramic grain boundaries

A large fraction of grain boundaries imparts high strength to nanocrystalline (NC) metals but causes the poor thermal stability of microstructures and a significant reduction of strength with increasing deformation temperature. Stabilizing the microstructure is thus desired. In this work, we self-patterned 1–2 nm thick amorphous ceramic SiOC as grain boundaries in NC Ni with grain size of 13 nm through co-sputtering 75 at.%-Ni and 25 at.%-SiOC, producing Ni-SiOC amorphous ceramic reinforced metals (Ni-SiOC ACRMs) with a core-shell structure. Microscopic characterizations of Ni-SiOC ACRMs before and after 400 °C annealing and after compression deformation confirmed the thermally and mechanically stable core-shell structure. Mechanical properties, such as stress-strain responses and strain rate sensitivity of Ni-SiOC ACRMs over a range of temperatures from room temperature (RT) to 400 °C were measured using in situ micropillar testing in a scanning electron microscope (SEM). We demonstrated that amorphous ceramic SiOC grain boundaries impart a high strength (2.5 GPa at RT and 1.6 GPa at 400 °C), large compressive plasticity (uniform compression strain greater than 35% without shear instability) and thermal stability to Ni-SiOC ACRMs. High strain rate sensitivity (0.015 at RT to 0.042 at 400 °C), small activation volume (about 13b3) and no obvious strain hardening/softening behavior suggest that plastic deformation is accommodated by dislocation slips in Ni grains. Amorphous ceramic grain boundaries act as strong barriers and sinks for impeding and trapping dislocations, strengthening the ACRMs and promoting plastic co-deformation between Ni nanograins and amorphous ceramic grain boundaries; the accumulated dislocations are smeared along the amorphous-crystal interfaces, preventing formation of localized shear bands across multiple nanograins.

Dislocation-mediated shear amorphization in boron carbide.

The failure of superhard materials is often associated with stress-induced amorphization. However, the underlying mechanisms of the structural evolution remain largely unknown. Here, we report the experimental measurements of the onset of shear amorphization in single-crystal boron carbide by nanoindentation and transmission electron microscopy. We verified that rate-dependent loading discontinuity, i.e., pop-in, in nanoindentation load-displacement curves results from the formation of nanosized amorphous bands via shear amorphization. Stochastic analysis of the pop-in events reveals an exceptionally small activation volume, slow nucleation rate, and lower activation energy of the shear amorphization, suggesting that the high-pressure structural transition is activated and initiated by dislocation nucleation. This dislocation-mediated amorphization has important implications in understanding the failure mechanisms of superhard materials at stresses far below their theoretical strengths.