Recent developments in test methodologies for nanoindenter-based small-length-scale mechanical characterization are overviewed, such as micropillar compression, cantilever beam bending, and tensile ...tests. Emphasis is placed on the possibilities offered by
in situ
testing in transmission and scanning electron microscopes, as well as examining strain rate and temperature dependence of mechanical strength. The versatility and growing impact of new nanomechanical characterization tools is highlighted through selected recent examples, such as indentation (sample) size effect, crack-tip plasticity, radiation damage, indentation creep, laser additive manufacturing, and crystalline/amorphous high-entropy alloys.
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Growth Twins and Deformation Twins in Metals Beyerlein, Irene J; Zhang, Xinghang; Misra, Amit
Annual review of materials research,
07/2014, Letnik:
44, Številka:
1
Journal Article
Recenzirano
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This article reviews recent basic research on two classes of twins: growth twins and deformation twins. We focus primarily on studies that aim to understand, via experiments, modeling, or both, the ...causes and effects of twinning at a fundamental level. We anticipate that, by providing a broad perspective on the latest advances in twinning, this review will help set the stage for designing new metallic materials with unprecedented combinations of mechanical and physical properties.
We quantify the effects of refraction in transit transmission spectroscopy on spectral absorption features and on temporal variations that could be used to obtain altitude-dependent spectra for ...planets orbiting stars of different stellar types. We validate our model against altitude-dependent transmission spectra of the Earth from ATMOS and against lunar eclipse spectra from Palle et al. We perform detectability studies to show the potential effects of refraction on hypothetical observations of Earth analogs with the James Webb Space Telescope NIRSPEC. Due to refraction, there will be a maximum tangent pressure level that can be probed during transit for each given planet-star system. We show that because of refraction, for an Earth-analog planet orbiting in the habitable zone of a Sun-like star only the top 0.3 bars of the atmosphere can be probed, leading to a decrease in the signal-to-noise ratio (S/N) of absorption features by 60%, while for an Earth-analog planet orbiting in the habitable zone of an M5V star it is possible to probe almost the entire atmosphere with minimal decreases in S/N. We also show that refraction can result in temporal variations in the transit transmission spectrum which may provide a way to obtain altitude-dependent spectra of exoplanet atmospheres. Additionally, the variations prior to ingress and subsequent to egress provide a way to probe pressures greater than the maximum tangent pressure that can be probed during transit. Therefore, probing the maximum range of atmospheric altitudes, and in particular the near-surface environment of an Earth-analog exoplanet, will require looking at out-of-transit refracted light in addition to the in-transit spectrum.
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•This works reveals novel strain accommodation mechanisms unique to fibrous Si in LRS Al-20Si.•After high load indentation, LRS morphologies show no cracking and less materials ...pile-ups, suggesting higher strain hardening than as-cast morphology.•S/TEM HRTEM studies show no dislocation storage capabilities within Si fiber.•Three deformation stages observed: dislocation build up around Si fiber, stress relieve by Si segmentation, and well-recovered Al subgrain formation.
Indentation plastic zone of Al-20 wt% Si laser rapid solidified (LRS) alloy with nanoscale, fibrous, fully eutectic microstructure is characterized using a combination of atomic force microscopy (AFM), scanning electron microscopy (SEM) and scanning/transmission electron microscopy (S/TEM). In contrast to cracking around the indents in the as-cast Al-Si alloy, the LRS nano-fibrous eutectic microstructure demonstrated higher strength, strain-hardening, and uniformly distributed plasticity around indents with no cracks. A transition in morphology from long nano-fibrous Si to short nano-fibrous Si is observed underneath the indent in LRS eutectics. The mechanism for this morphological evolution with increasing plastic strain is postulated to involve three stages of the deformation process: First, the formation of closely-spaced dislocation arrays in the nano-channels of Al matrix confined by Si fibers. Second, fiber segmentation when the stress field of the dislocation array exceeds the critical stress for fracture in Si nano-fibers. Third, recovery of dislocation substructures and sub-grain formation in Al leading to a rotation of fractured Si segments. The final microstructure consisted of short Si nano-fibers along triple junctions of the subgrains in Al. The implication of this unusual microstructure evolution with increasing plastic strain on the plastic co-deformation in high-strength LRS nano-eutectic microstructures was discussed in this report.
We demonstrate design of Cu/Mo nanocomposite thin films with bicontinuous intertwined morphologies for high strength and plastic deformability. Through co-sputtering Cu and Mo at different ...temperatures, nanocomposites with lateral and random concentration modulations (LCMs and RCMs) were fabricated. Ligament size and coherency of the Cu/Mo interfaces was also controlled via the deposition conditions. It was discovered that nanocomposites with LCM structure and coherent interfaces have high strength but limited deformability. Enhanced deformability was achieved when the interface was altered to semi-coherent, although plasticity was localized in kink bands. The RCM structure design with semi-coherent interfaces showed an excellent combination of high flow strength and plastic deformability due to suppression of shear and kink bands.
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•A novel approach in addressing the strength-deformability trade-off through morphological and interface design in metallic nanocomposites is presented.•Cu-Mo nanocomposites with tailorable morphology and interface properties have been fabricated by co-sputtering two immiscible metals at high temperatures.•The bicontinuous intertwined with semi-coherent Cu-Mo interfaces provides an unprecedented combination of high strength and enhanced plastic deformability.
Laser remelting was employed to refine the interlamellar spacing (λ) to approximately 30nm on an arc-cast Al-32.7wt%Cu eutectic. λ scaled with A ′(h)-°0.25 where h is the distance from the trace ...bottom, and A ′ is a factor dependent on laser processing conditions. A′ increased with the laser power (at a constant spot diameter) but decreased with the spot diameter (at a constant laser power). The nanoindentation measured hardness increased up to 4.68GPa with decreasing interlamellar spacing, following the Hall-Petch relation. The as-cast eutectics showed microcracks around indents, while cracking was suppressed in the laser remelted nanoscale eutectics. A dislocation theory based mechanism is proposed for the enhanced plastic co-deformation of fine scale laser-remelted Al-Al2Cu eutectics.
The effects of 5 MeV Fe2+ ion irradiation at 300°C on the microstructure evolution and deformation behavior of a FeCrAl C26M alloy are presented. It has been found that dislocation loop density ...increases an order of magnitude from 1 dpa to 16 dpa irradiations, whereas, the dislocation loop size saturates with increasing damage. Micropillars, 600 nm in diameter and 1.3 µm in height, were fabricated and compressed inside grains with , and crystallographic orientations, respectively. {112} has been identified as the primary slip system in both unirradiated and irradiated alloys. The increase in yield stress after irradiation is observed with measurable variation along and vs. along . By applying the Orowan dispersed barrier model, the increase of yield stress is found mainly due to the slip resistance of radiation generated defect loops. Detailed transmission electron microscopy (TEM) studies were performed to quantify the Burgers vector and the distribution of irradiation induced dislocations at elevated strains. It is revealed that localized shear instability is caused by avalanche slip events of ½ dislocations gliding out of tested pillars. Simultaneously, a large number of sessile/immobile dislocations formed in the vicinity of slip band, leading to the hardening at elevated strains.
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A novel interface engineering strategy is proposed to simultaneously achieve superior irradiation tolerance, high strength, and high thermal stability in bulk nanolayered composites of a model ...face‐centered‐cubic (Cu)/body‐centered‐cubic (Nb) system. By synthesizing bulk nanolayered Cu‐Nb composites containing interfaces with controlled sink efficiencies, a novel material is designed in which nearly all irradiation‐induced defects are annihilated.
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