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Bulk, polycrystalline (Co, Cu, Mg, Ni, Zn)O was synthesized using solid-state sintering. Micropillars were prepared and mechanically deformed along three crystallographic orientations: (001), (101), and (111). Pillars (001) and (111) cracked, while Pillar (101) remained intact. Pillars (001) and (101) exhibited activated slip systems, confirmed by a large stress drop, and the presence of slip bands, respectively. Schmid factor (SF) analysis was performed to examine the effect of grain orientations on dislocation activity and slip behavior. SF values range from 0 to 0.5, with non-zero values indicating potential for slip. Six slip systems exist in the (Co, Cu, Mg, Ni, Zn)O rock salt crystal structure: 1 / 2 ⟨ 110 ⟩ 1 1 ¯ 0. For the (001) orientation, four slip systems are potentially active (SF = 0.5). For the (101) orientation, there are four potentially active slip systems (SF = 0.25). For the (111) orientation, no potentially active slips systems exist (SF = 0). Dislocation structures, which were observed post-compression via transmission electron microscopy, demonstrated variations in size, number, and distribution across the pillar, depending on micropillar orientation. Entangled dislocations created misorientation in Pillar (001), which led to the possible formation of subgrains, while singular dislocations were observed in Pillar (101), and a lack of dislocations was observed in Pillar (111). Zener–Stroh type dislocation entanglement-mediated cracking is the proposed cause of the transgranular-type cracks in Pillar (001). The possible subgrain formation, or lack of formation, respectively, caused intergranular-type cracks to additionally form in Pillar (001), while Pillar (111) only exhibited transgranular-type brittle fracture. In combination, these findings highlight the importance of dislocation activity, without the need for elevated temperature, and grain orientation in controlling the mechanical deformation response in single-phase (Co, Cu, Mg, Ni, Zn)O. [ABSTRACT FROM AUTHOR]

Despite their vast composition space and promising properties, little is known about the fundamental mechanical behavior of high entropy oxides (HEOs). Here we provide experimental evidence of the nanoscale origin of room temperature deformation in (Co,Cu,Mg,Ni,Zn)O HEOs with different grain sizes. We find that micron-grain (Co,Cu,Mg,Ni,Zn)O deforms predominately through extensive 1 2 <110>{1–10} dislocation slip manifesting as discrete slip bands. Nano-grain (Co,Cu,Mg,Ni,Zn)O, in addition to moderate dislocation slip activity, deforms through grain boundary sliding and intergranular cracking. The significant dislocation-mediated deformation and grain size dependent behavior demonstrates that HEOs have the potential for diverse and highly tailorable mechanical behavior. High entropy oxides (Co,Cu,Mg,Ni,Zn)O exhibit grain size dependent room temperature deformation, with micron-grain samples displaying significant potential for dislocation-mediated deformation and nanocrystalline samples exhibiting grain boundary sliding and micro-cracking. [ABSTRACT FROM AUTHOR]

We report on the novel application of nanoscratch characterization to provide insight into the plasticity mechanisms responsible for the behaviour of composites. Accordingly, we conduct deformation characterization with nanoscratch testing (DCNT) to study the deformation behaviour of two B4C reinforced ultrafine grained Al alloy tri-modal composites with average B4C particle sizes of ~1–6 μm and ~500 nm, respectively. To highlight the type of mechanistic information revealed in a DCNT study of composites, we concentrate on the influence of B4C particle size on deformation mechanisms. [ABSTRACT FROM AUTHOR]

The nanoindentation and nanoscratch behavior under ramp loading conditions of three dense bulk alumina ceramic samples with average grain size regimes of ~290 nm, 750 nm, and 1.3 μm, respectively, has been investigated using a nanoindenter XPS system in indentation and scratch modes. The mechanical behavior, including hardness from nanoindentation tests, scratch hardness, scratch resistance, and coefficient of friction from nanoscratch tests, can be improved with grain refinement. Moreover, the surface roughness of the scratch grooves and the distinct morphologies of the nanoscratch debris from each sample reveal that alumina ceramic samples with ultrafine grain size exhibit an increase in plastic deformation capability at the nano- and micro-scale. [ABSTRACT FROM AUTHOR]

Abstract: A mechanism which is based on the formation of residual internal stresses caused by dislocation creation and absorption at grain boundaries is proposed to explain the unique strain softening phenomenon in nanocrystalline and ultrafine-grained metals. With the residual internal stresses caused by plastic deformation in individual grains, in order to create new dislocations at grain boundaries for further plastic deformation, lower external stress is required when compared with that without the residual internal stress field, which results in strain softening. [Copyright &y& Elsevier]

The mechanical behavior, with particular emphasis on the damage mechanisms, of SiCp/Al composites was studied by both experiments and finite element analysis in this paper. A 3D microstructure-based finite element model was developed to predict the elasto-plastic response and fracture behavior of a 7vol.% SiCp/Al composite. The 3D microstructure of SiCp/Al composite was reconstructed by implementing a Camisizer XT particle size analysis device and a random sequential adsorption algorithm. The constitutive behavior of the elastoplastic-damage in the metal matrix, the elastic-brittle failure for the particle reinforcement, and the traction-separation for interfaces, were independently simulated in this model. The validity of the modeling results were validated by the agreement of the experimental stress-strain curve and the morphology of fracture section with those predicted by the simulation. The visual elasto-plastic deformation process, along with crack generation and propagation was well simulated in this model. The numerical results were used to provide insight into the damage mechanisms of SiCp/Al composites, and the effects of interfacial strength and particle strength on material properties were also discussed in detail. [ABSTRACT FROM AUTHOR]

It is well known that micrometer-sized and/or submicrometer-sized metallic crystals exhibit “smaller is stronger” size effect: the yield strength σ varies with sample dimension D roughly as a power-law σ ∼ D − m . For some materials, near-theoretical strength values can be attained by reducing the dimensions of crystals to sub-micrometric or nanometric values. At these size scales, however, plastic instabilities, such as strain bursts, strain softening or low strain hardening rates, become operative due to the avalanche-like dislocation generation and escape; such instabilities contribute to disappointing flow intermittency. From a scientific standpoint, the onset of plastic instabilities has hindered fundamental study of the deformation behavior of materials near theoretical strength values. From a technological standpoint, these instabilities limit potential applications in microelectromechanical devices, for example. In this study we demonstrate that by concurrently introducing grain boundaries and secondary phase particles, plastic instabilities can be dramatically suppressed in submicron Al pillars at large strain and that this behavior is attributable to substantial dislocation storage and subsequent grain boundary (GB) mediated plasticity. Consequently the crystals possess superior strength with flow stress larger than 1.0 GPa. [ABSTRACT FROM AUTHOR]

In the last decade, the commercially available heat-treatable aluminum alloy (AA) 2139 (Al–Cu–Mg–Ag) has generated interest within the aerospace and defense communities because of its high strength and damage tolerance as compared to those of other AA 2XXX alloys. In this work we investigate the possibility of enhancing the performance of AA 2139 via a nanostructuring approach involving the consolidation of cryomilled powders. For comparison purposes, two types of feedstock powders (cryomilled and unmilled, gas-atomized powder), were consolidated via dual mode dynamic forging. Our results show that, following heat treatment (HT), the strength of the cryomilled material increases in the range of ∼25% to ∼200% relative to that of the unmilled counterparts, depending on specific processing parameters. We present microstructural data, including grain size and precipitate chemistry, to provide insight into the underlying strengthening mechanisms. Vickers microhardess tests are used to evaluate peak heat treatments, and tensile testing is performed to characterize mechanical behavior. The kinetics of precipitation, strengthening mechanisms and deformation behavior are discussed. It is proposed that the combination of elemental segregation with the presence of oxides along grain boundaries, both facilitated by enhanced diffusion paths, are responsible for the observed change in HT kinetics in the cryomilled material. [ABSTRACT FROM AUTHOR]

Abstract: Nanoscratch tests have been conducted at room temperature on M-plane sapphire under a ramp loading condition from 100μN to 200mN along a scratch length of 200μm at a scratch velocity of 1μms−1 using a Berkovich nanoindenter XPS system in scratch mode. Plastic deformation features, including short shaving scratch debris, linear surface features along the scratch groove, pile-up, and fish-bone features, indicating a stick–slip mechanism, and brittle deformation features, i.e. microcracking, chipping and tearing, were observed by optical, scanning electron, and atomic force microscopy. Applying the focused ion beam (FIB) technique, site-specific cross-sectional transmission electron microscopy further revealed nanoscratch-induced deformation behavior. Basal twinning was observed in the region below the scratch. The evolution of plastic deformation mechanisms can be summarized as lattice disorder, dislocation loops, stacking faults, dislocation glide and then basal twin formation. The observed features were rationalized using elastic and residual stress field calculations. [Copyright &y& Elsevier]

The nanoscratch behavior of two bulk α-alumina samples with 1.3μm and 290nm average grain sizes, respectively, was investigated using a nanoindenter in scratch mode, in combination with atomic force and scanning electron microscopy. A ductile to brittle transition was observed in the fine-grained sample, while the ultrafine-grained sample exhibited predominantly ductile deformation with a fish-bone feature indicative of a stick–slip mechanism. These findings suggest that grain refinement can increase the potential for plastic deformation in ceramics. [Copyright &y& Elsevier]

Abstract: The effect of cryomilled bond coat material (MCrAlY) on the improvement of service life of Thermal Barrier Coatings (TBCs) was investigated. Creep resistance of the bond coat was greatly increased by the dispersoids formed during cryomilling. The creep mechanisms were found to be based on dislocation detachment from dispersoids for the cryomilled sample and grain boundary sliding for the conventional one. The results suggest that creep behavior has a great influence on the thermo-mechanical properties of the TBC. [Copyright &y& Elsevier]

The effect of Y concentration on the slip and twinning mechanisms in binary Mg–Y alloys are investigated using transmission electron microscopy, electron backscattered diffraction, and visco-plastic self-consistent polycrystal constitutive modeling. Four concentrations of Y are studied in hot-rolled and recrystallized sheet material. The materials were deformed in tension and compression in the rolling direction and compression in the normal direction in order to invoke distinct proportions of slip and twin mechanisms with each test. Within the single crystal hardening model used in polycrystal modeling, a slip-twin interaction law is introduced to account for dislocation density reductions due to dislocation absorption during twin boundary migration. We show that increasing Y concentration reduces the intensities of both the initial and deformation textures. During deformation, the plastic anisotropy in yield stress, the tension-compression asymmetry, and amount of 10 1 ‾ 2 ⟨ 1 ‾ 011 ⟩ twinning is shown to decrease with increasing Y. For each alloy, the model identifies a single set of material parameters that successfully reproduced all measured stress-strain curves and achieved agreement with measured deformation textures and twin area fractions. Transcending texture effects, the model interpretation of the flow responses suggests that increased concentrations of Y increase the critical resolved shear stress for basal slip but have negligible effects on the other slip modes. The reduced plastic anisotropy with increases in Y is explained by a concomitant decrease in the prismatic-to-pyramidal slip critical resolved shear stress ratio. The model suggests that their nearly equivalent critical resolved shear stress values lead to the enhanced non-basal activity of Mg–Y alloys, which was confirmed by transmission electron microscopy. The calculations suggest that beyond any texture differences, this reduction in twinning can be attributed to a slightly increased resistance for 10 1 ‾ 2 ⟨ 1 ‾ 011 ⟩ twin propagation, particularly in the binary with the highest Y content. [ABSTRACT FROM AUTHOR]