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The effect of length scale on mechanical strength is a significant consideration for semiconductor materials. In III-V semiconductors, such as InSb, a transition from partial to perfect dislocations occurs at the brittle-to-ductile transition temperature (~150 °C for InSb). High temperature micro-compression reveals InSb to show a small size effect below the transition, similar to ceramics, while in the ductile regime it shows a size effect consistent with fcc metals. The source truncation model is found to agree with the observed trends in strength with size once the change in Burgers vector and bulk strength are taken into account.

The size dependence of the mechanical behavior of a single crystalline equiatomic FeCrCoMnNi single phase high entropy alloy was studied using in situ SEM microcompression. Electron back-scattered diffraction was used in conjunction with high-resolution scanning electron microscopy to identify the dominant slip system activated for accommodating plastic flow. The scaling of the yield strength with the size of the micropillar is discussed in comparison with the size dependence observed in face-centered and body-centered cubic single crystalline metals.

Based on a combination of micropillar compression experiments and modelling of the secondary cell wall (cw) using continuum micromechanics, the shear yield stress of the polymer matrix is identified for both normal and compression wood of Norway spruce. It is shown that the model is able to capture the differences in mechanical properties between the two tissues based on the knowledge of composition of the samples, microfibril angle, as well as phase properties on the nanometer scale. By testing an isolated piece of the cell wall with a homogeneous and uniaxial stress field on the micrometer scale and using the micromechanical model to determine average stress fields on the nanometer scale, it is possible to identify the shear yield stress of the polymer matrix in wood, which was found to be in the range of 14.9–17.5 MPa for normal and compression wood. It was shown that this corresponds to a stress in the lignin phase of approx. 17 MPa. This combined study thus demonstrates a new approach for validating ...

Nanolaminates of Cu and W with a bilayer thickness ratio of 1:1 and individual layer thicknesses of 5, 20 and 100 nm were studied by using elevated temperature microcompression. Failure in all the multilayers with different interlayer thicknesses occurs by rupture of the W interlayers, resulting in local strain softening and shearing at ambient temperature and lateral plastic flow of the Cu layers confined between the harder W layers at lower stresses with increasing testing temperatures.

The deformation and failure mechanism of a multilayered thin film consisting of alternating soft Cu and hard TiN interlayers has been studied by in situ SEM compression of micro-pillars and finite element simulations. While the yielding of the multilayer is governed by the 'size-dependent' strength of Cu, the failure was found to occur by shearing of the columnar grains of TiN. At elevated temperatures of 200 and 400 °C, the yielding of the multilayers is governed by the stress-assisted diffusion of the Cu interlayers, which coalesce into microcrystals and grow into larger faceted crystals.

The elevated temperature performance of a wide range of chromium nitride-based hard coatings was evaluated using in situ micro-compression in the scanning electron microscope. This allows the first direct measurement of the high temperature compressive strength, rather than the hardness, of such coatings. The microstructure of the coatings was analyzed using X-ray diffraction and focused ion beam cross sectioning followed by electron microscopy. Micropillars were examined using electron microscopy before, during and after compression. Trends in deformation behavior and failure stress with temperature are discussed in relation to the coatings' microstructures and their room temperature mechanical properties.

The single crystal deformation behaviour of orthorhombic Al13Co4 hasbeen studied below the brittle-ductile transition temperature observedin bulk material from room temperature to 600 degrees C, usingindentation, microcompression and transmission electron microscopy. Atroom temperature, slip occurred most easily by dislocation motion on the(0 0 1)[0 1 0] slip system, as observed in the ductile regime at hightemperatures. However, as the temperature was increased towards 600degrees C, the slip pattern changed to one consisting of linear defectsrunning perpendicular to the loading axis. Serrated flow was observed atall temperatures, although at 600 degrees C the magnitude of theserrations decreased. Anomalous yielding behaviour was also observedabove 226 degrees C, where both the yield and the 2\% flow stressincreased with temperature, almost doubling between 226 and 600 degreesC. It has been suggested that this might arise due to the increasingstability of orthorhombic Al13Co4 with respect to the monoclinic formwith increasing temperature. This is shown to be consistent with thetheoretical predictions that exist.

The elevated temperature mechanical response of a Zr-based bulk metallic glass (BMG) was examined using in situ microcompression in a scanning electron microscope at temperatures up to 387 °C. The flow stress was found to remain constant with temperature at ∼2 GPa below the glass transition temperature, T g . The magnitude of the stress drops/serrations in the stress–strain curve was found to increase with temperature. Above T g , plastic flow was observed to be homogeneous without any shear band formation.

Diamond ⟨100⟩- and ⟨111⟩-oriented nanopillars were fabricated by focused ion beam (FIB) milling from synthetic single crystals and compressed using a larger diameter diamond punch. Uniaxial compressive failure was observed via fracture with a plateau in maximum stress of ∼0.25 TPa, the highest uniaxial strength yet measured. This corresponded to maximum shear stresses that converged toward 75 GPa or ∼ G/7 at small sizes, which are very close to the ultimate theoretical yield stress estimate of G/2π.

The elevated temperature indentation response of a Zr-based bulk metallic glass (BMG) was examined using a novel in situ SEM indentation system modified with tip and sample heating. Hardness was found to linearly increase with temperature from 7 GPa at 25 °C to 7.6 GPa at 200 °C, while the modulus remained constant within measurement accuracy. Adhesion between indenter and the BMG was observed to increase with increasing temperature. Both the number and pointed morphology of the surface shear offset displacements were found to decrease with increasing temperature. Conversely, the magnitude of the load drops/serrations in the load displacement curve was found to increase dramatically with temperature. The transitions from smooth to serrated flow were observed to fall within the temperature–strain rate ranges observed by other researchers in uniaxial compression.

In situ Raman micro-pillar compression has been shown to be capable of determining the onset of plastic deformation as well as characterizing plastic deformation and fracture. The compression experiments were carried out on 10 µm diameter single crystalline GaAs micro-pillars at room temperature. The deformation and fracture events were revealed by a shift in position and broadening of the characteristic Raman peak in agreement with the load drops measured by the load cell and post-mortem observations from scanning electron microscopy.

We demonstrate that irradiation may enhance the plasticity in metallic glasses by increasing the free-volume content via micropillar compression experiments on an ion-irradiated bulk metallic glass (BMG). Results show that irradiation decreases the flow stress and enhances the shear band formation by lowering the magnitude of stress serrations in plastic flow regime. These results highlight that amorphous alloys can mitigate the deleterious affects of severe ion irradiation as compared to their crystalline counterparts.

Journal of Materials Research, 24 (8), ISSN:0884-2914, ISSN:2044-5326

Uniaxial compression experiments on 0.3, 1 and 3 mu m diameter micropillars of a Zr-based bulk metallic glass in as-cast, shot-peened and structurally relaxed conditions were conducted. Shear band formation and stable propagation is observed to be the plastic deformation mode in all cases, with no detectable difference in yield strength according to either size or condition. The limitations of uniaxial compression tests in assessing the influence of various material conditions on plasticity, when it is inhomogeneous in nature, are illustrated.

The dry sliding friction coefficient μ and the wear volume loss W, in a zirconium-based bulk metallic glass (BMG) under high-frequency (50 Hz) reciprocating conditions, were investigated with the objective of assessing the influence of free volume and crystallization on the wear behavior of amorphous metals. The BMG samples were annealed either below the glass transition temperature Tg to induce structural relaxation and hence reduce the free volume that controls plasticity through shear-band formation or above Tg to crystallize the amorphous BMG prior to wear testing. Results show that the wear behavior of both the as-cast and relaxed glasses was dominated by the oxidation of the surface layers. A sharp transition in the contact electrical resistance complemented by a marked increase in μ was noted. This was attributed to the formation of a thick tribo film with high oxygen concentration and its subsequent delamination. The μ values, before as well as after the transition, in the relaxed glasses were similar to those for the as-cast alloy. However, a gradual decrease in W with annealing temperature was observed. A good correlation between W and nanohardness was noted, implying that the intrinsic hardness in the BMGs controlled the wear rate.

Nanocrystalline and ultrafine-grained materials show enhanced strain rate sensitivity (SRS) in comparison to their coarse grained counterparts. Majority of SRS measurements on nanocrystalline thin films reported in literature have focused on nanoindentation-based approaches. In this paper, micropillar strain rate jump tests were demonstrated on an electrodeposited nanocrystalline nickel film from 25 to 100 °C. SRS exponent, m, and activation volume, V, values were determined as a function of temperature. The measured values were found to be in good agreement with previously reported literature on bulk and nanoindentation measurements. Apparent activation energy for deformation was found to be about 100 kJ/mol, which is close to that for grain boundary diffusion in nickel. Grain boundary sliding was observed in the deformed pillars from scanning electron microscopy images.

Nanoindentation experiments were conducted on a Ni+ ion-irradiated Zr-based bulk metallic glass (BMG). The irradiation was carried out using 2.5, 5, 10 and 15 MeV ions and a flux of similar to 10(16) ions/cm(2). Post mortem imaging of the indents reveals a transition in the deformation mechanism of the irradiated regions from heterogeneous shear banding to homogeneous flow. Additionally, the load-displacement curves exhibit a transition from serrated to continuous flow with increasing severity of irradiation damage. The stress-strain response obtained from micro-pillar compression experiments complements the indentation response exhibiting a decrease in the flow stress and an `apparent' strain hardening at the lowest irradiation damage investigated, which is not observed in the as-cast alloy. (C) 2011 Elsevier B.V. All rights reserved.

Structural relaxation behavior of a rapidly quenched (RQ) and a slowly cooled Pd40Cu30Ni10P20 metallic glass was investigated and compared. Differential scanning calorimetry was employed to monitor the relaxation enthalpies at the glass transition temperature, T g , and the Kolrausch–Williams–Watts (KWW) stretched exponential function was used to describe its variation with annealing time. It was found that the rate of enthalpy recovery is higher in the ribbon, implying that the bulk is more resistant to relaxation at low temperatures of annealing. This was attributed to the possibility of cooling rate affecting the locations where the glasses get trapped within the potential energy landscape. The RQ process traps a larger amount of free volume, resulting in higher fragility, and in turn relaxes at the slightest thermal excitation (annealing). The slowly cooled bulk metallic glass (BMG), on the other hand, entraps lower free volume and has more short-range ordering, hence requiring a large amount of perturbation to access lower energy basins.

The fatigue lives of a Zr-based bulk metallic glass (BMG) in the as-cast and shot-peened conditions were evaluated. Results suggest that shot peening does not cause significant enhancement of the fatigue performance of the BMG. Cracks were observed to nucleate in subsurface regions. This was attributed to the domination of the compressive residual stress field on the surface over deformation-induced plastic flow softening that otherwise leads to easy nucleation of shear bands which act like microcracks.

The mechanical behavior of single crystalline hydroxyapatite is examined through instrumented nano- and microindentation experiments on prism and basal planes. The results indicate that the c-axis orientation is stiff and hard, and significant anisotropy in properties was found. Nanoscale plasticity is observed in the form of pile-up along the nanoindentation edges and manifests as discrete displacement bursts in the load vs. depth of penetration curves. Toughness also shows significant anisotropy and is lower compared with the values reported for the polycrystalline hydroxyapatite.

The constraint factor, C , given by the hardness to the yield strength ratio and an indirect measure of the pressure sensitivity of plastic flow in amorphous alloys, of a Zr-based bulk metallic glass was measured as a function of temperature. Results show that, contrary to expectations, C increases linearly from ∼3.2 to 3.5 within the 300–525 K range. Gradual transition in the shear band-dominated deformation mechanism was suggested as a possible reason for the observed increase in C .