Effect of hydrogen on the nucleation and motion of dislocations

Conventional mechanical tests are costly, time consuming, and due to their large scale, not very successful in obtaining mechanistic information. In contrast, the local method like nanoindentation, compression or bending test of micro pillars have comprehensive ranges of possibilities to achieve an essential understanding about the influence of substitutional atoms and/or interstitial atoms (e.g., hydrogen and nitrogen) on the mechanical properties like Young´s modulus, Gibbs free energy for homogeneous dislocation nucleation, dislocation line energy and also friction stress. These methods allow us to measure the mechanical behavior in simulated environments and atmospheres close to the routine industrial applications. Nanoindentation was applied for studying the sensitivity of various materials like nickel, Cu, steels and iron aluminides to hydrogen embrittlement, as it offers sufficiently high resolution in determining load and displacement and works effectively non-destructive. Nevertheless, the method of in-situ nanoindentation suffers from the complexity of the stress field below the nanoindenter. Furthermore, a novel method was developed where miniaturized compression samples are machined using focused ion beam (FIB) milling and loaded in a nanoindenter system equipped with a flat diamond punch. This method is able to probe mechanical properties on the micrometer and sub-micrometer scale under nominally uniaxial loading. Additionally, very small volume of pillar guarantees a fast and homogeneous distribution of hydrogen. More recently the influence of hydrogen on the elastic properties and interaction of dislocations was studied using the in-situ bending test of micro pillars (see Figure 1). The advantage of the bending test is the presence of high tensile stress in the pillar during the test. It is in contrast to other techniques like in-situ nanoindentation or micropillar compression tests with the compressive stress field which works as driving force for the hydrogen diffusion out of the highly stressed region

Temperature-dependent size effects on the strength of Ta and W micropillars

The strength of metals increases with decreasing sample size, a trend known as the size effect. In particular, focused ion beam-milled body-centered cubic (BCC) micropillars exhibit a size effect known to scale with the ratio of the test temperature to the critical temperature (Tc) of the BCC metal, a measure of how much the yield stress is governed by the lattice resistance. In this paper, this effect is systematically studied by performing high-temperature compression tests on focused ion beam-manufactured Ta and W single crystal pillars ranging in diameter from 500 nm to 5 μm at temperatures up to 400 °C, and discussed in the context of bulk strength and size dependent stresses. Both metals show larger size effects at higher temperatures, reaching values that are in the range of FCC metals at temperatures near Tc. However, it is demonstrated that size effects can be considerably affected by material parameters such as dislocation density and lattice friction, as well as by the yield criterion used. Furthermore, for W, a change from uniform wavy deformation to localized deformation is observed with increasing temperature and pillar size, further indicating that the temperature ratio strongly influences the relative motion of screw and edge dislocations

Plastic Deformation Modes of CuZr/ Cu Multilayers

We synthesized CuZr/Cu multilayers and performed nanoindentation testing to explore the dependence of plastic deformation modes on the thickness of CuZr layers. The Cu layers were 18 nm thick and the CuZr layers varied in thickness from 4 nm to 100 nm. We observed continuous plastic co-deformation in the 4 nm and 10 nm CuZr − 18 nm Cu multilayers and plastic-induced shear instability in thick CuZr layers (\u3e20 nm). The plastic co-deformation is ascribed to the nucleation and interaction of shear transformation zones in CuZr layers at the adjacent interfaces, while the shear instability is associated with the nucleation and propagation of shear bands in CuZr layers. Shear bands are initialized in the CuZr layers due to the accumulated glide dislocations along CuZr-Cu interfaces, and propagate into adjacent Cu layers via slips on {111} plane non-parallel to the interface. Due to crystallographic constraint of the Cu layers, shear bands are approximately parallel to {111} plane in the Cu layer

Surviving the surf: The tribomechanical properties of the periostracum of Mytilus sp

We investigated the friction and wear behavior as well as the mechanical properties of the periostracum of Mytilus sp. Tribological properties were determined with a reciprocal sliding microtribometer, while mechanical characterization was performed using a nanoindenter. Measurements were performed in dry and wet conditions. On the dry periostracum we found a low friction coefficient of 0.078 ± 0.007 on the young parts and a higher one of 0.63 ± 0.02 on the old parts of the shell. Under wet, saline, conditions we only observed one average coefficient of friction of 0.37 ± 0.01. Microscopic ex situ analysis indicated that dry periostracum wore rather rapidly by plowing and fatigue, while it exhibited a high wear resistance when immersed in salt water. The Young’s modulus and hardness of the periostracum were also investigated in both dry and wet conditions. Under dry conditions the Young’s modulus of the periostracum was 8 ± 3 GPa, while under wet conditions it was 0.21 ± 0.05 GPa. The hardness of dry periostracum samples was 353 ± 127 MPa, whereas the hardness of wet samples was 5 ± 2 MPa. It was found that, in the wet state, viscous behavior plays a significant role in the mechanical response of the periostracum. Our results strongly indicate that the periostracum can provide an important contribution to the overall wear resistance of Mytilus sp. shell

Micro-mechanical response of ultrafine grain and nanocrystalline tantalum

In order to investigate the effect of grain boundaries on the mechanical response in the micrometer and submicrometer levels, complementary experiments and molecular dynamics simulations were conducted on a model bcc metal, tantalum. Microscale pillar experiments (diameters of 1 and 2 μm) with a grain size of ~100–200 nm revealed a mechanical response characterized by a yield stress of ~1500 MPa. The hardening of the structure is reflected in the increase in the flow stress to 1700 MPa at a strain of ~0.35. Molecular dynamics simulations were conducted for nanocrystalline tantalum with grain sizes in the range of 20–50 nm and pillar diameters in the same range. The yield stress was approximately 6000 MPa for all specimens and the maximum of the stress–strain curves occurred at a strain of 0.07. Beyond that strain, the material softened because of its inability to store dislocations. The experimental results did not show a significant size dependence of yield stress on pillar diameter (equal to 1 and 2 um), which is attributed to the high ratio between pillar diameter and grain size (~10–20). This behavior is quite different from that in monocrystalline specimens where dislocation ‘starvation’ leads to a significant size dependence of strength. The ultrafine grains exhibit clear ‘pancaking’ upon being plastically deformed, with an increase in dislocation density. The plastic deformation is much more localized for the single crystals than for the nanocrystalline specimens, an observation made in both modeling and experiments. In the molecular dynamics simulations, the ratio of pillar diameter (20–50 nm) to grain size was in the range 0.2–2, and a much greater dependence of yield stress to pillar diameter was observed. A critical result from this work is the demonstration that the important parameter in establishing the overall deformation is the ratio between the grain size and pillar diameter; it governs the deformation mode, as well as surface sources and sinks, which are only important when the grain size is of the same order as the pillar diameter

Micro-mechanical response of ultrafine grain and nanocrystalline tantalum

In order to investigate the effect of grain boundaries on the mechanical response in the micrometer and submicrometer levels, complementary experiments and molecular dynamics simulations were conducted on a model bcc metal, tantalum. Microscale pillar experiments (diameters of 1 and 2 μm) with a grain size of ~100-200 nm revealed a mechanical response characterized by a yield stress of ~1500 MPa. The hardening of the structure is reflected in the increase in the flow stress to 1700 MPa at a strain of ~0.35. Molecular dynamics simulations were conducted for nanocrystalline tantalum with grain sizes in the range of 20-50 nm and pillar diameters in the same range. The yield stress was approximately 6000 MPa for all specimens and the maximum of the stress-strain curves occurred at a strain of 0.07. Beyond that strain, the material softened because of its inability to store dislocations. The experimental results did not show a significant size dependence of yield stress on pillar diameter (equal to 1 and 2 um), which is attributed to the high ratio between pillar diameter and grain size (~10-20). This behavior is quite different from that in monocrystalline specimens where dislocation 'starvation' leads to a significant size dependence of strength. The ultrafine grains exhibit clear 'pancaking' upon being plastically deformed, with an increase in dislocation density. The plastic deformation is much more localized for the single crystals than for the nanocrystalline specimens, an observation made in both modeling and experiments. In the molecular dynamics simulations, the ratio of pillar diameter (20-50 nm) to grain size was in the range 0.2-2, and a much greater dependence of yield stress to pillar diameter was observed. A critical result from this work is the demonstration that the important parameter in establishing the overall deformation is the ratio between the grain size and pillar diameter; it governs the deformation mode, as well as surface sources and sinks, which are only important when the grain size is of the same order as the pillar diameter.Fil: Yang, Wen. University of California at San Diego; Estados UnidosFil: Ruestes, Carlos Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Interdisciplinario de Ciencias Básicas. - Universidad Nacional de Cuyo. Instituto Interdisciplinario de Ciencias Básicas; ArgentinaFil: Li, Zezhou. University of California at San Diego; Estados UnidosFil: Abad, Oscar Torrents. Leibniz Institute for New Materials; AlemaniaFil: Langdon, Terence G.. University of Southern California; Estados UnidosFil: Heiland, Birgit. Leibniz Institute for New Materials; AlemaniaFil: Koch, Marcus. Leibniz Institute for New Materials; AlemaniaFil: Arzt, Eduard. Leibniz Institute for New Materials; Alemania. Universitat Saarland; AlemaniaFil: Meyers, Marc A.. University of California at San Diego; Estados Unido

Surface softening in metal-ceramic sliding contacts: An experimental and numerical investigation

This study investigates the tribolayer properties at the interface of ceramic/metal (i.e., WC/W) sliding contacts using various experimental approaches and classical atomistic simulations. Experimentally, nanoindentation and micropillar compression tests, as well as adhesion mapping by means of atomic force microscopy, are used to evaluate the strength of tungsten?carbon tribolayers. To capture the influence of environmental conditions, a detailed chemical and structural analysis is performed on the worn surfaces by means of XPS mapping and depth profiling along with transmission electron microscopy of the debris particles. Experimentally, the results indicate a decrease in hardness and modulus of the worn surface compared to the unworn one. Atomistic simulations of nanoindentation on deformed and undeformed specimens are used to probe the strength of the WC tribolayer and despite the fact that the simulations do not include oxygen, the simulations correlate well with the experiments on deformed and undeformed surfaces, where the difference in behavior is attributed to the bonding and structural differences of amorphous and crystalline W-C. Adhesion mapping indicates a decrease in surface adhesion, which based on chemical analysis is attributed to surface passivation

Thermal Treatment of c-BN compounds

Cubic boron nitride (c-BN) possesses excellent hardness, being the second hardest material after diamond. Thanks to its high abrasion resistance with ferrous alloys and oxidation resistance, c-BN is used for machining elements where diamond is weak. When sintering c-BN powder with other binders, some swelling and shrinkage occur at the pre-sintering step in the production of grinding and cutting tools at the company Element Six (E6), but the complicated physicochemical reactions taking place are not fully understood. Its study is then of great interest from the technological point of view, as the production of such tools could be improved.Two different c-BN powders provided by E6 were studied in the current project. Heat treatments of both batches (called B and C) were carried out by means of a dilatometer at different temperatures up to 1100 ºC at different heating rates, in two different furnaces, graphite and alumina furnace, and in argon atmosphere and vacuum in order to study the influence of the heating rate, temperature, furnace and atmosphere on both batches. Dilatometry was first used to determine length changes and temperatures where reactions occur, and afterwards X-ray diffractometry (XRD) was used to find out the compounds formed at different temperatures.On the one hand, the study performed on batch B, initially containing c-BN, tungsten (W), tungsten carbide (WC), cobalt (Co), aluminium (Al) and organic additives, revealed that ditungsten carbide (W2C), cobalt tungsten boride (CoWB), ditungsten boride (W2B) and dicobalt boride (Co2B) are formed when heated between 800 ºC and 1100 ºC, thus indicating that c-BN reacts with the hard compounds. On the other hand, the study carried out on batch C, initially containing c-BN, titanium nitride (TiN), titanium carbide (TiC), Al, another unidentified compound, and organic additives, showed the formation of new compounds at temperatures between 800 ºC and 1100 ºC. Such compounds are: aluminium nitride (AlN) and titanium boride (TiB2). Moreover, some unidentified compounds present in the raw powder react at high temperatures and therefore no traces of them are found at 1100 ºC.The study also showed that burnout and pyrolysis of the organic binders present in both batches occur between 250 ºC and 450 ºC. Thus, carbonaceous materials are left in the sample and they further react with the other compounds.Further investigation is needed in order to confirm the reactions suggested in the current project.Validerat; 20110713 (anonymous

Größeneffekte in kleinen kubisch-raumzentrierten metallischen Strukturen

Small-scale metal structures play a crucial role in a broad range of technological applications. However, knowledge of mechanical properties at this size scale is lacking. Size strengthening effects are generally experienced at the microscale. Compression of non-free defect body centered cubic (BCC) metal micropillars has revealed that the size effect of these metals scales with a temperature ratio that signifies how much the yield strength is governed by screw dislocation mobility. So far, no effort has been made to systematically study the effect of screw dislocation mobility and lattice resistance on the size effect in BCC-based metals. Thus, this work investigated this in BCC tungsten (W) and tantalum (Ta), as well as B2 beta-brass (β-CuZn) and nickel aluminide (NiAl). The influence of temperature on the size effect in W and Ta was studied up to 400 °C, whereas the room-temperature size effect in β-CuZn and NiAl was studied as a function crystal orientation and deformation rate. It was found that the size effect scaled with the magnitude of the lattice resistance, which is strongly related to the screw dislocation mobility. Direct evidence of the mobility of screw dislocations was observed for the first time. The results also showed that plastic anisotropy vanishes with decreasing sample size and that ductility is considerably improved, thus highlighting the importance of dislocation-nucleation controlled deformation and screw dislocation mobility at the sub-micron scale.Metallische Strukturen auf Mikroskala spielen bei einer Vielzahl von technologischen Anwendungen eine wichtige Rolle – jedoch sind die Kenntnisse über mechanische Eigenschaften in dieser Größenordnung begrenzt. Größeneffekte kommen i. d. R. auf Mikroskala zum Tragen. Druckversuche mit krz-Mikropillars konnten zeigen, dass der Größeneffekt auf Mikroskala hauptsächlich vom Temperaturverhältnis abhängt, die Fließspannung also durch die Mobilität der Schraubenversetzungen bestimmt wird. Bisher liegen keine systematischen Untersuchungen des Einflusses der Mobilität von Schraubenversetzungen und des Gitterwiderstandes auf den Größeneffekt von krz-Metallen vor. In der vorliegenden Arbeit wurde diese Fragestellung anhand von W und Ta sowie β-CuZn und NiAl aus der Phase B2 untersucht. Hierbei wurden der Temperatureinfluss auf den Größeneffekt von W und Ta bis 400°C und der Größeneffekt von β-CuZn und NiAl bei Raumtemperatur als Funktion der Kristallorientierung und Deformationsrate betrachtet. Es konnte gezeigt werden, dass der Größeneffekt mit dem Gitterwiderstand skaliert, somit also eng mit der Mobilität der Schraubenversetzungen in Zusammenhang steht. Die Ergebnisse von NiAl haben offengelegt, dass die plastische Anisotropie mit kleiner werdenden Proben bis in den Submikrometerbereich verschwindet und sich die Duktilität beträchtlich verbessert. Die Untersuchungen zeigen die Bedeutung von Deformation und Mobilität der Schraubenversetzungen bedingt durch Versetzungsnukleation

Small-scale mechanical behavior of zirconia

The surface stability of yttria-doped tetragonal polycrystalline zirconia is critical for load-bearing biomedical applications. In this work, the small-scale mechanical behavior of this material is probed by employing the in situ micro-cantilever bending technique to near-surface regions. Micro-cantilevers are milled by the focused ion beam technique in the as-sintered condition as well as after hydrothermal degradation by water vapor and tested in order to investigate the effect of degradation on the local flexural response. Results demonstrate that the technique is reliable for assessing the mechanical properties of thin superficial layers and their dependence on orientation. In the non-degraded material, the flexural strength is surprisingly higher than in standard-size specimens and transformation-induced plasticity takes place during testing, inducing defects that become critical at the failure stress. The strength and stiffness of cantilevers obtained from the degraded surface are indeed much lower, and the magnitude of the effect clearly depends on their orientation with respect to the surface. These results are discussed in terms of the presence and spatial distribution of microcracks nucleated during hydrothermal degradation.Peer Reviewe