On the contribution of nanomechanical testing to the study of Earth mantle deformations

Nanogeodynamics is based on the belief that some answers concerning the dynamic evolution of our planet find their origin and their explanation in microscopic mechanisms within rocks and their constituent minerals. The particularity of these deformations is that they develop over time scales that exceed those accessible to humans. Building constitutive equations necessary for the modeling of terrestrial deformations must be based on very rigorous physics. Nanomechanical tests offer the possibility to isolate elementary mechanisms and to quantify their efficiency. We present examples that focus on a magnesium-iron silicate: olivine. This mineral is the most abundant (\u3e 60 % in volume) and weakest phase in the Earth’s upper mantle of which it controls the rheology. The lithospheric mantle (where plate tectonics couples with mantle convection) can be subject to temperatures as low as 500 °C. Experimental deformation of mantle rocks at such low temperatures is a major challenge in mineral and rock physics, since the strain rates necessary to achieve steady state dislocation creep are too low to be performed in the laboratory with standard techniques. The use of small-scale specimens shifts the brittle ductile transition and allows to activate ductile mechanisms which can be quantitatively studied in situ in a transmission electron microscope (TEM). In this presentation, we will present two studies on two different deformation mechanisms of olivine. One is dislocation glide which can be activated and followed in situ in the TEM . The quantification of dislocation velocities provides a new approach to evaluate the low-temperature rheology of olivine. The second one is grain boundary sliding resulting from stress-induced amorphization. Nanomechanical testing sheds light on the stress-induced amorphization mechanisms. It also provides a unique opportunity to study the mechanical properties of amorphous olivine which shear deformation controls grain boundary slidin

Effect of hydriding on nanoscale plasticity mechanisms in nanocrystalline palladium thin films

Thin palladium (Pd) membranes constitute an enabling material in hydrogen permeation and sensing applications. During hydriding of Pd, as long as the H/Pd (atomic ratio) stays below αSSmax≈0.02, the α-Pd with face centered cubic (fcc) lattice will expand from 3.889 Å to 3.895 Å. When the ratio reaches 0.02 a β-phase, again fcc based, having a lattice constant near 4.025 Å appears which induces a 10% volume change. In the present work, nanoscale plasticity mechanisms activated in sputtered nanocrystalline (nc) Pd thin films subjected to hydriding at different hydrogen pressures have been investigated for the first time using advanced TEM. The in-situ measurement of the evolution of the internal stress during hydriding shows that the internal stress increases rapidly and reaches a constant value of 120 MPa tensile stress for α phase and 920 MPa compressive stress for β phase transformation. The automated crystallographic orientation mapping in TEM (ACOM-TEM) before and after hydriding to α and β phase did not reveal significant changes of the grain size and the crystallographic texture, excluding grain boundary mediated processes as dominant hydrogen induced plasticity mechanisms. High resolution TEM (HRTEM) investigation of ∑3 {111} coherent twin boundaries (TBs) in Pd films shows clear loss of the coherency of these boundaries after hydriding to β phase. However, significant changes of microstructure have not been observed in Pd films hydrated to α phase. These results confirm that hydrogen induced plasticity is mainly controlled by dislocation activity at higher hydrogen pressures. Surprisingly, an fcc→9R phase transformation at Σ3 {112} incoherent TBs as well as a high density of stacking faults (SFs) (Fig. 1a) have been observed after hydriding to β phase indicating a clear effect of hydrogen on the stacking fault energy of Pd. Shear type faulted loops rarely reported in nc materials were also observed within the Pd grains after hydriding to β-phase (Fig. 1b). In order to investigate the stability of this shear type loops, different internal stress fields originating from the neighboring dislocation (dislocation d3 ) and surface effects (image forces) have been computed using a Finite Element method (Fig. 1c). Such calculations confirm that high attractive forces exist between the dislocation “d2” and “d3” forming the dipole. On the other hand, although the Peach Koehler force on the dislocation “d1” tends to extend the SF, the force magnitude is much smaller than the force induced by the fault on the partial segments. Therefore, an extra shear stress of +385MPa (τdis.) acting on the glide plane of the dislocation “d1” is required in order to counter balance the attractive force of the SF which thus explains the stability of this dislocation in the TEM thin foil after dehydriding. This shear stress can not be compensated by the negligible image force in such thin foil. Moreover, no residual hydrides were detected using high resolution electron energy loss spectroscopy. Therefore, the stability of glissile intrinsic SF loops in nc Pd films after dehydriding can thus be attributed to the presence of large internal stress heterogeneities typical of nc materials

Graphene effect on mechanical response of copper film

This research is investigated the effect of the presence of a single layer graphene on the development of the contact plasticity inside a copper underlying substrate. As a matter of fact, a film of copper (deposited on a Si wafer) is the substrate used in the CVD process for graphene production, there is no need for transferring graphene which avoids any possible artifacts. Moreover, the adhesion between CVD-grown graphene and the underlying Cu film is larger than transferred graphene, since during transfer, wrinkles and ripples may form, thus weakening the interaction between graphene and the substrate. The bare Cu-film in the same condition as to produce graphene except that no methane was introduced into the chamber (the last step in graphene production). Nanoindentation was performed on the Cu-film with and without graphene. Nanoindentation was performed on the bare Cu-film also Cu-film with graphene. The same process, as the growth of graphene on Cu-film, was performed on bare Cu just without introducing the methane flow at the last step. The analysis of the force-displacement curves indicates that the presence of graphene modifies the onset of plasticity which appears in the form of a burst which is called pop-in. The first pop-in occurs at lower loads and the pop-in lengths are smaller with graphene in comparison to the bare Cu-film. The magnitude of the effect of the presence of a graphene cap layer varies also with respect to the orientation of the indented Cu grain. In order to understand the root causes of these effects of the presence of graphene on the plastic flow, transmission electron microscopy is used to compare samples after nanoindentation in terms of dislocation structures. 3D discrete dislocation dynamics simulations are performed to analyze the long-range back stress that are generated by the dislocation arrangements with and without graphene. To further extend this research and investigate the known effect of hardening by graphene insertion into metals, another system has been addressed which involves the deposition of a Cu film on top of the graphene layer, lying itself on top of the annealed Cu substrate. The presence of graphene caused marked effect on the indentation response in this case, even larger than in the first configuration

Production and Marketing of Spring Greenwrap Tomatoes in the Lower Rio Grande Valley.

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Controlled precipitation in a new Al-Mg-Sc alloy for enhanced corrosion behavior while maintaining the mechanical performance

peer reviewedThe hot working of 5xxx series alloys with Mg ≥3.5 wt% is a concern due to the precipitation of β (Al3Mg2) phase at grain boundaries favoring Inter Granular Corrosion (IGC). The mechanical and corrosion properties of a new 5028-H116 Al-Mg-Sc alloy under various β precipitates distribution is analyzed by imposing different cooling rates from the hot forming temperature (i.e. 325 °C). The mechanical properties are maintained regardless of the heat treatment. However, the different nucleation sites and volume fractions of β precipitates for different cooling rates critically affect IGC. Controlled furnace cooling after the 325 °C heat treatment is ideal in 5028-H116 alloy to reduce susceptibility to IGC after sensitization

Etude par MET avancée des mécanismes de plasticité dans des films minces Palladium nanocristallin sous chargement en Hydrogène

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Study of twinning mechanisms in Fe-Mn-C TWIP steels using transmission electron microscopy

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Étude des mécanismes élémentaires de plasticité et de fracture dans des films minces d’aluminium nanocristallin par des tests nanomécaniques in situ ACOM-TEM

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TEM characterization of twinned nanocrystalline palladium freestanding thin films deformed using lab-on-chip technique

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