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Concept

Grosseur de grains

Résumé

vignette| Table de classification de Wentworth d'après United States Geological Survey. Notons la présence de deux fautes de frappe 33,1 mm au lieu 38,1 mm et 0,545 mm au lieu de 0,594 mm. vignette| Plage de cailloux (en l'occurrence des galets) à Nash Point, dans le sud du Pays de Galles. La taille ou grosseur des grains ou de particules (ou granulométrie ) est le diamètre des grains individuels de sédiment ou des particules lithifiées dans les roches clastiques. Le terme peut également être appliqué à d'autres matériaux granulaires. Un seul grain peut être composé de plusieurs cristaux. Le matériau granulaire peut aller des très petites particules colloïdales, aux plus grosses en passant par l'argile, le limon, le sable, les graves, les cailloux ou les blocs. Échelle de Krumbein Phi Les plages de taille définissent les limites des classes qui reçoivent des noms dans l'échelle de Wentworth (ou échelle Udden – Wentworth) utilisée aux États-Unis . L'échelle de Krumbein phi

Source officielle

https://fr.wikipedia.org/wiki/Grosseur_de_grains

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Nanocrystalline (NC) metals have attracted widespread interest in materials science due to their high strength compared to coarse-grained counterparts. It is well know that during uniaxial deformation, the stress-strain behaviour exhibits an extraordinary work-hardening followed by an early observation of constant flow stress. Information on possible deformation mechanisms have been gathered by extensive research combining in situ deformation experiments, electron microscopy observations and computational modelling. Generally, these mechanisms are categorized into two types: dislocation slip and grain boundary (GB) accommodation processes, as for instance, GB sliding based mechanisms and GB migration eventually coupled to the shear stress. However, the interplay between these mechanisms resulting in the constant deformation resistance is not fully understood. Transient testing has proven to be a suitable tool to gather information on the rate limiting deformation mechanisms that are activated during the deformation path. In particular in stress reduction experiments, after an intermediate/large stress drop thermally activated dislocation slip is suppressed so that other underlying mechanisms are brought into foreground during subsequent transient creep. Those mechanisms may play a minor role in the determination of the flow stress but might still be essential to the development of a constant deformation resistance. Within this thesis, transient testing is combined with in situ X-ray diffraction at the Swiss Light Source. Therefore, the transient responses are captured in terms of evolution of macrostrain as well as diffraction peak broadening. Since dislocation slip and GB accommodation have an opposite footprint on the peak broadening, the presence of these two types of mechanisms can be distinguished. Three electrodeposited NC materials with different grain sizes are investigated: two NC Ni batches and one NC Ni50Fe50 batch. The results reveal that the constant flow stress reached during uniaxial deformation of electrodeposited NC metals reflects a quasi-stationary balance between dislocation-based mechanisms and GB-mediated accommodation. The latter plays an important role in producing plastic strain and recovering the defects and internal stress. Depending on the magnitude of the stress drop, a non-monotonic behaviour of the diffraction peak width is observed, suggesting an alternation of mechanisms. Also, by comparing transient responses among different NC materials, the relative contributions of dislocation slip and GB accommodation mechanisms are discussed in terms of grain size and alloying. Finally, different magnitudes of stress reduction are carried out by molecular dynamics (MD) simulations with the aim to verify in situ experimental results and explore the mechanisms responsible for GB accommodation. MD simulations confirm that dislocation slip is reduced after a moderate stress drop, however can continue to operate after adaption of the GB structures by a variety of GB accommodation mechanisms explaining the non-monotonic behaviour of the peak broadening during transient creep.

EPFL2016

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Grain Boundary Relaxation in 18-carat Gold Alloys

Ann-Kathrin Audren

Grain boundaries (GBs) play an important role for the mechanical properties of metals. In addition to dislocation motion inside the grains, some metal alloys deform along the GBs at high temperatures. GBs can also be the cause of a brittle behaviour of some metallic alloys, where crack propagation along the GBs is observed. This thesis aims to understand the role of GBs on the thermo-mechanical behaviour of metals and especially to define the microscopic mechanism, which leads to a deformation at the GBs. In this thesis, polycrystals, single crystals and bi-crystals of a yellow gold alloy are studied in detail, primarily by mechanical spectroscopy. The analysis and interpretation of the experimental data identifies different anelastic relaxations (internal energy dissipation processes), that produce peaks in the mechanical loss spectrum. The mechanical loss spectrum of polycrystals shows a relaxation peak P2 at about 780K, which is absent in single crystals made from the same alloy. Stepwise deformation of a single crystal causes an increase of the high temperature mechanical loss background and the appearance of a high temperature peak (P3). Above a critical deformation, P3 disappears and the peak P2, which is normally observed in polycrystals, appears. The increase of the exponential background is interpreted as due to the introduction of new dislocations whereas the peak P3 is attributed to a dislocation relaxation mechanism in the sub-grain boundaries. The peak P2 located at intermediate temperatures depends on the grain size: with grain growth, the peak position shifts to higher temperatures. It is shown that the relaxation time is proportional to the grain size d. Such a grains size dependence is in agreement with the Zener model based on geometrical considerations of an assembly of elastic grains, which can slide against each other. The investigations on bi-crystals containing a single GB with a specific misorientation between crystal lattices and a specific boundary plane orientation show that the relaxation peak P2 is closely related to a mechanism taking place at the boundary. The peak height is proportional to the GB density in a bi-crystal, whereas a single crystalline part cut from a bi-crystal does not show a relaxation peak. Molecular dynamics simulations are performed in order to illustrate the potential microscopic mechanisms responsible for the stress relaxation peak P2 in polycrystals. A Sigma5 grain boundary is submitted to a shear deformation parallel to the boundary plane. The grain boundary shows a migration perpendicular to the boundary plane coupled to shear for temperatures below 700K. Above 1000K, only grain boundary sliding is observed. Two models are developed that provide expressions for the relaxation strength and the relaxation time that are compared to experimental measurements performed on polycrystals. The observed grain size dependence favours the sliding model over the migration model. Measurements as a function of stress allow a refinement of the sliding model. The stress amplitude dependence of the peak P2 indicates a depinning mechanism, which is interpreted as due to GB dislocations between zones where sliding of different amount occurred. Obstacles to the sliding movement like steps or extended coincidence sites in the GB plane act as pinning points. The pinning points can be overcome at high temperatures close to the melting point, which results in the onset of local microscopic creep.

EPFL2014

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Deformation behavior of nanoporous polycrystalline silver layers sintered by low temperature bonding: experiment and simulation

Saba Zabihzadeh

Integrated circuit packaging technology has become a prime design consideration for the develop-ment of electronic system concepts. One key issue is the bonding layer between chip and substrate. Currently, high-lead solder materials are being used, which apart from their environmental problem, limit the reliable operation temperature of devices to temperatures lower than 175ºC. Low-temperature bonding by sintering nanograin sized silver is emerging as a promising replacement in power electronics modules working at high temperatures. Although silver has been studied as bonding material since three decades, it has not been widely used in the packaging industry which might be ascribed to the limited knowledge on its mechanical behavior. In this work, the microstructure of thin layers of nanoporous silver sintered under different conditions are studied and linked to the mechanical behavior (at room temperature) using X-ray diffraction in-situ deformation tests and finite element simulations. First the microstructure of several samples such as grain size distribution, defect structures, porosity, pore morphology and ligament size distribution are investigated and are related to the corresponding sintering conditions. Electron microscopy, high resolution X-ray imaging, and X-ray diffraction techniques are the employed characterization methods. We found a strong dependency of the microstructure on the sintering conditions. Afterward, series of X-ray diffraction in-situ continuous, load-unload and stress relaxation tensile tests are performed to study the deformation mechanisms at room temperature. All samples exhibit small elongation and a large hysteresis in the stress-strain curve. From the change in the peak position upon unloading the residual elastic strain inside the sample is calculated while the evolution of the peak broadening provides information on the recovery mechanisms and induced dislocations. A significant recovery in the peak position and peak broadening after unloading and during relaxation is observed for all specimens. This is attributed to the presence of the pores. We found that the density of generated dislocations during tensile deformation depends on the porous structure. The results suggest that there exist two contributions to deformation: one related to the pores and other to intra-granular dislocations. The competition between both mechanisms is discussed in terms of porosity and grain size. Finally, 3D FE microstructure-based simulations are performed to allow a better correlation between the porous morphology and mechanical behavior. The 3D reconstructed images of nanoporous silver obtained from X-ray ptychography are used as an input to create finite element meshes of the actual porous morphology. For homogenization purpose, a representative volume element approach is chosen. The apparent elastoplastic response of the bulk silver is extracted from the porous samples using inverse homogenization method. The simulations allow distinguishing between the interplay and role of the ligament size, pore morphology and porosity, and provide a better comprehension on the experimental observations. We show that the proposed model has a predictive character for general nanoporous silver microstructures.

EPFL2016

Chargement

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Deformation behavior of nanoporous polycrystalline silver layers sintered by low temperature bonding: experiment and simulation

Saba Zabihzadeh

EPFL2016

Grain Boundary Relaxation in 18-carat Gold Alloys

Ann-Kathrin Audren

EPFL2014

EPFL2016