Elucidating the Mechanical Response of Metallic Glasses Prepared in Different Structural States at Sub-Micron Length Scales

Metallic glasses (MGs) exhibit both high yield stresses and elastic strain limits owing to their metallic bonding character and lack of long-range order. Yet the structural state (i.e. local atomic packing), and the corresponding elastic and plastic mechanical response, of MGs is nuanced and dependent on processing history. Moreover, the interplay between small length scales and glass processing routes have produced seemingly conflicting results. Here, the influence of processing on MG mechanical behavior at sub-micron length scales is explored, revealing extreme sensitivity to ion irradiation, enhanced control over the mechanical response, and an underpinning of yield strength in thermodynamic properties. Using in situ testing methods, the deformation response of individual thermoplastically molded MG nanowires is studied. In contrast with previous literature reports the nanowire behavior is observed to be consistent with bulk deformation, exhibiting brittle fracture and shear banding at room temperature. To determine the role of processing at the nanoscale, ion irradiation is used to systematically alter the glassy structure of molded nanowires, leading to enhanced tensile ductility and reduced strength. A model for MG strength and ductility rationalizes the observations based on the glass transition temperature and a structure-dependent excess energy term. In addition, studying deformation at elevated temperature provides insight into the role of size and processing history on the mechanical properties in MGs. The Newtonian to non-Newtonian flow transition occurs at higher strain rates in nanoscale specimens. This suggests a more relaxed nanowire structural state, potentially owing to thermal processing, and a wider range of thermally accessible structures at the nanoscale. Finally, the range of structural states in MG thin films is explored by sputter deposition at different substrate temperatures. The maximum hardness increases more than 30% with deposition temperature, revealing a wide range of achievable glass structures. Together, the nanowire and thin film results emphasize the need to quantify glass structural state and suggest a potential processing -- structural state -- property relationship in MGs, correlating mechanical properties with thermodynamic quantities

CARACTERISATION GEOLOGIQUE ET GEOTECHNIQUE DES GLISSEMENTS DE LA VILLE DE BOUGAA. EST ALGÉRIEN.

La région de Bougaa est affectée par un glissement déplaçant un volume considérable de matériaux détritiques formés par des colluvions, des éboulis à gros blocs et des éboulis de pente argilo-limoneux rougeâtre reposant sur un substratum imperméable d’argilite schisteuse. Ce glissement est façonné dans des formations géologiques particulièrement sensibles à ce type de mouvement à cause de l'hétérogénéité de leurs faciès, de l'imperméabilité de certaines couches et de l’effet de la tectonique cassante et de la karstification. Le but de cette étude est de caractériser ces glissements à l’aide d’outils géologiques, géotechniques et géomorphologiques. Le rôle des eaux d’infiltration est primordial dans le déclenchement de ces glissements où la lithologie, la disposition structurale,le climat sont des facteurs favorables. La présence de pentes > 20 %, de sources émergeant du réservoir karstique de Guergour, créant des zones humides qui permettent le déclenchement du mouvement du sol dans cette région.  Bougaa region is affected by a shift moving a considerable amount of detritus formed by colluvial deposits, scree to boulders and scree slope reddish clay loam resting on a substratum of impermeable shale shale. This shift is shaped in geological formations particularly sensitive to this type of movement because of the heterogeneity of their facies, the impermeability of certain layers and the effect of tectonics and karstification. The purpose of this study is to characterize these shifts using tools geological, geotechnical and geomorphological features. The role of seepage is essential in triggering these shifts where lithology, structural arrangement, the climate is favorable factors. The presence of slopes> 20%, emerging sources of karst reservoir Guergour, creating wetlands that allow triggering of ground motion in this region

Quantifying the commonalities in structure and plastic deformation in disordered materials

The nonequilibrium nature of kinetically frozen solids such as metallic glasses (MGs) is at once responsible for their unusual properties, complex and cooperative deformation mechanisms, and their ability to explore various metastable states in the rugged potential energy landscape. These features coupled with the presence of a glass transition temperature, above which the solid flows like a supercooled liquid, open the door to thermoplastic forming operations at low thermal budget as well as thermomechanical treatments that can either age (structurally relax) or rejuvenate the glass. Thus, glasses can exist in various structural states depending on their synthesis method and thermomechanical history. Nanocrystalline (NC) metals, also considered to be far-from-equilibrium materials owing to the large fraction of atoms residing near grain boundaries (GBs), share many commonalities with MGs both in terms of plastic deformation and its dependence on processing history. Despite these similarities, the disorder intrinsic to both classes of materials has precluded the development of structure-property relationships that can capture the multiplicity of energetic states that glasses and GBs may possess. Here, we report on experimental studies of MG and NC materials and novel synthesis and processing routes for controlling the structural state – and as a consequence, the mechanical properties. A particular focus will be on strategies for rejuvenation of disorder with the goal of suppressing shear localization and endowing damage tolerance. We also describe a microscopic structural quantity designed by machine learning to be maximally predictive of plastic rearrangements and further demonstrate a causal link between this measure and both the size of rearrangements and the macroscopic yield strain. We find remarkable commonality in all of these quantities in disordered materials with vastly different inter-particle interactions and spanning a large range of elastic modulus and particle size

Structure-property relationships from universal signatures of plasticity in disordered solids

When deformed beyond their elastic limits, crystalline solids flow plastically via particle rearrangements localized around structural defects. Disordered solids also flow, but without obvious structural defects. We link structure to plasticity in disordered solids via a microscopic structural quantity, “softness,” designed by machine learning to be maximally predictive of rearrangements. Experimental results and computations enabled us to measure the spatial correlations and strain response of softness, as well as two measures of plasticity: the size of rearrangements and the yield strain. All four quantities maintained remarkable commonality in their values for disordered packings of objects ranging from atoms to grains, spanning seven orders of magnitude in diameter and 13 orders of magnitude in elastic modulus. These commonalities link the spatial correlations and strain response of softness to rearrangement size and yield strain, respectively

Rejuvenation of metallic glasses by non-affine thermal strain.

When a spatially uniform temperature change is imposed on a solid with more than one phase, or on a polycrystal of a single, non-cubic phase (showing anisotropic expansion-contraction), the resulting thermal strain is inhomogeneous (non-affine). Thermal cycling induces internal stresses, leading to structural and property changes that are usually deleterious. Glasses are the solids that form on cooling a liquid if crystallization is avoided--they might be considered the ultimate, uniform solids, without the microstructural features and defects associated with polycrystals. Here we explore the effects of cryogenic thermal cycling on glasses, specifically metallic glasses. We show that, contrary to the null effect expected from uniformity, thermal cycling induces rejuvenation, reaching less relaxed states of higher energy. We interpret these findings in the context that the dynamics in liquids become heterogeneous on cooling towards the glass transition, and that there may be consequent heterogeneities in the resulting glasses. For example, the vibrational dynamics of glassy silica at long wavelengths are those of an elastic continuum, but at wavelengths less than approximately three nanometres the vibrational dynamics are similar to those of a polycrystal with anisotropic grains. Thermal cycling of metallic glasses is easily applied, and gives improvements in compressive plasticity. The fact that such effects can be achieved is attributed to intrinsic non-uniformity of the glass structure, giving a non-uniform coefficient of thermal expansion. While metallic glasses may be particularly suitable for thermal cycling, the non-affine nature of strains in glasses in general deserves further study, whether they are induced by applied stresses or by temperature change.This research was supported by the World Premier International Research Center Initiative (WPI), MEXT, Japan, by NSF China and MOST 973 China, and by the Engineering and the Engineering and Physical Sciences Research Council, UK (Materials World Network project). Y.H.S. acknowledges support from a China Scholarship Council (CSC) scholarship.This is the author accepted manuscript. The final version is available from Nature Publishing Group via http://dx.doi.org/10.1038/nature1467

Nanometallic Glasses: Size Reduction Brings Ductility, Surface State Drives Its Extent

We report tensile experiments on Ni80P20 metallic glass samples fabricated via a templated electroplating process and via focused ion beam milling, which differed only in their surface energy states: Ga-ion-irradiated and as-electroplated. Molecular dynamics simulations on similar Ni80Al20 systems corroborate the experimental results, which suggest that the transition from brittle to ductile behavior is driven by sample size, while the extent of ductility is driven by surface state