Vincent Keryvin

Indentation creep of Ge–Se chalcogenide glasses below Tg: elastic recovery and non-Newtonian flow

Abstract Chalcogenide glasses from the Ge–Se system behave viscoelastically at room temperature. It follows that indentation measurements are time- or rate-dependent. The study of the dependence of hardness ( H ) on the loading duration for Ge x Se 1− x glasses with x between 0 and 0.4 shows that the penetration displacement is the sum of an elastic component which reaches values as high as 60% of the total displacement, and a creep one, which is strongly non-Newtonian (shear thinning), and leads to a significant decrease of H with an increase of the loading time. The apparent viscosity and activation energy for flow were derived from the H ( t ) data on the basis of a theoretical analysis of the indentation process, and the results are in good agreement with those obtained from conventional viscosity measurements.

Changes in the mechanical properties of a Zr55Cu30Al10Ni5 bulk metallic glass due to heat treatments below 540 °C

Abstract The influence of heat treatments on the structure and the mechanical properties of a Zr 55 Cu 30 Al 10 Ni 5 bulk metallic glass is studied. Cyclic treatments above 480 °C lead to quite unchanged elastic moduli and hardness, while the thermal expansion coefficient keeps decreasing with rising annealing temperature above 460 °C, whereas a monotonous treatment rises respectively the elastic moduli and the hardness by 30% and 13%.

Poisson's Ratio and the Glass Network Topology - Relevance to High Pressure Densification and Indentation Behavior

Although Poissons ratio (ν) is a macroscopic elastic parameter it depends much on the fine details of the atomic packing. Glasses exhibit a wide range of values for  from 0.1 to 0.4 which correlate to the glass network polymerisation degree, hence reproducing at the atomic scale what is observed in cellular materials at the macroscopic scale[1]. As for pure oxide glasses, we found in various multi-component glasses built on ionic-, covalent- or Van der Waals bonds that an increase of Poisson’s ratio corresponds to a decrease of the atomic network crosslink degree[2]. Noteworthy, an extension of this analysis to the case of metallic glasses correlate the recently proposed cluster-like network structure for these glasses[3,4]. A general feature is that a highly cross-linked atomic network results in a glass with a low atomic packing density (large free volume fraction), as exemplified with the case of amorphous silica. The lower the atomic packing density is and the larger the volume change the glass experiences under high pressure (1 to 25 GPa). Indentation experiments with sharp indenters (such as the Vickers one) give birth to hydrostatic stresses of the same order of magnitude and thus induce glass densification. There is hence a direct correlation between ν (reflecting the packing density) and the indentation behavior[5].

Indentation as a probe for pressure sensitivity of metallic glasses

The question of hydrostatic pressure or normal stress dependence of the flow and fracture behaviour of bulk metallic glasses is addressed. Data from several types of mechanical tests indicate that flow is hardly sensitive to pressures or normal stresses, while fracture is normal stress sensitive and not pressure dependent. Besides, instrumented indentation adequately probes a low pressure dependence of flow, while a normal stress dependence is hardly noticeable.

Densification as the only mechanism at stake during indentation of silica glass

Silica glass is known to exhibit permanent changes in density under very high pressures. These changes may reach 21%. The sharp indentation test develops pressures underneath the indenter that trigger densification. Recently, we have proposed a constitutive modeling of the pressure-induced process accounting for its salient features: densification threshold, hardening, saturation of densifica- tion and permanent increase in elastic moduli. We examine in this paper the possibility that densi- fication could be the only mechanism for creating an imprint by indentation. We consider different models with growing complexity that we implement in a finite element software. Results indicate that the combination of shear and pressure as a driving force to densification may account for the mechanical response of the indentation test as well as the presence of densified zone underneath the imprint.

Photoinduced aging and viscosity evolution in Se-rich Ge-Se glasses

We propose here to investigate the non-equilibrium viscosity of Ge-Se glasses under and after light irradiation. Ge10Se90 and Ge20Se80 fibers have been aged in the dark and under ambient light, over months. During aging, both the relaxation of enthalpy and the viscosity have been investigated. The viscosity was measured by shear relaxation-recovery tests allowing the measurement of non-equilibrium viscosity. When Ge10Se90 glass fibers are aged under irradiation, a relatively fast fictive temperature decrease is observed. Concomitantly, during aging under irradiation, the non-equilibrium viscosity increases and reaches an equilibrium after two months of aging. This viscosity increase is also observed in Ge20Se80 fibers. Nevertheless, this equilibrium viscosity is far below the viscosity expected at the configurational equilibrium. As soon as the irradiation ceases, the viscosity increases almost instantaneously by about one order of magnitude. Then, if the fibers are kept in the dark, their viscosity slowl...

Vickers microhardness of oxidized and nonoxidized porous silicon

In this work we present our recent investigation on characterizing mechanical properties of porous silicon (PS) by using instrumented micro-indentation. Hardness and elastic modulus for oxidized and nonoxidized PS were measured. Experimental results revealed that hardness and elastic modulus are significantly lower than that of silicon substrate and decrease with increasing porosities. After oxidation an increase of the hardness and elastic modulus were observed. The task of stabilization of PS mechanical parameters can be solved with the help of oxidation.

Pressure Sensitivity of Plasticity in Metallic Glasses below Glass Transition: A Literature Review

This paper deals with the pressure dependence of plasticity in metallic glasses below glass transition. Recent results indicate that some metallic glasses have such a dependence and that it increases with temperature (Keryvin et al., Phil. Mag., 88, 1773, 2008). We investigate the possibility that such a situation could be a common feature for all metallic glasses by performing a literature review. Results indicate that it is not strightforward to draw decisive conclusions.