Discussion on the Structural Origins of the Fracture Toughness and Hardness Changes in Rapidly Quenched Borosilicate Glasses: A Molecular Dynamics Study (original) (raw)
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Ecf19, 2013
Molecular dynamics simulations were implemented to model fracturing in simplified glasses representative of actual nuclear glasses. The application of a faster thermal quenching algorithm allowed us to simulate a disordered and depolymerized structure constituting a model of a glass irradiated by deposited nuclear energy. At the same time, Vickers indentation tests were performed to measure the fracture toughness of a glass specimen of the same composition that had received a large dose of elastic energy. Numerical calculations showed an increase in the plasticity of the more rapidly quenched glass that can be attributed to a reduction in the average degree of polymerization of the glass, especially around boron atoms. The increased plasticity delays crack propagation because a larger number of plastic dislocations is required. This observation at the scale of the glass nanostructure is proposed to account for the experimentally observed fracture toughness.
MRS Proceedings, 2012
ABSTRACTClassical molecular dynamics simulations were used to compare the fracture behavior of pristine and disordered specimens of a simplified nuclear glass. The disordered specimen is prepared in order to mimic the effects of accumulating displacement cascades. It is characterized by a decreasing Boron coordination and an increasing Na concentration in a modifying role. We observe an enhancement of the plasticity of the disordered glass and a decrease of the elastic limit, resulting in greater fracture toughness. The simulation findings are consistent with experimental results.
HAL (Le Centre pour la Communication Scientifique Directe), 2011
Classical molecular dynamics simulations were used to compare the fracture behavior of pristine and disordered specimens of a simplified nuclear glass. The disordered specimen is prepared in order to mimic the effects of accumulating displacement cascades. It is characterized by a decreasing Boron coordination and an increasing Na concentration in a modifying role. We observe an enhancement of the plasticity of the disordered glass and a decrease of the elastic limit, resulting in greater fracture toughness. The simulation findings are consistent with experimental results.
Journal of Non-Crystalline Solids, 2012
This article describes the fracture processes simulated by classical molecular dynamics in three alkali borosilicate glasses of different compositions. Applying an external tensile load results in glass fracturing through processes of nucleation, growth, and coalescence of cavities. The cavity nucleation processes begin during the elastic phase and differ depending on the glass composition and especially on the [Na 2 O]/[B 2 O 3 ] ratio. The cavity growth and coalescence phases are associated with the plastic phase. The concentration of BO 3 entities has a strong influence on the cavity growth rate because these entities limit the accumulation of local stresses. Glass specimens with the same compositions but disordered by the application of a higher quenching rate were also fractured. These glasses are considered as models of irradiated structures and analyzing their fracture behaviors gives interesting information as to the understanding on why the fracture toughness evolves under irradiation.
Effect of chemical composition on borosilicate glass behavior under irradiation
Journal of Non-crystalline Solids, 2010
Assessing the behavior under irradiation of oxide glasses used for nuclear waste immobilization is essential for qualifying the long-term behavior of the wasteform. This study focuses on a series of borosilicate glasses of increasing chemical complexity. The effects of irradiation in these materials were investigated through multi-energy external irradiation by gold ions to obtain a constant nuclear damage level to a depth of about 2 μm. The macroscopic behavior of the glass was estimated from Vickers hardness measurement. The mechanical properties of all the borosilicate glasses studied were observed to improve, their hardness decreasing with the dose down by 30–35%. This evolution is also associated to an increase of the fracture toughness of the glass under irradiation. Analysis of the structural changes in the sodium borosilicate glass common to all the compositions studied with respect to dose revealed a progressive shift to higher wavenumbers in the vibration Raman band near 495 cm−1 indicating a drop in the mean Si–O–Si angle. At the same time, modifications in the Qn band between 850 and 1200 cm−1 are observed and characterize increasing depolymerization of the silicate network.
Journal of Non-Crystalline Solids, 2015
We provide a comprehensive description of the defect tolerance of sodium-borosilicate glasses upon sharp contact loading. This is motivated by the key role which is taken by this particular glass system in a wide variety of applications, ranging from electronic substrates, display covers and substrates for biomedical imaging and sensing to, e.g., radioactive waste vitrification. The present report covers the mechanical properties of glasses in the Na 2 O-B 2 O 3 -SiO 2 ternary over the broad range of compositions from pure SiO 2 to binary sodium-borates, and crossing the regions of various commercially relevant specialty borosilicate glasses, such as the multi-component Duran-, Pyrex-and BK7-type compositions and typical soda-lime silicate glasses, which are also included in this study. In terms of structure, the considered glasses may be separated into two groups, that is, one series which contains only bridging oxygen atoms, and another series which is designed with an increasing number of nonbridging oxygen ions. Elastic moduli, Poisson ratio, hardness as well as creep and crack resistance were evaluated, as well as the contribution of densification to the overall amount of indentation deformation. Correlations between the mechanical properties and structural characteristics of near-and mid-range order are discussed, from which we obtain a mechanistic view at the molecular reactions which govern the overall deformation reaction and, ultimately, contact cracking.
Borosilicate glass potentials for radiation damage simulations
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2014
Three borosilicate glass (SiO 2-B 2 O 3) fixed charge potentials from the literature are compared [1, 2, 3] and their suitability for use in simulations of radiation damage is assessed. For a range of densities, we generate glass structures by quenching at 5 × 10 12 K/s using constant volume Molecular Dynamics. In each case, the bond lengths, mean bond angles, bulk modulus, melting point and displacement energy thresholds are calculated and where possible compared to experimental data. Whereas the bond lengths and mean bond angles are reasonably well predicted, we find that the potentials predict melting temperatures, bulk moduli and densities that are higher than experimental data. The displacement energy thresholds are generally lower than those for ionic crystalline materials but show a wider spread of values. However the barriers for atomic rearrangements after atoms have been displaced in the equilibrium structures are very high. This indicates that the radiation damage produced in the ballistic phase of a collision cascade is likely to persist for extended times scales in contrast to crystals where interstitials and vacancies can diffuse rapidly between successive radiation events.
The influence of a changing glass topology on local mechanical properties was studied in a multitechnique nanomechanical approach. The glass response against sharp contacts can result in structural densification, plastic flow, or crack initiation. By using instrumented indentation testing, the mechanical response was studied in different strain rate regimes for a sodium borosilicate glass (NBS) exhibiting altering structures due to varying processing conditions. Comparison with data from former studies and with literature data on other glass structures helped to elucidate the role of the borate and silicate subnetworks and to understand the overall mechanical properties of the mixed glass systems. A peculiarity of some of the NBS glasses tested in this study is the fact that the connectivity of the borate and silicate entities depends on the sample's thermal history. Although the influence on macroscopic material properties such as E and H is minor, the onset of cracking indeed is influenced by those structural changes within the glass. Rapidly quenched glass shows an improved crack resistance, which is even more pronounced at high strain rates. Studies on various processing conditions further indicate that this transition is closely related to the cooling rate around Tg. The strain rate dependence of cracking is discussed in terms of the occurrence of shear deformation and densification.
Crack paths in a borosilicate glass under triaxial loading
The growth of 3D star-like cracks in a porous borosilicate glass, with a "mirror-mist-hackle" aspect due to the dynamic character of their propagation was induced in cylindrical specimens by "cold-to-hot" thermal shocks inducing triaxial tension. Pores were identified as the crack initiation sites. Thermo-mechanical simulations were done to analyse the stress field. Crack initiation at mid-height, from the center of the specimens was predicted, in accordance with the observations. The pore-induced stress concentration was found to depend on the local stress triaxiality, as well as K I for an annular crack initiated from a pore. INTRODUCTION. Blocks of vitrified nuclear waste for deep underground storage are prepared by pouring a mixture of waste with molten sodium-borosilicate glass into steel canisters. During cooling, sharp temperature gradients produce tri-axial tensile stresses, responsible for multiple cracking. When stored underground for hundreds of years, the canisters might not remain watertight , allowing leaching of the fractured glass by water and the release of radionucleides in the environment. Crack initiation and growth in cylindrical specimens of an inactive analogous glass during "cold-to-hot" thermal shocks that induce a triaxial tension field representative of the loading conditions during glass blocks cooling was thus investigated. EXPERIMENTAL AND NUMERICAL PROCEDURES Experimental procedures. The material investigated is a non-translucent SON68 glass, an inactive analogue of the industrial product, containing mainly SiO 2 (45,5weight %), B 2 O 3 (14%), Na 2 O (9,9%), many other oxides, plus Ruthenium and Palladium particles, to replace some heavy radionucleides. It is flawless, thanks to a slow cooling process, but, like the industrial glass, it contains a distribution of more or less spherical pores, issued from gas bubbles trapped in the solidifying liquid, whose diameter ranges from 100 to 800µm. The thermo-elastic properties were measured between 20°C and the glass-transition temperature, which is 502°C [1]. Relaxation tests have shown that below 350°C, viscous effects are negligible during the short time fracture tests reported below. 36 to 45mm-high and 80mm-high cylinders, 40mm in diameter were machined. These cylinders were equipped with three thermocouples glued in the center of each base and on the side, at mid-height. They were cooled in a freezer during more than 12h and then submitted to
Journal of Non-crystalline Solids, 2015
Plastic deformation, densification, and cracking of sodium borosilicate (NBS) glasses were examined during indentation with a three-sided pyramidal indenter. Compositions of 74.0SiO 2-10.0B 2 O 3-16.0Na 2 O (NBS1), and 74.0SiO 2-20.7B 2 O 3-4.3Na 2 O-1.0Al 2 O 3 (NBS2) (mol%) were investigated. The effect of thermal history was additionally considered for the NBS2 composition, which lies near the boron anomaly line. Hardness, elastic modulus, and fracture toughness were estimated with instrumented indentation techniques. Atomic force microscopy (AFM), Raman micro-spectroscopy, and post-indent annealing experiments were used to analyze surface topographies, densification, and recovery of deformed material. The results indicate that NBS1 exhibits a greater hardness and elastic modulus, and undergoes less densification than the NBS2 glasses. Different casting conditions influenced the plastic deformation and onset of crack initiation in NBS2. Interpretation of fracture toughness measured by indentation is complicated by residual stresses, densification during contact, and model assumptions. However, distinct differences in elastic modulus, plastic deformation and cracking between the glasses were noticeable. Such results and observations are discussed in terms of structural changes in the glass.