Abstract
The transient (or permanent) structural modifications which occur during local deformation of oxide glasses are typically studied on the basis of short-range data, for example, obtained through vibrational spectroscopy. This is in contrast to macroscopic observations, where variations in material density can usually not be explained using next-neighbor correlations alone. Recent experiments employing low-frequency Raman spectroscopy have pointed-out this issue, emphasizing that the deformation behavior of glasses is mediated through structural heterogeneity and drawing an analogy to granular media. Here, we provide additional support to this understanding, using an alternative experimental method. Structural modification of vitreous silica in the stress field of a sharp diamond indenter tip was monitored by in-situ small-angle X-ray scattering. The influenced zone during loading and after unloading was compared, demonstrating that changes in the position of the first sharp diffraction peak (FSDP) directly in the center of the indent are of permanent character. On the other hand, variations in the amplitude of electron density fluctuations appear to fully recover after load release. The lateral extent of the modifications and their relaxation are related to the short- to intermediate-range structure and elastic heterogeneity pertinent to the glass network. With support from Finite Element Analysis, we suggest that different structural length scales govern shear and isotropic compaction in vitreous silica.
Highlights
Investigations of liquids and glasses by Small-Angle X-ray Scattering (SAXS) usually address the characterization of physical heterogeneity, e.g., dispersed nanoparticles or pores
While early research in this field focused on polymers, the technique was successfully adapted for the characterization of inorganic glasses (Pierre et al, 1972; Rathje and Ruland, 1976; Fischer and Dettenmaier, 1978; Wiegand and Ruland, 1979; Golubkov et al, 1980; Roe and Curro, 1983; Tanabe et al, 1984): spatial density fluctuations are a universal feature of glassy materials
We presented in-situ SAXS as a complementary method to study structural modification induced by local material deformation on intermediate length scale
Summary
Physical fluctuations such as in network topology manifests in variations within the atomic arrangement, e.g., local network rigidity or packing density The latter may depend on the thermal history of the glass, for example, quenching rates (Levelut et al, 2002, 2005, 2007; Brüning and Cottrell, 2003; Watanabe et al, 2003; Brüning et al, 2005, 2007) or pressure (Reibstein et al, 2011; Cornet et al, 2019). Their assessment provides useful insight at the intermediate- to long-range structural characteristics of glasses (Greaves et al, 2008) As such density fluctuations are not strictly defined in shape or size (Golubkov, 1996; Bakai and Fischer, 2004; Ozhovan, 2006; Huang et al, 2018), they do not cause distinct, sharp scattering signals. Their presence manifests in the overall scattering intensity at low scattering angles q, i.e., the extrapolated SAXS intensity to zero scattering angle I(0)
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