Deformability of Mg-aluminosilicate glass under high pressure and shear stress: dynamic coordination change of Al3.

Deformation experiments were conducted on 20MgO-20Al2O3-60SiO2 glass, which possesses superior mechanical properties, using a simple shear geometry under confining pressure. The glass exhibited densification, analogous to quasi-uniaxial compression, up to a shear strain of γ ≈ 1, and flow-dominated deformation at γ > 2. In the low-strain regime, densification was associated with a reduction in ring sizes and an increase in the coordination number of Al. Distinctive structural changes specific to shear deformation were also observed, including broadening of the T-O-T (T = Al or Si) bond-angle distribution and the formation of dangling bonds. A key mechanism proposed for flow involves higher-coordinated Al, formed during densification, dynamically changing its coordination number at the onset of flow. This transition from higher- to lower-coordinated Al is inferred to promote structural fluidity, while the concomitant transfer of non-bridging oxygens further enhances network flexibility. Overall, the results suggest that glasses capable of readily initiating local structural rearrangements can cooperatively enhance the fluidity of the entire network. This mechanism serves as an effective stress-dissipation mode, underpinning the observed high fracture toughness.