Plastic deformation of glass
When the stress of the glass and ceramic materials exce […]
When the stress of the glass and ceramic materials exceeds the elastic ratio limit, the fracture will occur and the brittleness will be exhibited. However, for the metal and polymer materials, when the stress exceeds the elastic ratio limit, plastic deformation often occurs. A large amount of permanent deformation occurs before breaking. The permanent deformation is caused by the flow between the mass points in the material, which can be regarded as the shearing process in which the mass point of the substance is exchanged for its adjacent mass point. There are two types of plastic deformation, one is that the atomic planes slide to each other to cause plastic flow of the crystal, and the other is the amorphous flow that occurs due to the transposition of the atomic groups.
Whether the plastic deformation of the material is closely related to the crystal structure, different crystals have different slip systems. The so-called slip system refers to the combination of the slip surface and the slip direction. The more the slip system, the more likely it is plastic deformation, and vice versa. The material shows brittleness. Such as face-centered cubic metal, there are 12 slip systems, which are prone to plastic deformation; while corundum Al2O3 has only one slip surface and two slip directions, the number of slip systems that can be composed is small, and the energy required for misalignment slip occurs. Large, showing brittleness.
Glass is a brittle material, there is no slip system like crystal, but glass can also be deformed at high temperature. This is because glass is a metastable substance. Many atoms are not in the energy valley of potential energy curve. The bond strength is weak. As long as the small stress can break the bond between the atoms, the atom jumps to a nearby position, causing the displacement and rearrangement of the atom, thereby causing plastic deformation. This deformation is also called viscous flow, and viscosity and temperature. related.