Glass deformation

The plastic deformation exhibited by nonductile crystalline ceramics is not significant enough for deformation processes to be of much use in the fabrication of bulk articles. Noncrystalline materials (e.g. glasses) deform by the same mechanism as liquids, viscous flow. However, in a glass the effect is only pronounced at temperamres elevated enough to decrease the viscosity. 

Glass at 1100 K. Assume that the glass deforms by movement of single oxygen atoms. 

Softening point 10 - retains its shape Glass deforms under its own weight 700... 

Argon, A. S. (2001) Modeling of polymer glasses deformation, in Encyclopedia of Materials Science and Technology, edited by Buschow, K. H. J., Cahn, R. W., Flemings, M. C., Ilschner, B., Kramer, E. J., Mahajan, S., and Veyssiere, P., vol. 5, Polymers and Materials Chemistry, edited by Kramer E. J., Section 5.2, Amorphous and liquid crystalline polymers, edited by Windle A. H., Amsterdam Elsevier, pp. 5712-5724. 

Fracture of wires Fracture of metallic glasses Deformation texture Drawing defects Polycrystalline strengthening Fatigue of micro-wires Fatigue of metallic glasses Bonding wires... 

Some ceramics may have a glass phase cormecting the individual crystallites. Above the glass transition temperature, glasses deform by viscous flow that increases exponentially with temperature. Therefore, these ceramics as well as metallic glasses and polymers can creep by viscous flow. 

Polymers will be elastic at temperatures that are above the glass-transition temperature and below the liquiflcation temperature. Elasticity is generally improved by the light cross linking of chains. This increases the liquiflcation temperature. It also keeps the material from being permanently deformed when stretched, which is due to chains sliding past one another. Computational techniques can be used to predict the glass-transition and liquiflcation temperatures as described below. 

The kinetic nature of the glass transition should be clear from the last chapter, where we first identified this transition by a change in the mechanical properties of a sample in very rapid deformations. In that chapter we concluded that molecular motion could simply not keep up with these high-frequency deformations. The complementarity between time and temperature enters the picture in this way. At lower temperatures the motion of molecules becomes more sluggish and equivalent effects on mechanical properties are produced by cooling as by frequency variations. We shall return to an examination of this time-temperature equivalency in Sec. 4.10. First, however, it will be profitable to consider the possibility of a thermodynamic description of the transition which occurs at Tg. 

Elasticity. Glasses, like other britde materials, deform elastically until they break in direct proportion to the appHed stress. The Young s modulus E is the constant of proportionaUty between the appHed stress and the resulting strain. It is about 70 GPa (10 psi) [(0.07 MPa stress per )Tm/m strain = (0.07 MPa-m) / Tm)] for a typical glass. 

Traditionally, production of metallic glasses requites rapid heat removal from the material (Fig. 2) which normally involves a combination of a cooling process that has a high heat-transfer coefficient at the interface of the Hquid and quenching medium, and a thin cross section in at least one-dimension. Besides rapid cooling, a variety of techniques are available to produce metallic glasses. Processes not dependent on rapid solidification include plastic deformation (38), mechanical alloying (7,8), and diffusional transformations (10). 

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