Amorphous superplasticity

In some instances, subtle changes in the precursor architecture can change the composition and microstructure of the final pyrolysis product. For example, pyrolysis of —[MeHSiNH] — leads to amorphous, silicon carbide nitride (SiCN) solid solutions at >1000°C (see SiCN section). At ca 1500 °C, these material transform to SisN SiC nanocomposites, of interest because they undergo superplastic deformation20. In contrast, chemically identical but isostructural — [F SiNMe] — transforms to Si3N4/carbon nanocomposites on heating, as discussed in more detail below21. 

At room temperature plastic flow of amorphous metals occurs in the form of highly localized shear deformation bands. Multiple irregularly spaced shear bands appear in the deformed region. Fig. 46 gives an example of an almost completely back bent ribbon. Similar observation can be made in uniaxial compression and after rolling (Davies, 1978). Shear bands are less numerous in traction tests even when observed after fracture (Fig. 48a). Since these shear bands are extremely thin, TEM observations indicate a thickness of 5 to 20 nm (Masumoto and Maddin, 1971 Sethi et al., 1978 Donovan and Stobbs, 1981), and the surface step heights are on the order of micrometers, shear strains comparable to superplastic metals occur in this very small volume of the band. Pampillo... 

The natiocrystalline Ceo specimen exhibits a behaviour similar to that of superplasticity and demonstrates a large strain at room temperature, which may be due to grain boundary sUding that originates from the weak bonding of molecular forces. In the present experiments, however, the stress-strain curve and the fractured surface indicate an absence of the superplastic behaviour. This may be due to the decomposition of the C o phase into the amorphous phase during the heat treatment. 

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