Amorphous critical sample thickness

The second condition to validate the scheme B is that embrittlement must correspond to a critical morphological state that is the only approach to explain its sudden character. The extensive and careful work of Kennedy et al. (//) on relationships between fracture behavior, molar mass and lamellar morphology, shows that this condition is fulfilled in the case of PE. Comparing various samples of different molar masses with different thermal histories, they found that the thickness of the amorphous layer (la) separating two adjacent lamellae is the key parameter (Fig. 6). As a matter of fact, there is a critical value lac of the order of 6-7 nm. For la > lac the samples are always ductile whatever their molar mass, whereas for U < laC the samples are consistently brittle. As a result, lac appears to be independent of the molar mass. Indeed, there is a specific molar mass, probably close to 70 kg.mof for PE below which crystallization is so fast that it is impossible to have la values higher than lac whatever the processing conditions. 

Deformation behavior of amorphous polymers has been studied for years, and explained in terms of structure of polymers. Especially, great progress has been made by using the concept of " network (strand) density" to understand the deformation behavior (such as crazing and shear deformation) [1,2]. When the concept of a critical thickness of the polymer layer, below which a sample behaves in a ductile manner even for normally brittle polymers like polystyrene (PS), has been added to the network density concept, more comprehensive understanding has become possible [3-8] and it provides a great opportunity for developing ductility of otherwise brittle... 

The control of the second critical step, crystallization of the amorphous alloy, is the focus of the following discussion. Iron-silicon and iron-aluminum systems are discussed. The critical lengthscales in the iron-aluminum and iron-silicon systems are much larger than that observed in the molybdenum-selenium systems. By critical lengthscale, we refer to the thickness of the repeat unit in the multilayer below which the multilayer evolves completely into an amorphous material without the nucleation of any crystalline phase. The samples discussed are all layered on a lengthscale which is less than this critical value. That is, they evolve from a layered initial state, through a distinct amorphous intermediate, to a crystalline compound. 

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