Plastic-flow-controlled processes

In aU wrought processes, the flow of metal is caused by application of an external force or pressure that pushes or pulls a piece of metal or alloy through a metal die. The pressure required to produce plastic flow is determined primarily by the yield stress of the material (cf. Section 5.1.4.3) which, in turn, controls the load capacity of the machinery required to accomplish this desired change in shape. The pressure, P, used to overcome the yield stress and cause plastic deformation is given by... 

Since an understanding of the importance of any one process contributing to the failure in thermoplastics and the control over these processes is only partly attainable, a knowledge and understanding of the nature of endurance Hmits is of extreme importance for successful use of plastics, in particular engineered thermoplastics [27]. In terms of the failure type, polymer fracture may occur as a rapid extension of an initial defect, plastic flow of the matter and the thermally activated flow of the macromolecules. In all these cases, however, fracture is a localized phenomenon characterized by a large inhomogeneity of deformations. 

Proper melt flow and compatibility of the plastics is required in order to provide the proper adhesion. The type of the available plasticator and mold process control adjustments can compensate some of the melt flow variable factors. 

In the EFG process, commercialised at Schott Solar (under development in the past by Mobil Tyco, ASE and RWE), the silicon ribbon is pulled to heights of up to 7 m from the top of a graphite die. The molten silicon is fed through the die by capillary action (shape of meniscus Ml). Extensive temperature control by radiation shields, cold shoes and afterheating realises maximum temperature gradient where plastic flow is possible, to allow for a maximum growth rate as described above [30,31] (Fig. 7.2). 

Inelastic deformation can occur in crystalline materials by plastic flow . This behavior can lead to large permanent strains, in some cases, at rapid strain rates. In spite of the large strains, the materials retain crystallinity during the deformation process. Surface observations on single crystals often show the presence of lines and steps, such that it appears one portion of the crystal has slipped over another, as shown schematically in Fig. 6.1(a). The slip occurs on specific crystallographic planes in well-defined directions. Clearly, it is important to understand the mechanisms involved in such deformations and identify structural means to control this process. Permanent deformation can also be accomplished by twinning (Fig. 6.1(b)) but the emphasis in this book will be on plastic deformation by glide (slip). 

---