Dislocation forest

In earlier sections, the characteristics of single dislocations were discussed. It was indicated that a dislocation moves quite freely in certain glide planes under applied shear stress. However, even well-annealed crystals contain many dislocations at a level of 10". Therefore, it is quite probable that moving dislocations will encounter others, known as forest dislocations , which will hinder their freedom to glide. Even the most favored planes will contain dislocations and, thus, moving dislocations will interact with those others present in the material. The term dislocation intersection refers to an interaction occurring between a moving dislocation and the others encountered while in motion. For the sake of simplicity, this section will describe the interactions between two dislocations. 

The possibility of APB tubes being produced by dislocation mechanisms was proposed in 1962 (Vidoz and Brown, 1962), but they were not observed until the early 1980s (Chou et al., 1982 Chou and Hirsch, 1983). In the mechanism of Vidoz and Brown (1962) the cross-section of the APB tube is a parallelogram. In this mechanism, when a superdislocation consisting of two APB-coupled superpartials is intersected by a forest dislocation or a dislocation in a secondary slip system. 

Fig. 11.36. Schematic of the forest hardening process. Dislocation gliding in the primary slip plane forms junctions as a result of encounters with dislocations piercing that plane.

During deformation, the material is hardened. Further away from the crack, the emitted dislocations interact with the forest obstacles previously formed. Hence, two zone form a diffusion zone (enhanced plasticity) near the crack tip and... 

The gradual move from green environments to deserts as a result of humanity-caused environmental dislocations is not a new phenomenon. Many old archeological sites have been unearthed in the Sahara desert, all over the Middle East, and in Takramakam. Desert areas have increased in just 1000 years. However, the deforestation that has been of reeent concern is due to increased meadowlands, sudden increases in population, reductions of forests to obtain fuel and fields, and abandonment of soil maintenance as a result of local conflicts. With such multifaceted problems, to what extent ean seience and technology, in particular polymer science, eontribute to reforestation It is perhaps difficult for reforestation to sueeeed onee the area is eompletely deforested. However, it is possible to produee vegetables in desert areas as well as to reproduce plants that help prevent deforestation. 

---