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发表于 2005-8-30 09:43:32
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来自 辽宁沈阳
Re:[讨论]:影响材料塑性的因素
Plastic deformation is characterized by a permanent deformation of the material. Unlike elastic deformation, it does not reverse on unloading but leaves the material with a permanent shape. This is called the plastic region. Between these two regions, there is a limiting stress, called the yield stress of the material, or the critical resolved shear stress for a single crystal.
The crystallinity of the structure is the prime cause of this behavior, for it enables whole slabs of crystal to glide past one another. Each slip is a displacement, in certain glide direction, generally the crystal direction of closest atomic packing on certain crystal planes which is called the slip plane. In FCC. and HCP metals, these are mainly close-packed planes, but in BCC metals, the situation is complicated. It will be discussed elsewhere. Slip begins on some small area of the surface. The slip-front line between the slipped and unslipped areas is by definition a dislocation line. The glide motion of a dislocation is a property of a periodic crystal. The transition from the slipped to the unslipped region is spread over several atomic distances which is the width of the dislocation. Every atom in this transition region is pushed only a little further out of its original equilibrium site when it moves forward. This is the reason why dislocations can move easily in the crystal. Thus, the yield stress is much lower than the theoretical strength of crystals. Dislocation theory plays important role in understanding the microscopic processes in plastic deformation. Even the elastic theory of dislocations may explain many phenomena, such as yielding, work hardening, etc. It provides not only a deeper qualitative physical picture of plastic deformation but also to a certain degree a quantitative analysis of it.
Plastic deformation can also occur by twinning. The atoms slide, layer by layer to bring each deformed slab into mirror-image lattice orientation relative to the undeformed material. The critical stress of twinning is usually higher. Twins form at low temperature and under rapid deformation, e.g. bcc iron strained quickly at room temperature and slowly at 100K |
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