Stress-strain curve of mild steel
Plastic deformation is a permanent unrecoverable deformation. When the load that caused the deformation is removed, the material will not return to its original shape but will maintain its newly deformed shape．
Plastic deformation and the following heat treatment have significant effects on the microstructures and properties of materials, especially metal materials．
Therefore, it is important to understand the mechanism of plastic deformation and the effect of plastic deformation on the microstructures.
(1) Plastic Deformation of Single Crystals and Strain Strengthening单晶体的塑性变形及应变强化
Plastic deformation of a single crystal metal. (a) schematic illustration of normal and shear stresses on a plane when a pure tensile force is applied. (b) schematic illustration of plastic deformation due to the shear stress τ. (c) a photography of a plastically deformed zinc single crystal.
A single crystal can be deformed by the application of a tensile force F
Tensile stress σ (正应力)：may cause elastic deformation and cleavage fracture
Shear stress τ (切应力)：may result in slip, and consequently plastic deformation
The mechanism of plastic deformation for most metals is slip and twinning.
Slip takes place only under shear stress.
The slip deformation forms small steps on the surface of the single crystal, which is termed slip band
The slip planes are usually the most densely packed planes(密排面); the slip directions are usually close-packed directions(密排方向).
A combination of the slip plane and the slip direction is termed the slip system.
The slip system depends on the crystal structure.
Slip systems for FCC，BCC and HCP structures
The figure shows schematically an edge dislocation moves in response to a shear stress and produce a unit of slip under a low shear stress. It can be seen that only a small group of atoms slip over each other at any particular instant.
Twinning occurs in a specific direction called the twinning direction. The displacement magnitude within the twin region is proportional to the distance from the twin plane.
Comparing with slip, twinning requires relatively large shear stress, and the deformation by twinning is very fast.
Twinning easily takes place in HCP crystal structures because there are few operable slip systems. In FCC structure, there is generally no twinning deformation. The twin structure observed in FCC structure usually is the result of phase transformation during an annealing heat treatment, which is called annealing twin(退火孪晶).
(2) Plastic Deformation of Polycrystalline Metals 多晶体金属的塑性变形
There are grain boundaries between the grains. Moreover, because the orientations of the grains are different from each other, the direction of slip varies from one grain to another. For each grain, dislocation motion occurs along the slip system.
During plastic deformation under an external force, dislocations move to grain boundaries. If dislocations cannot traverse the grain boundaries, they tend to pile up at the grain boundaries.
Plastic deformation cannot occur in all the grains at the same time. Polycrystalline metals are stronger than their single crystal equivalents．
The procedure of plastic deformation of a polycrystalline metal
When an external force is applied on a polycrystalline metal and plastic deformation occurs, slip first takes place in the grains, in which the angle between the slip system and the external force direction is close to 45°. The dislocation pileups introduce stress concentrations ahead of the slip planes．
When this stress increases to a certain level, the dislocations in adjacent grains begin to glide. When slip occurs in a large amount of grains, the specimen of polycrystalline metal shows plastic deformation in macro scale．
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