1. Basic mode of single crystal plastic deformation - slip
2. Slip system
A slip plane and a slip direction on this plane form a slip system.
Generally speaking, the slip plane is usually the atomic close packed plane, and the slip direction is the direction where the atoms are arranged most closely. The more slip systems in the metal, the better its plasticity.
3. Rotation of crystal during sliding
When the crystal slips under the action of tensile force F, if it is not restricted by the collet to slip, the slip plane and slip direction remain unchanged, and the orientation during stretching does not change. In order to keep the direction of the tensile extension axis fixed, the orientation of the single crystal must rotate relative to each other, that is, the slip plane and the slip direction change when there is a restriction of the chuck.
4. Multi system slip
The case that only one slip system is activated (single system slip) generally occurs in metals with closely spaced hexagonal structures with few slip systems. For crystals with many slip systems, the initial slip is first carried out in the slip system with the most favorable orientation. However, as a result of crystal rotation, the shear stress in other slip systems may reach a critical value sufficient to cause slip, so the slip process will be carried out simultaneously or alternatively in two or more slip systems
1. Dislocation motion and crystal slip
The theoretical calculated strength of copper crystal is 1500MPa, while the measured strength is only 0.98MPa. This shows that the actual crystal sliding is not the result of the rigid movement of one part of the crystal relative to the other, but the gradual movement of dislocations along the sliding plane under the action of shear stress.
When a dislocation line moves to the crystal surface, it leaves a slip deformation of atomic spacing on the surface.
2. Slip mechanism
The theoretical critical shear stress, which is assumed as a rigid whole sliding, is 3-4 orders of magnitude larger than the actual measured critical shear stress. The slip is realized by the movement of dislocations on the slip plane.
3. Dislocation multiplication
A large number of slip bands are produced during the plastic deformation of crystals, which requires a large number of dislocations. The number of dislocations in real crystals does not decrease, but increases, indicating that there is a dislocation propagation mechanism.
4. Delivery and stacking of dislocations
During multi system slip, dislocations on different slip planes meet and form a cut (a new dislocation line). On the one hand, it increases the length of the dislocation line, and on the other hand, it may form a fixed cut that is difficult to move, becoming an obstacle to the movement of subsequent dislocations.
During the movement of dislocations under shear stress, if they encounter obstacles such as fixed dislocations, impurity particles and grain boundaries, the leading dislocations are stopped in front of the obstacles, and the subsequent dislocations are piled up to form a pile up group of dislocation planes, and a high stress concentration is formed in the front of the obstacles.
Twining is a uniform shear process in which one part of a crystal has a certain angle relative to the other along a certain crystal plane (called twin plane). The crystals on both sides of twin boundary are mirror symmetrical.
Twinning is also a form of plastic deformation.
The orientation of crystal lattice is changed by twinning;
The required shear stress is much larger than slip, and the deformation velocity is close to sound velocity;
The relative displacement of adjacent atomic planes is less than one atomic spacing
There are few slip systems in closely arranged hexagonal lattice metals, which are often deformed in a twinning manner.
The body centered cubic lattice metal only undergoes twinning deformation at low temperature or under impact.
For the face centered cubic lattice metal, twinning deformation does not occur in general, but twins are often found. This is due to the dislocation of atoms when they are rearranged during the phase transition process. It is called annealing twin
In polycrystalline deformation, only the slip system with the most favorable orientation (the largest orientation factor) can start first.
When polycrystal deformation occurs, the deformation of one grain must be coordinated with the deformation of adjacent grains to avoid fracture between grains. The plastic deformation of polycrystals is coordinated by the multi system slip of each grain.
During polycrystalline deformation, the deformation amount of each grain is different, and the deformation in each grain is uneven because the grain boundary strength is higher than that in the grain.
Effect of plastic deformation on metal structure
1) Formation of fibrous tissue
During metal plastic deformation, grains are elongated along the deformation direction, and become fibrous stripes when the deformation is large.
2) Forming deformation texture
With the occurrence of deformation, it is also accompanied by the rotation of the crystal. When the deformation is large, the orientation of each grain will be consistent. This kind of structure with preferred orientation of grains due to deformation is called deformation texture
3) Substructure refinement
Cold deformation will increase the dislocation density in the grains. With the increase of deformation, dislocations intertwine and tangle, forming cellular substructure in the grains.
4) The lattice is severely distorted
Effect of plastic deformation on metal properties
1) Effect of plastic deformation on mechanical properties of metals
Due to the formation of fiber structure and deformation texture, the metal has obvious anisotropy.
Due to the increase of dislocation density, the mutual intersection of dislocations is intensified during their movement, resulting in dislocation packing group, cut step, entanglement network and other obstacles, which hinder the further movement of dislocations, cause the increase of deformation resistance, and improve the strength of the metal.
2) Effect of plastic deformation on physical and chemical properties of metals
With the increase of plastic deformation, the conductivity, resistance temperature coefficient and thermal conductivity of the metal decrease, the permeability and magnetic saturation decrease, the coercive force increases, the internal energy and chemical activity increase, and the corrosion resistance decreases.