slip

slip

Under the action of shear stress, a part of the crystal moves along a certain crystallographic direction on a certain crystallographic plane relative to another part of the crystal, so that the atoms on the crystal plane move from a stable equilibrium position to another process crystal The slip process is shown in Figure 1. Slip is the main way of plastic deformation of metal crystals. During the slip process, the orientation of the crystal does not change, and the slipped and unslided parts still maintain the same orientation; each time The amount of slip is an integral multiple of the atomic distance of the crystal in the slip direction, and the amount of slip cannot be restored after the stress is removed. The accumulation of a large number of slips constitutes the macroscopic plastic deformation of crystals. The slips of crystals are divided into single crystal slips and polycrystalline slips.



Single crystal sliding single crystal test rod after stretching and deformation is shown in Figure 2. In addition to the elongation of the test rod, many can be seen on its polished surface at 45 to the stretching direction. The parallel lines at the corners, which are called slip bands, are observed under an electron microscope, and it can be seen that each slip band is composed of many densely packed slip line groups. The slip line distance d is about 100 atomic distances, However, the slip amount s of each slip line is about 1000 atoms, and the crystal sheets between the slip lines or slip bands are not deformed, and only relative displacements are made between each other, as shown in Figure 3 .

The crystal plane and crystal orientation that produce slip are called slip plane and slip direction, respectively. The slip plane is often the plane with the most densely packed atoms, and the slip direction is always the direction in which the atoms are densely packed. A slip plane and a possible slip direction on this plane are collectively called a slip system. Systematic face-centered cubic metals have four equivalent {111} planes, and each slip plane contains three different possible slip directions <110>, so there are 12 slip systems, denoted by (hkl) [UVW]- means; close-packed hexagonal metals contain (0001) [1120] three slip systems.

Experiments have shown that when a certain external force acts on the crystal, not all slip systems in the crystal can be activated. Only when the shear stress τ of one or several slip systems reaches a certain value τc under the external force to start. The minimum shear stress τc that is sufficient to start the crystal slip is called I critical shear stress. Based on the tensile yield stress and the orientation of the slip system relative to the tensile axis of the single crystal, the critical shear stress value can be determined. For common pure metals, they are about 10-4---10-5G. Crystal composition, temperature and deformation rate have a great influence on the critical shear stress.

The face centered cubic crystal has 12 slip systems, and their critical shear stress is the same. Therefore, which slip system whose shear stress reaches the critical value first, the slip will be carried out on that slip system first; but as the slip process progresses, the crystal will be subjected to additional torque, so that the slip occurs The rotation of the system to the direction of the force axis makes the shear stress on some non-slip systems reach a critical value, so more slip systems also participate in the slip. In this way, double slip, triple slip, quadruple slip, etc. will occur, collectively called multi-slip. During the double-slip process of the crystal, the orientation remains unchanged. Such two slip systems are called conjugate slip systems, also called the first slip system as the main slip system, and the second slip system as Conjugate slip system. The experimental results show that the double slip of pure metals such as aluminum and copper occurs simultaneously, while the slip of certain alloys such as a brass occurs alternately. In addition, for a system with the same slip direction and different slip planes as the main slip system, such as (111){101} is called cross-slip system.

The critical shear stress value of pure metal crystals when slipping is generally 1×10-2--102MPa. According to the complete crystal model, the τc value calculated by electrostatic theory is dozens or even hundreds of times larger than the experimental value. This contradiction reveals that there are defects in the crystal, which has contributed to the emergence and development of the dislocation theory. The reason why the crystal slips under the action of stress is that the actual sliding process of the crystal is not that all the atoms on both sides of the slip surface move at the same time, but that they move first in a local area (at a part of the atomic group), and then sweep across the entire slip surface. The local area can move first , it is caused by the concentration of stress caused by some kind of defect in this area. Although the stress acting on the entire slip surface is relatively low, the stress on this local area has reached a value sufficient to cause slip. This kind of crystal Defects are called dislocations. A dislocation is a tubular region in a crystal in which atoms are arranged irregularly, forming a defect. Since the diameter of this "pipe" is very small (only a few atomic distances), it can be regarded as a line, so dislocation is a line defect. It is assumed that the slip starts from the local area of the crystal, and the atoms on both sides of the slip plane in this local area have undergone relative displacement, while the atoms on both sides of the slip plane have not yet undergone relative displacement in the area where the displacement line has not been swept. Therefore, the definition of dislocation is The borderline between the slipped and unslided regions of a crystal.

There are two basic types of dislocations: edge dislocation and screw dislocation. Edge dislocation can be seen as a half-atom plane inserted into a certain part of an ideal complete crystal, and the dislocation line is the edge of the half-atom plane; because it is like a knife edge, it is called an edge dislocation. There are two types: when the inserted semi-atomic plane is above the slip plane, it is called a positive edge dislocation, generally represented by the symbol "丄"; when the inserted semi-atomic plane is below the slip plane, it is called a negative edge dislocation, The characteristic of edge dislocation is represented by "T" is that the dislocation line is perpendicular to the slip direction of the crystal. The edge dislocation model and the atomic configuration near the dislocation line are shown in Figure 4a`4c.

Figure 4b shows that part of the upper and lower halves of the crystal has moved half the atomic distance in opposite directions, and a dislocation line is also formed at the boundary of the slipped region and the non-slided region, and the atoms above and below the slip surface around it have obvious dislocations misaligned (see Figure 4d). The characteristic of this atomic dislocation is that the atomic plane perpendicular to the dislocation line becomes a helical plane, and this type of dislocation is called a screw dislocation. Screw dislocations are also divided into two types: when the rotation direction and forward direction of the helix conform to the right-handed screw rule, it is called a right-handed dislocation, or right-handed dislocation; When regular, it is called a left-handed dislocation. The characteristic of screw dislocation is that the dislocation line is parallel to the crystal slip direction.

Since the atoms near the dislocation line deviate from the stable equilibrium position and bear high distortion energy, the force that prevents the dislocation movement and the crystallization force that promotes the dislocation movement tend to be in balance. In this way, the starting force required for slip Much smaller. In addition, only the dislocation line or atoms near the dislocation need to move a small distance to achieve one interatomic distance slip (Fig. 5).

The vector representing the lattice distortion near the dislocation line and the slip direction of the slipped region in the crystal is called the Burgers vector (abbreviated as the Burgers vector), and is generally represented by the letter b. When the value of Burgers vector b is one lattice spacing, it is defined as the unit dislocation intensity; when b is several lattice spacings, it is a high-intensity dislocation, and when the value of b is a fraction, it is a partial or incomplete dislocation.

According to the relative position of the Burgers vector and the dislocation line, the type of dislocation can be expressed: when the Burgers vector is perpendicular to the dislocation line, it is an edge dislocation; when the Burgers vector is parallel to the dislocation line, it is a screw dislocation ; When the Burgers vector and the dislocation line form any angle, it is a mixed dislocation. At this time, according to the vector decomposition rule, the vector can be drawn as a vertical component vector and a parallel component vector; it can be seen that the mixed dislocation is composed of an edge component and a spiral component.

After a dislocation sweeps across the slip plane, a step of height b is formed on the crystal surface. Thousands of slip lines on the same slip surface successively move out of the crystal surface, forming microscopic slip steps (see Figure 3).

An edge dislocation may also move along its slip plane perpendicular to it through the climbing process. Because climbing is a diffusion process, two conditions must be met to realize dislocation climbing: there are vacancies in the crystal and the temperature is high enough.

In a fully annealed metal, the dislocation density ρ is about 1×106---108/cm2 After deformation, the dislocation density can increase to 1×1011---1012/cm2, which is due to the dislocation during plastic deformation As shown in Figure 6a, if a dislocation line DD' is fixed by an obstacle in the crystal, after the stress τ is applied, the dislocation line can only move forward along the slip surface (paper surface) from two points Bow out to form an arc shown in Figure 6b, and the radius of curvature of the arc decreases with the increase of stress. When the driving force increases so that the radius of curvature of the dislocation line is less than half of the distance between the obstacles, the dislocation becomes unstable, and continuing to move will increase the radius of curvature of the dislocation line instead (Figure 6c, d), forming a The expanded ring returns to its back, and finally separates a closed ring dislocation and a new dislocation line (Figure 6d, 6e). This process can be repeated indefinitely until another obstacle is encountered and a new dislocation line appears. The reaction force enough to stop its motion is the Frank-Read dislocation proliferation mechanism. Due to the continuous proliferation of dislocations in the slip, the dislocation density in the crystal increases after the metal undergoes cold deformation.

Polycrystalline slip polycrystal is an aggregate composed of many grains (equivalent to a single crystal) with different orientations, shapes and sizes. Generally speaking, at room temperature, the basic deformation mode of each grain in a polycrystal is still the same as that of a single crystal, but since there are grain boundaries between each grain and the orientations of adjacent grains are different, the plasticity of the polycrystal In addition to its commonality with single crystals, deformation has its particularity. Theoretical analysis points out that in order to make the polycrystal deform continuously and undamaged by slipping, there must be at least five independent slip systems in each grain. Experiments demonstrate that individual grains clearly slip on several slip systems even at small strains, especially in regions close to grain boundaries. Grain boundaries have a retarding effect on slip (referred to as grain boundary retardation effect). In polycrystalline metals, more than 90% of the grain boundaries are large-angle grain boundaries, and the lattice distortion is serious. The orientation of the grains on both sides is different, and the slip direction and slip planes do not coincide with each other. Therefore, it is extremely difficult for slip to continue directly from one grain to the next. After the polycrystalline test bar is stretched, the slip band in each grain terminates near the grain boundary, so that dislocations accumulate on the grain boundary, preventing the dislocation source inside the grain from generating new dislocations and causing The metal is visibly hardened. Therefore, the work hardening rate of polycrystalline materials is many times greater than that of single crystals.

Metal Slip

Metal slip is the basic mechanism of plastic deformation in metal materials and is of great significance for the study and application of metal materials. In this article, we will explore the phenomenon of metal slip and related knowledge in depth.

I. Crystal structure and crystal defects of metals

Metals are crystalline materials with ordered crystal structures. The crystal structure of a metal is formed by atoms arranged in a certain way, and the distances and relative positions between the atoms are regular. The crystal structure of a metal can usually be divided into several types, such as face-centered cubic, body-centered cubic, hexagonal close-packed, and so on, each with its specific characteristics and applications.

Crystal defects are common phenomena in metal crystal structures. Crystal defects include point defects, line defects, and surface defects. Point defects refer to irregularities in atomic positions in the crystal, such as vacancies and interstitial atoms. Line defects refer to dislocation lines on crystal surfaces caused by mismatched crystal structures. Surface defects refer to defects on grain boundaries and dislocation surfaces caused by irregularities in crystal structures.

II. Metal slip phenomenon

Metal slip is the phenomenon that when a metal material is subjected to external force, atoms inside the crystal will slide along the crystal planes, resulting in plastic deformation of the material. Metal slip is one of the basic mechanisms of metal plastic deformation.

Metal slip occurs on the planes inside the lattice of the metal, called slip planes. Atoms on the slip plane slide and deform in a certain way. The choice of slip plane is closely related to the metal crystal structure, and different metal crystal structures correspond to different slip planes. For example, slip planes of face-centered cubic metals are usually {111} planes, while slip planes of body-centered cubic metals are usually {110} planes. The slip direction is the same as the normal direction of the slip plane, and is called the slip direction.

Metal slip requires overcoming the binding force and inertia force between the lattice, so it requires a certain stress to trigger the slip phenomenon. Metal slip usually occurs at room temperature, but as the temperature rises, the slip activity of metal will increase.