Left Slip occurs when dislocations glide through the material, causing crystal planes to slide along each other. Right Twinning occurs via a concerted shift of atomic positions in the twinning direction. This critical resolved shear stress CRSS is related to the stress required to move the dislocations on the slip plane.
Where n is the normal vector to the slip plane and s is the slip direction. Crystal orientation can make the movement of dislocations easy or difficult. Under identical conditions, more the slip systems in the crystal, more will be the spatial orientations that can be assumed during the slip process, which is beneficial for slip and, therefore, leads to high plasticity. Therefore, the face-centered cubic crystal has 12 slip systems.
Thus, there are 48 potential slip systems for the BCC crystal. There are three slip systems of HCP. Table 1 shows the critical resolved shear stress of some metal single crystals with different crystal structures at room temperature [ 33 ]. As can be seen from the table, impurities can affect the CRSS of the material significantly.
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Srinivasan et al. Critical resolved shear stresses of some metal single crystals with different crystal structures at room temperature. In addition to impurities, many factors affect the CRSS of materials. Spitzig et al. In addition, the slip of the BCC crystal and its dependence on crystal orientation and temperature are much more complicated than those of the FCC crystal. Wu et al. The orientation dependence of Al single crystals maintains good agreement with the Schmid factor relationship and the extracted critical resolution shear stress even at the micron scale and nanoscale.
Moreover, higher the energy of the stacking fault, higher will be the dislocation mobility and lower will be the influence of the sample size. Twinning is another type of plastic deformation that occurs when it is not possible for the crystal to slip. Under the action of shear stress, some parallel crystal planes in a local area of the crystal produce uniform shearing at a distance relative to each other along a certain direction.
The deformed and undeformed two-part crystal is referred to as a twin crystal. The interface between the uniform shear region and the nonshear region is called the twin boundary and the moving direction of the twin plane becomes the twinning direction Figure 2 a [ 29 ]. In the shear zone, the relative displacement of each layer of the crystal plane parallel to the twin plane is constant, which is common in HCP and BCC crystals.
The twin deformation also occurs under the action of shear stress, usually in the stress concentration zone caused by the slip resistance. Therefore, the critical shear stress required for twinning is much larger than that for slip. Twinning is uniform shear compared to the uneven deformation caused by slip.
Sliding directly produces plastic deformation, and twinning changes the crystal orientation, resulting in a new slip system, which has an indirect effect on plastic deformation. Yu et al. It was found that the stress required for deformation twinning increases sharply with the decrease in sample size until the sample size is reduced to 1 micron. In this range, the deformation twins are completely correlated. The stress is replaced by a less common general dislocation plasticity.
In some HCP metals, such as Mg, under the very low stress of c -axis stretching, there will be tension twinning or stretching twins. It has a significant contribution to structural evolution and a substantial effect on plastic flow during large deformations. Rawat et al. The effects of multiaxial loads on the evolution of different twin volume fractions and twin numbers were also reported. In this review, we compared five loading conditions.
The results showed that all of the samples exhibited the characteristic of twin binding, and, in addition to the binding between the twins with the same variant, binding occurred between the twins belonging to the conjugated variant.
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The activation of twin mutations is consistent with the Schmid factor. However, variants with the same Schmid factor are not necessarily activated simultaneously. Furthermore, active twin variants with the same Schmid factor do not evolve at the same rate; thus, they contribute differently to the entire twin volume fraction.
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Narayanan et al. Strain-hardening in five-fold double-crystal AgNWs does not give rise to single-crystal nanowires or other nanostructures. The smaller wire is significantly hardened and its ultimate tensile strength is higher than that of the larger Ag wire. In addition, surface toughness [ 40 ], impurities [ 41 , 42 ], or orientations [ 43 , 44 ] can have a significant effect on twin deformation. The plastic deformation of crystals is a complicated process and it is of significance to study the deformation mechanism. Yoshinaga et al. The CRSSs of these two structures were determined to be very high approximately twice the non-base slip and they decreased rapidly as the test temperature increased.
In general, low stacking faults can catalyze cracking metals and alloys with high twinning activity, whereas high stacking faults can catalyze slippage of cracked metals and alloys. Kim et al. In situ transmission electron microscopy showed that the thin smear of the Shockley local dislocations formed at the grain boundaries indicates the nucleation process of the deformed twins at the grain boundary defects.
The observed twinning process is similar to the deformation twinning process produced by the continuous emission of partial dislocations at the grain boundaries in the nanocrystalline material. In addition, high-density Frank partial dislocations were observed inside the deformed twins. These defects affect the growth of deformed twins and may result in high work-hardening of TWIP steel. Karaman et al. The overall stress—strain response is strongly dependent on the crystal orientation and applied stress direction.
Studies have shown that twins are the main deformation mechanism in the compression of crystals at the beginning of crystal and inelastic deformation. The main deformation mechanism of crystal deformation and crystal tensile deformation is multislip. In addition, the crystal undergoes a single slip during stretching and compression, and planar dislocations occur.
Wang et al. The results show that the twin crystal is the main deformation mechanism of BCC tungsten nanocrystals. This deformed twin is pseudoelastic and exhibits reversible decrystallization during unloading. In addition, the synergy between twinning and dislocation slip can be adjusted by the loading orientation, which is related to the nucleation mechanism of nanoscale BCC crystal defects Figure 3.
Table 2 lists the largest Schmid factors on the dislocation slip and deformation twinning systems for the four loading orientations tested in BCC W nanowires. This work provides new insights into the deformation mechanism of BCC nanostructures. Reversible deformation twinning and detwinning processes in a W bicrystal nanowire under cyclic loading. A deformation twin nucleates at the same place in subsequent deformation cycles.
The stress is estimated based of the lattice strain, and the overall compressive strain is measured from the change of the nanowire length. The error bars represent the variations of the estimated stresses at different locations of the nanowire. All scale bars: 5 nm. The largest Schmid factors on the dislocation slip and deformation twinning systems for the four loading orientations tested in body-centered cubic BCC W nanowires.
The plastic deformation of each of the crystal grains in a polycrystal is substantially similar to that of a single crystal, and the polymorphous plastic shape is still slip-like. Kocks et al. The mechanism of plastic deformation is that dislocations occur in the crystal under the action of shear stress, sliding along a certain crystal plane to achieve plastic deformation of the metal. As the lattice orientation of each crystal grain in the polycrystal is different, the arrangement of atoms at the interface between the crystal grains is extremely irregular, and the grain deformation is both intragranular and intergranular.
Thus, the plastic deformation of the polycrystal is smaller than that of the single crystal. When polycrystalline materials are plastically deformed, not all grains are slipped simultaneously—the crystal grains are staged and slipped in batches through the application of external force.
The main mechanism is the slip inside each grain, accompanied by the slip and rotation between the grains. Owing to the unevenness of the deformation, various internal stresses are generated in the deformed body, which do not disappear after the deformation is completed and ultimately become residual stress.
Residual stress can play a role in strengthening or weakening the working stress. If the residual stress is a large tensile stress, the metal will be destroyed under the condition of small load, and the residual tensile stress of the surface layer will promote the formation of fatigue cracks, which will lead to considerable damage to the parts under alternating load working conditions. However, if the existence of residual stress reduces the working tensile stress, the service life of the part can be improved, for example, by intentionally manufacturing compressive residual stress, shot peening, and nitriding on the surface of the work piece.
The mechanical properties of polycrystalline materials can be adjusted by varying the degree of nucleation, propagation, and interaction of dislocations and shear bands.POTOLOKROSTOV.RU/cache/help/repa-the-middle-ages.php
Twin Deformation Mechanisms in Nanocrystalline and Ultrafine-Grained Materials
As metals and alloys are mostly polymorphic, the interaction between defects and grain boundaries is particularly significant. The grain boundaries accord higher strength and hardness to polycrystals than that of single crystals. The finer the crystal grains in polycrystals, the larger the ratio of grain boundary regions and the strength and hardness of metals and alloys.
In bulk polycrystalline materials, the grain boundary acts as a barrier to dislocation motion, expressed by the classical Hall—Petch relationship, which describes the increase in the yield strength of polycrystalline metals as the grain size decreases.