Omar Khalid
Plastic deformation is the irreversible deformation of a solid material in response to applied forces. When a material is stressed, it first undergoes elastic deformation, but once the yield limit is reached, it deforms plastically, which in metals is achieved through dislocation motion. Dislocations are line defects in materials that occur when atoms are displaced from their regular positions in a crystal lattice, disrupting uniformity and causing stress and distortion in the surrounding structure. Dislocations play a significant role in plastic deformation, as they can increase or decrease material strength and ductility, affecting the material's ability to withstand stress. The processes of dislocation generation and recovery are not yet fully understood quantitatively, but basic models have been developed over the past 50 years.
| Characteristics | Values |
|---|---|
| Plastic deformation | Deformation of a solid material that undergoes non-reversible changes of shape in response to applied forces |
| Dislocation motion | When stress is applied to a material, it first deforms elastically, then plastically through dislocation motion |
| Dislocation generation | Observed in ceramic single crystals, semiconductors, metals, intermetallics, and quasicrystals during in situ deformation |
| Work hardening | Increase in the stress level necessary to continue plastic deformation |
| Dislocation strengthening | Increased dislocation density that leads to more interactions between dislocations, making it harder for them to move and increasing material strength |
| Solid-solution strengthening | Introduction of foreign atoms into the base metal, distorting the lattice and creating stress fields that strengthen the material |
| Glide or slip | Movement of dislocations along the slip plane in response to shear stress |
| Climb | Influenced by temperature or stress, allowing plasticity by enabling dislocations to bypass obstacles in their glide paths |
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What You'll Learn
Dislocation motion and plastic deformation
Plastic deformation refers to the non-reversible deformation of a solid material in response to applied forces. When a stress is applied to a material, it first undergoes elastic deformation. However, once the yield limit is reached, the material will deform plastically, which, in metals, occurs through dislocation motion.
Dislocations refer to line defects in materials that occur when atoms are displaced from their regular positions in a crystal lattice. This displacement disrupts the uniformity of the lattice, causing stress and distortion in the surrounding structure. The motion of these dislocations is responsible for plastic deformation. Any dislocation motion, regardless of distance, constitutes plastic deformation. If the stress is removed during dislocation motion, the dislocations do not disappear, and the atoms remain in their new positions, resulting in a permanent change.
The mechanisms of dislocation generation during plastic deformation have been studied through in situ straining experiments using high-voltage electron microscopy (HVEM) on various materials, including ceramics, semiconductors, metals, intermetallics, and quasicrystals. One mechanism of dislocation generation is the Frank-Read source, proposed by Frank and Read in the 1950s. This mechanism involves the multiplication of dislocations through cross-slip, where each source can produce large amounts of slip before becoming immobilized.
The interaction of dislocations plays a crucial role in material strength. An increased number of dislocations leads to more interactions, impeding their motion and enhancing the material's strength. This phenomenon is known as dislocation strengthening. Additionally, processes like solid-solution strengthening further reinforce the material by introducing impurities that act as obstacles to dislocation motion.
The glide or slip of dislocations along the slip plane in response to shear stress contributes to plastic deformation. The climb, influenced by temperature or stress, enables plasticity by allowing dislocations to bypass obstacles in their glide paths. The intersection of screw dislocations, as illustrated by G.E. Dieter, results in the formation of jogs, which can move by slip along the axis of the screw dislocation.
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Work hardening and plastic deformation
Work hardening, also known as strain hardening or cold working, is a phenomenon that occurs during the plastic deformation of metallic materials. It is the strengthening of a metal or polymer by plastic deformation. This strengthening is caused by dislocation movements and dislocation generation within the crystal structure of the material.
When a material is deformed, numerous dislocations are emitted and forced to interact with each other. This interaction increases the stress required to overcome further plastic deformation, leading to increased strength and hardness of the material. The rate at which the strength increases with strain is critical for understanding and predicting the mechanical behaviour of the material.
The work hardening process can be influenced by factors such as the initial dislocation density, the rate of dislocation generation, and the rate of dislocation annihilation. In most crystalline materials, the dislocation density increases significantly during plastic deformation, leading to work hardening. This increase in dislocation density may be described by an evolution law of dislocation density, which considers the rate of dislocation generation and annihilation.
Work hardening is particularly notable in ductile materials such as metals. Ductility is the ability of a material to undergo plastic deformation before fracture, such as bending a steel rod until it breaks. While work hardening can be desirable in some applications, specialised alloys are used to avoid it in metal objects designed to flex, such as springs.
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Dislocation strengthening
Dislocations are major carriers of plastic deformation in most metals. Plastic deformation occurs when a large number of dislocations move and multiply, resulting in macroscopic deformation. The movement of dislocations in a material allows for deformation. When stress is applied to a material, it first undergoes elastic deformation. Once the yield limit is reached, it undergoes plastic deformation, which, in the case of metals, means by dislocation motion.
One solution to this trade-off is a strengthening strategy that not only impedes dislocation motion but also provides extra dislocation storage capacity. Metastable austenitic steels are an example of this, where a one-step thermal mechanical treatment (TMT), i.e. hot rolling, can enhance yielding strength while retaining formability and hardenability. The success of this method lies in the decoupled strengthening and toughening mechanisms, where the yield strength is controlled by the initial dislocation density, and ductility is retained by the ability to nucleate new dislocations.
The ease of dislocation movement is critical to the hardness and strength of a material. Pinning points, or locations in the crystal that oppose dislocation motion, can be introduced into the lattice to increase mechanical strength. These pinning points can be created by stress field interactions with other dislocations and solute particles, forming physical barriers along grain boundaries. Additionally, dislocation line entanglements can act as pinning points, opposing dislocation motion and increasing shear strength.
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Solid-solution strengthening
Plastic deformation occurs when a material is stressed beyond its yield limit, causing it to deform through dislocation motion. This motion results in an imperfect lattice structure, and the dislocations can be generated through various mechanisms, such as Frank-Read sources and cross-slip.
In substitutional solid solutions, some of the solvent lattice atoms are replaced by solute atoms of a different size and electronic structure. This replacement creates distortions in the lattice, impeding the motion of dislocations and increasing the material's strength. The degree of distortion depends on the size and electronic mismatch between the solvent and solute atoms. According to Hume-Rothery rules, higher solubility is achieved when solvent and solute atoms are similar in size and adopt the same crystal structure in their pure form. Examples of alloys strengthened by substitutional solid solutions include high-entropy alloys and titanium alloys, where the addition of oxygen enhances the material's properties.
Interstitial solid solutions, on the other hand, form when the solute atoms are small enough to fit into the interstitial sites between the solvent atoms. The presence of these small atoms in the interstitial sites creates strong lattice distortions, substantially strengthening the metal. Interstitial solid solutions are particularly effective in impeding dislocation nucleation and motion, leading to improved mechanical properties. Alloys that utilize interstitial solid solutions include massive interstitial solid solution (MISS) alloys, which can incorporate high amounts of interstitials without forming brittle ceramics.
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The Frank-Read source
Plastic deformation is caused by the motion of dislocations. When a stress is applied to a material, it first undergoes elastic deformation, but once the yield limit is reached, it deforms plastically, which in metals is achieved through dislocation motion.
The Frank-Read process has inspired other theories, such as that of Bardeen and Herring (1952), who proposed a configurationally similar type of source, where the active dislocation segment bows out by climb.
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Frequently asked questions
Plastic deformation is the deformation of a solid material that undergoes non-reversible changes of shape in response to applied forces.
Dislocations are line defects in materials that occur when atoms are displaced from their regular positions in a crystal lattice. This disrupts the uniformity and causes stress and distortion in the surrounding structure. During plastic deformation, the dislocation density increases, leading to work-hardening and an increase in the material's plastic deformation.
When stress is applied to a material, it first deforms elastically. Once the yield limit is reached, it deforms plastically, which in metals is by dislocation motion. If the stress is removed during dislocation motion, the dislocations do not vanish, and atoms remain in their new locations, resulting in plastic deformation.
The two main mechanisms of dislocation generation during plastic deformation are localized Frank-Read sources and multiplication by the double-cross slip mechanism. The Frank-Read source was proposed in the 1950s and refers to the generation of large amounts of slip by each source before it becomes immobilized. Cross-slip is required for dislocation multiplication and is observed in various materials during in situ deformation.
