Alexei Volkov
Plastic deformation is the ability of a solid material, such as metal, to undergo irreversible changes in shape in response to applied forces. It is a common phenomenon observed in metals, soils, rocks, concrete, and foams. In the context of metals, plastic deformation occurs when external stress exceeds the yield point, causing permanent deformation. This is achieved by the movement of dislocations, which are defects in the lattice structure of metals, allowing planes to shear over each other without affecting the atomic arrangement. The force required to cause plastic deformation can be reduced by localizing deformation through line defect movement, such as slip and twinning mechanisms.
| Characteristics | Values |
|---|---|
| Definition | Plastic deformation is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. |
| Cause | The fundamental mechanism of plastic deformation is the movement of dislocations. |
| Types of dislocations | Edge and screw. |
| Types of dislocation movement | Glide and climb. |
| Types of deformation | Slip and twinning. |
| Slip | A shear deformation that moves atoms through many interatomic distances relative to their initial positions. |
| Twinning | Plastic deformation that takes place along two planes due to a set of forces applied to a given metal piece. |
| Types of processes | Hot and cold deformation. |
| Examples of processes | Rolling, extrusion, wire-drawing, stretch-forming, etc. |
| Yield stress | The stress needed to initiate global plasticity in a sample. |
| Work hardening | Continuation of plastic deformation requires a progressively increasing level of applied stress. |
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What You'll Learn
Plastic deformation is a permanent change in shape
Plastic deformation is a fundamental aspect of metalworking, involving the application of external stress to induce a permanent change in shape. This process is commonly observed in metals and various other materials, including soils, rocks, concrete, and foams. However, the specific mechanisms underlying plastic deformation can vary across different substances.
In the context of metals, plastic deformation is predominantly facilitated by the movement of dislocations. Dislocations refer to defects in the lattice structure, where an extra half plane is inserted, resulting in two primary types: edge dislocations and screw dislocations. These dislocations can move through glide or climb. During glide, dislocations traverse along a surface defined by their Burgers vector, while climb involves movement outward from the glide surface.
The plastic deformation of metals typically occurs due to the glide of dislocations, driven by shear stresses. This process is known as slip, where atoms slide over each other within the crystal lattice when the applied stress surpasses the critical resolved shear stress of the material. Slip occurs through the movement of dislocations along close-packed planes, allowing for the sliding of lattice planes over each other. This results in macroscopic changes without disrupting the atomic arrangement.
In some cases, deformation twinning may also contribute to plastic deformation. Twinning involves a portion of the crystals assuming an orientation related to the rest of the lattice in a symmetrical manner. While twinning requires shear stresses similar to slip, it does not involve volume change. Both slip and twinning mechanisms play a significant role in the plastic deformation of polycrystalline metals, where individual grains must deform cooperatively, undergoing complex shape changes consistent with their neighbours.
The plasticity of metals is influenced by various factors, including crystal structure, grain size, crystallographic texture, composition, and phase constitution. Additionally, the physical state of the metal, such as whether it is hot or cold, can impact its plasticity. Most metals exhibit greater plasticity when hot, making them more amenable to shaping and forming operations.
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It occurs when stress exceeds the yield point
Plastic deformation of metals occurs when stress exceeds the yield point, causing a permanent change in shape. This phenomenon is known as plasticity, and it is characterised by the breaking of atomic bonds and the movement of dislocations within the material. Dislocations are defects in the crystal lattice where an extra half plane of atoms is inserted, allowing for slip and twinning deformation mechanisms. In the case of slip, atoms slide over each other along slip planes, while twinning involves a portion of the crystal taking up a new orientation relative to the rest of the lattice. These mechanisms enable metals to undergo macroscopic changes without affecting the atomic arrangement, resulting in plasticity.
The transition from elastic behaviour to plastic behaviour, or yielding, occurs when the applied stress surpasses the yield strength of the material. In the elastic region, stress and strain increase proportionally, following Hook's Law. However, once the stress exceeds the yield point, the material enters the plastic deformation stage, where deformation occurs much faster and becomes permanent. This critical stress value, known as the yield stress, marks the onset of global plasticity in the metal.
The yield stress is influenced by various factors, including crystal structure, grain size, composition, and prior dislocation density. As more dislocations are created and interact with each other through mechanisms like glide and climb, they can become less mobile, leading to a phenomenon known as work hardening or strain hardening. Work hardening complicates the definition of yield stress, as the required stress for further deformation progressively increases.
The plasticity of metals is crucial in metalworking processes such as rolling, extrusion, and wire drawing. These processes involve the application of controlled heat and pressure to achieve the desired shape changes while altering the mechanical properties of the material. Hot deformation techniques are commonly employed, as most metals exhibit higher plasticity when heated. During hot deformation, metals may recrystallize, resulting in a microstructure that is nearly free of dislocations.
While plasticity is commonly associated with metals, it is also observed in other materials such as soils, rocks, concrete, and foams. The mechanisms underlying plasticity can vary across different materials, but it generally involves the rearrangement or realignment of internal structures in response to applied forces. Understanding the plastic deformation of metals and other materials is essential for engineering and manufacturing applications, allowing for the design of durable and functional products.
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It is caused by the movement of dislocations
Plastic deformation of metals occurs when the material is subjected to stress beyond its yield strength, causing it to undergo a permanent change in shape or size. This phenomenon is primarily driven by the movement of dislocations within the crystalline structure of the metal.
Dislocations are defects or irregularities in the crystal lattice, where an extra half-plane of atoms is introduced. These dislocations can be categorised into two main types: edge dislocations and screw dislocations. In edge dislocations, an additional half-plane of atoms is inserted into the lattice, while screw dislocations involve a shift along a plane, resulting in a spiral arrangement of atomic planes.
The movement of dislocations within the metal plays a crucial role in plastic deformation. There are two primary types of dislocation movement: glide and climb. During glide, dislocations move along a surface defined by their Burgers vector, while in climb, they move outward from the glide surface. The glide of dislocations is driven by shear stresses, allowing the sliding of lattice planes over each other without affecting the atomic arrangement. This results in macroscopic changes to the metal's shape.
The ease of dislocation movement is influenced by several factors, including crystal structure, temperature, dislocation density, and applied stress. Face-centred cubic (FCC) structures, for example, exhibit higher plasticity compared to other crystal structures due to the increased number of directions available for dislocation movement. Higher temperatures facilitate dislocation motion by providing thermal energy, thereby increasing the rate of plastic deformation.
As more dislocations are created and interact with each other, they can become less mobile, leading to a phenomenon known as "work hardening" or "strain hardening". This complex process depends on various factors, including crystal structure, grain size, composition, and prior dislocation density. By understanding the mechanics of dislocations and their role in plastic deformation, scientists can gain valuable insights into the strength and malleability of materials.
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Slip and twinning are two deformation modes
Plastic deformation in metals occurs when the applied stress exceeds the yield point, causing atomic bonds to stretch and break and resulting in permanent deformation. This process involves the movement of dislocations, which are defects in the lattice structure of metals. The two primary deformation modes are slip and twinning.
Slip is the dominant deformation mode in many metals and involves the movement of atoms slipping over each other within the crystal lattice when the applied stress surpasses the critical resolved shear stress of the material. Dislocations glide along close-packed planes and directions, allowing for macroscopic changes without disrupting the atomic arrangement. The slip system refers to the set of slip planes and directions that require lower energy for dislocation movement.
Twinning, on the other hand, comes into play when slip is not possible. Deformation twinning occurs in certain alloys, such as nickel-based alloys, and enables room-temperature deformation and increased strain hardenability. The presence of specific precipitates, such as Pt2Mo-structured Ni2Cr-typed precipitates, influences the activation of slip versus twinning.
The choice between slip and twinning as deformation modes depends on various factors, including the crystal structure, grain size, composition, and the presence of precipitates or impurities. In a polycrystal, individual grains must deform cooperatively, requiring the operation of multiple slip systems to maintain consistency with neighbouring grains.
While slip and twinning are fundamental deformation modes, more complex deformation mechanisms exist, such as grain boundary sliding in polycrystalline metals, which further contribute to plastic deformation. The specific deformation mode employed depends on the metal's microstructure and the applied stress conditions.
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Work hardening is required to continue deformation
Plastic deformation of metals is an intrinsic part of the processing of most metals. It involves the application of external stress to achieve a shape change, while also altering the material's mechanical properties. This process is generally divided into hot and cold deformation, with the latter being more commonly known as work hardening or cold working.
Work hardening is a critical phenomenon in the context of plastic deformation, as it enables the continuation of deformation. It is the process by which a material's load-bearing capacity or strength increases during plastic deformation. This increase in strength is what distinguishes ductile materials from brittle ones. Work hardening may be desirable or undesirable, depending on the specific application.
During plastic deformation, the number and density of dislocations within the material increase, leading to their entanglement and interaction. These interactions impede the motion of other dislocations, resulting in a higher stress requirement to overcome further plastic deformation. Consequently, the strength and hardness of the material increase. The rate at which strength increases with strain is crucial for understanding and predicting the mechanical behaviour of the material.
The hardening rate is influenced by factors such as the initial dislocation density, the rate of dislocation generation, and the rate of dislocation annihilation. Work hardening significantly impacts the mechanical properties of metals, enhancing their strength, ductility, and toughness. However, the increased strength due to work hardening is often accompanied by a decrease in ductility as the dislocation movement becomes increasingly hindered.
Work hardening is particularly relevant in metalworking processes that intentionally induce plastic deformation to achieve a desired shape change. These processes are conducted at temperatures below the recrystallization temperature, typically at ambient temperature. By manipulating the material through work hardening, its properties can be mechanically advantageous, especially in the context of metals, where work hardening is used to make them much stronger.
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Frequently asked questions
Plastic deformation of metal is the ability of a metal to undergo a permanent change of shape in response to applied forces. This occurs when the applied stress exceeds the yield point, causing atomic bonds to stretch and break, and planes to shear over each other.
Plastic deformation in metals can occur through two main mechanisms: slip and twinning. Slip involves the sliding of crystal blocks over one another, while twinning is the plastic deformation that occurs along two planes due to a set of forces applied to the metal.
The plasticity of a material is influenced by its ductility and malleability. It is also dependent on the deformation speed, with higher stresses required to increase the rate of deformation. In metals, the presence of dislocations, grain size, and temperature can also impact plastic deformation.
Plastic deformation is an integral part of metalworking processes, where shape changes are achieved through the application of controlled external stress. This can involve various techniques such as rolling, extrusion, and wire-drawing, which may be performed at hot or cold temperatures to alter the material's properties.
