Chloe Zhang
Plasticity, also known as plastic deformation, is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. Materials that exhibit high plasticity are typically ductile and malleable, allowing them to be deformed without rupture. This property is commonly observed in metals during metal-forming processes such as rolling, pressing, and forging. Additionally, geologic processes involving rock folding and rock flow under high pressures and temperatures also showcase plastic deformation. While brittle materials like cast iron typically cannot be plastically deformed, certain materials like glass can undergo plastic flow at elevated temperatures. The plasticity of a material is crucial in engineering and manufacturing, contributing to its toughness and ability to withstand hard blows without rupturing.
| Characteristics | Values |
|---|---|
| Definition | The ability of certain solids to flow or to change shape permanently when subjected to stresses |
| Occurrence | Observed in most materials, particularly metals, soils, rocks, concrete, and foams |
| Plastic deformation | Occurs in many metal-forming processes (rolling, pressing, forging) and in geologic processes (rock folding and rock flow within the earth under extremely high pressures and at elevated temperatures) |
| Perfect plasticity | The property of materials to undergo irreversible deformation without any increase in stresses or loads |
| Plastic deformation speed | Higher stresses usually have to be applied to increase the rate of deformation |
| Plastic deformation in crystals | Caused by two modes of deformation in the crystal lattice: slip and twinning |
| Slip | A shear deformation that moves the 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 |
| Brittleness | An unwanted property in mechanical engineering where a material breaks without noticeable plastic deformation |
| Toughness | A combination of strength and plasticity, allowing a material to take hard blows without rupturing |
| Ductility | An aspect of plasticity that signifies a material's ability to deform under tension |
| Malleability | An aspect of plasticity that describes how a material deforms under compression |
| Yield point | The stress level at which a material begins to deform plastically |
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What You'll Learn
Metals, soils, rocks, concrete and foams
Plasticity is a property of materials that allows them to undergo irreversible deformation without any increase in stress or load. Materials that exhibit notable plasticity include metals, soils, rocks, concrete, and foams.
Metals
Plasticity in metals is often a result of crystal lattice defects, such as dislocations, which are relatively rare in most crystalline materials. However, in some metals, these defects are numerous and become part of their crystal structure, leading to plastic crystallinity. Additionally, ductile metals can exhibit plasticity when subjected to tensile loading, where each increment of load results in a proportional increase in extension. The plasticity of a metal is also influenced by prior deformation, with materials like cold-formed steel requiring higher stresses for further deformation.
Soils
Soils, particularly clays, exhibit significant plasticity due to their unique microstructure, chemical composition, and water content. The plasticity of soil is primarily caused by the rearrangement of adjacent grain clusters. Soils with high plasticity indices, indicating a high capacity to hold water, tend to contain larger amounts of clay minerals. Organic soils, on the other hand, have lower plasticity indices due to their high liquid and plastic limits.
Rocks
Rocks are typically associated with brittleness and fracture, but they can also exhibit plasticity. Rock mass plasticity refers to the study of rock behaviour under loads beyond the elastic limit. Rocks under high hydrostatic stresses may undergo plastic deformation before ultimately failing. Experiments have demonstrated plasticity in rocks at high confining pressures, indicating a transition from elastic to plastic behaviour. Certain rock specimens subjected to compression and tension tests exhibit necking, while wedge penetration tests result in lip formation, showcasing localized plasticity.
Concrete
Concrete is classified as a brittle material, and its plasticity is predominantly caused by slip at microcracks. Additionally, the formation of microcracks and sliding motions relative to these cracks contribute to the inelastic deformation of concrete.
Foams
Foams, particularly open-cell foams, can exhibit plasticity when the bending moment exerted on the cell walls exceeds the fully plastic moment. This plasticity arises from the rearrangement of bubbles or cells within the foam structure. The material's amorphous nature allows for plastic deformation, accommodating tension by creating regions of free volume and resulting in a hazy appearance due to crazing.
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Crystalline materials
Plasticity, or plastic deformation, is the ability of a solid material to undergo irreversible changes in shape in response to applied forces. In crystalline materials, plasticity is primarily caused by two modes of deformation in the crystal lattice: slip and twinning. Slip is a shear deformation that moves atoms through many interatomic distances relative to their initial positions. Twinning is plastic deformation that occurs along two planes due to a set of forces applied to a given metal piece.
Crystal plasticity refers to the study of plastic deformation in single-crystal and polycrystalline materials while taking into account the physics and geometry of deformation at the crystal (or grain) level. At low homologous temperatures, the dominant mode of plastic deformation in crystalline materials is slip on specific crystallographic planes in specific directions. Deformation twinning is another mode of plastic deformation that occurs on specific crystallographic planes and plays a significant role in the stress-strain response and the evolution of the underlying microstructure in the material.
The crystal plasticity (CP) method is a constitutive theory based on the dislocation slip mechanism of crystalline materials and mesoscopic medium mechanics theory. The plastic deformation of crystalline materials at room temperature is mainly achieved by dislocation movement along the slip systems. The CP theory introduces the concept of plastic shear strain to describe this dislocation movement as a continuous plastic deformation process.
The plasticity of a crystalline material is influenced by factors such as temperature, stress, and crystal structure. At higher stress and a higher crystal fraction, crystalline materials exhibit a larger amount of strain accommodated by crystal plasticity. Additionally, the crystal plasticity of materials like metals is influenced by their low-stacking fault energy and low-symmetry crystal structures.
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Amorphous materials
The fundamental building blocks of plasticity in amorphous materials are local reorganizations, but their collective behaviour is still a topic of research. The plastic yielding of amorphous solids occurs through power-law distributed deformation avalanches, and the universality of this process is debated. A tensorial mesoscale model has been introduced to capture the complex shear patterns and avalanche scaling behaviour, which differs from mean-field models.
Amorphous ceramics exhibit superior properties such as high strength, good resistance to oxidation and creep, and microstructural stability at high temperatures and under radiation. However, they are brittle at room temperature due to strong ion-covalent bonds, which limit plasticity mechanisms and can result in sudden, catastrophic fractures. To address this, patterning nanoscale metal-rich heterogeneities can be used to improve the plasticity of amorphous ceramics.
Additionally, amorphous materials have been explored in pharmaceuticals, where some drugs may exist in a total or partially amorphous state. Understanding the physicochemical principles behind this state is crucial for drug development and stability. Overall, amorphous materials offer unique advantages and are an active area of research for various applications.
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Ductile and malleable solids
Plasticity is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. Ductility and malleability are two mechanical properties of solids that are directly proportional to plasticity.
Ductility is the property of a solid material that allows it to be drawn out or stretched into a thin wire without breaking. It is the ability of a material to deform plastically under tensile stress. The most ductile metal is platinum. Ductility is of utmost importance in metalworking as materials that crack, break, or shatter under stress cannot be manipulated using metal-forming processes such as hammering, rolling, drawing, or extruding. The ductility of steel varies depending on the alloying constituents. For example, increasing the levels of carbon decreases ductility.
Malleability is a similar property to ductility and is characterized by a material's ability to deform plastically without failure under compressive stress. Historically, materials were considered malleable if they were amenable to forming by hammering or rolling. Malleable materials can be formed cold using stamping or pressing. Gold is the most malleable metal. Many plastics and amorphous solids, such as Play-Doh, are also malleable.
The minimum temperature at which a metal transitions from brittle behavior to ductile behavior, or vice versa, is known as the ductile-brittle transition temperature (DBTT). Below the DBTT, the material will not be able to deform plastically and will instead rapidly undergo brittle failure. Therefore, the DBTT is an important factor in determining the plasticity of a material.
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Plastic deformation methods
Plastic deformation is a fundamental concept in materials science and engineering, essential for understanding how materials behave under stress. It refers to the permanent change in shape that occurs when a material is subjected to forces beyond its elastic limit. This is also known as yielding. Plastic deformation is observed in most materials, particularly metals, soils, rocks, concrete, and foams.
There are several plastic deformation methods, including:
- Slip: This is the most common mode of plastic deformation, where dislocations move along specific planes and directions known as slip planes and slip directions. The choice of slip system is influenced by the crystal structure of the material. For example, Face-Centered Cubic (FCC) crystals have multiple slip systems, making them highly ductile.
- Twinning: This occurs when a portion of the crystal lattice shifts to create a mirrored region of the crystal structure. Twinning is less common than slip but can be significant in certain materials.
- Dislocation movement: This is one of the primary mechanisms of plastic deformation in crystalline materials. Dislocations are defects in the crystal lattice that allow atoms to slide past each other more easily. When a material is stressed, dislocations move through the crystal lattice, facilitating plastic deformation.
- Forging: This involves shaping metal by applying compressive forces, often at high temperatures.
- Pressing and rolling: These are metal-forming processes that can induce plastic deformation.
The plasticity of a material is directly proportional to its ductility and malleability. The rate at which stress is applied also affects plastic behaviour, with higher strain rates leading to increased strength but reduced ductility.
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Frequently asked questions
Plasticity is the ability of a solid material to undergo irreversible deformation without rupture. In other words, plasticity enables a solid to change shape under external forces without breaking.
Metals, soils, rocks, concrete, and foams all display plastic deformation. Metals are particularly noteworthy for their plasticity, which is often a result of dislocations in their crystal structure. Soils, especially clays, also exhibit significant plasticity due to the rearrangement of adjacent grain clusters.
The plasticity of a material is influenced by its ductility and malleability. Additionally, the deformation speed and the magnitude of applied stresses play a role in determining the level of plasticity. In the case of metals, prior deformation, such as cold forming, can increase the stresses required for further deformation, thus affecting plasticity. For soils, plasticity is dependent on microstructure, chemical composition, and water content.
