Omar Khalid
Plasticity, or plastic deformation, is the ability of a solid material to undergo irreversible changes in shape in response to applied forces. In the context of metals, plasticity is observed when a load is applied beyond the elasticity limit, causing the metal to adopt a new shape and size. This phenomenon is influenced by factors such as temperature, strain rates, and the presence of dislocations or defects in the crystal lattice of the metal. The plasticity of metals has been a subject of research since 1864, with various theories and models developed to understand and predict their behaviour under different conditions. The complexity of metal plasticity, particularly at the microscale, has led to the widespread use of empirical macroscale plasticity theories in applications.
| Characteristics | Values |
|---|---|
| Definition | Plasticity is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. |
| Materials | Metals, soils, rocks, concrete, foams, polymers, rubbers, and more. |
| Causes | In crystalline materials, plasticity is 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 applied forces. |
| Temperature | Most metals show more plasticity when hot than when cold. Lead shows plasticity at room temperature, while cast iron does not show sufficient plasticity for forging even when hot. |
| Deformation | Plastic deformation is observed when stress exceeds the critical value, causing a non-reversible change in shape. Elastic deformation is reversible, with objects returning to their original shape after the removal of stress. |
| Yielding | The transition from elastic to plastic behaviour is known as yielding. Yielding occurs when stress exceeds the yield strength, causing a rapid increase in extension. |
| Ductility | The plasticity of a material is directly proportional to its ductility and malleability. Ductile metals can exhibit elastic behaviour under tensile loading, but once the load exceeds the yield strength, plastic deformation occurs. |
| Anisotropy | Plastic deformation results in anisotropic material properties due to the rotation and alignment of slip planes. This makes it challenging to model and calculate in 3D. |
| Models | Macroscale models treat metals as isotropic continuums, starting with 1-D applications and progressing to 3-D. Microscale descriptions of polycrystal plasticity are complex, involving stress-strain curves, isotropic and kinematic hardening, and the Bauschinger effect. |
| Limitations | Plasticity theories provide rough values and do not always accurately predict behaviour, especially for high-temperature creep and strain rate sensitivity. |
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What You'll Learn
Plastic deformation
Twinning is another mode of plastic deformation that occurs when slip is not possible. It takes place along two planes due to a set of forces applied to a given metal piece. Deformation twinning also requires shear stress, similar to slip. Both slip and twinning contribute to the complex shape changes observed in individual grains during plastic deformation.
The fundamental mechanism of plastic deformation in metals is the generation and movement of dislocations. This movement can occur through glide, where dislocations move along a surface defined by their Burgers vector, or climb, where dislocations move outward from the glide surface. The presence of dislocations increases the likelihood of planes slipping past each other, leading to a permanent change in the shape of the crystal.
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Ductile metals
Ductility is the ability of a metal to receive permanent deformation without fracturing. Metals that can be formed or pressed into another shape without breaking are ductile. In general, all metals are ductile at high temperatures. However, at room temperature, metals can only withstand a small amount of deformation before breaking and are thus classified as brittle.
The plasticity of a material is directly proportional to its ductility and malleability. Plasticity, or plastic deformation, is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. For example, a solid piece of metal being bent or pounded into a new shape displays plasticity as permanent changes occur within the material itself. In engineering, the transition from elastic behaviour to plastic behaviour is known as yielding.
Most plastics and metals are ductile, but some metals, like high-carbon steel, are not. Materials that aren't ductile are considered brittle; instead of deforming under pressure, they simply break, chip, or snap. The ductility of steel varies depending on the alloying constituents. Increasing the levels of carbon decreases ductility.
Ductility is especially important 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. Malleable materials can be formed cold using stamping or pressing, whereas brittle materials may be cast or thermoformed.
Gold, silver, platinum, and most aluminium alloys are ductile metals. Tantalum, copper, and brass are also ductile.
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Slip and twinning
Plasticity in metals is caused by two modes of deformation in the crystal lattice: slip and twinning. Slip is a shear deformation that occurs when crystal blocks slide over one another along crystallographic planes called slip planes. This movement of atoms results in a permanent change in the shape of the crystal, leading to plastic deformation. The slip process occurs rapidly, usually within milliseconds, and is influenced by factors such as temperature, strain rate, and applied stress.
Slip is facilitated by the presence of dislocations, which are defects in the crystal lattice. Dislocations allow planes to slip past each other along their close-packed directions, resulting in plastic deformation. As the atoms slide during slip, the number of dislocations increases, raising the critical resolved shear stress required to maintain plastic deformation. This leads to a gradual increase in stress with continued deformation.
Twinning, on the other hand, involves a portion of the crystals adopting an orientation that is symmetrically connected to the direction of the remaining untwined lattice. In other words, the portion of crystals in twinning takes on a mirror-image orientation relative to the rest of the lattice. Twinning is determined by factors such as defects, crystal axis shift, visibility, and the occurrence of a threshold value in stress slip. Compared to slip, twinning involves less movement of atoms and occurs on a smaller scale, typically within a few microseconds.
The primary process of deformation in metals is typically slip, as it is the more prominent mechanism. However, both slip and twinning contribute to the overall plasticity of metals. Most metals exhibit greater plasticity when heated, making them more amenable to shaping and forming operations. The increased temperature enhances the mobility of dislocations, facilitating slip and promoting plastic deformation.
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Plasticity in nanoscale metals
In physics and materials science, plasticity is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. Plasticity in nanoscale metals is an emerging area of research, with promising findings in certain gradient metallic materials. Severe plastic deformation (SPD) methods are becoming popular due to their ability to refine the grain size of materials to the nanoscale. This process has been applied to the commercial production of nanostructured metals and alloys, such as thin strips of stainless steel.
At the nanoscale, the primary plastic deformation in simple face-centered cubic metals is reversible, provided there is no material transport in the form of cross-slip. This is because the presence of dislocations, or glitches in the atomic stack-ups, increases the critical resolved shear stress required to maintain plastic deformation. As a result, the nanoscale grains induce high strength but also degrade tensile ductility.
However, recent studies have shown that certain metallic materials with gradient microstructures produced by surface severe plastic deformation (SSPD) techniques exhibit a combination of high strength and ductility compared to their homogeneously structured counterparts. Gradient nanostructured steel, for example, has been found to exhibit exceptional plasticity, with a significant improvement in yield strength and uniform elongation. This is attributed to the prominent grain coarsening in the surface NC layers during deformation, as well as heterodeformation-induced (HDI) hardening generated by the pileup of geometrically necessary dislocations (GNDs).
The generation of gradient layers in these materials is often facilitated by deformation twinning or phase transformation. While tensile deformation behavior in body-centered cubic (BCC) steel systems, such as ferritic/martensitic (F/M) steels, is much less understood. Most studies on gradient metallic materials are still limited to pure metals such as copper and nickel, low stacking fault energy alloys, and face-centered cubic (FCC) steels.
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Plasticity theories
Plasticity, or plastic deformation, is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces. The transition from elastic behaviour to plastic behaviour is known as yielding. Plastic deformation is observed in most materials, particularly metals, soils, rocks, concrete, and foams.
Plasticity in a crystal of pure metal is primarily caused by two modes of deformation in the crystal lattice: slip and twinning. Slip is a shear deformation that moves the atoms through many interatomic distances relative to their initial positions. Twinning is the plastic deformation that takes place along two planes due to a set of forces applied to a given metal piece. Most metals show more plasticity when hot than when cold, and are rendered plastic by heating and hence shaped hot.
The mathematical theory of plasticity, flow plasticity theory, uses a set of non-linear, non-integrable equations to describe the set of changes on strain. The key fundamental mechanism of metal plasticity is the movement of atoms along atomic slip planes. The presence of other defects within a crystal may entangle dislocations or otherwise prevent them from gliding. When this happens, plasticity is localized to particular regions in the material.
In 1864, Tresca published his maximum shear stress criterion for yielding, marking the beginning of metal plasticity research. In 1934, Egon Orowan, Michael Polanyi, and Geoffrey Ingram Taylor realized that the plastic deformation of ductile materials could be explained in terms of the theory of dislocations.
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Frequently asked questions
Plasticity is the ability of a solid material to undergo permanent deformation, a non-reversible change of shape in response to applied forces.
Metal plasticity is the study of how metals respond to applied forces. Most metals show more plasticity when hot than when cold. Metals can be rendered plastic by heating and are shaped when hot.
Elasticity is the tendency of solid objects and materials to return to their original shape after the external forces (load) causing a deformation are removed. Materials exhibit plastic behaviour when stress is larger than the elastic limit.
