Plasticity (physics)
Permanent deformation of solids under applied forces.
Plasticity, also known as plastic deformation, is the ability of a solid material to undergo permanent, non-reversible change of shape in response to applied forces. In physics and materials science, it describes how materials like metals, soils, rocks, concrete, and foams can be bent or pounded into new shapes, with permanent changes occurring within the material itself. The transition from elastic behavior to plastic behavior is known as yielding.
The physical mechanisms behind plastic deformation vary widely depending on the material. In metals, plasticity at the crystalline scale is primarily a consequence of dislocations—defects in the crystal lattice that allow planes of atoms to slip past each other along close-packed directions. Slip and twinning are the two main modes of deformation in pure metal crystals. Most metals become more plastic when heated, which is why shaping and forming operations are often performed hot; lead is notably plastic at room temperature, while cast iron remains insufficiently plastic for forging even when hot. In brittle materials such as rock, concrete, and bone, plasticity is predominantly caused by slip at microcracks. In cellular materials like liquid foams or biological tissues, it results from bubble or cell rearrangements, particularly T1 processes. Amorphous materials, lacking long-range order, undergo plastic deformation through crazing, where fibrils form in regions of high hydrostatic stress, creating a hazy appearance. For many ductile metals, tensile loading initially produces elastic deformation, where extension is proportional to load and fully recoverable. Once the load exceeds the yield strength, extension increases more rapidly, and some permanent extension remains after unloading. Perfect plasticity describes irreversible deformation without any increase in stress, though materials hardened by prior cold forming may require higher stresses for further deformation. Plastic deformation is also generally dependent on deformation speed, with higher stresses needed to increase the rate, a behavior known as visco-plasticity. On the nanoscale, primary plastic deformation in simple face-centered cubic metals can be reversible if no material transport occurs, while shape-memory alloys exhibit a reversible form called pseudoelasticity. The presence of other defects can entangle dislocations, localizing plasticity i
- field
- Physics and materials science
- known_for
- Permanent deformation of solids under applied forces
- related_concept
- Yielding
- key_mechanisms
- Dislocations, slip, twinning, microcracks, T1 processes
Lore & Background
Plastic deformation is observed in most materials, particularly metals, soils, rocks, concrete, and foams. The physical mechanisms causing plastic deformation vary widely: in metals, it is usually a consequence of dislocations; in brittle materials like rock and concrete, it is caused predominantly by slip at microcracks; in cellular materials such as liquid foams, it is mainly due to bubble or cell rearrangements. For many ductile metals, tensile loading causes elastic behavior until the load exceeds the yield strength, after which extension increases more rapidly and some degree of extension remains when the load is removed.
Reader's Guide
Plasticity is a fundamental concept in engineering and materials science, governing how materials are shaped and formed. The property is directly proportional to ductility and malleability. Most metals show more plasticity when hot than when cold, which is important in forming, shaping, and extruding operations. Perfect plasticity describes materials that undergo irreversible deformation without any increase in stresses, while materials hardened by prior deformation may need increasingly higher stresses to deform further. Plastic deformation is also dependent on deformation speed, with higher stresses usually required to increase the rate of deformation, a behavior known as visco-plasticity. The study of plasticity includes various mechanisms such as slip systems in crystals, shear banding, microplasticity, crazing in amorphous materials, and time-independent yielding defined by critical resolved shear stress.
Did You Know?
- Plasticity in metals is usually a consequence of dislocations at the crystalline scale.
- In brittle materials such as rock, concrete, and bone, plasticity is caused predominantly by slip at microcracks.
- Most metals show more plasticity when hot than when cold.
- On the nanoscale, primary plastic deformation in simple face-centered cubic metals is reversible as long as there is no material transport in form of cross-slip.
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