Alexei Volkov
Plastic deformation of metals occurs when the external stress applied exceeds the yield strength of the material, causing it to deform quickly and permanently. This deformation is driven by shear stresses and occurs through the glide of dislocations, where lattice planes in the material slide over each other, allowing the shape to change without affecting the arrangement of atoms. The stress required for plastic deformation can be reduced by localizing deformation through line defect movement, which is classified as edge-type or screw-type dislocations. This process is known as work hardening or strain hardening, and it plays a crucial role in understanding the mechanical behaviour of metals under stress.
| Characteristics | Values |
|---|---|
| Type | Time-dependent and time-independent |
| Cause | Shear stress |
| Deformation | Predominantly by shearing, i.e., sliding of lattice planes over each other |
| Dislocations | Edge, screw, or mixed |
| Dislocation movement | Glide and climb |
| Yield stress | Dependent on crystal structure, grain size, crystallographic texture, composition, phase constitution, grain boundary structure, prior dislocation density, and impurity levels |
| Work hardening | Progressive decrease in the work hardening rate with increasing strain |
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What You'll Learn
- Plastic deformation is irreversible and consists of time-dependent and time-independent components
- Plastic deformation occurs when stress exceeds the yield strength of the material
- Dislocations and slip planes are important factors in plastic deformation
- Creep refers to the time-dependent component of plastic deformation
- Work hardening and strain hardening refer to the need for progressively increasing stress levels to continue plastic deformation
Plastic deformation is irreversible and consists of time-dependent and time-independent components
Plastic deformation is a permanent and irreversible deformation of a body. It occurs when tensile, compressional, torsional, or other forms of stress are applied to a body, causing it to become elongated, shortened, twisted, or respond in other ways to various stresses.
Plastic deformation consists of both time-dependent and time-independent components. Creep, or time-dependent deformation, is a slow and continuous deformation of materials over extended periods under load. It occurs at all temperatures above absolute zero Kelvin, but is traditionally associated with elevated temperatures, often higher than roughly 0.4Tm (where Tm is the absolute melting temperature), as diffusion can assist creep at higher temperatures. Time-independent plastic deformation, on the other hand, occurs above the athermal yield stress and is followed by time-dependent deformation.
The deformation of metals occurs predominantly by shearing, where lattice planes in the material slide over each other, allowing the shape to change without significantly affecting the ordering and arrangement of atoms within the structure. The stress required to cause plastic deformation can be reduced by localizing deformation through the movement of line defects, or dislocations. Metals, even in an annealed state, contain a statistical density of dislocations, which is sufficient to allow plastic deformation to occur.
Dislocations can take on various geometries and are classified into edge-type and screw-type. Edge-type dislocations cause deformation normal to the line defect, while screw-type dislocations result in deformation parallel to the line defect. By understanding the mechanics of dislocations, it is possible to predict the deformation mechanism that will occur under specific conditions, such as in the case of heat-resistant steel.
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Plastic deformation occurs when stress exceeds the yield strength of the material
Plastic deformation occurs when stress exceeds the yield strength of a material. It is a type of deformation that is irreversible and distinct from elastic deformation, which is nonpermanent and where the material returns to its original size and shape once the stress is removed.
In the context of metals, plastic deformation occurs predominantly through the shearing of lattice planes. This involves the sliding of lattice planes over each other, allowing the metal's shape to change macroscopically without significantly altering the arrangement of atoms within its structure. This is in contrast to elastic deformation, where the applied stress is lower than the yield point, and atomic bonds only stretch temporarily.
The onset of plastic deformation in metals is marked by the activation of dislocation movement. Dislocations are line defects that can be classified into two types: edge-type and screw-type. Edge-type dislocations result in deformation normal to the line defect, while screw-type dislocations cause deformation parallel to the line defect. The movement of these dislocations can be either glide, where they move along a surface defined by their Burgers vector, or climb, where they move outward from the glide surface. By moving dislocations along planes, atoms are able to slip over each other at a lower stress compared to breaking all atomic bonds simultaneously.
The stress required to initiate plastic deformation in metals, termed yield stress, depends on various factors such as crystal structure, grain size, composition, and prior dislocation density. As more dislocations are created and interact with each other through processes like work hardening or strain hardening, the stress needed to continue plastic deformation progressively increases. This progressive increase in stress can be observed in a stress-strain curve, which illustrates the complex relationship between true stress and true strain as plastic deformation occurs.
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Dislocations and slip planes are important factors in plastic deformation
Plastic deformation in metals occurs when a metal undergoes persistent deformation due to the application of force. This deformation is irreversible and consists of time-dependent and time-independent components. Metals, even in their annealed state, contain a statistical density of dislocations, which are line defects. These dislocations are crucial in the plastic deformation process as they allow for the movement of lattice planes in the metal, resulting in a change of shape without significantly affecting the arrangement of atoms within the structure.
Dislocations are line defects that occur when there is a disturbance in the regular arrangement of atoms in a crystal lattice. They can be classified into two types: edge-type and screw-type. Edge-type dislocations occur when there is a partial plane of atoms missing in the lattice, resulting in a gap or edge. On the other hand, screw-type dislocations occur when there is a distortion in the lattice, causing the planes of atoms to be stacked in a helical pattern.
The movement of dislocations is an essential factor in plastic deformation. When a force is applied to a metal, dislocations can move through the material, producing shear that facilitates the sliding of lattice planes over each other. This movement of dislocations along densely packed planes, known as slip planes, results in plastic deformation. The slip planes provide the easiest path for dislocations to move, requiring the least amount of energy for their motion.
Slip planes are specific crystallographic planes along which crystal blocks slide over one another during plastic deformation. The sliding of these crystal blocks allows for the macroscopic change in shape of the metal without significantly altering the arrangement of atoms. Twinning, where a portion of the crystals adopts a symmetrical orientation, also contributes to plastic deformation by affecting the plane's orientation and promoting slipping.
The intersection of screw dislocations, a type of dislocation with a particular orientation, is an important factor in plastic deformation. When two screw dislocations intersect, they create jogs of edge orientation in each other. These jogs are constrained to move along specific planes, and their movement contributes to the overall deformation of the metal.
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Creep refers to the time-dependent component of plastic deformation
Plastic deformation in metals occurs predominantly by shearing, where lattice planes in the material slide over each other, allowing the macroscopic shape to change without affecting the arrangement of atoms within the structure. This deformation is irreversible and consists of time-dependent and time-independent components.
Creep deformation occurs both above and below athermal yield stress. Below the athermal yield stress, creep starts after elastic deformation upon loading. Above the stress, time-independent plastic deformation occurs upon loading before time-dependent plastic deformation. Since plastic deformation upon loading alters the major obstacle to creep deformation, the behaviour of creep is different above and below the athermal yield stress.
The temperature at which creep begins depends on the alloy composition. At elevated temperatures and high stresses, metals undergo permanent plastic deformation called creep. Creep deformation occurs by grain-boundary sliding, where adjacent grains or crystals move as a unit relative to each other.
The effects of creep deformation generally become noticeable at approximately 35% of the melting point for metals. Creep behaviour can be split into three main stages: primary or transient creep, secondary or steady-state creep, and tertiary creep. In primary creep, the strain rate is a function of time and decreases over time due to increasing dislocation density or evolving grain size. In secondary creep, the dislocation structure and grain size have reached equilibrium, so the strain rate is constant.
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Work hardening and strain hardening refer to the need for progressively increasing stress levels to continue plastic deformation
Plastic deformation in metals occurs predominantly by shearing, where lattice planes in the material slide over each other, allowing the shape to change without affecting the arrangement of atoms within the structure. This process is irreversible.
Work hardening, also known as strain hardening or cold working, is a phenomenon that occurs during plastic deformation. It describes the increase in stress levels required to continue deforming a material plastically. This is due to the accumulation and entanglement of dislocations within the material, which impede the motion of other dislocations. The strength and hardness of the material increase as a result, but ductility decreases.
The rate of work hardening is influenced by factors such as the initial dislocation density, the rate of dislocation generation, and the rate of dislocation annihilation. It can be predicted by analyzing a stress-strain curve or through hardness tests.
As an example, consider a tensile test where a steel bar is strained until it almost fractures. When the load is released, the bar decreases in length, exhibiting elastic recovery. The work-hardened bar has a high number of dislocations, and their interaction prevents further plastic deformation. If the applied stress exceeds the fracture stress, the bar will fracture, and the amount of plastic deformation possible decreases to zero.
To achieve large shape changes during forming operations, work hardening dictates that the process should be conducted at elevated temperatures to allow the work-hardened dislocation structures to relax.
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Frequently asked questions
Plastic deformation is a slow and continuous irreversible deformation of materials over extended periods under load.
Elastic deformation is non-permanent, meaning that when the applied stress is removed, the material reverts to its original size and shape. Plastic deformation, on the other hand, is permanent and occurs when the applied stress exceeds the yield point, causing atomic bonds to stretch and break.
Plastic deformation in metals is predominantly caused by the glide of dislocations driven by shear stresses. Dislocations are line defects that move along lattice planes, allowing the planes to slide over each other and resulting in macroscopic shape change without affecting the atomic arrangement.
The yield stress is the stress required to initiate plastic deformation. As more dislocations are created and interact with each other, the stress required to continue plastic deformation increases, which is known as work hardening or strain hardening. The yield stress depends on various factors such as crystal structure, grain size, composition, and prior dislocation density.
