Maya Ellison
Plastic deformation is a change in shape of a material at high stress that is irreversible after the stress is removed. It involves the breaking of a limited number of atomic bonds by the movement of dislocations. The force required to break the bonds of all the atoms in a crystal plane all at once is very large. Plastic deformation in metals develops by predominantly shearing, which means sliding of lattice planes over each other by allowing macroscopic changes without affecting atomic arrangement. The fundamental mechanism that plastic deformation relies on is the movement of dislocations. In many metals, the basic mechanism of plastic deformation is slip movement. However, in cases where slip is not possible, twinning becomes the basis of plastic deformation.
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What You'll Learn
The movement of dislocations
Dislocations are defects in the crystal lattice that allow atoms to slide past one another more easily than they would in a perfect crystal structure. When a material is stressed, dislocations move through the crystal lattice, facilitating the plastic deformation process. The movement of dislocations can be influenced by factors such as temperature and the presence of impurities or alloying elements, which can either hinder or enhance dislocation movement.
Dislocations are linear crystallographic defects or irregularities within a crystal structure that contain an abrupt change in the arrangement of atoms. The movement of dislocations allows atoms to slide over each other at low stress levels, and this process is known as glide or slip. The crystalline order is restored on either side of a glide dislocation, but the atoms on one side have moved by one position. A dislocation can be characterised by the distance and direction of movement it causes to atoms, defined by the Burgers vector.
Dislocations can be formed when interstitial atoms or vacancies cluster together, resulting in locally high densities of interstitial atoms and vacancies. This can happen due to single or multiple collision cascades. In most metals, prismatic dislocation loops are the most energetically preferred clusters of self-interstitial atoms. Geometrically necessary dislocations are arrangements of dislocations that can accommodate a limited degree of plastic bending in a crystalline material.
The presence of alloying elements and impurities can significantly impact plastic deformation by either strengthening the material or enhancing its ductility. For example, alloying elements can hinder dislocation movement, while providing additional slip systems can enhance ductility. The size, shape, and distribution of grains and phases within a material can also influence its plastic behaviour. Materials with fine grains or specific phase distributions may exhibit different deformation characteristics compared to those with coarser grains.
Dislocation motion as a result of external stress on a crystal lattice can be described using virtual internal forces acting perpendicular to the dislocation line. The Peach-Koehler equation can be used to calculate the force per unit length on a dislocation as a function of the Burgers vector. When a dislocation line intersects the surface of a metallic material, the associated strain field locally increases the relative susceptibility of the material to acid etching, resulting in etch pits of regular geometrical format. By repeatedly deforming and re-etching the material, a series of etch pits can be produced that trace the movement of the dislocation.
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Slip and twinning
Plastic deformation is the permanent change in shape, form, or texture of a material due to the action of stress. This differs from elastic deformation, where the material returns to its original shape when the force is removed.
Dislocations are defects in the crystal lattice of a material, and their movement results in the breaking of atomic bonds, leading to plastic deformation. As more dislocations are created and interact with each other, they become less mobile, a phenomenon known as work hardening or strain hardening. Work hardening is characterized by equations such as the Ludwik-Hollomon and Voce equations, which consider applied stress, yield stress, plastic strain, and work hardening coefficients.
Twinning, on the other hand, involves a portion of the crystals adopting a symmetrical orientation connected to the direction of the remaining untwined lattice. In twinning, atoms in each slip plane within a block move different distances, causing half of the crystal lattice to become a mirror image of the other half. Twinning is influenced by factors such as defects, crystal axis shift, visibility, and the occurrence threshold value in stress slip.
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Stress-strain curve
The stress-strain curve illustrates the relationship between stress and strain in a material, and it is used to determine Young's modulus in tensile tests. It is characterised by several stages, each indicating different mechanical properties.
The first stage is the linear elastic region, where the material undergoes elastic deformation. The stress is proportional to the strain, and the slope of this region is Young's modulus. The end of this stage is marked by the initiation of plastic deformation, and the stress component is defined as the yield strength or upper yield point (UYP).
The second stage is the strain hardening region, where the stress surpasses the yielding point and reaches a maximum at the ultimate strength point, or ultimate tensile strength (UTS). During this stage, the material becomes stronger through the movement of atomic dislocations, and the cross-sectional area decreases uniformly along the gauge length.
The third stage is the necking region, where the material can no longer withstand the maximum stress and the strain increases rapidly. The cross-sectional area of the specimen decreases, and the material becomes weaker.
The final stage is fracture, where the material breaks. This is often a ''cup and cone' fracture, characteristic of ductile materials.
The work hardening rate, or the gradient of the true stress/true strain plot, tends to decrease progressively with increasing strain. This effect is termed 'work hardening' or 'strain hardening'. It occurs because as more dislocations are created and interact with each other, they become less mobile.
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Yield strength
Plastic deformation is the permanent distortion of a material. It occurs when a material is subjected to tensile, compressive, bending, or torsion stresses that exceed its yield strength, causing it to elongate, compress, buckle, bend, or twist. The point at which this nonlinearity in the stress-strain relationship begins is known as the proportional limit. Yield strength, therefore, is the stress level at which a material begins to undergo permanent deformation.
The yield strength of a material can be determined through experimental testing. One method involves applying a load to a material and measuring the deviation from a straight line in a plot of contact pressure versus contact area. This deviation indicates the onset of microcracking, which is a precursor to plastic deformation. Another method involves the use of the PLX-Benchtop, a tool that extracts metal stress-strain curves from small and irregular specimens.
The yield strength of a material is influenced by various factors, including crystal structure, grain size, crystallographic texture, composition, and phase constitution. Work hardening, also known as strain hardening, is a common phenomenon where the yield strength of a material increases as plastic deformation proceeds. This is due to the creation and interaction of dislocations within the material, which makes them less mobile. However, given enough time or increased temperatures, dislocations can move in a way that reduces strain energy, causing a decrease in yield strength.
The relationship between stress and strain is generally linear and reversible up to the yield point, after which the deformation becomes plastic. This plastic deformation can be characterized by two regions: the strain hardening region and the necking region, which ultimately leads to fracture. During the strain hardening region, the material becomes stronger through the movement of atomic dislocations. In the necking region, the cross-sectional area of the specimen decreases, and a smaller load is required to produce greater deformation.
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Work hardening
Plastic deformation in metals is a permanent change in shape, distinct from elastic deformation, which is reversible. Work hardening, also known as strain hardening or cold working, is a consequence of plastic deformation. It occurs when a material undergoes plastic deformation, increasing its strength and hardness due to the accumulation and interaction of dislocations.
The process of work hardening can be achieved through various cold forming techniques, including squeezing, bending, drawing, and shearing. These techniques are commonly used in applications such as the heading of bolts and cap screws and the finishing of cold-rolled steel. During cold forming, metal is formed at high speed and pressure using tool steel or carbide dies, increasing its hardness, yield strength, and tensile strength.
The prediction and understanding of work hardening are essential in engineering applications. While analytical expressions such as the Ludwik-Hollomon and Voce equations are used to characterise work hardening, accurately predicting the mechanical properties of metallic alloys remains challenging due to the complexity of the crystalline structure and various influencing factors.
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Frequently asked questions
Plastic deformation is when a material undergoes a permanent change in shape or size due to an applied force.
Plastic deformation in metals is caused by the movement of dislocations, which are defects in the crystal structure of the metal. When sufficient stress is applied, these dislocations move, allowing atomic planes to slip past each other and resulting in a permanent change in shape.
Elastic deformation is temporary and occurs when the applied stress is below the yield point of the material. In this case, the atomic bonds stretch but return to their original state when the stress is removed. Plastic deformation, on the other hand, occurs when the applied stress exceeds the yield point, causing the atomic bonds to break and resulting in permanent deformation.
The amount of plastic deformation in metals is influenced by factors such as crystal structure, grain size, composition, temperature, and prior treatments. For example, most metals show greater plasticity when hot than when cold, and treatments such as annealing can restore ductility.
Plastic deformation is typically measured through stress-strain curves, which plot the relationship between stress and strain. Mathematical theories, such as flow plasticity theory, also describe the changes in strain and stress during deformation. The work hardening of metals can be characterised by equations such as the Ludwik-Hollomon and Voce equations.
