Plastic flow, also known as plastic deformation, is the mechanism by which solid materials undergo permanent changes in shape in response to an applied force. This phenomenon is distinct from elastic deformation, where a material temporarily changes shape but returns to its original form once the stress is removed. In elastic deformation, atoms are merely displaced slightly; however, in plastic flow, atomic bonds are broken and reformed, leading to an irreversible alteration of the material’s internal structure. Understanding this process is fundamental to engineering, as it determines how materials can be shaped, such as through forging or rolling, and what stress limits they can withstand before failing permanently.
The Transition from Elasticity: Yielding
The onset of plastic flow is marked by a specific physical threshold known as the yield point. Before this point, a material behaves elastically, meaning the deformation is temporary and fully recoverable. The moment the applied stress exceeds the yield strength, the internal structure begins to change permanently, and the material starts to deform plastically.
The yield strength represents the maximum stress a component can endure before experiencing lasting deformation. For materials without a sharp, distinct yield point, such as many metals, the yield strength is often defined using the “offset method,” typically marking the stress that results in a permanent strain of 0.2%. Once this threshold is surpassed, the material’s atomic arrangement is fundamentally altered, and it will not return to its original shape.
Microscopic Cause: Movement of Dislocations
In crystalline materials, such as metals and ceramics, the underlying cause of plastic flow is the motion of atomic-level defects called dislocations. These are linear irregularities within the otherwise perfectly ordered crystal lattice structure. Plastic deformation occurs not by the simultaneous sliding of entire planes of atoms, which would require impossibly high theoretical stress, but through the much easier movement of these defects.
An applied shear stress forces these dislocations to glide or slip along specific crystallographic planes, known as slip planes. The two primary types are edge dislocations, visualized as an extra half-plane of atoms inserted into the lattice, and screw dislocations, which resemble a helical ramp. The movement of a dislocation involves a local breaking and reforming of atomic bonds one row at a time, which requires significantly less energy than moving a whole plane at once.
The collective motion of a large number of these dislocations results in the macroscopic plastic deformation observed in the material. Factors like grain boundaries and the density of other dislocations act as obstacles, impeding this movement and thereby increasing the material’s yield strength. The crystal structure itself, such as Face-Centered Cubic (FCC) or Body-Centered Cubic (BCC), dictates the number of available slip systems, which in turn influences the material’s ductility.
Influence of Temperature and Strain Rate
External factors like temperature and the rate of deformation, known as the strain rate, significantly modulate the ease and speed of plastic flow. An increase in temperature generally lowers the resistance to dislocation motion because the heat provides the necessary thermal energy to assist defects in overcoming obstacles within the lattice. This thermal assistance leads to increased ductility and can induce phenomena like creep, which is the slow, continuous plastic flow of a material under a constant stress over an extended period.
The strain rate, the speed at which the material is being deformed, also plays a crucial role. For most materials, a higher strain rate, or faster loading, requires a higher stress to maintain the plastic flow; this effect is known as strain-rate hardening. Conversely, a very low strain rate can have a similar effect to an increase in temperature, allowing more time for thermally assisted processes to occur.
Plastic Flow in Amorphous Materials
Materials that lack a regular, long-range crystalline structure, such as glasses, polymers, and amorphous metals, do not contain dislocations. Consequently, the mechanism for plastic flow in these amorphous solids is fundamentally different. Instead of the long-range sliding motion of defects, deformation is often conceptualized as the rearrangement of small, localized clusters of atoms.
This process is frequently modeled using the concept of “shear transformation zones” (STZs), where a small volume of material undergoes a sudden, irreversible change in shape when the local stress reaches a certain level. The macroscopic plastic flow is the result of a vast number of these localized rearrangements occurring throughout the material. Under high stress, these localized events can combine to form macroscopic features called “shear bands,” which are narrow zones of intense plastic deformation that propagate through the material.