High Temperature Creep
High temperature creep is a type of creep usually defined as deformation of solids (metals, ceramics and polymers) due to applied stresses at temperatures where creep is significant. Temperature plays a key role in causing and controlling creep, but other factors such as applied stress, chemical composition, and grain structure must also be considered.
It is a time-dependent nonlinear process, where stresses deform materials slowly, over a long period of time. Typically, even high stresses do not cause large amounts of creep strain in materials, until higher temperatures are reached. Polymers and ceramics tend to creep at much lower temperatures then metals. It is rarely observed in materials at room temperature.
Creep is a form of plastic deformation, usually caused by the slow plastic flow of materials due to a combination of applied stresses and high temperatures. It is usually observed in metals and alloys as they are heated. It is closely related to other reversible solid-state changes such as viscoplasticity, and both phenomena can co-exist in a material.
At large enough stresses, the creep curves for all materials follow the same shape. Creep strain increases linearly with time for stresses below the saturation stress. If a stress level is larger than the saturation stress, the creep curve has an abrupt drop. Once the stress level exceeds the saturation stress, creep rates increase rapidly and significantly.
Metals and alloys, such as austenitic stainless steels, aluminum alloys, copper alloys and zirconium alloys are commonly used in elevated temperature applications, and are subject to high temperature creep. Therefore, it is important to understand the effects of high temperature on creep behavior and mechanisms in these materials. High temperature creep is associated with several mechanisms, such as grain boundary evacuation, stress relaxation, dislocation-diffusion mechanisms, and diffusion-controlled grain boundary sliding.
Grain boundary evacuation occurs when mobile edge-dislocations accumulate and diffuse along the grain boundaries. Mobile dislocations have enough energy to move through lattice planes and can easily cross grain boundaries, kinetically creating vacancy and interstitial sites. This process causes an increase in vacancy concentrations and suppresses precipitation at the grain boundary as temperature increases.
Stress relaxation is one of the most commonly observed modes of high temperature creep in metals and alloys. Stress relaxation refers to the process by which stress initially increases as the material deforms plastically, and then gradually decreases with increasing deformation until the stress level reaches zero, at which point no further deformation occurs. Stress relaxation is the result of mobile dislocations carrying stress away from the materials surface in the presence of an externally applied stress. As these dislocations move away, they reduce the applied stress and thus reduce the rate of creep.
The dislocation-diffusion-controlled creep model is an extension of the stress relaxation model. In this model, the accumulation of solute atoms at grain boundaries, which reduces the grain boundary diffusivity, replaces the grain boundary evacuation of dislocations. This model also predicts that the creep rate rates vary as a power law, as grain boundary barriers arise due to solute atom binding, which hinders the motion of dislocations across grain boundaries.
Atomal diffusion across grain boundaries is also responsible for grain boundary sliding. In this case, atoms tend to move from one grain boundary to another, resulting in a net reduction in the area of a single grain boundary, resulting in much higher creep rates.
In summary, high temperature creep is a time-dependent nonlinear process and is associated with several mechanisms such as grain boundary evacuation, stress relaxation, dislocation-diffusion mechanisms, and diffusion-controlled grain boundary sliding. Temperature plays a key role in causing and controlling creep, but other factors such as applied stress, chemical composition, and grain structure must also be considered. It is important to understand the effects of high temperature on creep behavior and mechanisms in materials used in elevated temperature applications.
