Time-dependent deformation: Creep

Abstract

Creep is time-dependent deformation under constant stress. It may occur at relatively moderate temperatures. Most ceramics are intended for use at high temperatures, where they are ductile and creep deformation might occur. For ceramics with low-temperature ductility, creep may occur at ~0.5 Tm or even at lower temperatures. Creep generally is a function of the stress applied, the time of load duration and the temperature. Many ceramics are characterized by a high melting point even above 2000 °C(MgO, Al2O3, SiC, etc.) which makes them natural candidates for high temperature applications without the risk of creep failure during their lifetimes. Single and polycrystals creep, but to eliminate grain boundary sliding single crystals are preferred in certain important applications, despite the cost factor involved. Although small grain size enhances most of the mechanical properties, for creep resistance large grained materials are preferred. Mechanisms of creep that can act individually or simultaneously (depending on conditions) are Nabarro-Herring Creep, Dislocation Creep and Climb, Climb-Controlled Creep, Thermally-Activated Glide via Cross-Slip and Coble Creep, involving Grain-Boundary Diffusion. Creep may terminate in rupture which has to be avoided by choosing the proper ceramics and the safe temperature use for the desired life time. The presence offlaws (cracks) in ceramics intended for high-temperature applications can be controlled by the manufacturing process, which should be reduced to a minimum. It is essential for design purposes to estimate the life time in service of a ceramics to avoid failure, which is evaluated by some parametric method. The most popular methods to predict life time are the Larson-Miller and Monkman–Grant methods.