Summary

This TLP covers the mechanisms involved in creep deformation, how it can be modelled and how it can be investigated experimentally. There is also coverage of the need for creep-resistant materials and how they can be designed.

Firstly, the fundamental mechanisms of creep are covered, including the dependence of each on levels of stress and (homologous) temperature.  The key role of diffusion is emphasized, leading to a dependence on time that is not exhibited by conventional plastic deformation.

Secondly, the concept of (empirical) constitutive laws to characterize creep was introduced.  The emphasis is often on Stage II (steady state) creep, but in practice the primary regime often constitutes an important part of the overall behaviour and a law is presented that covers both regimes.  Mention is also made of the so-called tertiary regime (immediately before final rupture), and the possible ways in which it can arise.

Thirdly, there are descriptions of the experimental procedures that can be used to obtain creep characteristics.  The most common of these are the conventional uniaxial (tensile or compressive) tests, which need to be carried out with a series of different applied stresses in order to obtain the values of parameters in constitutive laws.  These procedures are rather cumbersome and time consuming.  It is also shown that test procedures exist in which the stress and strain fields are more complex, but can be analysed – either using a set of equations or via Finite Element Modelling.  The creeping coil experiment and the Indentation Creep Plastometry procedures are described as examples of these, the latter having the potential to at least partly replace conventional creep testing.

Finally, the importance of creep resistance in technological applications was illustrated using the well-known example of Ni-based superalloys in turbine blades for aero-engines and power generation plants.  The microstructure of these components, and the ways in which their design and production have been approached, are related to the optimisation of creep resistance, based on various principles outlined earlier in the TLP.