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What determines the creep strength of an alloy?


Creep strength is essentially a measure of the resistance to creep deformation, and is thus determined by the resistance to dislocation movement within the material. Several factors contribute to the creep strength of an alloy. These are summarised as follows:

  • Dislocation substructure, i.e. a stable network of dislocations, provides an inherent resistance to movement of dislocations throughout the material. The substructure can develop and coarsen with creep strain, as is the case for example in nickel base alloys. As the substructure coarsens creep strength is accordingly reduced. The substructure is often stabilised by a fine dispersion of precipitates such that the inter-particle spacing and particle stability are again important.
  • The grain size of the material, in the absence of other factors, contributes to the creep strength. A fine grain size is generally beneficial in impeding long range dislocation movement. However, this effect is usually outweighed by precipitate dispersion or substructural contributions to creep strength.
  • The transformation structure of the alloy also provides a contribution to creep strength. This is inherited from the initial heat treatment as in the case of low alloy ferritic steels where the austenite transformation product, e.g. bainite, has a lath structure which in itself imparts creep strength.
  • Solid solution strengthening restricts the movement of dislocations through lattice distortion. For example, molybdenum in ferrite provides significant solution strengthening in 2.25%CrMo and 9%CrMo steels. In these materials, after over-ageing or extended service, when the dispersion strengthening is lost due to precipitate coarsening, it is the solid solution strengthening which dominates inherent creep strength.
  • A fine dispersion of hard precipitates such as carbides or intermetallics, e.g. VC, MO 2C in ferritic steels, and gamma prime in nickel-base alloys, obstruct the movement of dislocation climb and glide. The precipitate particle spacing is the key factor. There is an inverse relationship such that closely spaced precipitates confer greater creep strength than coarsely-spaced dispersions. The stability of the precipitate at creep temperatures, i.e. the resistance to particle coarsening, is thus important in conferring high long term creep strength. Loss of creep strength in many cases occurs after over-tempering or extended service exposure at elevated temperatures when the precipitate particles have coarsened. The size, shape, mechanical properties and interface characteristics of the precipitate are also important in determining the creep strength level.

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