Strain, recovery and work-hardening during creep due to dislocations

Abstract

Development of a model of creep in which work-hardening and recovery are attributed to specific types of dislocation movement has continued. Relationships for the time dependence of strain, for the strain-rate dependence of the normalized Bailey-Orowan recovery parameter and for the work-hardening rate have been produced by integrating the set of differential equations which describe the model processes. The strain-rate equation so obtained is valid for any sequence of stresses and temperatures. It fits Garofalo's data for the primary creep of stainless steel and correctly predicts the recovery rate measured in stress-reduction creep experiments on a similar material. The theoretical value of the recovery parameter has the same activation energy as has secondary creep whilst the work-hardening rate is predicted to be independent of temperature: published data on nickel and aluminium exhibit both of these characteristics. Reloading, after removal (for period t ₂) of the stress, should regenerate a fraction of the initial primary creep strain which is exponentially related to t ₂: data for the creep of lead confirm this. Dorn's observation that the relationship between creep strain and temperature-compensated time (i.e. t exp (-Q/RT)) is, for a given stress, independent of temperature is mirrored by the model equations: so too is his finding (for aluminium) that the graph ε → ε exp (+Q/RT) is independent of temperature.