An improved description of creep in dispersion-strengthened metals

Abstract

An improved model based on thermally activated dislocation glide is proposed to describe the dislocation detachment-controlled creep of extruded and annealed GlidCop Al-15 and extruded GlidCop Al-60 at 973 K as well as the threshold behavior observed at lower stresses. Unlike the customary analysis using the Rösler-Arzt approach, both steady-state and strain-transient stress reduction creep tests were performed to obtain the activation energies and the obstacle strengths for creep. In contrast to pure copper, the constant-structure creep rates of GlidCop were greater than the subsequent steady-state creep rates in all tests. This result supports the concept that dislocation detachment from particles, rather than dislocation-dislocation interactions, is rate-controlling. In addition, the constant-structure creep rates at the same reduced stresses were found to be greater at higher initial stresses. This observation is interpreted as evidence that a forward internal stress arising from inter-dislocation interactions near the particles acts on the ready-to-detach dislocations and scales with the initial stress. This forward stress and an explicit treatment of the stress dependence of mobile dislocation density were incorporated in the improved model. Furthermore, the operational activation area for dislocation glide was determined at constant temperature and structure, and the values are consistent with the interparticle distances multiplied by the Burgers vector, which further supports the interpretation of detachment controlled glide. Finally, the true threshold stress for creep is modeled as originating from thermodynamic back jumps of just-detached dislocations due to the attractive nature of the particle/matrix interface.