Thermal creep mechanisms in V–4Cr–4Ti pressurized tube specimens

Pressurized thermal creep tubes of V–4Cr–4Ti have been examined following testing in the range 650–800 °C for tests lasting ∼104 h to provide comparison with tests on similar tubes following irradiation. It is found in all cases that creep results from dislocation motion. But the mechanism changes with increasing temperature and lower stress from one controlled by the climb and interaction of individual dislocations, to one controlled by sub-grain boundary structure that is created by relaxation of the interacting dislocations to lower energy planar arrays. This change in mechanism corresponds to a change from power law creep to Newtonian creep such that the stress exponent drops from ∼4 to ∼1. Although it is possible to explain the Newtonian response as Nabarro–Herring or Coble creep, it appears more likely that behavior is due to Harper–Dorn creep, in which case the change in response occurs at the Peierls stress.