Transient Creep Mechanism in Pure Aluminum at High-Temperature

In order to clarify the deformation mechanism in the transient creep region, the instantaneous plastic strain and the strain rate have been measured by using pure aluminum from a very early stage of transient creep to the steady state. The creep test has been carried out at temperatures from 623 to 823 K under stresses from 0.81 to 6.7 MPa.It is found that the instantaneous plastic strain does not depend on temperature; it depends exclusively on stress. At a very early stage of transient creep, the Zener-Hollomon parameter, Z, depends strongly on temperature in the lower temperature region (623–723 K), but it is independent of temperature in the higher temperature region (773–823 K). At a later stage, the Z vs strain curves at lower temperatures converge to a single one, which is the same as that in the higher temperature region, and finally comes into the steady-state.From the temperature-independence, it is concluded that the main part of the instantaneous plastic strain is produced by the athermal motion of dislocations. From the theoretical analysis based on a dislocation-network model, it is inferred that the creep mechanism in the lower temperature region changes from a process, in which the thermally activated glide of some long dislocation links takes part, to the well-known recovery process at an early stage of transient creep.