In order to clarify the deformation mechanism in the transient creep region, the instantaneous plastic strain and 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 prameter, Z, depends on temperature in the lower temperature region (623-723 K), but 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. It is concluded that the main part of the instantaneous plastic strain is produced by the a thermal motion of dislocations.
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