ANALYSIS OF CREEP TRANSIENTS IN PURE METALS FOLLOWING STRESS CHANGES

The analysis of creep transients associated with stress change tests i s reviewed, with an emphasis on using the results of these experiments to characterize the kinetics of deformation under conditions of nomin ally constant structure. In order to develop a common framework for th e description of results obtained by various authors, operational defi nitions of the characteristic strain rates observed after stress chang es are adopted. The data for aluminum reported by numerous investigato rs provide a consistent picture over a broad range of temperatures and initial creep stresses. These results show that transient creep after stress reductions occurs by two parallel processes of dislocation gli de within subgrain interiors and dynamic recovery associated with subg rain boundary migration. Following relatively large stress reductions, the creep transient is dominated by the subgrain boundary migration p rocesses. After relatively small changes in stress, thermally activate d motion of dislocations within subgrain interiors is the predominant mechanism of deformation. In this regime, the creep transients can be described by a thermally activated rate law, thereby enabling various activation parameters to be evaluated from the data. In particular, th e true activation areas are found to be equal to the dislocation spaci ng within subgrain interiors, hence are consistent with thermally acti vated cutting of forest dislocations. Limited results for other f.c.c. metals and related materials are shown to follow the trends establish ed for aluminum. In particular, it is demonstrated that the data for p ure copper and LiF at high temperatures and after small stress changes are also consistent with a description based on thermally activated g lide. However, the true activation areas in copper are about five time s greater than the dislocation spacing. This difference between copper and aluminum is attributed to the fact that the former has a substant ially lower stacking fault energy. It is argued that the resulting wid er separation of partials makes the thermally activated cutting of for est dislocations more difficult.