SIGMOIDAL CREEP IN A CU-16AL SOLID-SOLUTION ALLOY

The sigmoidal creep in a Cu-16Al solid solution alloy at temperatures ranging from 689 It to 798 K is investigated. The 'characteristic' cre ep strain rates. i.e. the initial and maximum creep strain rates in th e primary creep stage as well as minimum creep strain rate, are measur ed. The internal stress levels an estimated under conditions in which the initial, maximum and minimum creep strain rates appear. The initia l creep strain rats exhibits an apparent applied stress exponent incre asing with temperature, although its mean value is close to that typic al for Alloy Class creep behaviour, i.e. 3. Accordingly, the apparent activation energy of the initial creep strain rate increases with appl ied stress, its value being approximately one-half of that of the acti vation enthalpy of lattice diffusion at the highest applied stress und er consideration. Moreover, the internal stress levels associated with the initial creep strain rate are surprisingly high? reaching values as high as similar to 0.95. These results may suggest a strong locking of dislocation at the very beginning of the inverse primary creep. Th e nature of this locking, the unlocking mechanism, as well as the mech anism controlling the creep strain rate in the early inverse primary c reep stage, remain to be identified. The transition from inverse to 'n ormal' primary creep is considered to be due to an accumulation of pro per structural changes in the course of the inverse primary creep stag e. This is strongly supported by the temperature and applied stress de pendence of maximum creep strain rate as well as by internal stress me asurements. The results strongly suggest the onset of recovery creep. The minimum creep strain rate is found to be lattice diffusion control led and to depend on the fifth power of applied stress, as is typical for Metal Class creep behaviour.