Sigmoidal creep in a Cu16Al solid solution alloy

Abstract

The sigmoidal creep in a Cu16Al solid solution alloy at temperatures ranging from 689 K to 798 K is investigated. The ‘characteristic’ creep strain rates, i.e. the initial and maximum creep strain rates in the primary creep stage as well as minimum creep strain rate, are measured. The internal stress levels are estimated under conditions in which the initial, maximum and minimum creep strain rates appear. The initial creep strain rate exhibits an apparent applied stress exponent increasing with temperature, although its mean value is close to that typical for Alloy Class creep behaviour, i.e. 3. Accordingly, the apparent activation energy of the initial creep strain rate increases with applied stress, its value being approximately one-half of that of the activation enthalpy of lattice diffusion at the highest applied stress under consideration. Moreover, the internal stress levels associated with the initial creep strain rate are surprisingly high, reaching values as high as ∼0.95. These results may suggest a strong locking of dislocation at the very beginning of the inverse primary creep. The nature of this locking, the unlocking mechanism, as well as the mechanism controlling the creep strain rate in the early inverse primary creep stage, remain to be identified. The transition from inverse to ‘normal’ primary creep is considered to be due to an accumulation of proper structural changes in the course of the inverse primary creep stage. This is strongly supported by the temperature and applied stress dependence of maximum creep strain rate as well as by internal stress measurements. The results strongly suggest the onset of recovery creep. The minimum creep strain rate is found to be lattice diffusion controlled and to depend on the fifth power of applied stress, as is typical for Metal Class creep behaviour.