“…One explanation for this bifurcation could be dissolution of the solute clusters into smaller more complex objects, as described by Chen et al [50] for Al-Cu, which ultimately decreases the apparent activation volume via a decrease in both the activation distance, d, and spacing of the obstacles, l, such that V' = bdl decreases. An alternative mechanism for the decrease could be dislocation interactions in the solute cluster field during straining generating dislocation debris (small loops), which are weak obstacles that recover rapidly during the downchanges, though this effect is unexpected at such low strains [51], [52] Based on the calculated S values shown for the three alloys in Figure 7, it is possible to estimate the activation distance, representative of the size of the rate-controlling obstacles, based on the relationship:…”