  Longevity of materials, especially alloys, is crucial for enhancing the
sustainability and efficiency of various applications, including gas turbines.
Jerky flow, also known as dynamic strain aging effect, can indeed have a
significant impact on the fatigue life of high-temperature components in gas
turbines. In general, three jerky flow types, i.e., A, B and C, existed in
superalloys. Type A and B, occurring at low temperature, were proved to be
caused by interstitial elements, such as carbon. However, Type C serration at
high temperature has not been verified directly and remains unresolved, which
was unanimously agreed it is caused by the interaction between solute elements
and dislocations. In this study, our new discovery challenged this mainstream
axiom. We proposed that vacancies play a dominant role in inducing jerky flow
instead of solute atoms. By transmission electron microscopy, we observed
directly that dislocations are pinned by vacancy-type dislocation loops (PDLs)
or dipoles. Our findings suggest that vacancies located at dislocation jogs are
primarily responsible for pinning and unpinning of superlattice dislocations.
As dislocations move and interact with vacancies, PDLs or dipoles are left
behind in their path. This pinning and unpinning process is repeated as
successive dislocations encounter the newly formed PDLs or dipoles, leading to
the recurring fluctuations in the stress, i.e., serrated flow. Additionally,
antisite defects created by Co, Cr and Ti sitting on Ni positions is postulated
to facilitate vacancy formation and migration towards dislocations via nearest
neighbor jumps. This study provides a new avenue to show how future alloy
design can improve the fatigue life of superalloys in extreme environments.
