  The order-disorder transition in Ni-Al alloys under irradiation represents an
interplay between various re-ordering processes and disordering due to thermal
spikes generated by incident high energy particles. Typically, ordering in
enabled by diffusion of thermally-generated vacancies, and can only take place
at temperatures where they are mobile and in sufficiently high concentration.
Here, in-situ transmission electron micrographs reveal that the presence of He,
usually considered to be a deleterious immiscible atom in this material,
promotes reordering in Ni3Al at temperatures where vacancies are not effective
ordering agents. A rate-theory model is presented, that quantitatively explains
this behavior, based on parameters extracted from atomistic simulations. These
calculations show that the V2He complex is an effective agent through its high
stability and mobility. It is surmised that immiscible atoms may stabilize
reordering agents in other materials undergoing driven processes, and preserve
ordered phases at temperature where the driven processes would otherwise lead
to disorder.
