Effect of He on the Order-Disorder Transition in ${\mathrm{Ni}}_3\mathrm{Al}$ under Irradiation

The order-disorder transition in Ni-Al alloys under irradiation represents an interplay between various reordering processes and disordering due to thermal spikes generated by incident high energy particles. Typically, ordering is 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 ${\mathrm{Ni}}_3\mathrm{Al}$ 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 ${\mathrm{V}}_2\mathrm{He}$ 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.

Figure 1

The order parameter of the ${\mathrm{Ni}}_3\mathrm{Al}$ precipitates, as a function of He concentration, irradiation dose, and temperature. Red triangles and blue circles indicate the experimental presence of ordered and disordered structures, respectively. The lines are the predictions of a rate-theory model, calibrated using atomistic simulations. Results at three He concentrations are reported: (a) zero, (b) 200 ppm, and (c) 400 ppm. ${\mathrm{T}}_{\mathit{c}}\sim 740\mathrm{K}$, corresponding to the dashed line in the “0 ppm He" graph, beyond which disordering was not observed.

Figure 2

Left: a reaction path from the disordered sublattice to the ordered sublattice, involving a ${\mathrm{V}}_2\mathrm{H}$ complex. The formation and migration energies of the complex are reported in Table 1, among other plausible paths. The ${\mathrm{V}}_2\mathrm{He}(1\mathrm{nn})\stackrel{\mathrm{Ord}}{\to }{\mathrm{V}}_2\mathrm{He}(3\mathrm{nn})\stackrel{\mathrm{Ord}}{\to }{\mathrm{V}}_2\mathrm{He}(1\mathrm{nn})$ mechanism is illustrated here. The displacement vector is colored red. The moves result in ordering without changing the configuration of the ${\mathrm{V}}_2\mathrm{He}$ complex. He and V partially dissociate to the 3 nn in the first jump and rebond in the second jump. Right: a schematic representation of the minimum energy path along this two-step reaction path.

Figure 3

(a) The temperature-dependent, steady-state monovacancy concentration predicted by the model. (b) The temperature-dependent, steady-state ${\mathrm{V}}_2\mathrm{He}$ concentration predicted by the model. (c) The time (or, equivalently, dose) dependence of the ${\mathrm{Ni}}_3\mathrm{Al}$ precipitate order parameter, as predicted by the model. The values are reported at 350 K, 400 K, 450 K, and 500 K in the system without He and with 200 appm He.