Investigating Dislocation Loop Nucleation and Growth under Stress by In-Situ TEM in Al

Irradiation creep that occurs both under stress and irradiation flux, even at moderate temperature, has a profound impact on mechanical properties in structural materials. For instance, it leads to deleterious permanent deformation in austenitic steel vessel internals in pressurized water nuclear reactors. Although this phenomena has been documented in the 70’s and 80’s, experimental and theoretical investigations proposed have never found consensus so far. It was proposed that irradiation would contribute to dislocation climb but the role of anisotropic growth of irradiation loops was the subject of debates and contradictory results. From early microstructural observations, showing a bias in the development of Frank loops in austenitic steels, two models have emerged. They postulate that the applied stress biases either loop nucleation or their growth which lead to different loop populations density and size. Experimental results, provided by post-mortem TEM observations at high doses, were not able to discriminate clearly between the two models, while in-situ observations carried out in high voltage microscopes reported opposite trends.<br/><br/>To gain knowledge in these phenomena, we studied loop nucleation and growth during in-situ TEM experiments under stress and irradiation at low doses. To that purpose, we take advantage of the low energy displacement threshold of aluminum, to both irradiate and observe samples at the same time, in-situ in a conventional transmission electron microscope.<br/>We show that close to the elastic limit, almost only interstitial Frank loops are formed in the {111} planes normal to the tensile direction. Deep learning and tracking approaches were used to follow the kinetics and distribution of loops during these experiments. They reveal that loop growth is in average linear with time, i.e. with electron fluence, but with numerous cases of stagnation or accelerated growth. More importantly, we demonstrate that the anisotropy of loop population is determined at early stage, probably at the nucleation, and that the growth kinetic only plays a minor role in the development of the observed loop population. These findings echo simulations performed both at atomic scale for the nucleation processes (molecular dynamics Frenkel-pair accumulation), and at mesoscale (Object Kinetic Monte-Carlo) for elastic-diffusion mechanisms of loop growth.