The reliability assessment and service life prediction of critical nuclear reactor materials under extreme operating conditions have become central issues for ensuring the safe and sustainable development of nuclear energy, as well as key factors in enhancing the economic efficiency of reactor operations. The dynamic evolution of irradiation-induced nanoscale dislocation loops is a critical factor governing the macroscopic mechanical properties of reactor structural materials during long-term service. Dislocation loops, composed of clusters of point defects, exhibit one-dimensional migration. Furthermore, dislocation loops with different orientations possess varying degradation capabilities; however, a mechanistic understanding of the dynamic coupling between dislocation loop size and orientation during evolution remains lacking.

The research team employed a two-step irradiation technique to focus on the short-term evolution of irradiation defects following their long-term evolution. First, bulk samples were subjected to high-dose, high-temperature pre-irradiation to generate irradiation defects such as dislocation loops; Next, transmission electron microscopy thin-film samples were prepared from the pre-irradiated bulk samples using a focused ion beam. By developing a millisecond-scale electrochemical flash-polishing technique and apparatus, the damage layer introduced by the focused ion beam during sample preparation was completely removed, thereby avoiding confusion with defects generated by the pre-irradiation; Finally, the thin-film samples were subjected to secondary ion irradiation under high-temperature conditions using an accelerator-transmission electron microscope coupling facility. Simultaneously, the electron beam enabled in-situ observation of the dynamic evolution of dislocation loops generated by high-dose pre-irradiation under secondary irradiation. This established analytical techniques for the one-dimensional migration, size, and orientation evolution of dislocation loops under the combined effects of irradiation and high temperature.

In this study, a model alloy for reactor pressure vessels was used, with Ni and Mn content set as control variables. The study reported the first discovery of non-edge-type 1/2<111> dislocation loops in this material, while <100> dislocation loops were found to be purely edge-type, and a new mechanism for the transformation of dislocation loop habit planes was revealed. Through this mechanism, the 1/2<111>{110} dislocation loop first splits to form two smaller dislocation loops, 1/2<111>{110} and 1/2<111>{111}; subsequently, through one-dimensional migration of the dislocation loop array, the 1/2<111>{110} dislocation loop is gradually and completely absorbed by the 1/2<111>{111} dislocation loop, thereby achieving the transformation of the dislocation loop’s habit plane. At the same time, the study observed that dislocation loops can grow linearly by absorbing free-moving defects, or through one-dimensional migration and merging. Merging growth drives the one-dimensional migration of dislocation loops; during this process, larger loops absorb smaller ones, maintaining the Burgers vector of the larger loop unchanged and thereby violating the law of Burgers vector conservation. The study further analyzed the influence of alloying elements on the dislocation loop’s ability to absorb free-moving defects, the concentration of free-moving defects, and the diffusion of irradiation-induced defects. It was found that Ni primarily increases the concentration of free-moving defects, while Mn primarily inhibits the diffusion of irradiation-induced defects, elucidating the patterns and mechanisms by which Ni promotes the formation and growth of dislocation loops and Mn suppresses their growth. This study provides a theoretical basis and experimental support for the development of service behavior models for component materials, as well as for material composition design and evaluation.

Figure 1: TEM images of the same region of the sample before irradiation, at the start of in-situ irradiation (0 dpa), and after in-situ irradiation up to 0.15 dpa: (a)–(c) Fe-0.6Ni, (d)–(f) Fe-1.4Mn, (g)–(i) Fe-0.6Ni-1.4Mn

Figure 2: dislocation loop growth rates in alloys of different compositions under in-situ irradiation

Figure 3: Evolution of dislocation loop habit planes in Fe-0.6Ni-1.4Mn alloy under in-situ irradiation: (a1) 0.02 dpa, (a2) 0.03 dpa, (a3) 0.05 dpa, (a4) 0.06 dpa, (a5) 0.12 dpa; (b) Total number of defects contained in the dislocation loop

Figure 4: One-dimensional migration process of <100> dislocation loops in Fe-0.6Ni alloy under in-situ irradiation: (a1–a3), (b1–b3) TEM images and schematic diagrams; (c) Change in dislocation loop diameter

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