The three-dimensional nature of twins, especially the atomic structures and motion mechanisms of the boundary lateral to the shear direction of the twin, has never been characterized at the atomic level, because such boundary is, in principle, crystallographically unobservable. We thus refer to it here as the dark side of the twin. Here, using high-resolution transmission electron microscopy and atomistic simulations, we characterize the dark side of deformation twins in magnesium. It is found that the dark side is serrated and comprised of coherent twin boundaries and semi-coherent twist prismatic–prismatic boundaries that control twin growth. The conclusions of this work apply to the same twin mode in other hexagonal close-packed materials, and the conceptual ideas discussed here should hold for all twin modes in crystalline materials.

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Recent studies show that immiscible metallic multilayers with incoherent interfaces can effectively reduce defect density in ion irradiated metals by providing active defect sinks that capture and annihilate radiation induced defect clusters. Although it is anticipated that defect density within the layers should vary as a function of distance to the layer interface, there is, to date, little in situ TEM evidence to validate this hypothesis. In this study monolithic Cu films and Cu/Fe multilayers with individual layer thickness, h, of 100 and 5 nm were subjected to in situ Cu ion irradiation at room temperature to nominally 1 displacement-per-atom inside a transmission electron microscope. Rapid formation and propagation of defect clusters were observed in monolithic Cu, whereas fewer defects with smaller dimensions were generated in Cu/Fe multilayers with smaller h. Furthermore in situ video shows that the cumulative defect

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Any attempt to produce nuclear materials with improved mechanical properties depends heavily on understanding the root causes of radiation damage, which are individual and clustered vacancies and interstitials (self or external) produced during collision cascades between energetic particles and target atoms [1,2]. The subsequent diffusion and clustering of these defects, along with the associated transport of impurities, will dramatically change the microstructure and lead to accelerated degradation of properties during irradiation. In order to effectively suppress irradiation-induced damage, significant research is being conducted to synthesize materials containing stable sinks for trapping and recombining irradiation-induced point defects [3,4]. However, experimental evidence through in situ studies is seldom provided [5] due to both the tediousness of identifying boundary types and the difficulty of directly performing experiments observing the interaction of boundaries with those defects.In order to unravel the evolution of irradiation damage with the dose, we performed in situ ion irradiation experiments in a transmission electron microscope (TEM). The present study will focus on a common grain boundary in Cu, the Σ3 {112}||{112} incoherent twin boundary (ITB). Through in situ Cu 3+ ion irradiation at room temperature in a TEM, we have investigated the evolution of defect clusters as a function of the radiation dose at different distances from the Σ3 {112} ITB in Cu. During irradiation, defect clusters evolve through 4 stages: (i) incubation, (ii) non-interaction, (iii) interaction and (iv) saturation; and the corresponding density was observed to initially increase with irradiation dose and then approach saturation (in Fig. 1). No denuded zone is observed along the Σ3 {112} ITB and the configuration of defects at the boundary displays as truncated SFTs (in Fig. 2). Several defect evolution models have been proposed to explain the observed phenomena [6]. In summary, current study emphasis will be placed on the defect evolution as a function of the radiation dose at different distance to the boundary. These results will provide a useful basis for analyzing the influence of interface sink strength on the reduction of radiation-induced defects [7].[1] M