Nucleation of Nano-sized Prismatic Dislocation Loop from Spherical Vacancy Clusters in <i>α</i>-iron: An Atomic-scale Study

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  • Vijendran Mugilgeethan
    Graduate Student, Department of Mechanical and Electrical Systems Engineering, Kyoto University of Advanced Science Faculty of Engineering, University of Jaffna
  • Matsumoto Ryosuke
    Department of Mechanical and Electrical Systems Engineering, Kyoto University of Advanced Science

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<p>The concentration of vacancy-type defects in α-iron is increased by plastic deformation and the presence of hydrogen. This leads to the accumulation of monovacancies, either in the form of planar vacancy clusters (VCs) or small voids. The prismatic dislocation loop (PDL) can nucleate from VC as vacancies agglomerate into two-dimensional (2D) VCs and collapse due to attractive force between two interior surfaces. The transition between 2D-VC and PDL is comparatively more straightforward, requiring only a short displacement without the need for atom diffusion to reach stability. However, the most stable VC configuration is three-dimensional (3D) (spherical cluster), which have lower formation energy than 2D-VCs. Despite their stability, the transformation from 3D-VC to PDL is complex, involving the diffusion of multiple atoms. A quantitative energy barrier is established for transitioning from 3D-VC to nano-sized 1/2<111> PDLs using an approach that combines the reaction rate theory and molecular dynamics (MD) simulations. The nucleation of PDL from a spherical cluster composed by 15 vacancies is a rare event at room temperature, even under considerable compressive strain since the activation energy is 1.33 eV. In contrast, 2D-VC with 37 vacancies can be nucleated to PDL with an energy barrier of 0.61 eV.</p>

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