Dislocation Loop Transformation in Metals: Computational Studies, Theoretical Prediction and Future Perspectives

Abstract

Dislocation loops (DLs), characterized by closed dislocation lines, are a category of defects of vital importance in determining the mechanical properties of metals, particularly under extreme conditions, such as irradiation, severe plastic deformation, and hydrogen embrittlement. These loops, more intricate than simple dislocations, exhibit far more intricate reaction and evolution pathways arising from the loop type transformation and the associated planar fault transition. This can significantly alter dislocation activities contributing to dislocation channels and complex dislocation networks, which are closely linked to crack initiation and propagation during fracture. Understanding the transformation of DLs is crucial for the development of materials capable of withstanding harsh environments, including those encountered in nuclear reactors, aerospace applications, and hydrogen-rich environments. This Account delves into the computational advancements in studying DL transformations in FCC, HCP, and BCC metals. Traditional simulations often struggle to capture the complexity of DL structures and interactions. To overcome these limitations, a novel computational approach has been developed, enabling precise construction and analysis of DLs. Not only does it automatically account for necessary atom addition or deletion, it is also generic and versatile, applicable for any arbitrary DL morphology with planar fault or fault combination in both pristine metal and complex alloy systems. The new construction approach of DLs provides a critical enabler for studying the transformation of DLs across different crystal structures. In high-symmetry FCC metals, these transformations involve complex unfaulting driven by Shockley and Frank loop interactions, influenced by variations in stress, temperature, and radiation. Meanwhile, HCP metals, with a lower crystal symmetry, exhibit more complex DL transformations due to high anisotropy in the slip systems, variation in Burgers vectors, and different planar faults. Unlike pristine FCC and HCP lattices, ordered intermetallic systems like L1₂-Ni₃Al experience a disruption of translational symmetry within the lattice. The ordered nature of these alloys complicates DL interacting with line dislocation, causing asymmetrical shearing and looping mechanisms. BCC metals, in contrast, exhibit different DL evolution due to the lack of stable stacking faults, leading to stronger interactions with impurities such as carbon and hydrogen. In particular, the interaction between DLs and hydrogen in BCC metals is a critical aspect worth investigating as it can cause severe damage in BCC materials under irradiation, hydrogen embrittlement, and intense deformation. This Account highlights the complex nature of DL transformation in metals under extreme environments and recent computational advances. Differences in the evolution of DLs across crystal structures and their interactions with cracks and solute elements are critical areas for future research. Key challenges include extending DL transformation theories to ordered lattice structures, developing machine-learning-based interatomic potentials, and refining multiscale models to better capture the dynamic behavior of DLs. These efforts will help develop more accurate predictive models, leading to materials with improved resistance to deformation and fracture in harsh environments.