Iranian Journal of Materials Science and Engineering

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Abstract: The theoretical calculation of dislocation density in different regions of a deformed workpiece of 99.99% pure copper has been carried out using different procedures consisting of Finite Element Method (FEM) and hardness measurement. To assess the validity of the results pertaining to these procedures, the dislocation density is experimentally measured utilizing the Differential Scanning Calorimetry (DSC). Comparing the predicted and experimental results, it was found that the average error in prediction of the dislocation density by the hardness measurement and FEM is 12% and 2.5%, respectively. Also, for further confirmation of the evaluated dislocation density of each region of the deformed workpiece, the annealing process was carried out and in the region of higher dislocation density, a finer grain size was observed.

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Twin-boundary engineering provides an effective approach for tailoring the mechanical response of nickel-based superalloys; however, the density-dependent atomistic mechanisms remain insufficiently understood in explicit dual-phase γ/γ′ microstructures. Molecular dynamics simulations were performed on a single-crystal model and models containing one, two, and eight twin boundaries oriented perpendicular to the loading direction. The tensile response was analyzed in conjunction with dislocation-density evolution, microstructural changes, and Constructed-surface-mesh analysis to characterize crack initiation and growth. The TB1 model exhibited higher yield stress and strain than the single-crystal model because the isolated twin boundary impeded dislocation motion. In contrast, TB2 and TB8 yielded at lower stresses and strains because the increased twin-boundary density introduced additional preferential nucleation sites at the twin boundaries and twin-boundary/γ–γ′ interface intersections. Despite its earlier yielding, TB2 exhibited the highest ultimate stress among all models and the highest ultimate strain among the twinned models. This behavior was attributed to deformation partitioning between two comparatively stable twin boundaries, which promoted distributed precipitate shearing, dislocation storage, and sustained strain hardening. TB8 exhibited the highest initial dislocation density, followed by a decrease during plastic deformation associated with twin-boundary migration, defect rearrangement, and strain localization. Constructed-surface-mesh analysis further showed that crack initiation was delayed to a strain of approximately 0.07356 in TB2, compared with approximately 0.066 in TB1 and TB8. Crack propagation occurred predominantly along the twin boundaries and γ/γ′ phase interfaces. These findings reveal a non-monotonic, density-dependent transition from barrier-controlled strengthening in TB1 to deformation-partitioning-assisted hardening in TB2 and boundary-migration-assisted, localization-dominated softening in TB8.