PSI - Issue 57

Wilmer Velilla-Díaz et al. / Procedia Structural Integrity 57 (2024) 461–468 Velilla-D´ıaz & Zambrano / Structural Integrity Procedia 00 (2023) 000–000

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Fig. 3. Cyclic deformation with R = 0.5.

2.2. Deformation mechanisms and Fracture behavior

In order to analyze the deformation mechanism, the stress-strain curve response of the bicrystals is ploted cycle by cycle in Figure 3. On the other hand, the elasto-plastic fracture mechanics parameter: Crack tip opening displacement (CTOD) is estimated to study the fracture resitence of the Al bic-rystals. The global stress is estimated based on the Virial stress tensor Thompson et al. (2009), and the deviation of the volume box due to the atoms in the free surfaces is corrected by using VORO ++ Rycroft (2009). The distance between atoms on the crack tip to estimate the CTOD is computed using OVITO Stukowski (2010) as shown in Figure 2. To analyze the fatigue crack growth in the simulations the ∆ CTOD is used, which is defined as Anderson (2005): ∆ CTOD = CTOD max − CTOD min (1) The crack growth is computed from the MD simulation by means of OVITO, in order to analyze the resistance to crack propagation due to misorientation GBs as is shown in Figure 5. Figure 4 demonstrates the significant e ff ects observed for both low and high tilt and twist GBs angles. When low angles, viz.: 5° and 10°, are considered the GB has a minimal e ff ect on the crack growth, suggesting that the required stress to fracture the bicrystal exceeds the strength of the first crystal. Conversely, at high tilt angles, the GB acts as an e ff ective barrier, preventing further crack propagation. In such cases, the stress necessary to resume crack propagation must exceed a critical threshold, typically it is reached at the initial crack tearing along the GB. The results of our study reveal a significant e ff ect of twist grain boundaries (GBs) with angles higher than 5° on fatigue crack growth in aluminum bi-crystals. However, at higher tilt angles the GBs act as an e ff ective barriers, arresting crack propagation. Thus, the presence of GBs with certain angles provide a substantial improvement in crack propagation resistance for nanocrystalline materials. In the case of the twist misoriantation and its e ff ect on the fracture behavior of the bicrystals, GB has a notable impact on arresting the crack growth as shown in Figure 5. Specifically, the high angles alter the path of crack propagation. As a consequence, cracks encounter considerable resistance and face greater challenges when attempting to propagate through the GB region. The altered crack propagation path, combined with the stress concentrations, e ff ectively arrest and hinder the advancement of fatigue cracks. Consequently, the presence of Twist GBs plays a crucial role in impeding crack growth and enhancing the material’s fatigue resistance. Figure 6 presents the relationship between the crack length and the ∆ CTOD as a function of the number of cycles for the 20° Tilt GB configuration. By tracking the crack length over multiple cycles, we can observe how the crack evolves and assess its growth rate. 3. Results and discussion