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1. Unraveling the Role of Interfaces on the Spall Failure of Cu/Ta Multilayered Systems

   https://doi.org/10.1038/s41598-019-57048-9  

   Chen, Jie; Mathaudhu, Suveen N.; Thadhani, Naresh; Dongare, Avinash M. (December 2020, Scientific Reports)

   Abstract Molecular dynamics (MD) simulations are carried out to investigate the effects of the type and spacing of FCC/BCC interfaces on the deformation and spall behavior. The simulations are carried out using model Cu/Ta multilayers with six different types of interfaces. The results suggest that interface type can significantly affect the structure and intensity of the incoming shock wave, change the activated slip systems, alter dislocation slip and twinning behavior, affect where and how voids are nucleated during spallation and the resulting spall strength. Moreover, the above aspects are significantly affected by the interface spacing. A transition from homogeneous to heterogeneous dislocation nucleation occurs as the interface spacing is decreased to 6 nm. Depending on interface type and spacing, damage (voids) nucleation and spall failure is observed to occur not only at the Cu/Ta interfaces, but also in the weaker Cu layer interior, or even in the stronger Ta layer interior, although different mechanisms underlie each of these three distinct failure modes. These findings point to the fact that, depending on the combination of interface type and spacing, interfaces can lead to both strengthening and weakening of the Cu/Ta multilayered microstructures. 

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2. In Situ Study of Twin Boundary Stability in Nanotwinned Copper Pillars under Different Strain Rates

   https://doi.org/10.3390/nano13010190  

   Chang, Shou-Yi; Huang, Yi-Chung; Lin, Shao-Yi; Lu, Chia-Ling; Chen, Chih; Dao, Ming (January 2023, Nanomaterials)

   The nanoscopic deformation of ⟨111⟩ nanotwinned copper nanopillars under strain rates between 10^−5/s and 5×10^−4/s was studied by using in situ transmission electron microscopy. The correlation among dislocation activity, twin boundary instability due to incoherent twin boundary migration and corresponding mechanical responses was investigated. Dislocations piled up in the nanotwinned copper, giving rise to significant hardening at relatively high strain rates of 3–5×10^−4/s. Lower strain rates resulted in detwinning and reduced hardening, while corresponding deformation mechanisms are proposed based on experimental results. At low/ultralow strain rates below 6×10^−5/s, dislocation activity almost ceased operating, but the migration of twin boundaries via the 1/4 ⟨10-1⟩ kink-like motion of atoms is suggested as the detwinning mechanism. At medium strain rates of 1–2×10^−4/s, detwinning was decelerated likely due to the interfered kink-like motion of atoms by activated partial dislocations, while dislocation climb may alternatively dominate detwinning. These results indicate that, even for the same nanoscale twin boundary spacing, different nanomechanical deformation mechanisms can operate at different strain rates. 

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3. Twinning in rolled AZ31B magnesium alloy under free-end torsion

   https://doi.org/https://doi.org/10.1016/j.msea.2020.140405  

   Carneiro, L.; Culbertson, D.; Yu, Q.; Jiang, Y. (January 2021, Materials science engineering)

   null (Ed.)

   The mechanical response and microstructure evolution in a rolled AZ31B magnesium alloy were experimentally characterized using companion thin-walled tubular specimens under free-end monotonic torsion. The tubular specimens were made with their axes along the normal direction of the rolled magnesium plate. The shear stress-shear strain response shows a subtle sigmodal shape that is composed of four distinctive stages of strain hardening. Basal slips and tension twinning are operated throughout the shear deformation. Both tension twinning and compressing twinning are favored. Growth and interaction of tension twins with multiple variants lead to formation of twin-twin boundaries (TTBs). The collective hardening effects by twin boundary (TB) and TTB result in a unique rise of the strain hardening rate in Stage II and III. In addition to primary twins, tension-compression double twins and tension-compression-tension tertiary twins with detectable sizes are observed in the tension-twin favorable grains whereas compression-tension double twins are detected in the tension-twin unfavorable grains; all of which become more observable with the increasing shear strain. During Stage IV deformation where TTB formation exhausts, non-basal prismatic slips become more significant and are responsible for the progressive decrease in strain hardening rate in this stage. Swift effect, which is commonly observed in textured materials, is evidenced under free-end torsion. The origin of Swift effect is confirmed to be dislocation slips at a shear strain less than 5% but is predominantly due to tension twinning at a larger plastic strain. 

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4. Copper effects on the microstructures and deformation mechanisms of CoCrFeNi high entropy alloys

   https://doi.org/10.1063/5.0201647  

   Amalia, Lia; Li, Yongkang; Bei, Hongbin; Chen, Yan; Yu, Dunji; An, Ke; Lyu, Zongyang; Liaw, Peter K; Zhang, Yanwen; Ding, Qingqing; et al (April 2024, Applied Physics Letters)

   In situ neutron diffraction experiments have been performed to investigate the deformation mechanisms on CoCrFeNi high entropy alloys (HEAs) with various amounts of doped Cu. Lattice strain evolution and diffraction peak analysis were used to derive the stacking fault probability, stacking fault energy, and dislocation densities. Such diffraction analyses indirectly uncovered that a lower degree of Cu doping retained the twinning behavior in undoped CoCrFeNi HEAs, while increasing the Cu content increased the Cu clusterings which suppressed twinning and exhibited prominent dislocation strengthening. These results agree with direct observations by transmission electron microscopy. 

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5. Atomic-level study of twin–twin interactions in hexagonal metals

   https://doi.org/10.1557/jmr.2020.65  

   Gong, Mingyu; Wu, Wenqian (July 2020, Journal of Materials Research)

   null (Ed.)

   Twin–twin interactions (TTIs) take place when multiple twinning modes and/or twin variants are activated and interact with each other. Twin–twin junctions (TTJs) form and affect subsequent twinning/detwinning and dislocation slip, which is particularly important in determining mechanical behavior of hexagonal metals because twinning is one major deformation mode. Atomic-level study, including crystallographic analysis, transmission electronic microscopy (TEM), and molecular dynamics (MD) simulations, can provide insights into understanding the process of TTIs and structural characters associated with TTJs. Crystallographic analysis enables the classification of TTIs and the prediction of possible interfaces of twin–twin boundaries (TTBs), characters of boundary dislocations, and possible reactions of twinning dislocations and lattice dislocations at TTBs. MD simulations can explore the process of TTIs, microstructures of TTJs, atomic structures of TTBs, and stress fields associated with TTJs. The predictions based on crystallographic analysis and the findings from MD can be partially verified by TEM. More importantly, these results provide explanation for microstructural characters of TTJs and guidance for further TEM characterizations. 

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