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1. 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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2. Decoupling between Shockley partials and stacking faults strengthens multiprincipal element alloys

   https://doi.org/10.1073/pnas.2114167118  

   Pei, Zongrui; Zhang, Siyuan; Lei, Yinkai; Zhang, Fan; Chen, Mingwei (December 2021, Proceedings of the National Academy of Sciences)

   Mechanical properties are fundamental to structural materials, where dislocations play a decisive role in describing their mechanical behavior. Although the high-yield stresses of multiprincipal element alloys (MPEAs) have received extensive attention in the last decade, the relation between their mechanistic origins remains elusive. Our multiscale study of density functional theory, atomistic simulations, and high-resolution microscopy shows that the excellent mechanical properties of MPEAs have diverse origins. The strengthening effects through Shockley partials and stacking faults can be decoupled in MPEAs, breaking the conventional wisdom that low stacking fault energies are coupled with wide partial dislocations. This study clarifies the mechanistic origins for the strengthening effects, laying the foundation for physics-informed predictive models for materials design. 

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3. Shear‐induced amorphization in boron subphosphide (B ₁₂ P ₂ ): Direct transition versus stacking fault mediation

   https://doi.org/10.1111/jace.18629  

   Li, Jun; An, Qi (July 2022, Journal of the American Ceramic Society)

   Abstract The shear‐induced amorphization has been observed in many strong ceramics and is responsible for their cracking and fragmentation. But its underlying mechanism remains elusive due to the complex structure and bonding environment in strong ceramics. To illustrate the deformation mechanism of local amorphization in strong ceramics, we employed molecular dynamics simulations with a deep‐learning force field to examine the shear‐induced amorphization in B₁₂P₂. Surprisingly, we identified a stacking‐fault‐mediated amorphization mechanism along the most plausible slip system (1 1 1)/[1 1 ]. This mechanism is even more favorable at a higher temperature than room temperature. In contrast, the direct crystal to amorphization transition, due to the icosahedral slip, is observed for the other most plausible slip system (0 1 1)/[2 ]. We report the activation volume and the activation free energy for the amorphization along the (1 1 1)/[1 1 ] slip system. The derived activation volume is only 41.47 A³, which is roughly 2–3 icosahedra, suggesting that the localized amorphization in B₁₂P₂is mediated by stacking fault formation. The previous results suggest complex amorphization mechanisms in strong ceramics. 

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4. Inferring fault zone structure from the azimuthal variation in the stacked P-spectra of earthquake clusters

   Neo, Jing Ci; Huang, Yihe; Yao, Dongdong (April 2023, 2023 SSA Annual Meeting)

   Fault damage zones can influence various aspects of the earthquake cycle, such as the recurrence intervals and magnitudes of large earthquakes. The properties and structure of fault damage zones are often characterized using dense arrays of seismic stations located directly above the faults. However, such arrays may not always be available. Hence, our research aims to develop a novel method to image fault damage zones using broadband stations at relatively larger distances. Previous kinematic simulations and a case study of the 2003 Big Bear earthquake sequence demonstrated that fault damage zones can act as effective waveguides, amplifying high-frequency waves along directions close to fault strike via multiple reflections within the fault damage zone. The amplified high-frequency energy can be observed by stacking P-wave spectra of earthquake clusters with highly-similar waveforms (Huang et al., 2016), and the frequency band which is amplified may be used to estimate the width and velocity contrast of the fault damage zone. We attempt to identify the high-frequency peak associated with fault zone waves in stacked spectra by conducting a large-scale study of small earthquakes (M1.5–3). We use high quality broadband data from seismic stations at hypocentral distances of 20-80 km in the 2019 Ridgecrest earthquake regions. First, we group the Ridgecrest earthquakes in clusters by their locations and their waveform similarity, and then stack their velocity spectra to average the source effects of individual earthquakes. Our results show that the stations close to the fault strike record more high-frequency energies around the characteristic frequency of fault zone reflections. We find that the increase in the amount of high-frequencies is consistent across clusters with average magnitudes ranging from 1.6-2.4, which suggests that the azimuthal variation in spectra is caused by fault zone amplification rather than rupture directivity. We will apply our method to other fault zones in California, in order to search for fault damage zone structures and estimate their material properties. 

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5. Crustal Architecture Across Southern California and Its Implications on San Andreas Fault Development

   https://doi.org/10.1029/2022GL101976  

   Sui, Siyuan; Shen, Weisen; Holt, William; Kim, Jeonghyeop (April 2023, Geophysical Research Letters)

   Abstract In this study, we perform a 2‐frequency sequential receiver function stacking investigation in Southern California. The resulting Moho depths exhibit similar patterns to previous studies while the crystalline crustal Vp/Vs values show more regional variations. Most Vp/Vs variations can be explained by compositional differences. We observe a dichotomy in Moho depth, Vp/Vs, and crustal strain rates between the Peninsular Ranges and Southern San Andreas Fault system. Comparisons between strain rates, Vp/Vs, and temperature suggest that crustal compositional variations may have played a more critical role in influencing the crustal strain rate variations in the Peninsular Ranges and Southern San Andreas than temperature. The structural and compositional variations provide a new insight into the causes of the migration of the Southern San Andreas Fault system and the formation of the “Big Bend.” 

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