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As a typical body-centered cubic material, Ta has both high strength and high temperature resistance and is, thus, widely applied in the field of high-energy physics. In this work, the spallation behavior and its underlying physical mechanism of nano-polycrystalline Ta was systematically studied by non-equilibrium molecular dynamics simulations, with special attention to the internal grain size effect vs external shock intensity. The results reveal that the grain size effects on void evolution, spallation strength, and corresponding mechanical and thermodynamic responses are different under different shock intensities. Under the piston velocity of 0.75 km/s, when the grain size decreases from 30 to 2 nm, the spallation mechanism switches from intergranular fracture (d ≥ 10 nm) to cavitation failure (d ≤ 5 nm), and the correlation between spallation strength and grain size also switches from an inverse Hall–Petch to a Hall–Petch relation at a critical grain size dc ∼ 10–20 nm. As the piston velocity increases to 1.5 or 1.8 km/s, a failure mode transition from classical spallation to micro-spallation is observed, leading to a significantly weakened grain size effect on the spallation strength. Through thermodynamic analysis, melting is detected in the tensile region, which is responsible for the micro-spallation. These results can help to understand the effects of internal grain size and external shock intensity on the spallation behavior of Ta and make a leap in the design of shock-resistant materials. [ABSTRACT FROM AUTHOR]

The Geza arc is an important part of the Sanjiang tectono-magmatic belt and is a newly discovered copper polymetallic ore concentration area in northwest Yunnan province, Southwest China. The area comprises numerous metal ore deposits, including one super-large deposit, three large deposits, etc. The formation of these deposits was closely related to intermediate–acidic magmatic intrusions. Based on previous studies, the “big data” analysis technique was used for a comparative study of large geochemical datasets of granite related to ore-formation in the Geza porphyry copper deposit and global adakites. As a result, 1313 element combinations and 127,765 overlap ratios were obtained. The results show that the Geza porphyry has similar geochemical characteristics to global adakites (the ratios of REE and Ga to major elements are in the range of global adakites). However, the Cu, Mo, and Zn contents of the porphyry are significantly higher than those of global adakites, and the porphyry may, therefore, represent an end-member of the global range of adakite composition. In addition, the geochemistry of adakites associated with the porphyry copper deposits overlaps in part with that of global adakites, although most of the data lie outside of the range of global adakites (i.e. low Mn/Cu, Sr/Cu, Na/Cu, and Zr/Cu values, and high Th/Cu, Ba/Cu, Na/Mo, Rb/Mo, Th/Mo, Ta/Mo, Ba/Mo, Mn/Zn, and Ba/Zn values). The samples with characteristics that deviate significantly from the geochemistry of global adakites show more advanced mineralization and alteration, and a stronger relationship with Cu and Mo mineralization. The results of geochemical data mining can be used as a prospecting indicator, and provide a new scientific basis for geological prospecting of the deep levels and periphery of the Geza Cu polymetallic ore belt.

Engineering structures or materials are often subjected to multiple shock loadings. Mechanical response and its physical mechanism under such loadings are extremely complex and need to be studied in depth. To reveal double shock-induced deformation and microstructural evolution in the key structural material Ta, large-scale non-equilibrium molecular dynamics simulations of monocrystalline and polycrystalline Ta under double shock loading were performed. The results show that the activation and re-evolution of twins and dislocations introduced by the first shock dominate the plastic deformation during the second one. Some crystallographic orientation dependent mechanisms of plastic deformation under the second shock are revealed. Twin-dislocation conversion is dominant in the ⟨100⟩-orientated monocrystalline and polycrystalline Ta, while dislocation slipping is dominant in the ⟨110⟩- and ⟨111⟩-orientated Ta. The dependence of flow strength on the loading-paths of single and double shocks was also investigated. Shock-induced amorphization and recrystallization are observed in the single shock-loaded Ta models, leading to lower flow strengths than those of the double shock-loaded ones. These results help understand the complex relationship between the dynamic strength and intrinsic deformation mechanism of Ta under multiple shock loadings. [ABSTRACT FROM AUTHOR]