The use of mechanical metamaterials for the purpose of controlling wave propagation is made possible by their flexibility and adjustability. In this paper, a one-dimensional metamaterial composed of compound pendulums and inverted pendulums with high-frequency parametric excitation is proposed. The pendulum system can be equivalent to a nonlocal metamaterial and effective mass and effective stiffness can be obtained. The high-frequency parametric excitation provides an equivalent additional stiffness. This additional stiffness can provide an antigravity effect to the inverted pendulum. It makes the inverted pendulum into a stable system. The stability, bandgap structure and wave velocity can be manipulated by adjusting the frequency of the parametric excitation. And a zero-frequency bandgap can be obtained. Moreover, the pendulum system is capable of splitting a wavepacket into two distinct wavepackets within the dual wavevector region. The theoretical results are corroborated by the finite element numerical method. Due to the presence of parametric excitation, the proposed model is more tuneable than conventional metamaterials. This study has the potential to establish a novel approach for the control of ultra-low frequency vibration. The wave splitting phenomenon has the capacity to provide a theoretical foundation for the design of wave separator. It is anticipated that the findings will also furnish novel insights for the design of other devices that control wave propagation.

To clarify the anomalous corrosion behavior in Cu-containing CoCrNi-based medium-entropy alloys, in which an enhanced corrosion driving force is accompanied by a reduced overall corrosion rate, the phase constitution, microstructure, electrochemical behavior, post-corrosion morphology, and surface chemical states of CoCrNi, CoCrNiCu, and CoCrNiCuFe alloys were systematically compared. The results show that Cu addition induces pronounced phase separation in the CoCrNi matrix, leading to the formation of a Cu-depleted FCC1 phase, a continuous Cu-rich FCC2 intergranular network, and dispersed nanoscale Cu-rich precipitates, with an FCC2 area fraction of about 0.145. In 3.5 wt.% NaCl solution, CoCrNiCu exhibits a stronger thermodynamic tendency for corrosion, whereas its overall corrosion rate does not increase, but instead shows the lowest corrosion current density and higher impedance, indicating an anomalous electrochemical response. Post-corrosion SEM morphology, EDS elemental mapping, and XPS valence-state analyses further reveal that corrosion is mainly concentrated in the Cu-rich phases and their adjacent narrow regions, while the Cu-rich phases themselves remain relatively stable as non-sacrificial cathodes. Semi-quantitative thermodynamic and mass-transport calculations indicate that although Cu-induced phase separation enhances the micro-galvanic corrosion driving force, with an estimated interphase potential difference of about 0.337 V, the overall corrosion rate remains constrained by the oxygen diffusion supply during cathodic oxygen reduction on the Cu-rich regions. Therefore, the anomalous corrosion response of CoCrNiCu can be attributed to the synergistic effect of the enhanced micro-galvanic corrosion driving force caused by Cu-induced phase separation and the restricted cathodic oxygen supply.

The anomalous corrosion behavior of Cu-containing CoCrNi-based medium-entropy alloys, in which an increased corrosion driving force is accompanied by a reduced overall corrosion rate, was investigated with emphasis on the underlying microstructural origin and corrosion mechanism. The results show that Cu addition induces pronounced phase separation in the CoCrNi alloy, leading to the formation of a Cu-depleted FCC1 matrix and a Cu-rich FCC2 intergranular phase, together with dispersed Cu-rich nanoscale precipitates within the grains. Electrochemical measurements in 3.5 wt.% NaCl solution reveal that the CoCrNiCu alloy exhibits a more negative corrosion potential, a lower corrosion current density, and higher polarization resistance and apparent charge-transfer resistance, indicative of an anomalous electrochemical response. Post-corrosion surface morphology and XPS analyses show that corrosion is mainly localized in the regions adjacent to the Cu-rich phase, whereas the Cu-rich phase itself remains relatively stable during corrosion. Thermodynamic and mass-transport calculations further demonstrate that a pronounced interphase potential difference exists between FCC1 and FCC2, confining anodic dissolution to the vicinity of phase boundaries. Meanwhile, the oxygen reduction process on the Cu-rich cathodic phase exhibits response characteristics consistent with cathodic oxygen-supply limitation, so that the overall corrosion rate is constrained by the oxygen availability at the cathodic sites. Therefore, the anomalous electrochemical response of the CoCrNiCu alloy essentially arises from the synergistic regulation of phase-separation-induced micro-galvanic driving force and cathodic oxygen-supply limitation.