Investigating the dynamic mechanical behavior of single-crystal aluminum oxynitride (AlON) is fascinating and crucial for understanding material performance in relevant applications. Nevertheless, few studies have explored the dynamic mechanical properties of AlON single crystals. In this study, a series of nanoimpact experiments (representative strain rate ${\dot{\epsilon }}_{\text{r}}\text{≈}{\text{10}}^{\text{2}}{\text{s}}^{-\text{1}}$) were performed on three principal orientations ((010), (101), and (111)) of grains to extract the dynamic mechanical responses of AlON single crystals. Our results reveal that the dynamic plasticity of an AlON single crystal is governed by a combination of mechanisms, including dislocation motion and amorphization. Significantly, the localized amorphization induced by mechanical deformation has a softening effect (a lower dynamic hardness). The crystallographic orientation affects the dynamic hardness similarly to the static hardness. In particular, the (111) orientation results in the highest hardness, whereas the (010) orientation is the softest among the three principal orientations. This dependency aligns with the expectations derived from applying Schmid law. Furthermore, both the dynamic and static hardnesses exhibit typical indentation size effects (ISEs), which can be effectively described via the strain gradient theory associated with the geometrically necessary dislocations. In addition, the size and rate dependencies of the dynamic hardness can be decoupled into two independent terms.

With the rapid development of high-end industries, the demand for high-temperature piezoelectric materials is significantly increasing. However, realizing the ultra-high performance to meet more applications still faces major scientific and engineering challenges of our time. Here, a new Nb/Mn co-doped CaBi₄Ti₄O₁₅ (CBT) high-temperature piezoelectric material system of CaBi₄Ti(Nb_2/3Mn_1/3)O₁₅ was synthesized by the conventional solid-state sintering method. The results show that the addition of the dopants tends to break the long-range ferroelectric chain and soften the flexibility of polarization, resulting in more distorted crystal structure and better ferroelectric properties of CBT ceramics. The ultra-high piezoelectric constant (d₃₃ = 26.8 pC/N) is thus attained in CBT-based ceramics with x = 0.12, which is about several times larger than that of pure CBT ceramics. Moreover, numerous nano-sized layered domain structures that lie on the lateral plane of grains are observed in ceramics, with lower domain wall energy and better dynamic features under electric fields, mainly responsible for the origin of enhanced performance. Besides, excess dopants could make the conductivity mechanism of CBT ceramics transform from p-type to n-type, and also result in a shift of conduction relaxation mechanism from defect dipole rotation polarization to electron relaxation polarization. The work not only provides a promising candidate for high-temperature piezoelectric materials, but also opens a window for optimizing performance by tailoring domain structures using chemical modification.

Coupling effects of fretting wear and cyclic stress could result in significant fatigue strength degradation, thus potentially causing unanticipated catastrophic fractures. The underlying mechanism of microstructural evolutions caused by fretting wear is ambiguous, which obstructs the understanding of fretting fatigue issues, and is unable to guarantee the reliability of structures for long-term operation. Here, fretting wear studies were performed to understand the microstructural evolution and oxidation behavior of an α/β titanium alloy up to 10⁸ cycles. Contact surface degradation is mainly caused by surface oxidation and the generation of wear debris during fretting wear within the slip zone. The grain size in the topmost nanostructured layer could be refined to ~40 nm. The grain refinement process involves the initial grain rotation, the formation of low angle grain boundary (LAGB; 2°–5°), the in-situ increments of the misorientation angle, and the final subdivision, which have been unraveled to feature the evolution in dislocation morphologies from slip lines to tangles and arrays. The formation of hetero microstructures regarding the nonequilibrium high angle grain boundary (HAGB) and dislocation arrays gives rise to more oxygen diffusion pathways in the topmost nanostructured layer, thus resulting in the formation of cracking interface to separate the oxidation zone and the adjoining nanostructured domain driven by tribological fatigue stress. Eventually, it facilitates surface degradation and the formation of catastrophic fractures.

Achieving full densification of some ceramic materials, such as Y₂O₃, without sintering aids by spark plasma sintering (SPS) is a great challenge when plastic deformation contributes limitedly to the densification as the yield stress of the material at an elevated temperature is higher than the applied sintering pressure. Herein, we demonstrate that particle fracture and rearrangement is an effective strategy to promote the densification during the pressure-assisted sintering process. Specifically, Y₂O₃ nanocrystalline powders composed of nanorod and near-spherical particles were synthesized and sintered at various temperatures by the SPS. The results show that the relative density of the ceramics prepared by the nanorod powders is higher than the density of the ceramics from the near-spherical powders after 600 ℃ due to the fracture and rearrangement of the nanorods at low temperatures, which leads to the decrease of particle size and the increase of density and homogeneity. Based on this novel densification mechanism, ultrafine-grained Y₂O₃ transparent ceramics with good optical and mechanical properties were fabricated successfully from the nanorod powders.

In this paper, both the 1D radial mode and the equivalent circuit of a piezoceramic disk resonator were theoretically analyzed based on IEEE standards. And then, the radial resonance frequency spectra of the PZT-based (Nb/Ce co-doped Pb(Zr_0.52Ti_0.48)O₃, abbreviated as PZT-NC) piezoceramic circular disks were measured by an impedance analyzer. A set of resonance frequency spectra including six electrical parameters: Z, R, X, Y, G, and B, were used for making a value distinction between three possible resonance frequencies, and between three possible antiresonance frequencies. A new-form Nyquist diagram was depicted to describe the position relations of these characteristic frequencies. Such a complete resonance frequency spectrum was used to perform the accurate calculation of some material constants and electromechanical coupling parameters for the PZT-NC piezoceramics. Further, the frequency dependence of the AC conductive behavior of the specimen was characterized by the complex impedance measurement. The values of AC conductivity at resonance/antiresonance were deduced from the equivalent circuit parameters. Moreover, the Van Dyke circuit model was assigned to each element contribution and the simulated curves showed a nice fitting with the experimental results. Finally, an additional impedance analysis associated with resonance frequency calculation revealed a complicated coupled vibration mode existing in the annular disk specimen.

In this work, we present a new piezoelectric solid solution consisting of two typical alkali niobate-based materials, K_0.5Na_0.5NbO₃ (KNN) and Li_0.15Na_0.85NbO₃ (LNN). Although KNN and LNN have the same perovskite structure, they exhibit extremely different electrical properties and mechanical behaviors. The phase structures, electrical and mechanical evolutions of the new lead-free piezoelectric materials with different ratios of KNN and LNN are comprehensively and theoretically investigated. According to the X-ray diffraction patterns and curves of permittivity versus temperature, a series of complicated phase transitions can be found with varied LNN content. Rietveld refinement results based on XRD patterns reveal an oxygen octahedron tilting in the LNN-rich crystal structure, and simultaneously the reasons for octahedron tilting are discussed. The distorted crystal structure is accompanied by extremely decreased electric properties but increased mechanical properties, which reveals electrical and mechanical properties of alkali niobate-based piezoelectric ceramics strongly depend on their inner structures, and the enhancement of intrinsic hardness results in the deterioration of piezoelectric properties. Our work exhibits the detailed evolutions of structure, electrical and mechanical properties from KNN to LNN, which provides experimental and theoretical basis for development of new alkali niobate-based piezoelectric materials.