Materials Characterization最新文献

In this study, the Gasar process and vapor phase dealloying (VPD) method were combined to fabricate a massive three-level hierarchical porous silver (MTHPS), which composed of interconnected micron-sized, submicron-sized, and nanoporous structures. The key aspect of this study lies in the preparation of a hypoeutectic Gasar Mg_91.6Ag_8.4 alloy with the regular micron-scale pore structure as the dealloying precursor, improving the size of the hierarchical porous metal. Under high vacuum conditions, by adjusting the dealloying time and temperature, MTHPS samples with various morphologies were obtained. After dealloying, the MTHPS exhibited Gasar pores (diameter 376 ± 86 μm), submicron pores (diameter 465 ± 125 nm), and a continuous ligament/channel structure (pore size less than 70 ± 40 nm). The coarsening index was 3.6258, and the activation energy was measured to be 0.95 eV, indicating that the formation and coarsening of the pores during VPD are attributed to surface diffusion. The calculated evaporated mass of the precursor alloy after dealloying was very close to the experimental value (±0.004 g), indicating that the evaporation of Mg occurred simultaneously in both solid and liquid phases during the VPD process. This study elucidates the phase transformation characteristics, structural control, and diffusion mechanisms of MTHPS prepared by VPD, providing valuable insights for further research and applications of MTHPS.

The good interfacial bonding between reinforcements and magnesium (Mg) matrix is the essential requirement for the development of magnesium matrix composites (MMCs) with high performance. Herein, this work tries to improve the interfacial bonding of MMCs. Here, the pre-dispersed Ti nanoparticles were introduced in AZ31 composites to form in-situ TiAl nanoparticles, and the AZ31 composites were synthesized by semisolid stirring assisted ultrasonic vibration method. Microstructural analysis reveals that the TiAl nanoparticles are in-situ formed by the Al atoms in composites diffusing into Ti nanoparticles, and the composites obtain a strong interfacial bonding between the TiAl nanoparticles and matrix owing to the formation of TiAl/Mg semi-coherent interface. The composites achieve simultaneous improvement of strength and plasticity. The composite with 0.2 wt% Ti nanoparticles addition possesses the best comprehensive tensile properties. The corresponding ultimate tensile strength and elongation reach 315 MPa and 21.3 %, respectively. The enhanced strength is owing to dislocation strengthening, grain refinement strengthening, and Orowan strengthening. The good plasticity is the result of the activation of non-basal dislocations, refined grains, weakened texture, and good interfacial bonding.

Cobalt (Co) and cobalt-graphene oxide (Co-GO) coatings were electrodeposited on mild steel. The Co-GO coating with an optimum volume fraction of GO (Co-GO-3) displayed significantly reduced corrosion current density (5.3 μA/cm²) compared to pristine cobalt coating (21.4 μA/cm²) in 3.5 wt% NaCl solution. Formation of a stable CoO passive oxide layer and a higher proportion of low energy grain boundaries (low-angle grain boundaries and CSL boundaries) contributed to the enhanced anti-corrosion properties. The Co-GO-3 coating also exhibited the lowest hydrogen permeation current, attributed to its higher fraction of relatively closed grain boundaries and presence of compressive strain within the coating.

This investigation focused on the thermal stability of nanocrystalline (NC) binary and ternary Ni-Y-Zr alloys synthesized through ball-milling. The microstructural changes following annealing, conducted up to 1200 °C, were studied using various techniques, including X-ray-line-broadening, micro-hardness, and transmission electron microscopy. The results revealed that the rate of grain growth observed in the Ni-Y-Zr ternary alloy at 600 °C resembled that in pure NC-Ni at 100 °C. Moreover, the Ni-1.4Y-1.1Zr ternary system exhibited a maximum hardness of 753 HV (average) at 600 °C, which was approximately 60 HV higher than the Ni-1.2Y/1.9Y and Ni-1.5Zr/2.7Zr binary alloys and more than double that of pure NC-Ni for similar grain sizes. These characteristics were linked to the formation of nano-sized oxides and nitrides of Y and Zr within the microstructure. In summary, this study emphasizes the notable high-temperature microstructure stability of Ni-based ternary alloys, attributed to the additive effects of Y and Zr. Due to this extended stability, this ternary system could find potential applications at elevated temperatures in areas such as jet engine turbine blades and power plants.

Joining Al₂O₃ ceramics to Cu heat pipes should be conducted at low temperatures since Cu heat pipes may fail at temperatures higher than 320 °C. Due to the poor wettability of low-temperature solder on Al₂O₃ ceramics, it is essential to metallize Al₂O₃ ceramics before joining Al₂O₃ to Cu with a low-temperature solder. Surface metallization of Al₂O₃ has been conducted using Ag-Cu-Ti active metal filler at 900 °C. Microanalysis indicates that an interfacial reaction occurs between the active metal filler and the Al₂O₃, leading to the formation of a Cu₃Ti₃O reaction layer. The metal layer on the surface of Al₂O₃ is primarily composed of Ag (s, s) (solid solution), Cu (s, s) and Ti₂Cu. The polishing treatment of the surface metal layer in metallized Al₂O₃ results in a reduction of the oxide content and contaminants, which subsequently allows joining at low temperature. Low-temperature joining of metallized Al₂O₃ to Cu has been carried out using Sn-Ag-Cu solder at 280 °C. The joining area of the joint includes a Cu₃Ti₃O reaction layer, a metal layer and a solder layer. The solder layer mainly consists of Sn (s, s), Ag₃Sn and Cu₆Sn₅. The Al₂O₃/Cu joint fractures in the solder layer after the shearing test, indicating that the strength of the solder is relatively low. However, the interfacial bonding between the metallized Al₂O₃, solder layer and Cu base material is satisfactory.

Irradiation damage and thermal aging greatly affect the phase boundary microstructure and stress corrosion cracking of austenitic stainless steel weld metals (ASSWMs) in water-cooled nuclear reactors. However, the effects of irradiation plus thermal aging (I + A) on the phase boundary segregation and phase changes remain unclear. Phase changes and elemental segregation at the carbide and carbide-free phase boundaries of 308 L ASSWMs after I + A treatment were investigated using atom probe tomography and transmission electron microscopy. The I + A treatment induced Si depletion at the phase interfaces of δ-ferrite/austenite (δ/γ) and δ-ferrite/carbide (δ/C) and also induced Ni enrichment and Cr depletion with concentrations having the order Ni/Cr(δ/γ) > Ni/Cr(δ/C) > Ni/Cr(γ/C). The solute-defect binding model and the vacancy mechanism were applied to explain the phase boundary segregation. Furthermore, the I + A treatment affected microstructural evolution near the phase boundaries; this reduced spinodal decomposition and inhibited G-phase and Ni/Si-rich-cluster formation. The phase separation of δ-ferrite near δ/γ and δ/C phase boundaries differed with the distance from the boundary, forming a gradient microstructure from phase boundaries to δ-ferrite internally. The gradual precipitation of the G-phase was observed beyond about 15 and 10 nm from the δ/γ and δ/C interfaces, respectively; moreover, the growth of α'-phase in the δ-ferrite was accelerated with increasing distance from the δ/γ and δ/C interfaces.

A dual-phase high entropy alloy (DP-HEA) containing 7.82 wt% Al was synthesized to withstand corrosive supercritical water. The long-term corrosion and quasi-static tensile tests reveal superior resistance to such environmental-assisted degeneration. The outstanding corrosion resistance can be attributed to the high aluminum content in B2 phase, which forms a continuous Al₂O₃ layer for protection, while the FCC phase is selectively corroded. Under external stress, stress corrosion cracking (SCC) initiates from surface oxide and penetrates uncompact mixed Cr/Al-rich oxide in deteriorated FCC regions or corroded phase interface, though network-shaped B2 can inhibit SCC propagation, which supports the brittle film rupture model.

The controllable synthesis of high-entropy fluorite oxide (HEO) having large ionic radius mismatch remains a challenging due to poor understanding on nucleation. The (HfZrTiLn)-5HEO nanoparticles with 15 % ionic radius mismatch were synthesized via benzyl alcohol route at 220 °C-5 min in presence of PtCl₄ and Fe(acac)₃, exhibiting novel optical, electrical and magnetic properties. Nucleation pathways of the 5HEO at the critical temperature were elucidated by using a comparison study of conventional heating and microwave irradiation heating. Consistency of XRD patterns and STEM-EDX observation indicate that the resultant Hf-OBn monomers acted as the nucleation center of the 5HEO, determined by diffusion kinetics. The nucleation rate depended on the metal monomers assembly and esterification reaction, which was accelerated by water vapor pressure produced in-situ by $0.5\times {10}^{-4}\mathrm{mol}/l$ PtCl₄ catalyst. The Fe-metal organic cages derived from $1.5\times {10}^{-4}\mathrm{mol}/l$ Fe(acac)₃ additive served as the structure stabilizer of Zr/Ti monomers, and prevented early hydrothermal reaction route.