Reversible phase transformation in Ti2AlC films during He radiation and subsequent annealing

Ti₂AlC film can be used as a protective coating for fuel cladding materials and structural materials in nuclear reactors. However, the related radiation damage and the helium (He) effects have not been well understood. In this work, the He radiation effects on Ti₂AlC thin films, deposited by reactive magnetron sputtering, were studied. In addition to the detailed characterization of the radiation-induced defects and He bubbles, phase transformation was identified and investigated during film deposition, ion irradiation, and subsequent annealing. Results suggested that the hexagonal close-packed (hcp) Ti₂AlC was formed from a solid-solution face-centered cubic (fcc) (Ti₂Al)C phase during the film deposition process. A phase transformation from hcp-Ti₂AlC to fcc-(Ti₂Al)C happened during the He ion irradiation, while a reversible phase transformation from fcc-(Ti₂Al)C to hcp-Ti₂AlC occurred during the post annealings at temperatures above 600 °C. The reversible phase transformation indicates dynamic restoration of this material and provides insights into the design of new irradiation-damage-tolerant ceramic materials.     

石墨烯/氟碳涂层的制备及其耐蚀性能

目的为了增强氟碳涂层的耐蚀性,研究涂层在3.5%NaCl溶液中的失效过程。方法采用硅烷偶联剂对石墨烯进行接枝改性,将改性后的石墨烯添加到氟碳树脂中,制成不同含量的石墨烯氟碳复合涂层。采用傅里叶变换红外光谱、拉曼光谱、透射电镜和扫描电镜,分析了石墨烯改性前后的结构及在涂层中的分散性。采用交流阻抗谱和动电位极化曲线,研究了涂层在模拟海水中的电化学腐蚀行为和失效过程,并考察了涂层的耐盐雾性能。结果石墨烯表面成功接枝官能团,在涂层中分散较均匀。石墨烯对腐蚀介质具有良好的屏障作用。涂层的防护性能随着石墨烯含量的增大先增加后降低,当含量为0.4%(质量分数)时,涂层的腐蚀电流密度为2.209×10~(–10) A/cm~2,氟碳涂层的腐蚀电流密度为6.026×10~(–6) A/cm~2,腐蚀电流密度大大降低,该涂层的耐蚀性能最好,且浸泡360 h内均为浸泡前期,能有效隔绝腐蚀液体的渗透,对Q235钢基底的防护性能最佳。石墨烯含量过高时易团聚,容易引起缺陷,降低涂层的防护作用。结论石墨烯显著提高了氟碳涂层的耐蚀性能。     

Mechanical properties and friction−wear characteristics of VN/Ag multilayer coatings with heterogeneous and transition interfaces

The heterogeneous multilayer interface of VN/Ag coatings and transition multilayer interface of VN/Ag coatings were prepared on Inconel 781 and Si(100), and the microstructures, mechanical and tribological properties were investigated from 25 to 700 °C. The results showed that the surface roughness and average grain size of VN/Ag coatings with transition multilayer interface are obviously larger than those of VN/Ag coatings with heterogeneous multilayer interface. The coatings with transition multilayer interface have higher adhesion force and hardness than the coatings with heterogeneous multilayer interface, and both coatings can effectively restrict the initiation and propagation of microcracks. Both coatings have excellent self-adaptive lubricating properties with a decrease of friction coefficient as the temperature increases, but their wear rates reveal a drastic increase. The phase composition of the worn area of both coatings was investigated, which indicates that a smooth Ag, Magnéli phase (V₂O₅) and bimetallic oxides (Ag₃VO₄ and AgVO₃) can be responsible to the excellent lubricity of both coatings. To sum up, the coatings with transition multilayer interface have excellent adaptive lubricating properties and can properly control the diffusion rate and release rate of the lubricating phase, indicating that they have great potential in solving the problem of friction and wear of mechanical parts.     

The surface performance of shot peened and ion implanted NiTi shape memory alloy

Equiatomic NiTi shape memory alloy in austenitic and martensitic forms has been modified by shot peening with glass media and by N⁺ ion implantation. The effect upon microstructure, surface hardness and coefficient of friction was measured, along with the effect of the two treatments upon the shape memory behaviour. Examination using transmission electron microscopy (TEM) showed amorphous regions within the surfaces treated by both peening and ion implantation. In addition, peened and ion implanted samples showed similar increases in surface hardness and reduced surface friction coefficient. Differential scanning calorimetry showed that both surface treatments had no effect upon the bulk shape memory phase transformation behaviour.     

Quantification of corrosion mechanisms under immersion and salt-spray environments on an extruded AZ31 magnesium alloy

The corrosion mechanisms of pitting, intergranular, and general corrosion were examined on an extruded AZ31 magnesium alloy subjected to immersion and salt-spray environments. The three mechanisms were quantified using optical microscopy and laser profilometry for over 60 h of testing. Although both environments showed similar trends, the immersion environment was more deleterious with respect to intergranular and general corrosion. On the other hand, the salt-spray environment did allow deeper pits to form throughout the entirety of the experiments, which then led to a substantial thickness drop (general corrosion) compared with the immersion environment.     

Development of FeNiNbSiBP bulk metallic glassy alloys with excellent magnetic properties and high glass forming ability evaluated by different criterions

Fe₃₈Ni₃₈Nb_2.5B_21.5−x−yP_xSi_y (x, y = 1–8) bulk metallic glassy alloys with high glass forming ability and excellent magnetic properties were developed. Bulk samples with maximum diameters of 3 mm are fabricated by copper mold casting method. The glassy alloys have large ΔT_x of 40–70 K. The alloys exhibit excellent magnetic properties like extremely low H_c of 0.5–0.8 A/m, high μ_e of 1.6–2.85 × 10⁴ and comparatively high B_s of 0.6–0.8 T which changes regularly with the content variations of P, B and Si. By ascertaining applicability of the empirical GFA criterions, T_rg, α, β and γ can be used in evaluating the GFA of FeNiBSiPNb system alloys.     

Nano-texture for a wear-resistant and near-frictionless diamond-like carbon

The effect of nano-scale surface texture on wear resistance of diamond-like carbon (DLC) films was studied using a reciprocating ball-on-flat tribometer in dry, humid, and liquid water environments. The nano-scale surface texture was produced by depositing ∼1 μm thick DLC films onto silicon substrates pre-textured with pyramidal wells and polystyrene spheres. The surface roughness of the textured DLC films was about 50 nm in both cases. The friction and wear behavior of the flat and nano-textured DLC films were tested with AISI 440C-grade stainless steel balls at a contact load creating about 360 nm deep Hertzian deformation which is significantly larger than the surface roughness. At this condition, nano-texturing did not affect the friction coefficient, but it significantly reduced the wear of DLC films in dry and humid nitrogen compared to flat DLC. In dry nitrogen, the nano-textured DLC films showed the ultra-low friction without substantial wear of DLC and deposition of thick transfer films onto the counter-surface. The wear reduction appeared to be related to the stress relief in the nano-textured DLC film. In liquid water, surface features on the nano-textured DLC films were diminished due to tribochemical oxidation and material removal at the sliding interface.     

Controlling the compactness and sp2 clusters to reduce interfacial damage of amorphous carbon/316L bipolar plates in PEMFCs

In proton exchange-membrane fuel cells, the compactness and composition of amorphous carbon (a-C) films directly affect the interface-induced degradation of a-C-coated metallic bipolar plates, but their roles and synergistic effects are ambiguous. In this study, by changing the working pressure, the compactness and composition of a-C are adjusted, and the relationship among the plasma state, microstructure, and performance of coated 316L stainless steel are studied. Results show that, at 2 mTorr, a high ionization degree of the plasma causes high compactness (highest density 2.52 g/cm³) and large sp² cluster size in a-C films, resulting in their best performance. At 11 mTorr, the low ionization degree causes their loose structure and large sp² cluster size. The possible galvanic coupling effect can result in serious interface damage and the highest Fe ions concentration of 13.05 ppm after a potentiostatic test. Thus, high compactness and large sp² cluster sizes should be obtained simultaneously to improve their performance.     

A ternary phase diagram for amorphous carbon

The sp² phase in amorphous carbon is modeled as coexisting nc-graphite, fused aromatic ring, and olefinic chain clusters. A phenomenological linear dispersion model is derived to calculate the G peak position from weighted relative contents of the sp² clusters and vice versa. Ternary phase diagrams based on the weighted relative contents provide quantitative predictions for G peak position, dispersion rate, sp³ content, H content, and other properties of amorphous carbon. The phase diagrams are used to classify and characterize amorphous carbon, and to track their structural changes due to modification procedures like annealing, doping, and ion irradiation. Numerous insights are drawn from patterns in the distributions or in the migration paths of the analyzed samples.