Damage development of a woven SiCf/SiC composite during multi-step fatigue tests at room temperature

The monotonic tensile and multi-step fatigue tests of 2D woven SiC_f/SiC composite were performed to explore the damage development, respectively. The acoustic emission-based technique was used to analyze the damage state and select the peak stresses for fatigue tests. The damage evolution after specific mechanical tests was characterized by optical microscopy and scanning electron microscopy. Cracks are prone to occur in the vicinity of flaws and boundaries of different matrix components under relatively low fatigue stress. The cyclic fatigue stress can do much harm to the interfaces and facilitate the interfacial debonding. The damage characteristics of five types of cracking, fiber breakage and pull-out, and interfacial debonding of the composite after specific mechanical tests are concluded and discussed in detail, which can offer help for deeper analysis of the oxidation mechanism in service and more reasonable design of SiC_f/SiC composite.     

Microstructure and properties of WC-12Co composite coatings prepared by laser cladding

A comprehensive study of the phase composition, microstructure evolution, microhardness and wear performance of WC-12Co composite coatings fabricated by laser cladding using coaxial powder-feed mode was presented. It was shown that a combination of high scan speed and high laser energy density made WC on the edge of WC-12Co composite powders partially melt in liquid Co and 304 stainless steel matrix, and then new carbides consisting of lamellar WC and herringbone M₃W₃C (M=Fe, Co) were formed. Meanwhile, WC-12Co composite coatings with no porosity, cracks and drawbacks like decarburization were obtained, showing high densification and good metallurgical bonding with the substrate. Furthermore, a considerably high microhardness of HV_0.3 1500-1600, low coefficient of friction of 0.55 and wear rate of (2.15±0.31)×10^-7 mm³/(N·m) were achieved owing to the synergistic effect of excellent metallurgical bonding and fine microstructures of composite coating under laser power of 1500 W.     

Influence of Nb content on martensitic transformation and mechanical properties of TiNiCuNb shape memory alloys

In present work, microstructure, martensitic transformation behavior and mechanical properties of (Ti₅₀Ni₄₀Cu₁₀)_100−xNb_x (x = 0, 5, 10, 15 at.%) alloys were investigated as a function of Nb content. The addition of Nb to TiNiCu alloy leads to the presence of β-Nb phase. During cooling and heating, the alloys show one-step B2 ↔ B19 transformation. As the Nb content increases, the transformation temperatures almost linearly decrease and the transformation hysteresis monotonously increases due to the decrease of middle eigenvalue of the phase transformation matrix. The addition of Nb is effective in improving the elongation because of the introduction of β-Nb phase. With the increase of Nb content, both the yield strength and the critical stress to induce martensitic transformation increase, resulting in the improved superelastic strain.     

孕育剂Ti和Zr元素对2219铝铜合金焊缝组织和性能的影响

采用5种焊丝对2219铝铜合金进行MIG焊,研究了孕育剂Ti和Zr元素对焊缝组织和性能的影响.结果表明,Ti和Zr元素的细化作用是相容的,当焊丝中单独添加孕育剂Ti或Zr元素时,焊缝几乎由粗大的柱状晶组成.焊丝中复合添加少量的孕育剂Ti和Zr元素时,Ti和zr元素在熔池中分别形成Al3Ti和Al3Zr,促进了α(Al)非均质形核.随着焊丝中Ti和Zr元素含量的增加,焊缝组织逐步细化.当焊丝中Ti和Zr元素含量较高时,Ti和Zr元素还在熔池中形成大量的Al3(Ti,Zr)质点,促进α(Al)非均质形核,焊缝组织由细小均匀的等轴晶组成,显著提高了接头强度和塑性.     

An empirical approach to explain the material removal rate for copper chemical mechanical polishing

In this paper, an empirical expression was deduced based on the experimental data for material removal rate of copper chemical mechanical polishing. The parameters of this expression includes the initial chemical corrosion rate(MRR₀), the corrosion inhibition efficiency(k) and the mechanical abrading rate(MRR_M). The deduced empirical expression revealed that under certain slurry systems, the corrosion inhibition efficiency may always keep unchanged, which may be useful to characterize the inhibition properties of different inhibitors.     

Ti-6Al-2Zr-1Mo-1V合金β锻动态再结晶(英文)

通过热压缩实验研究Ti-6Al-2Zr-1Mo-1V钛合金在变形温度为1000～1100°C,应变速率为10-3～1.0s-1的条件下的动态再结晶行为。结果表明:在变形温度高于1050°C、应变速率低于0.01s-1时,合金的动态再结晶机制以不连续动态再结晶为主;在变形温度低于1050°C、应变速率高于0.01s-1时,合金的动态再结晶机制以连续动态再结晶为主,同时存在少量的不连续动态再结晶。此外,降低应变速率和升高变形温度均能促进动态再结晶进程并使β变形晶粒细化。     

Controllable fabrication of superhydrophobic alloys surface on 304 stainless steel substrate for anti-icing performance

In this paper, 304 stainless steel-based ZnO (304SS-based ZnO) seed layer was prepared by using sol-gel method or electrochemical deposition. Superhydrophobic nano-ZnO (CSS–ZnO) surface were prepared on its surface by hydrothermal method. The results show that different structural morphologies of 304SS-based ZnO surface were prepared by varying different seed layer preparation methods. In the static icing test, compared with hydrophilic nano-ZnO (SS–ZnO) surface, hydrophobic nano-ZnO (QS-ZnO) surface and 304SS surface at −5 °C, −10 °C and −15 °C. The icing time of CSS-ZnO surface was prolonged by about 2.7 h at −5 °C, delayed by about 40 min at −10 °C and delayed by about 9 min at −15 °C. The CSS-ZnO surface is the most effective surface in static anti-icing. It is because that there has a residual air layer at the solid-liquid interface and the coating can still effectively retard ice formation in a partially wetted state. In the dynamic icing test, compared with QS-ZnO surface, SS-ZnO surface and 304SS surface at −16 °C, SS-ZnO surface and QS-ZnO surface have no anti-icing effect, and CSS-ZnO surface has a significant anti-icing effect. The mechanism for inhibiting condensation of water droplets by superhydrophobic surfaces was illustrated, which can be identified that the contact angle of the ice embryo will increase with the increase of the water contact angle. This work provides a practical application for promoting anti-icing ability of 304SS surfaces in industry.     

Early stage consolidation mechanisms during hot isostatic pressing of Ti–6Al–4V powder compacts

Mechanisms contributing to early stage compaction of metal powder compacts were identified in a series of Ti–6Al–4V powder compacts hot isostatically pressed to relative densities ranging from 71% to 100%. The partially dense compacts, consolidated from loose powder in thin-walled containers, were examined using optical microscopy of polished sections and stereo pair scanning electron microscopy of fracture surfaces. Relative particle motion, characterized by small relative movements of particles and clusters of particles, was found to contribute significantly to compaction over a broad range of relative densities (from 63% to 90%). A mechanism was identified by which the preferential deformation of small particles at large–small particle contacts enables rigid body motion of larger particles which, in turn, increases the relative density of the compact. Tensile fractures within the compact occurred between 82% and 90% relative density providing direct evidence of cooperative movement among clusters of particles. In comparisons with mechanistic powder compaction models, measurements of particle deformation, contact areas, and coordination numbers were found to substantiate the importance of the experimentally identified mechanisms.     

Mechanical and ablative properties improvement of HfC-SiC coatings upon introduction of TiC

A novel structural Hf-Ti-Si-C multiphase solid solution coating was designed and manufactured by chemical and solid solution reactions to improve the mechanical and ablation behavior of HfC-SiC coatings. Results show that, with TiC addition, the formed Hf_xTi_1?xC and (Ti_1?xHf_x)₃SiC₂ solid solutions can significantly enhance the micro-mechanical and ablation properties of the coating. The improved hardness and modulus, as well as the reduced ablation rates are mainly attributed to the optimized structure and solid solution reinforcing effect of coating. Moreover, the Ti-additives are conducive to restrain the active oxidation of SiC. Furthermore, HfTiO₂ can reduce the oxides cracking due to the inhibited crystal transformation of HfO₂ and its good self-healing ability, forming a dense and stable oxide scale with superior thermal protection.     

Characterization of cyclic loading/unloading damage behavior in fiber-reinforced ceramic-matrix composites using inverse tangent modulus

Under cyclic loading/unloading, the mechanical hysteresis appears in fiber-reinforced ceramic-matrix composites (CMCs) due to multiple micro damage mechanisms. In this paper, the cyclic loading/unloading damage evolution in different CMCs is analyzed using the inverse tangent modulus (ITMs). Experimental micro damage mechanisms are observed using the X-ray computed tomography (XCT) and scanning electron microscopy (SEM). Based on the damage mechanisms’ analysis, a damage-based micromechanical constitutive model is developed to predict the cyclic loading/unloading curves and related damage parameters. Effects of composite’s constitutive properties, peak stress, damage state and interface properties on the cyclic loading/unloading damage evolution are discussed. For the 1D and 2D SiC/SiC, and 3D C/SiC composites, the evolution curves of ITMs can be divided into two regions. In region I, the increasing rate of the ITMs is constant and depends on the composite’s constitutive properties; and in region II, the increasing rate of the ITMs decreases as the interface slip range approaches the interface debonding tip.