Characterisation of a surface treated AlN powder in aqueous media

The role of an anionic surfactant, which can act as both a deflocculant and a hydrolysis depressor, has been examined on the hydrolysis of a surface treated aluminium nitride, AlN, powder using a range of characterisation techniques. It has been determined that the initial hydrophobic surface treatment serves only to minimise the contact between water molecules and the particles and to enable the adsorption of the surfactant. The latter is the main factor preventing the hydrolysis of the powders during slip preparation. The amount of surfactant needed to achieve complete coverage of the AlN powder has been determined and found to be at a level of 1 wt% addition. At this level, the surfactant is capable of largely suppressing hydrolysis for periods of up to 60 days at room temperature. Without the surfactant, a substantial degree of hydrolysis occurs in under 50 h. The mechanism by which the surfactant depresses hydrolysis has also been defined and is explained in terms of a protecting shell which is formed as a result of adsorption of surfactant.     

Phases in La2O3 and NiO doped (Zr,Sn)TiO4 microwave dielectric ceramics

Dielectric ceramics with composition in the ZrO₂–SnO₂–TiO₂ system containing La₂O₃ and NiO as sintering aids were prepared and investigated by XRD, XPS, SEM, EDS, and microwave dielectric measurements. Ceramics prepared with defloculed slurry using a Dyno-Mill^®, and sintered at 1370°C exhibit very good microwave dielectric characteristics: ε=37 and QF up to 62 000 at 4 GHz. We have observed a matrix phase with at least two or three different secondary phases in function of the employed grinding media (balls of zircon or magnesium stabilised zirconia). So, we have synthesised and characterised these phases.     

The influence of transient strain-rate deformation conditions on the deformed microstructure of aluminium alloy Al–1% Mg

Constant and varying strain-rate deformation conditions have been applied to an Al–1% Mg alloy using plane strain compression (PSC) testing. Transmission electron microscope (TEM) investigations have shown significant variations in internal microstructural parameters after different strain-rate histories to a strain of 1.0, even though the final flow stress was the same. Detailed investigations have been carried out in the transient regime, and after further deformation at constant strain rate. Considerable attention is given to establishing the errors associated with the experimental observations. The observations illustrate the complexity of the evolution of microstructure with strain. The relevance of the measurements to modelling the evolution of the microstructure is discussed.     

Toughness of aluminium alloy foams

The fracture behaviour of closed cell aluminium-based foams (trade-name “Alulight”) is characterized for the compositions Al–Mg1–Si0.6 and Al–Mg1–Si10 (wt%), and for a relative density in the range 0.1–0.4. The toughness testing procedures are critically analysed, and the origins of the observed R-curve behaviour for metal foams are explored. A major contribution to the observed increasing crack growth resistance with crack advance is in the development of a crack bridging zone behind the crack tip. The crack bridging response is quantified in terms of a crack traction vs extra displacement curve by performing independent tests on deep notch specimens. The area under the bridging traction vs extra displacement curve from the deep notch tests is approximately equal to the measured initiation toughness J_IC, in support of the crack bridging concept. A line spring model is then used to interpret the fracture response. The effect of material composition and relative density upon the initiation toughness is measured, and the accuracy of an existing micromechanical model for the fracture toughness of a brittle foam is assessed. Finally, the reduction in tensile and compressive strengths due to the presence of an open hole is determined; it is found that the Alulight foams are notch-insensitive, with the net section strength equal to the unnotched strength.     

常用碳基固体润滑薄膜的研究现状与展望

首先从碳基固体润滑薄膜的应用需求与成本效益出发,探讨了研究碳基固体润滑薄膜的迫切要求和重要意义,然后对类金刚石(DLC)薄膜、类富勒烯(FLC)薄膜及石墨烯薄膜三类最常用的碳基固体润滑薄膜的研究现状进行了较详细的介绍。其中,重点介绍了DLC薄膜的三种减摩抗磨机理,探讨了掺杂元素改性对DLC薄膜硬度、摩擦系数和磨损率等多个方面的影响,并指出外部因素(基体材料、过渡层和应用环境等)对DLC薄膜性能的重要作用。探讨了掺氢、掺氟和掺氮对FLC薄膜构性转变和摩擦学性能的影响。总体来说,氟掺杂导致FLC结构变化,并显著改变薄膜硬度;掺氮会诱导类富勒烯微结构的增加;掺氢FLC薄膜热处理后可达到超润滑状态。总结了石墨烯薄膜制备工艺的发展、石墨烯基复合薄膜的摩擦学性能和石墨烯薄膜在不同基体材料的应用。最后,指出了碳基润滑薄膜领域亟待解决的关键难题,并对未来的研究方向做出了预测。     

Comparison of Cu–Al–Ni–Mn–Zr shape memory alloy prepared by selective laser melting and conventional powder metallurgy

This work aimed to investigate and critically analyze the differences in microstructural features and thermal stability of Cu−11.3Al−3.2Ni−3.0Mn−0.5Zr shape memory alloy processed by selective laser melting (SLM) and conventional powder metallurgy. PM specimens were produced by sintering 106−180 µm pre-alloyed powders under an argon atmosphere at 1060 °C without secondary operations. SLM specimens were consolidated through melting 32−106 µm pre-alloyed powders on a Cu−10Sn substrate. Mechanical properties were measured through Vickers hardness testing. Differential scanning calorimetry was conducted to assess the martensitic transformation temperatures. X-ray diffraction patterns were collected to identify the metallurgical phases. Optical and scanning electron microscopy was used to analyze the microstructural features. β′₁ martensite was found, irrespective of the processing route, although coarser martensitic variants were present in PM-specimens. In conventional powder metallurgy samples, intergranular eutectoid constituents and stabilized austenite also formed at room temperature. PM-specimens showed similar average hardness values to the SLM-specimens, albeit with high standard deviation linked to the porosity. The specimens processed by SLM showed reversible martensitic transformation (T₀=171 °C). PM-processed specimens did not show shape memory effects.     

Hydrogen storage property of as-milled La7RE3Mg80Ni10 (RE = Sm,Ce) alloys

Element replacement and mechanical milling are considered as the most effective ways to improve Mg-based alloys in their hydrogen storage performance. The as-milled La₇RE₃Mg₈₀Ni₁₀ (RE = Sm, Ce) alloys were prepared in this experiment by introducing both element replacement (replacing La by Ce or Sm partially) and mechanical milling technologies. The influence made by different replacing elements on the structure and hydrogen storage property of La₇RE₃Mg₈₀Ni₁₀ (RE = Sm, Ce) alloys was investigated in detail. X-ray diffraction, transmission electron microscope, automatic Sievert apparatus, thermogravimetry and differential scanning calorimetry were used to investigate the experimental alloys. The experiment reveals that a nanocrystalline and amorphous structure appears after mechanical milling. Moreover, comparing with the RE = Sm alloy, the RE = Ce alloy has a superior hydrogen desorption property, including larger hydrogen absorption capacity, faster hydriding/dehydriding rate, lower onset hydrogen desorption temperature, and lower dehydrogenation activation energy.     

喷涂功率对NiCr-Cr3C2涂层表面自由能及其摩擦性能的影响

利用超音速等离子喷涂技术制备了不同喷涂功率条件下的NiCr-Cr3C2涂层,结合表面自由能理论研究了其表面摩擦学性能。利用Owens-Wendt几何平均法计算了涂层的表面自由能及其分量。对比发现,涂层孔隙率、显微硬度和摩擦因数均随着喷涂功率的变化而变化,且其变化趋势与极性分量均表现出一定的相似性。分析表明,受喷涂功率的影响,喷涂粒子的温度和速度对涂层表面分子间作用力产生影响,从而间接改变了涂层表面的自由能,并且自由能及其分量的改变,会直接影响NiCr-Cr3C2涂层表面的摩擦学特性。     

Sliding wear behaviour of T6 treated A356–TiB2 in-situ composites

Dry sliding wear behaviour of A356–TiB₂ composites in T6 condition was tested using a pin-on-disc wear testing machine. The composites were prepared by the reaction of a mixture of K₂TiF₆ and KBF₄ salts with molten alloy. The wear tests were conducted at normal loads of 19.6–78.4 N and a sliding speed of 1 ms^?1. A detailed SEM study of wear surface and debris was carried out to substantiate the wear results. The results indicate that wear rate of the composites is a strong function of TiB₂ content rather than overall hardness of the composite. The role of Si and TiB₂ particles towards the overall mechanism has been discussed.