Effects of ZrO_2 Nanoparticles on the Microstructure and Thermal-protective Properties of PEO Coating on Al-12.5%Si Alloy

PEO ceramic coatings including ZrO_2-Al_2O_3-SiO_2 in three phases were prepared on an Al-12.5%Si alloy in electrolyte solutions containing ZrO_2 nanoparticles. The microstructures and phases of the coatings were analyzed by SEM and XRD, and the heat insulation performance and the thermal shock resistance of the coatings were investigated. The compactness of the coating increased significantly and the hindrance of the Si element on plasma electrolytic oxidation process was effectively weakened. The growth rate of the coating was improved substantially with the addition of ZrO_2 nanoparticles. The PEO ceramic coatings are primarily composed of SiO_2 and high temperature steady phases such as a-Al_2O_3 and c-ZrO_2. Both the content of c-ZrO_2 and the heat-insulating property of the coating increased significantly. The ceramic coatings with special microstructure and composition formed in the solutions containing ZrO_2 nanoparticles possess excellent heat insulation performance and thermal shock resistance.

MICROSTRUCTURE AND MACROSEGREGATION OF Al-7%Si ALLOY SOLIDIFIED UNDER COMPLEX EFFECTS OF ELECTROMAGNETIC AND CENTRIFUGAL FORCES

Macrosegregations and microstructures of Al-7%Si alloy solidified under complex of fects of magnetic field and centrifugal forces are studied by means of a set of selfdesigned electromagnetic centrifugal casting (EMCC) device. It is shown that electromagnetic field (EMF) has an important effect on the macrosegregation of centrifugal casting specimen of Al-7%Si alloy in two respects: one is that there exists always a kind of convection in the liquid in front of the S/L interface caused by effect ofthe electromagnetic force; the other is that different atomic clusters of solidparticles with different physical characteristics are subjected to quite different electromagnetic (Lorentz) force. Therefore, their movements get changed. In addition, the formation process of a complex band structure consisting of primary α-Al dendrites and (α-Al+β-Si) eutectics in hypoeutectic Al-Si alloys during EMCC and the effect of EMF are discussed.

Structural evolution of spray-formed Al-70wt.%Si alloys during iso-thermal holding in the semi-solid state

The grain growth behavior of spray-formed Al-70wt.%Si alloys was studied in the semi-solid state. The specimens were isothermally heat-treated at various temperatures between the solidus and liquidus of Al-Si alloys and then quenched in water. The microstructure of reheated specimens was characterized using optical and scanning electron microscopies. The isothermal holding experiment was carried out to investigate grain growth behavior as a function of holding time and temperature in the semi-solid state. The coarsening mechanism and the effect of porosity on microstructure were also studied.

STRUCTURAL TRANSFORMATION AND ITS REVERSIBILITY IN LIQUID Al-13%Si ALLOY AT HIGH TEMPERATURES

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Modification and aging precipitation behavior of hypereutectic AI-21wt.%Si alloy treated by P+Ce combination

In the present study, the tested hypereutectic Al-21 wt.%Si alloys were prepared by modifying the melt using different proportions of P and Ce, and then applying T6 heat treatment. The modif ication effects and mechanism of P+Ce complex modif ier on the Si phase of hypereutectic Al-21 wt.%Si alloy were studied, and the aging precipitation behavior after modif ication was characterized by means of tensile strength measurement, OM, SEM and TEM analysis. The results show that the massive primary silicon phase particles are signif icantly ref ined after modif ication, while the needle-like eutectic silicon crystals become f ibrous and short. It was found that the mechanism of phosphorus modifi cation on the primary silicon can be attributed to heterogeneous nucleation of Al P, while the modification mechanism of Ce can be explained by adsorbing-twinning theory. In the aged microstructure of the modif ied hypereutectic Al-21 wt.%Si alloy, there existed some strengthening phases such as Al4Cu9, Al2 Cu, AlC u3, and Al57Mn12. The P+Ce complex modif ier not only affected the size of primary silicon and eutectic silicon, but also the aging behavior of alloys under the heat treatment process. When Al-21 wt.%Si alloy was modif ied using 0.08%wt.P + 0.6wt.% Ce, the aging precipitates were dispersed uniformly in the alloy, and its mechanical properties at room and elevated temperatures are optimized(Rm = 287.6 MPa at RT, Rm = 210 MPa at 300 ℃).

ZrO_2纳米颗粒对Al-12.5%Si合金PEO陶瓷层微观形貌及热防护性能的影响

采用外加ZrO_2纳米颗粒的电解液体系在Al-12.5%Si合金表面制备ZrO_2-Al_2O_3-SiO_2三相PEO陶瓷层。利用SEM和XRD对陶瓷层微观结构和物相进行分析,并对其隔热及热冲击性能进行测试。结果表明:ZrO_2纳米颗粒显著提高了膜层的致密度和结合性,并有效减弱了Si元素对PEO成膜的抑制作用,提高了成膜速率;三相PEO陶瓷层的主要物相为SiO_2及高温稳定相α-Al_2O_3和c-ZrO_2,其独特的微结构和成分致使ZrO_2纳米颗粒改性的陶瓷层具有良好的热防护性能和热冲击性能。

Al-12.5%Si合金表面ZrO2溶胶改性PEO陶瓷层的形成机制

采用ZrO2溶胶改性的等离子体电解氧化(PEO)技术在Al-12.5%Si合金表面制备Al2O3-ZrO2陶瓷层,研究ZrO2溶胶对PEO陶瓷层形成机制的影响。采用扫描电镜(SEM)、能谱仪(EDS)、X射线衍射仪(XRD)、红外光谱仪(FTIR)和激光粒度仪等研究了ZrO2溶胶的微观组织结构、粒径及荷电性以及陶瓷层的微观形貌、成分及物相。结果表明:荷负电的ZrO2溶胶粒径为50~60 nm,烧结后以四方相t-ZrO2存在,含有少量m-ZrO2。ZrO2溶胶改性的PEO涂层较未改性的涂层均匀、致密,并且生长速率提高。由于ZrO2溶胶吸附在基体表面形成凝胶层,凝胶层的高电阻使PEO初期更易产生火花放电,促进了Al2O3膜的形成。ZrO2溶胶改性的PEO涂层由Al2O3、c-ZrO2、t-ZrO2和少量的SiO2组成,而溶胶改性前SiO2的含量较高。ZrO2凝胶颗粒在等离子体放电产生的高温下进入放电通道,与电化学反应形成的Al2O3烧结形成Al2O3-ZrO2复合陶瓷层。