【作者】 周衡志；

【导师】 李子全；

【作者基本信息】 南京航空航天大学 ， 材料加工工程， 2007， 博士

【摘要】 作为钢铁材料保护镀层的纳米CeO2/Zn-4.5%Al-RE-Mg-Ti(ZA)复合材料(ZACs),具有优良的耐蚀性及良好的可镀性等优点。本文首先介绍了颗粒增强金属基纳米复合材料(PRMMNCs)的研究方法及现状,并设计了一套高能超声搅拌制备装置,然后用SEM、EDS、TEM、XRD、DSC、TG、AES、XPS和EIS等多种现代分析和测试手段,着重研究了纳米CeO2的表面改性工艺、CeO2/ZA体系的润湿特性,进而较系统地研究了纳米CeO2p/ZA复合材料的机械搅拌与高能超声搅拌的复合制备工艺、凝固特征、力学性能和耐蚀性能及机理,最后探讨了复合材料在热镀方面的应用。试验结果表明所设计的高能超声搅拌装置具有防氧化、控温准确、超声搅拌效果佳的特点,能够制备MMNCs。计算结果表明Ti制带过渡段阶梯形超声变幅杆在熔体中的振幅为53.41μm,声强达到了1.30×106 W/m2,声压幅值为7.58 MPa。TEM和AES分析结果表明,超声表面活性剂包覆改性在纳米CeO2表面形成了一层厚度约为20 nm的物理包覆层,显著提高了微粒的分散性;TG分析结果表明CeO2表面的包覆层在ZACs制备过程中能够完全炭化,热力学分析证实该炭化层能与包裹在CeO2表面的氧化膜(主要成分为ZnO)反应,消除氧化膜对CeO2/ZA体系润湿的不利影响,提高体系的润湿性。两相润湿过程力学和动力学判据的讨论结果表明,在高能超声作用下纳米CeO2粒子很难直接进入到ZA熔体中。因而采用卷入能力强、分散能力差的机械搅拌法制备复合材料的初步混合体,然后再用高能超声对卷入的CeO2颗粒团进行分散,从而得到名义质量分数1~6%的ZACs。FE-SEM和TEM观察的结果表明,用该法制备的ZACs中纳米分散较均匀。OM和XRD结果表明,ZACs的室温金相组织主要包括呈小岛状分布的α(Al)+β(Zn)共析组织和呈层片状分布的α+β组织。TEM的研究结果表明,纳米CeO2与基体界面清晰、光滑,无反应产物;纳米CeO2分布在初生和共晶β相中,且与基体无明显位向关系,复合材料中的CeO2颗粒是在初生相长大过程中被捕获的。力学性能测试结果表明,适量纳米CeO2的加入改善了ZACs的力学性能。与基体合金相比,的抗拉强度和弹性模量明显提高,延伸率降低。但当纳米CeO2质量分数大于3%时,使ZACs的抗拉强度比3%时略有下降。ZACs的断裂机制为脆性断裂。腐蚀试验研究结果表明,ZACs的耐蚀性能明显优于基体合金的,主要是由于加入的纳米CeO2通过提供氧空位和变价作用来耗氧,阻碍了腐蚀过程的进行,从而提高复合材料的耐蚀性。热镀工艺的初步探讨结果表明,在本研究的热镀工艺条件下,纳米CeO2含量为2%的ZACs的热镀质量好,表面平整,没有漏镀、微裂、鼓包等表面缺陷,镀层厚度约为30μm,而且与基体的结合力较好。更多还原

【Abstract】 The steel protective covering materials nano-CeO2p/Zn-4.5%Al-RE-Mg-Ti (ZA) nanocomposites (ZACs), whose fabrication process has become one of the hot spots in metal matrix composites research, feature with good corrosion resistance, adhesion and excellent processing performance. In present study, the fabrication techniques of nano-particles reinforced metal matrix composites (PRMMNCs) were firstly introduced and compared. The high-intensity ultrasonic stirring technique, which could refine the molten alloy, disperse reinforcement particulates and enhance the wettability of the particulate/molten alloy couple, was selected to fabricate nano-CeO2 particulates reinforced Zn-based composites. Then, a suit of high-intensity ultrasonic stirring device was assembled. Moreover, the surface modification to nano-CeO2 particulates, wetting processes between particles and molten ZA alloy, mechanical-high intensity ultrasonic combination stirring processes, solidification characteristics, mechanical properties, as well as corrosion resistance and corrosion mechanism of the ZACs were systematically investigated by the help of SEM, EDS, TEM, XRD, DSC, TG, AES, XPS and EIS. Finally, the application of the ZACs in hot dipping is discussed.The tests show that the designed ultrasonic stirring device, which has the functions of antioxidation for molten metal and accurate temperature control, as well as a good stirring effect, can be used to fabricate MMNCs. The calculation results also exhibite that the vibration amplitude, ultrasonic intensity and pressure amplitude of the titanium stepped ultrasonic transformer with a transition section in molten are 53.41μm, 1.30×106 W/m2, 7.58 MPa, respectively.The TEM, FTIR and AES investigations show that nano-CeO2 particles are covered by a physical surface covering with its thickness about 20 nm by the help of ultrasonic agitation, and the distribution of the nano-particles is obviously improved. Besides, the TG result indicates that the carbonization of this covering layer can take place in the fabrication of ZACs. From classical thermodynamic point of view, reactions between the carbonization layer and zinc oxide may carry out at the interface, which promotes the wetting via getting rid of the block of the zinc oxide.According to the mechanics and dynamics discussion of the system’s wetting process, it is very difficult for nano-CeO2 particulates to be directly dispersed into ZA melt by high-intensity ultrasonic. Thus, nano-CeO2 particulates are mechanically engulfed into ZA by machine stirring process, and subsequently dispersed by high-intensity ultrasonic stirring technique. By doing this, the ZACs with the nominal mass fractions of 1, 2, 3, 4, 5 and 6%, were obtained. Based on the FE-SEM and TEM observations, nano-CeO2 particles could be homogeneously distributed in the ZACs.The optical microscope and SEM observations clarify that the microstructure of ZACs at room temperature consists of island-like eutectoid structure ofα(Al)+β(Zn) and lamellar structure ofα+β. The investigation of the solidification process of the ZACs indicates that no reaction production exists on the clear and smooth interface between CeO2 and the matrix. Furthermore, nano-CeO2 are dispered in primary and eutecticβ(Zn), but no ctystallographic orientation relations are found amongβ(Zn),α(Al) and CeO2. Thereby nano-CeO2 particulates in ZACs are arrested by growingβgrains.An optimized amount of nano-CeO2 addition could improve the mechanical properties of ZA matrix. Compared with the matrix alloy, the tensile strength and elastic modulus of the composites increase obviously with the increment of mass fraction of nano-CeO2 particles, while the elongation decreases. However, the increased large mass fraction deteriorates the distribution of CeO2 particles in matrix alloy, which leads to the decrease of mechanical properties of ZACs. The tensile fracture also shows that the damage mechanism of the composites vary into a brittle fracture pattern.Furthermore, the corrosion tests results display that the ZACs exhibits better corrosion resistance than the ZA alloy. The mainly reason is that the oxygen, which is crucial for the corrosion process of ZACs, is consumed by nano-CeO2 particulates via the storage and release capacity (OSRC) and redox properties.And by doing so, the ZACs’s corrosion resistance is improved.Finally, primary exploring works on of the ZAC layer with its mass fraction 2% reveal that hot dip quality is qualified in the condition of this study. The hot dip coating, which exhibits good adhesion with steel matrix, is about 30μm and almost has little surface defects, such as pretermission of plating, micro-cracks, and bubbles.更多还原