【作者】 詹肇麟；

【导师】 何业东； 高唯；

【作者基本信息】 北京科技大学 ， 材料学， 2005， 博士

【摘要】 本文研究了利用介质球的冲击效应,在金属材料表面制备纳米结构铝化物涂层、纳米结构ODS涂层和在钢管内壁制备铝覆层的规律和涂层的形成机理,评价了涂层的高温氧化性能。采用自行设计的振动渗铝实验装置,研究了在传统的粉末包埋渗铝过程中增加介质球的冲击作用,制备纳米结构铝化物涂层。在440～600℃处理0.25～4小时,可以分别在低碳钢、Cr5Mo合金、不锈钢和镍基高温合金表面制备出纳米结构铝化物涂层。这种涂层结构简单、均匀致密、没有粗晶组织和粗大的第二相沉积物,呈现纳米结构。涂层由富铝相的化合物组成,元素呈均匀的分布状态。涂层的厚度随处理温度的提高而增加,呈线性关系; 随处理时间的延长而增加,呈抛物线关系。本研究结果使铝化物涂层形成的温度大幅度降低,时间显著缩短,为在低温条件下制备扩散涂层和纳米涂层提供了一条新的途径。采用SEM、TEM、AFM、XRD、EDS以及Pt标记实验等,研究了介质球的冲击作用下纳米铝化物涂层的形成机理。研究结果表明,介质球的冲击对低温形成纳米结构铝化物涂层起主导作用,涂层的形成机制与传统的粉末包埋渗铝不同,是表面细化、纳米化过程和原子扩散过程共同作用的结果。涂层的形成过程包括四个步骤: (1)表面形成铝附着层:在介质球冲击作用下,吸附于基体表面和介质球表面细小的铝粉颗粒,发生塑性变形,产生钉扎、冷焊等效应,在基体表面形成铝附着层。(2)表面层组织细化与合金层的形成:介质球的冲击,使基体表面层产生塑性变形,通过晶粒内位错反应、滑移带的交互作用和晶粒的解理断裂形成解理细晶组织等过程,使表面层的组织细化,形成了大量的晶界和晶体缺陷,为原子扩散提供了快速通道,降低了扩散激活能,使原子在相对较低的温度下扩散成为可能。同时,介质球的冲击在撞击点产生严重的塑性变形,导致局部的温度升高,提高了原子的扩散能力。因此,在介质球的冲击作用下,铝附着层中的铝原子和基体中的合金原子沿大量的晶界发生扩散,富铝相金属间化合物形核、生长和转变,逐步形成合金层。(3)合金层组织细化、均匀化、致密化:富铝相的形核降低了表面能,在相对较低的温度下,晶粒的长大受到了抑制。同时,介质球的冲击,打碎合金层中趋于长大的富铝相化合物,使合金层组织细化,形成纳米结构的合金层。另外,介质球的冲击对表层产生压应力的作用,消除了原子扩散形成的孔洞,使合金层组织致密、成分均匀。更多还原

【Abstract】 A novel technique has been developed to produce Al-rich intermetallic compound coatings on metals. This technique applied mechanical vibration to a retort, which was loaded with Al powder, alumina filler, ammonium chloride activator and FeCrAl alloy balls. The operation temperature was below 600oC for 0.25～4h treatment. The Al-rich intermetallic compound coatings have been successfully produced on the carbon steel, Cr5Mo, stainless steel and Ni-base superalloy. The coatings appear to be homogeneous, with a high density and free of porosity, and have excellent adherence to the substrate. The coatings mainly consist of Al rich phases and greatly enhance the high-temperature resistance of metals. Microstructure studies suggest that the formation of the intermetallic phases at the relative low temperature has a complex mechanism. The growth process of the Al-rich intermetallic coatings consists of four steps: 1. Ball impact causes Al particles adhering onto the substrate. An Al-adhered layer is formed on the surface of the metals. 2. Severe plastic deformation induces refinement of top layer and local high temperature, which promote the atomic diffusion and result in the formation of the initial alloy layer. 3. Subsequent ball impact refines the Al-rich phases in the initial alloy layer, and causes a new Al adhered layer formed on the surface of metals. 4. Diffusion of Al and alloy elements takes place between the newly formed Al layer and the initial alloy layer. The inward diffusion of Al is hindered by the coarse-grains of the substrate, while the outward diffusion of alloy elements is enhanced along the large amount of grain boundaries and defects in the alloy layer. The intermetallic coatings grow outward by the repeated ball impact. With this mechanism, Al-rich intermetallic coatings are formed at a much lower temperature in a much shorter time, compared with the conventional aluminizing process. This concept provides a new approach that can be used in other diffusion coating processes. We apply this technique to produce nanocrystallite ODS coatings. Pure Al powder was mixed with 1wt% Y2O3 powder by ball milling. The ultrafine Y2O3 powder was well dispersed in the Al particles. The modified Al particles were welded onto the surface of metals by ball impact, causing the refinement of coarse grains and acceleration of atomic diffusion. Nanocrystallite ODS alloy layer with Y2O3 grew outward at a much low temperature and in a short treatment duration. We also apply this technique to clad inner surface of tubes with Al foils at room temperature. Al foils were welded onto the inner surface of steel tubes by ball attrition in planetary rotation, followed by annealing at 660°C for 20 min. Studies on the interface microstructure and bonding indicate that the bonding strength is a function of the rotation speed, diameter of alloy balls and treatment time. A large amount of plastic deformation increases the bonding degree but may break Al foils and separate them from the inner surface. Subsequent annealing allows diffusion of Al into the substrate, transfers the Al foil into multi-layers of intermetallic compound and alloy phases. This technique provides an easy and fast way to improve the properties of inner surface of steel tubes. The nanocrystallite Al-coatings and ODS coatings greatly improve the high temperature corrosion properties. In high temperature environment, Al2O3 scale is formed on the surface of the coatings with spinel of FeAl2O4、NiAl2O4, and Cr2O3、TiO2. Ball peening causes stress and cracks in the coatings, and results in internal oxide and intrusive oxide in high temperature oxidation. Dispersed Y2O3 in the coatings reduces the growth rate of oxide scale, and enhances the resistance to spallation.更多还原