【作者】 何美凤；

【导师】 胡文彬； 刘磊；

【作者基本信息】 上海交通大学 ， 材料学， 2009， 博士

【摘要】 镁合金具有比重小、绿色环保等优点,日益成为汽车、航空航天以及电子消费品等领域的重要材料。但是,镁合金的耐腐蚀性能较差,这一直是阻碍其进一步应用的主要因素,也是镁合金研究开发中存在的一个难题。因此,耐腐蚀与防护技术是镁合金应用基础研究的重要研究领域,开展该方面的基础研究工作具有重要的经济和社会意义。本文开展了镁合金表面熔盐置换扩渗铝涂层的制备及相关的耐腐蚀性能研究。通过把镁合金基体置于熔融的等摩尔AlCl3和NaCl的混和盐中,进行置换反应并且保温扩散,在镁合金表面形成扩渗铝涂层。其核心是在低于传统的固体扩渗铝的温度条件下利用镁合金自身高活性的特点,在低温熔盐中置换出活性铝原子及形成新的合金相,随着扩散过程的推进,最终形成合金相层状分布的铝涂层,提高镁合金基体的耐腐蚀性能。主要研究结论如下:（1）当扩渗温度较低或者扩渗时间较短时,镁合金表面只形成了单一相γ相(Al₁₂Mg₁₇)层,并伴随着δ（Mg）固溶体的析出;当扩渗温度较高或者扩渗时间较长时,AZ91D镁合金表面从内到外依次形成了过渡层δ（Mg）固溶体层,灰色渗层γ相(Al₁₂Mg₁₇)层,线状结构与亮灰色层ε相(Al_0.58Mg_0.42)层,白亮层β相（Al₃Mg₂）层,以及铝涂层的最外侧零星分布的亮层α（Al）的固溶体。扩渗温度、扩渗时间以及基体中的初始原子浓度是影响扩渗层形成的主要因素。（2）对镁合金表面熔盐置换扩渗制备铝涂层进行了热力学分析,其中包含了置换反应的热力学分析、扩散过程的热力学分析和合金相形成的热力学分析。镁合金与熔盐能否发生置换反应产生活性原子可分为二个阶段。第一阶段:镁合金与熔盐的初始接触阶段;第二阶段:镁合金表面形成了一定厚度的扩渗层阶段。上述两个阶段的热力学分析,说明在熔盐环境下镁合金表面置换反应的自发性和Al元素扩散的持续性。通过对扩散过程进行热力学分析,可以得出只要在镁合金表面置换反应在持续进行,镁合金表面与镁合金基体内部就存在Al元素的浓度梯度,那么就存在Al元素的扩散驱动力。对合金相形成过程的进行热力学分析,可以得到在115.86 <T <450°C条件下,只要有铝元素的持续提供,Al₁₂Mg₁₇的合金相就能够生成;在T <415.13°C条件下,Al_0.58Mg_0.42相能够形成;在228 .22< T <450°C条件下,Al₃Mg₂相能够形成。（3）建立了镁合金/熔盐界面置换扩渗的物理模型。该模型的核心为通过置换反应提供活性的Al原子以及新的合金相析出,扩散过程逐渐推进,在镁合金表面逐步形成了δ（Mg）层→Al₁₂Mg₁₇（γ）层→Al_0.58Mg_0.42（β）层→Al3Mg2（? ）层。其中,扩散过程占主导地位,置换反应提供活性原子始终伴随着扩散过程同时进行。（4）对镁合金表面熔盐置换扩渗制备铝涂层进行了动力学分析,通过对扩散过程进行动力学分析得到了镁合金表面熔盐置换扩散过程中不同扩渗温度和不同扩渗时间下Al在Mg中的表观扩散系数和Mg在Al中的表观扩散系数。其Mg在Al中的表观扩散系数比纯镁扩散纯铝过程Mg在Al中的扩散系数有显著提高,而Al在Mg中的表观扩散系数比纯镁扩散纯铝过程Al在Mg中的扩散系数大3⁴个数量级。拟合得到了扩渗层的生长活化能为51666 J·mol^-1;推导了每个相厚度的生长模型,验证了置换扩散过程中,其一相的生成是依靠消耗另一相与活性原子的扩散发生相变得到的,从而导致相界面的移动;建立了镁合金/熔盐界面置换扩渗的数学模型。（5）当熔盐置换扩散渗铝试样没有出现大的结构缺陷时,呈连续态的?相可完全隔离镁合金基体与腐蚀介质的接触,对电子的传输构成屏障,使电荷转移极化电阻增加,提高试样的耐蚀性;而当置换扩散渗铝层中出现了脆性β相,导致裂纹等结构缺陷时,由于?相的电极电位低于β相,因此,在腐蚀溶液中就会发生电偶腐蚀,γ相作为有效的微电偶阴极而加速置换扩散渗铝层的腐蚀导致耐腐蚀性能降低。本文的主要创新点如下:（1）尝试采用熔盐置换扩渗工艺方法,在镁合金表面成功制备了成分呈梯度分布和相结构层状分布的富铝涂层,扩渗温度明显低于传统的镁合金表面固体扩渗铝工艺,其耐腐蚀性能得到了显著提高,有望为镁合金表面防护提供一种新的改性方法。（2）通过熔盐置换扩渗热力学和动力学研究和理论分析建模,建立了镁合金熔盐置换扩渗过程机理模型,并对涂层的组织结构和相关材料学基础进行了系统研究分析,为涂层成分、结构的设计,工艺优化奠定了基础。更多还原

【Abstract】 Owing to their low density and be friendly to the environments, magnesium alloys have been increasingly used in the automobile industry, aerospace components communications. Unfortunately, their poor corrosion resistance not only prevents their further application in many fields, but also is a difficult problem on research and development of magnesium alloys. Therefore, it is very important region to improve the corrosion resistance of magnesium alloys. It also has important economic and social significance to investigate the corresponding basic research.This paper carried out a study on the magnesium alloy by molten salt surface treatment and corrosion resistance of the Mg-Al intermetallic coating. A new process for surface modification of magnesium alloy substrate called for dipping the alloy into molten bath of equal molar mixture of AlCl₃ and NaCl salt, and obtaining the diffusion aluminized coating. Using the characteristics of its high activity of magnesium alloy, active aluminum atoms and new phase were obtained at low molten salt temperatures. These temperatures are lower than the traditional solid diffusion temperatures. With the diffusion process progresses, the phase layered distribution of the Mg-Al intermetallic coating is observed. This coating improved the corrosion resistance of magnesium alloy. The main conclusions are shown as follows:（1） When the diffusion temperature is lower or the diffusion time is shorter, only the formation of a single-phase (Al₁₂Mg₁₇) layer, and accompanied byδ（Mg） solid solution precipitation on the magnesium alloy surface; when the temperature is higher or diffusion time is longer, from magnesium substrate to the surface the diffusion alloying layer was mainly composed of Mg-Al intermetallic compounds: a transitionδ（Mg） solid solution layer, gray phase (Al₁₂Mg₁₇) layer, linear structure and light gray phase (Al_0.58Mg_0.42) layer, white phase （Al3Mg2） layer and the scattered light distribution ofα（Al） solid solution at the outermost layer of intermetallic coating. Diffusion temperature, diffusion time and the initial atoms concentration in substrate were the main factors, which effected the formation of Mg-Al intermetallic coating. （2） Thermodynamic analysis on the formation of Mg-Al intermetallic coating of molten salt bath treatment is discussed, which includes thermodynamic analysis on the process of replacement reaction、diffusion and alloy phase formation. The replacement reaction between magnesium alloy and molten salt can produce active atoms, which was divided into two stages. First phase: the initial contact of magnesium alloy and molten salt; Second phase: the formation of a certain thickness Mg-Al intermetallic coating on magnesium alloy surface. The two-stage thermodynamic analysis shows that the displacement reactions spontaneously occur and Al element continually diffuse in the molten salt bath on the magnesium alloy. Thermodynamic analysis on diffusion process shows that Al element has the driving force of diffusion if Al element exist the concentration gradient. Thermodynamic analysis on phase formation shows that Al₁₂Mg₁₇ phase can be obtained at 115.86 < T <450°C, as long as there continue to provide the aluminum elements; At T <415.13°C condition, Al_0.58Mg_0.42 phase can be obtained; At 228 .22< T <450°C conditions, Al3Mg2 phase can be obtained.（3） The physical model of replacement and diffusion at magnesium alloy/molten salt interface has been established. The core of model is using the replacement reaction product active aluminum atoms and the new phase. With the diffusion process gradually advancing, from magnesium substrate to the surface the diffusion alloying layer was mainly composed of Mg-Al intermetallic coating:δ（Mg） layer→γ(Al₁₂Mg₁₇) layer→ε(Al_0.58Mg_0.42) layer→β（Al3Mg2） layer→α（Al） layer. Among them, the diffusion process is dominant. The displacement reactions which provide active atomics always accompanied the diffusion process to occur at the same time.（4） Dynamic analysis on the formation of Mg-Al intermetallic coating of molten salt bath treatment is discussed. At different temperatures and diffusion time, the apparent diffusion coefficients of Mg diffusing in Al and Al diffusing in Mg were obtained. The apparent diffusion coefficient of Mg diffusing in Al with molten salt treatment significantly increased than that of pure magnesium solid diffusion Al powder, while the apparent diffusion coefficient of Al diffusing in Mg with molten salt treatment is lager 3 4 orders of magnitude than that of pure magnesium solid diffusion Al powder. The activation energy for the growth of layer has been fitted that the value was 51666 J·mol^-1; Deducing the growth model of each phase. The growth model of each phase had been verified that one phase generation is consumed another phase, leading to the phase boundary movement. The mathematical model of replacement and diffusion at magnesium alloy / molten salt interface has been established.（5） When the sample of molten salt treatment without a major structural defects, the continuousγphase can be completely isolated from magnesium alloy substrate and the corrosive media. The coating was an effective barrier to hinder the transmission of electronic, so that the charge transfer polarization resistance increased and improved the corrosion resistance. The sample of molten salt treatment appeared brittleβphase, which would lead to cracks and other structural defects on the surface of magnesium alloy. The electrode potential ofγphase is lower than that ofβphase, the surface of sample would occur galvanic corrosion in the corrosion solution. Theγphase as an effective micro-galvanic cathode would accelerate the corrosion rate and decrease the corrosion resistance of the sample.The main innovations are shown as follows:（1） The distribution of gradient compositions and layered phases in Mg-Al intermetallic coating on magnesium alloy with molten salt surface bath treatment can be obtained at low temperatures. These temperatures are lower than the traditional solid diffusion temperature. The layered phases distribution of Mg-Al intermetallic coating can remarkably improve the corrosion resistance of magnesium alloy. Molten salt treatment is a new surface modification on magnesium alloy for the surface protection.（2） Thermodynamic and dynamic analysis and on the formation of Mg-Al intermetallic coating with molten salt bath treatment is discussed. The physical model of replacement reaction and diffusion at magnesium alloy/molten salt interface has been established. The research on the structure of Mg-Al intermetallic coating and the corresponding basic of materials science had been carried out, which can lay the foundation for structure design and process optimizing.更多还原