【作者】 罗群；

【导师】 周国治；

【作者基本信息】 上海大学 ， 钢铁冶金， 2013， 博士

【摘要】 全世界每年因钢铁腐蚀造成的直接经济损失高达数千亿美元，通过合金化镀层保护钢铁构件，延长其使用寿命，一直倍受国内外关注。55wt.%Al-Zn-1.6Si合金镀层是一种应用广泛的钢铁防护涂层，但合金元素Ti的添加对镀层的组织和高温氧化性影响的作用机理目前尚不清楚。有人认为添加Ti可促使Fe₄Al₁₃析出，成为形核核心，利于组织细化；而另一部分研究者认为，Ti与Al形成TiAl3，成为（Al）的形核核心，利于组织细化。这两者的分歧在于形成什么中间合金。该分歧可借助Al-Zn-Si-Ti-Fe体系相图数据库来澄清。另一方面，对于镀层板的氧化动力学，目前大多数的实验反复测定恒温条件下的反应分数与时间的关系，很少研究升温速率、样品形状及尺寸、氧化物体积变化等因素。故本文围绕55wt.%Al-Zn-Si-Ti镀层体系的相图热力学和氧化动力学开展研究。本论文通过CALPHAD方法优化和完善Al-Zn-Si-Ti-Fe体系富Al角的热力学数据库；计算不同Ti和Si含量的等温截面相图，结合实验分析微量Ti对镀层中金属间化合物种类和形成量的作用；通过X-ray衍射分析（XRD）、扫描电镜（SEM）和热天平（TG）等分析技术对比研究55wt.%Al-Zn-Si和55wt.%Al-Zn-Si-Ti镀层板的恒温氧化和变温氧化动力学行为，得到镀层板氧化的组织演变规律；建立具有实际物理意义的动力学方程，定量探讨氧化物与金属体积比，反应温度、氧分压、升温速率和样品形状及尺寸等对反应速率的影响。本论文得到的主要结果如下：1)通过HR-XRD和HR-TEM，结合结构精修的方法解析Al-Zn-Ti体系中τ-Ti₂₆Al₅₅Zn₁₉的晶体结构，为有序的面心立方结构，其中Ti占据1a（0,0,0）位置，Al/Zn共同占据3c（0,0.5,0.5）位置。采用扩散偶和平衡合金的实验方法，获得Al-Zn-Ti体系富Al-Zn侧、Al-Zn-Fe体系富Al-Fe侧和Al-Si-Ti体系富Al角在450⁷00°C的等温截面相图。2)根据实验相图的结果，通过CALPHAD方法优化Al-Zn-Ti、Al-Zn-Fe和Al-Si-Ti的热力学相图，结合已有的Al-Zn-Si-Ti-Fe体系其它子体系的热力学数据，外推得到可靠的Al-Zn-Si-Ti-Fe体系富Al角的热力学相图，据此阐明Ti添加细化组织的机理：一方面Ti添加形成的TiAl3，降低镀液中有效Si的含量，降低镀液中Fe的饱和溶解度，增加Fe₄Al₁₃的析出量；另一方面，TiAl₃和Fe₄Al₁₃都可以作为形核核心，增加镀液中形核密度，细化晶粒。3)通过XRD和SEM对55wt.%Al-Zn-Si和55wt.%Al-Zn-Si-Ti镀层板在550°C、650°C、700°C和750°C恒温氧化过程进行分析，得到镀层板在恒温氧化过程中组织演变规律。利用TG系统地研究了温度、气体分压、升温速率和粉末粒度对镀层板、纯Zn、纯Ni氧化反应速率的影响。4)引入金属体积与氧化物体积比参数，提出球形和平板形金属气-固相反应以扩散为控速环节时的动力学模型，定量地表达氧化物体积变化、温度、气体分压、升温速率、粉末粒度对反应速率的影响。利用模型分析镀层板在550⁷50°C的恒温氧化、350⁹50°C的变温氧化、纯Zn和纯Ni粉末的恒温氧化动力学，得到满意的结果。综上所述，本论文关于热浸镀镀层体系的材料设计和氧化动力学研究结果对钢铁热浸镀有重要的科学价值和应用价值。Al-Zn-Si-Ti-Fe体系数据库不仅可以应用在热浸镀层的设计，还可以用于其它铝合金的的材料设计。提出的金属气-固相反应模型，由于引入了氧化物与金属体积比参数，并采用一次回归的方法，使计算结果更为准确。该模型可以推广到冶金和材料中其它的气-固相反应动力学研究中。更多还原

【Abstract】 Every year, the direct economic loss caused by steel corrosion is up to severalhundred billion dollars around the world. How to extend the service life of steelthough alloy coating has been much concerned in domestic and international. The55wt.%Al-Zn-1.6Si alloy coating is a widely used steel protective coating. However,the mechanism that the addition of alloy element Ti causes the changes inmicrostructure and oxidation kinetics is still unclear. Some people thought that theaddition of Ti lead to the precipitation of Fe₄Al₁₃which acts as nucleation sites torefine the microstructure. But other researchers thought that Ti would combine withAl to form TiAl3which acts as nucleation sites. The problem is which intermetalliccompounds would form. This can be clarified by Al-Zn-Si-Ti-Fe thermodynamicdatabase. For the oxidation kinetics of coated steels, most of experimental worksdetermined repeatedly the relationship between reaction fraction and time inisothermal conditions recently. Few researches investigated the influence of heatingrate, sample shape and size and the volume change of scale. Therefore, this thesis willstudy the thermodynamic phase diagram and oxidation kinetics of the55wt.%Al-Zn-Si-Ti coating system.The Al-rich corner of the Al-Zn-Si-Ti-Fe system was optimized and improvedthrough CALPHAD method. The precipitated phases and precipitated quantities wereinvestigated through calculating the isothermal sections at different Ti and Si content.The effects of the Ti addition on the intermetallics in the alloy layer and themicrostructure of the55wt.%Al-Zn-Si coating were investigated by experiments. TheX-ray diffraction （XRD）, scanning electron microscopy （SEM） and thermogravimetry（TG） were used to investigate the isothermal oxidation and non-isothermal oxidationof55wt.%Al-Zn-Si and55wt.%Al-Zn-Si-Ti coatings. The evolution of the coatingmicrostructure during isothermal oxidation process was analyzed based on theexperimental results. A kinetic model with physical meaning was proposed, discussing quantitively the effect of oxidation induced volume change, temperature, oxygenpressure, heating rate, sample shape and size.The main results in this thesis can be summarized as following:1) The crystal structure of the ternary intermetallic τ-Ti26Al55Zn19was determinedthrough HR-XRD and HR-TEM, which showed that τ is an ordered face centeredcubic with Ti atoms occupying1a（0,0,0） positions and Al/Zn atoms occupying3c（0,0.5,0.5） positions. The Al-Zn riched side of Al-Zn-Ti system, Al-Fe riched sideof Al-Zn-Fe system and Al riched corner of Al-Si-Ti system at the temperature rangeof450⁷00oC were obtained by diffusion couple method and equilibrium alloys.2) Based on the experimental data, the Al-Zn-Ti, Al-Zn-Fe and Al-Si-Ti systemwere optimized through CALPHAD method and Pandat software. Then the reliable Alriched corner of Al-Zn-Si-Ti-Fe system was assessed. The calculated phase diagramsillustrated the effect of Ti addition on the55wt.%Al-Zn-Si coating: on the one hand,The TiAl₃forming in the bath makes the Si content decrease in the bath, leading to thesolubility of Fe in bath decrease and the amount of precipitated Fe4Al3increase. Onthe other hand, both the Fe4Al3and TiAl3can be the nucleation sites of （Al）, leadingto the nucleation density increase and refining the grains.3) The isothermal oxidation of55wt.%Al-Zn-Si and55wt.%Al-Zn-Si-Ti coatedsteels at550,650,700and750oC were investigated by XRD and SEM. The evolutionof the coating microstructure during isothermal oxidation process was observed. Theeffect of temperature, partial gas pressure and particle size on the reaction rate ofcoated steel, zinc powders and nickel powders were systematically studied by TG.4) The kinetic model was deduced to describe the isothermal and non-isothermalgas-solid reaction of metals in the form of sphere and flat plate with consideringoxidation induced volume change when diffusion is the rate controlling step. Theeffect of oxidation induced volume change, temperature, partial gas pressure, heatingrate and particle size on the oxidation reaction fraction were quantitively analyzedthrough an explicit function. Using the model to analyze the kinetics of isothermaloxidation of coated steel at550⁷50°C, non-isothermal oxidation at350⁹50°C,isothermal oxidation of zinc and nickel powders, the results showed that a good agreement has been obtained.In this thesis, the study of material design and oxidation kinetics on the hot-dipcoating system is significant and useful for practical application. The Al-Zn-Si-Ti-Fesystem database not only can be used to design the hot-dip coatings, but also can beused in designing other aluminum alloys. The proposed gas-solid model makes thecalculated results more accurate because of considering the oxidation induced volumechange and avoiding multi-fitting. This model can be used to other gas-solid reactionsin metallurgy and material fields to obtain an accurate prediction.更多还原