【作者】 彭彩霞；

【导师】 梁伟；

【作者基本信息】 太原理工大学 ， 材料加工工程， 2007， 硕士

【摘要】 γ-TiAl基合金具有密度低，高温比强度高、高温比刚度高及耐高温蠕变性良好等优点，已成为最具潜力的新一代航空航天用轻质高温结构材料之一。经过十多年的研究，TiAl基合金的室温塑性和韧性已得到明显改善，然而750℃以上高温抗氧化性能不足却严重制约着其实际应用。本人所在课题组提出并尝试了通过固体渗硅在γ-TiAl基合金表面形成Ti₅Si₃层来提高其抗高温氧化性能的思路及方法，近些年研究发现：对TiAl基合金表面渗硅处理后形成的以Al₂O₃为主并分布有少量Ti₅Si₃的较薄外层，和以Ti₅Si₃为主并分布有少量Al₂O₃的较厚内层组成的复合渗层，在900℃下长期循环氧化过程中不开裂，不剥落，与基体粘附性好，表现出优异的抗氧化性能。本文主要研究固体渗硅TiAl合金在900℃以上的高温抗氧化性能，并研究合金在更高温度下的氧化动力学规律和氧化机理，以期为TiAl基合金的高温应用提供一定的理论和实践依据。首先用混合均匀的固体粉末对Ti-46.5 Al-2.5 V-1 Cr（at％）合金在1250°C进行渗硅处理，并对未渗硅和渗硅的合金分别在900℃、1000℃、1100℃、1200℃的高温下进行周期为2h的循环氧化实验，用最小二乘法对实验数据进行拟合，研究了高温下渗硅后合金的氧化动力学规律和氧化机理。结果如下：未渗硅试样在900℃循环氧化时，氧化4h出现轻微剥落，1000℃、1100°C、1200℃氧化2h均呈现不同程度的黄色氧化皮，而且均出现了较为严重的剥落。渗硅处理后，合金在上述温度下的抗氧化均得到明显提高：在900°C氧化118h后，试样表面和截面结构与渗硅前相比没有明显变化，渗层与基体结合良好，增重仅0.1875mg／cm²；1000℃氧化118h后，出现轻微的剥落；1100℃氧化16h、1200氧化4h后，分别出现了不同程度的剥落。用最小二乘法对渗硅TiAl合金在900℃、1000℃、1100℃和1200℃的循环氧化数据进行了拟合。结果表明：渗硅TiAl基合金900℃氧化在所测试的118h内氧化动力学曲线呈抛物线规律；1000℃氧化0-50h之间呈抛物线型规律，50-118h之间呈直线型氧化规律；1100℃氧化在0-4h之间呈抛物线型规律，4-8h之间呈直线型氧化规律，8-16h呈二次型加速氧化的律，16h出现剥落现象；1200℃循环氧化呈二次型加速氧化规律，4h出现剥落现象。求得相应的氧化速率常数为：900℃时K_p≈4.924E-5mg²cm^-4h^-1，1000℃时K_p≈7.778E-5mg²cm^-4h，1100℃时K_p≈9.392E-2mg²cm^-4h^-1。即，当温度从900℃升高至1000℃时，K_p值虽有所增加，但仍保持在同一数量级；而当温度从1000℃升高至1100℃时，K_p值增加3个数量级以上。K_p值的如此巨大变化可能与TiAl基合金在1100℃以上循环氧化时升温过程中发生从α₂+γ到α-γ相变及降温过程中发生相反方向的相变以及由此引起的相变应力有关：二元TiAl基合金的等温共析转变温度为1125℃，而本课题采用的合金中添加了Cr和V合金元素，而且又通过渗硅处理，因而相变可能在更低的温度下发生。但尽管如此，渗硅仍然使合金在1100℃和1200℃的抗氧化能力得到明显提高。更多还原

【Abstract】 Gamma titanium aluminide （γ-TiAl） has attracted many scientific research interests as a kind of high temperature structural material, due to its low density, high specific strength, high specific stiffness and good creep resistance at high temperatures. The ductility and toughness of TiAl-based alloy at room temperature have been improved greatly during last one decade and a half, but its insufficient oxidation resistance above approximately 750℃is still one of the main barriers to its wider applications at elevated temperatures.The work of our research group shows that a composite coating can be formed by pack siliconizing with a mixture powder of silicon and alumina. The coating is composed of a thin Al₂O₃-dominated outer layer and a thick Ti₅Si₃-dominated inner layer, which bonds well with substrate. Neither cracks nor spallations occurred during the long-term cyclic oxidation at 900℃. The specimen siliconized at 1250℃for 2 h exhibits the best oxidation resistance in the siliconized specimens. This paper investigates the oxidation kinetics and the oxidation mechanisum of the siliconized TiAl-based alloy above 900℃and makes a comparation with that at 900℃, to supply certain theory and practical basis for its application at elevated temperatures.The specimens were pack sliconized with a mixture powder of silicon and alumina with the propertion of 15% to 85%, the siliconized specimens and the bare one were cyclically oxidated at 900℃, 1000℃, 1100℃, 1200℃. The experimental data were fitted by the least square method to study the oxidation kinetics that follows different laws at different oxidation stages above 900℃. The results are as follows:For the bare specimens oxided at 900℃for 4 h, a little spallation occurred and the weight gains are as high as 0.688mg/cm², which is three time more than 0.1875 mg/cm² that of siliconized specimens oxided for 118 h at the same temeperature. The spallation took place severely and the surfaces appereared yellow to diffferent degree for all of bare specimens oxided at 1000℃, 1100℃, 1200℃for 2 h. But oxidation resistance at 1100℃and 1200℃of the specimens siliconized at 1250℃for 2 h were improved greatly, and at 900℃and 1000℃, it behaved much better oxidation resistance. After oxidized at 900℃for 118 h, the surface and the cross-section showed no limbus changes, with the coating and the substrate bonded well with each other. The slight spal lation occurred for the siliconized specimen oxided at 1000℃for 118 h. And l l00^C for 16 h, and 1200℃for 4 h different degree spallation occurred.Three kinds of oxidation kinetics behavior, parabolic, linear and quadratic, were revealed by analyzing the experimental data of 900℃, 1000℃, 1100℃and 1200℃with the least square method. At 900℃, only parabolic oxidation behavior appears during the whole cyclic oxidation test of 118 h. At 1000℃, the oxidation behavior is parabolic at the initial stage up to 50 h, and becomes linear from 50 to 118 h. At 1100℃, the oxidation behavior is parabolic at the initial stage up to 4 h, and is linear from 4 h to 8 h, and then becomes quadratic from 8 until spallation at 16 h. At 1200℃, only quadratic oxidation behavior appears until spallation at 4 h. The corresponding oxidation constants obtained by fittingthe curves are K_p≈4.924×10^-5mg²cm^-4h^-1 at 900℃, K_p≈7.778×10^-5mg²cm^-4h at 1000℃, K_p≈9.392×10^-2mg²cm^-4h^-1 at 1100℃. That is to say, when the is to say, when the temeperature rises from 900℃to 1000℃, although Kp value increaces to a certain degree, it keeps in the same order. When the temeperature rises from 1000℃to 1100℃, K_p value increaces more than 3 orders of magnitude. This steep change of K_p values when temperature increases from 1000℃to 1100℃was associated with the phase transformation of TiAl-based alloy fromα₂＋γtoα＋γ, and the inverse process occurred during cyclic oxidation. This transformation maybe occur at a lower temperature in the siliconized. TiAl-based alloy with Cr and V addition than the eutectoid temperature 1125℃in binary TiAl-based alloys.更多还原