【作者】 钱旭坤；

【导师】 赫晓东；

【作者基本信息】 哈尔滨工业大学 ， 材料学， 2010， 博士

【摘要】 Ti 3 AlC2是MAX化合物的典型一员,由于其具有低密度、高电导、高热导、良好的抗热震性能和可加工性等,因而在众多领域有很大的潜在应用。本论文采用燃烧合成的方法制备了高纯度的Ti 3 AlC2,并对其燃烧合成机理、热物性能、导电性能和抗循环氧化性能进行了系统的研究。红外测温仪测得Ti-Al-C体系燃烧合成最高温度为1780°C,高于Al和Ti的熔点。对Ti-Al-C体系热力学分析表明:在低温区Ti3Al的生成趋势最大,而在高温区TiC的生成趋势最大。根据Ti-Al-C体系热力学分析,淬熄区域的XRD、SEM/EDS和TEM分析结果,提出Ti 3 AlC2的燃烧合成机理如下:Al首先在660°C发生熔化,随后包覆到Ti颗粒的表面。Ti开始向液态Al中溶解,并且在界面处生成TiAlx金属间化合物,并且释放出大量的热,引燃了自蔓延反应。体系的温度被进一步提高,Ti颗粒和先前生成的TiAlx发生熔化,形成Ti-Al熔体并开始在C颗粒表面铺展。碳元素开始溶解到Ti-Al熔体中,并且开始形成TiC 0.67 ,TiC0.67的形成导致体系温度进一步提高并能达到最高温度。当体系温度降低到1370°C时,TiC 0.67开始和液态的Al反应生成Ti 3 AlC 2。本论文分别研究了坯体的致密度以及Ti、Al和C粉末的初始配比对燃烧合成产物的物相组成的影响。结果表明前者对产物的物相组成影响很小,后者对产物的物相组成有较大影响。对合成产物在1100°C进行2 h的热处理,结果发现物相组成并没有发生明显变化。采用直流四端电极法测试了Ti 3 AlC2在51~900°C电阻率。结果发现在该温度区间内,其电阻率随温度升高呈线性增加的关系,表现出与金属相同的导电规律。在200~1200°C范围内测试了Ti3 AlC2的线膨胀系数,结果发现平均值为9.3×10-6 K-1。对Ti 3 AlC2在200~1200°C的热导率研究发现,热导率随着温度的升高而增大,而且电子和声子对Ti 3 AlC 2的热导均有贡献。Ti 3 AlC2在200~1200°C时的摩尔热容随着温度的升高而变大。通过对Ti 3 AlC2在550~1300°C的40次抗循环氧性能的研究,发现在550和650°C存在反常氧化现象。在550和650°C,其循环氧化动力学基本属于直线加速型的;而在750~1300°C,其循环氧化动力学属于抛物线型。XRD和XPS结果表明:在550和650°C,氧化膜由锐钛矿型和金红石型TiO2以及非晶态Al 2 O 3组成;在750~950°C,Ti 3 AlC 2表面氧化膜由α-Al 2 O3和锐钛矿型TiO2。表面和截面形貌观察均发现在550和650°C时形成的氧化膜疏松、表面充满裂纹,并且与Ti 3 AlC2基体结合处有裂纹存在;在750~1300°C时形成的氧化膜致密,无裂纹且和Ti 3 AlC2基体结合良好。在1000和1100°C时,氧化膜包含外层非连续的锐钛矿型TiO 2和内层连续的α-Al 2 O3;在1200°C时,氧化膜外层是非连续的Al 2 TiO 5 ,中间层是TiO 2和α-Al 2 O3的混合物,内层为连续的α-Al 2 O 3 ;在1300°C时,氧化膜的最外层为Al 2 TiO 5 ,中间层为α-Al 2 O3和Al 2 TiO 5的混合物,内层为连续的α-Al 2 O 3 ,并且像钉子一样深深扎入Ti 3 AlC2基体,形成机械钉扎,大大提高了氧化膜和基体的界面强度。利用XRD测试了Ti 3 AlC2在1000°C经历5、20和40次循环氧化后,表面氧化膜中的残余应力。结果发现氧化膜中存在压应力,分别为0.82,0.65和0.49 GPa。而Fe-Cr-Al高温合金在1100~1300°C经历40 h氧化后,表面氧化膜中的应力为4~5 GPa,因此Ti 3 AlC2表面氧化膜内的应力要比Fe-Cr-Al高温合金表面氧化膜中的应力小许多。为了提高Ti 3 AlC2在低温的抗循环氧化能力,采用了高温预氧化技术。结果发现在1100°C空气中预氧化2 h后,能有效提高其在550和650°C的抗循环氧化能力。在室温下对Ti 3 AlC2的氧化机理进行了研究,发现其氧化过程包含两个阶段。在第一个阶段只发生氧的吸附和溶解,发现没有钛、铝和碳的氧化。在第二个阶段铝发生选择性氧化;而Ti2p和C1s电子结合能的峰位在经历3600 s后仍然没有发生明显的变化,意味着钛和碳没有被氧化。更多还原

【Abstract】 Ti 3 AlC2 is a typical member of MAX phase materials, which has a lot of potential applications in many fields because it possesses a low density, high electric and thermal conductivities, good thermal shock resistance and ready machinability.In the present study, highly-pure Ti 3 AlC2 was prepared by combustion synthesis. The synthsis mechanism, the thermalphysical properties, the electrical properties and the cyclic-oxidation resistance were studied systematically.The maximum temperature of Ti-Al-C system during combustion syntheis was measured up to 1780°C, which was higher than the melting points of Al and Ti, respectively. The thermodynamic analysis indicated that the formation tendency of Ti3Al and TiC was the maximum at the low and high temperature range, respectively. The combustion synthesis mechanism of Ti 3 AlC2 was proposed based on the thermodynamic calculation of Ti-Al-C system and XRD, SEM, EDS and TEM results of the quenched samples. Al would first melt at 660°C and spread on Ti particles. Then Ti began to dissolve into liquid Al and form TiAlx intermetallics at the interface, which released large amounts of heat. As a result of the released heat, combustion synthesis was initiated and the temperature was elevated higher. Ti particles and previously formed TiAlx completely melted subsequently and formed Ti-Al melts which could wet and spread on C particles. Then C began to dissolve into Ti-Al melts and form TiC0.67, which again released large amount of heat and brought the system temperature to the maximum. After that the temperature began to decline and TiC 0.67 reacted with liquid Al to form Ti 3 AlC2 when the temperature was below 1370°C.Efforts were made on optimizing the relative density of the green compact and the initial molar ratio of Ti, Al and C powders. XRD results indicated that the former had little effect on the resulting phase, while the latter had greater effect on it. Heat treatment at 1100°C for 2 h was performed on the as-synthesized product and it was found the composing phases remained unchanged.The electrical resistivity of Ti3 AlC2 was measured by four-point probes at 51~900°C. The results indicated that the electrical resistivity was similar with a metal, increasing linearly with increasing temperature.The coefficient of thermal expansion of Ti 3 AlC 2 was examined at 200~1200 °C and yielded a mean value of 9.3×10 -6 K-1. The thermal conductivity and the molar heat capacity of Ti 3 AlC2 increased with increasing temperature at 200~1200°C and the results proved that both electron and phonon contributed to the thermal conductivity of Ti 3 AlC 2.A 40-cycle oxidation was performed on Ti 3 AlC2 at 550~1300°C and the results showed that abnormal oxidation occurred at 550 and 650°C. The cyclic-oxidation kinetics approximately obeyed an accelerated linear law at 550 and 650°C, while that at 750~1300°C basically followed a parabolic law. XRD and XPS results showed the scales on Ti 3 AlC 2 containedα-Al 2 O 3 and rutile TiO2 at 750~950°C, while at 550 and 650°C they contained anatase and rulite TiO2 and amorphous Al 2 O3. Surface and cross-section morphologies showed the scales formed at 750~950°C were dense, free of microcrack and well adhesive to Ti 3 AlC2 substrate. However, the scales formed at 550 and 650°C were loose and full of cavities, and cracks were found at the scale/Ti 3 AlC2 interface. At 1000 and 1100°C, the scales were composed of an outer and discontinuous layer of rutile TiO2, and an inner and continuous layer ofα-Al 2 O3. The scale formed at 1200°C contained an outmost layer of discontinuous Al 2 TiO 5 , an intermediate mixed layer of TiO 2 andα-Al 2 O3, and an inner layer ofα-Al 2 O3. At 1300°C, the scale contained an outmost layer of discontinuous Al 2 TiO 5 , an intermediate mixed layer of Al 2 TiO 5 andα-Al 2 O3, and an inner layer of continuousα-Al 2 O 3 which deeply intruded into the Ti 3 AlC2 substrate and greatly improved the interface strength.XRD results showed that the compressive stress within the scale formed on Ti 3 AlC2 was measured to be 0.82,0.65 and 0.49 GPa after 5, 20 and 40 thermal cycles at 1000°C. However, the compressive stress within the scale formed on Fe-Cr-Al superalloy was 4~5 GPa after 40-h oxidation at 1100~1300°C. It was obvious that the compressive stress within the scale formed on Ti 3 AlC2 was much smaller than that within the scale on Fe-Cr-Al superalloy.In order to improve the cyclic-oxidation resistance of Ti 3 AlC2 at low temperatures, preoxidation of Ti 3 AlC2 was employed. The results showed that the cyclic-oxidation resistance of Ti 3 AlC2 at 550 and 650°C could be greatly improved by preoxidation at 1100°C for 2 h.The oxidation mechanism of Ti 3 AlC2 was studied at room temperature. The oxidation process contained two stages. During the first stage, the adsorption and absorption of oxygen occurred and there was no evidence that titanium, aluminum and carbon were oxidized. During the second stage, the selective oxidation of aluminum occurred, whereas the binding energy of titanium and carbon remained unchanged after 3600 s exposure in air which meant they were not oxidized.更多还原