Effects of Nb and Si on high temperature oxidation of TiAl

The isothermal oxidation behavior of TiAl based alloys at 900℃ in air with a combination of Nb (5%-10%, mole fraction) and Si (1%-5%, mole fraction) was investigated. The microstructure and the composition of the oxidation scale were studied by using XRD, TEM and EPMA. The results show that the combination of Nb and Si can improve the oxidation resistance of the alloys significantly. The element Si can change the typical microstructure of oxidation scale on TiAl based alloys. In alloys with Si addition, the compact Al2O3 forms in the interior side of oxidation scale. When x (Si)3%, the Ti5Si3 phase forms and the coarse crystal TiO2 forms on Ti5Si3 phase after oxidation. The increase of Nb content in the TiAl based alloys impedes the growth of Ti5Si3 phase, and the formation of TiO2 on surface and on Ti5Si3 phase is also impeded.     

Effect of Cr on high temperature oxidation of TiAl

The oxidation behavior of TiAl-Cr（mole fraction of Cr：0-20%） was investigated at 1 173 K in air. The microstructure and composition of the oxide scale were studied by X-ray diffractometry（XRD）, scanning electron microscopy（SEM） and electro-probe micro-analyses（EPMA）. The results show that with the addition of Cr content from 0 to 8%, the oxidation resistance decreases, especially at 3%, which is mainly attributed to the doping effect of Cr^3＋. TiAI-Cr（mole fraction of Cr： 15%-20%） has good oxidation resistance, and the protective alumina layer is preferentially formed on the surface of TiAI alloy, which is due to an increase of mole ratio of Al to Ti in TiAl-Cr alloys.     

Ti对Nb基合金高温抗氧化性能的影响

采用热重分析(TG)、X射线衍射(XRD)、透射电镜(TEM)及能谱分析(EDS)等方法,研究Nb-Si-Ti(15%~26%,摩尔分数)-Hf-Al-Cr多元合金、Nb-Ti(0~50%)二元合金在1250℃大气中的高温氧化行为。通过Ti含量变化对合金氧化产物种类及氧化物PBR值的影响,探讨Ti含量对Nb基合金高温抗氧化性能的作用。结果表明:随着Ti含量的增加,合金的高温氧化产物从Nb2O5、Ti2Nb10O29、TiNb2O7和TiO2依次过渡。通过计算发现:Nb基合金的氧化产物Nb2O5、Ti2Nb10O29、TiNb2O7及TiO2的PBR值随着Ti含量的升高依次减小。氧化产物PBR值的下降,可以有效地降低Nb基合金氧化膜的生长应力、提高氧化膜的完整性、增大氧化膜的失效周期,提高Nb基合金的高温抗氧化性能。     

Ti-Al合金高温氧化膜的形态及形成

利用Pt丝标记法和定量成分分析等研究方法,研究Ti-Al(2%~75%Al,摩尔分数)合金在1 173 K的大气中经24 h氧化形成的氧化膜的组织形态和生长过程。结果表明:在氧化膜中观察到Al2O3存在3种不同分布形式,即当2%65%时,氧化膜由单一的Al2O3构成。Ti-Al合金高温氧化膜中Al2O3和TiO2的分布可分为4种组织形态。采用Pt标记可清晰观察到氧化膜内、外层,外层氧化膜的形成与Al离子向外扩散有关,内层氧化膜的形态受MO/M界面热力学平衡控制。     

超细YAl_2颗粒复合球磨表面改性研究

研究表明采用复合球磨表面改性方法对改善颗粒在复合材料中的分散性具有明显效果。进一步研究不同的Mg颗粒与超细YAl2颗粒进行复合球磨时对超细YAl2颗粒的表面改性效果的影响,并且研究了不同的球磨转速和球磨时间对超细YAl2颗粒与Mg颗粒复合球磨包覆效果的影响。分别采用扫描电镜(SEM)、显微硬度测试和X射线衍射(XRD)方法对超细YAl2颗粒和3种Mg颗粒的形貌、显微硬度及相组成作了测定和分析讨论,采用扫描电镜和能谱分析(EDS)方法对YAl2/Mg复合颗粒的形貌及包覆效果进行评价分析。试验结果表明:工业镁粉颗粒呈厚片状,近似椭圆形,当量直径(D)约为70μm;镁屑(锯态)呈薄片状,且有明显的尖角,当量直径约为350μm;镁屑(钻态)颗粒整体呈片状,颗粒表面有大量机械加工造成的"板筋状"微结构,当量直径约为1700μm。工业镁粉、镁屑(锯态)及镁屑(钻态)3种Mg颗粒的显微硬度值相近,但其厚径比(H/D)差别较大,分别为0.14,0.03,0.06~0.12。不同的Mg颗粒对超细YAl2颗粒复合球磨后表面改性的效果影响从高到低依次是:镁屑(锯态)、工业镁粉、镁屑(钻态),这种现象与3种Mg颗粒的厚径比值有着显著的反比关系。采用球磨转速为150 r·min-1时,超细YAl2颗粒与镁屑(锯态)混合球磨2 h后超细YAl2颗粒表面被Mg膜包覆,超细YAl2颗粒的分散性提高显著。