【作者】 侯猛；

【导师】 王小祥；

【作者基本信息】 浙江大学 ， 材料学， 2010， 硕士

【摘要】 本文对Ti6Al7Nb（TAN）和Ti6Al4V（TAV）两种钛合金的热氧化行为进行系统性的研究,通过不同的热氧化条件,对比两种钛合金在空气气氛下表面氧化层的生长规律和性质。采用金相显微镜、X射线衍射法以及维氏硬度计等测试和分析手段对合金的氧化层的厚度、致密度、相组成和表面硬化效果进行表征。利用XPS表面测试方法对合金氧化层最外层（1～5nm）的化学组分和合金元素的价态进行分析,探讨氧化层的形成机理及合金添加元素对氧化层的重要影响。研究表明,热氧化的温度和时间对氧化层的厚度和致密度有重要影响。通常随温度的升高和时间的延长,氧化层的厚度增加但致密度下降。从氧化增重曲线可知,相同条件Ti6Al7Nb表面形成的氧化层更薄,致密度更好,与基体的结合力更强。在800℃及以下温度氧化时,Ti6Al7Nb遵循抛物线氧化动力学规律,而Ti6Al4V在700℃氧化超过36h时其表面氧化速率由直线氧化动力学规律主导。表面硬度测试表明,表面硬度值随加载载荷增加而下降,氧化层对合金有明显的硬化效应,硬化效果随着氧化温度的升高和氧化时间的延长而明显增强。Ti6Al4V表面硬化效果略强于Ti6Al7Nb,但是900℃氧化时,Ti6Al4V合金的表面硬度反而有所下降,二者的表面硬度最大值都是在600℃氧化72h时获得。通过XRD对1h氧化的合金氧化层的相组成进行分析,发现两种合金的表面氧化层主要是由金红石型TiO₂组成,不同的是Ti6Al4V合金在600℃氧化时表面出现了锐钛矿型TiO₂,而在900℃氧化时则出现了Al₂O₃相。两种合金在800℃氧化24h时,Al₂O₃均出现在氧化层中,随着氧化温度的提高和氧化时间的延长,两种合金的氧化层都由金红石型TiO₂和Al₂O₃两相组成。XPS对外层氧化层（1～5nm）分析表明,Ti、Al、Nb均是以最高价态存在,V除了最高价态外还有4价V（稳态）出现。另外Al元素由于拥有最低的氧化还原电势,具有优先氧化性,并主要覆盖在氧化层的最外层。Nb元素有降低氧化层中金属离子扩散速率的作用,其本身并不能改变氧化机制,但可以使氧化层更具保护性;V元素在高温时氧化严重,并形成低熔点氧化物,在温度较高时会发生恶性氧化,极大地降低了Ti6Al4V合金在高于700℃时氧化层的抗氧化能力。更多还原

【Abstract】 This paper studied comparatively the oxide films on Ti6Al7Nb and Ti6Al4V alloys by thermal oxidation.The surface characterization and oxidation kinetics of the alloys were evaluated at different oxidation temperature and time.The thickness, density,phases and surface hardening were characterized with the testing methods such as optical microscopy（OM）,XRD and Vickers hardness tester.Using XPS, composition and chemical state of the surface oxide film were investigated.The mechanism of the formation of the surface oxide films and the effect of the adding elements were discussed.The results show that the thickness of oxide films on both alloys increases with temperature and time,while the density of the oxide films decrease.The weight gain curve and cross-sectional OM micrographs reveal that the oxide film on the surface of Ti6Al7Nb is thinner and denser than that on Ti6Al4V alloy and has stronger adhesion with substrate.Thermal oxidation behaviour of Ti6Al7Nb alloy obey parabolic and linear oxidation kinetics at 600～800℃and above 900℃,respectively.At 700℃parabolic oxidation is dominant up to 36h of oxidation,and prolonged oxidation yielded linear oxidation kinetics for themal oxidation behaviour of Ti6Al4V.Vickers hardness tester’s results reveal that the surface hardness values of both alloys decrease with increasing indenter loads,which is the typical behaviour of surface hardneing.The surface hardness increases with oxidation temperature or time, while the surface hardness of Ti6Al4V alloy decrease at 900℃.At same oxidation temperature and time,the surface hardness value of Ti6Al4V alloy is slightly larger than that of Ti6Al7Nb alloy.The Maximum surface hardness values of both alloys are obtained at 600℃for 72h.XRD results reveal that,after oxidation of 1h at temperature ranging from 600℃to 900℃,surfaces of both Ti6Al7Nb and Ti6Al4V are mainly covered by rutile TiO₂ （R-TiO₂）.The difference is that peaks of anatase TiO₂（A-TiO₂） and Al₂O₃ are detected in the oxide film of the Ti6Al4V at 600℃and 900℃,respectively.When the oxidation temperature is above 800℃and oxidation time is longer than 24h, surfaces of all specimens are covered by R-TiO₂ and Al₂O₃ together.XPS analysis confirmed that the surface oxide film only contain cations with the highest or most stable valences.The preferential oxidation of Al is due to its the minimum standard redox potential in all adding element.In the process of thermal oxidation,the outer most sections of the surface oxide film of both Ti6Al7Nb and Ti6Al4V are allways Al₂O₃.Nb dose not change fundamentally the corrosion mechanism itself,however,it plays,at least,a role on a mechanism active during the initial stages of corrosion,rendering the oxide film more protective.Above 700℃,severe oxidation of V causes a loss of corrosion resisitance of Ti6Al4V alloy.更多还原