Effects of Boron on the Surface Chemistry of Single Crystal Ni3 (Al, Ti)

9713052过去的研究表明，硼在金属间化合物合金(如Ni3Al)的延伸化中的主要作用是抑制湿气诱导的脆化。人们普遍认为，脆化是由于原子氢扩散到裂纹尖端而产生的，原子氢是由水蒸气和新暴露的金属间化合物表面之间的化学反应产生的。关于硼抑制脆化过程的机理还没有详细的探讨。本项目考察了硼对单晶Ni3(Al，Ti)表面对水蒸气的表面反应性的三种物理和化学效应。(1)物理位置阻塞；(2)氧存在下表面羟基的形成；(3)氢的脱附和表面扩散的相对速率的修正。为了理解最近一个令人惊讶的发现，即掺硼的Ni3Al在干氢中的延展性低于不掺硼的Ni3Al，我们研究了氢在Ni3(Al，Ti)表面的解离。这些研究是在单晶Ni3(Al，Ti)(100)和(111)表面上用X射线光电子能谱和热脱附法进行的，作为硼表面浓度和温度的函数。控制剂量的硼是使用固态、低能量的硼离子源来完成的。镍基铝化物，Ni3Al，属于一类具有潜在高温结构应用价值的铝基金属间化合物。这项工作有助于理解影响这些合金延展性的因素。***

9713052 Chung Past research has demonstrated that a major role of boron in ductilizing intermetallic alloys, such as Ni3Al, is to suppress moisture-induced embrittlement. It is generally agreed that the embrittlement arises from the diffusion of atomic hydrogen to the crack tip and that the atomic hydrogen is produced from the chemical reaction between the water vapor and the freshly exposed intermetallic surfaces. The mechanism by which the boron suppresses the embrittlement process has not been explored in detail. This project examines three physical and chemical effects of boron on the surface reactivity of single crystal Ni3(Al,Ti) surfaces towards water vapor, viz. (1) physical site blocking; (2) formation of surface hydroxyls in the presence of oxygen; and (3) modification of the relative rates of hydrogen desorption and surface diffusion. To understand a recent surprising finding that boron-doped Ni3Al is less ductile than boron-free Ni3Al in dry hydrogen, the dissociation of hydrogen on Ni3(Al,Ti) surfaces is investigated. These studies are performed in situ as a function of boron surface concentration and temperature using primarily x-ray photoemission and thermal desorption on single crystal Ni3(Al,Ti) (100) and (111) surfaces. Controlled dosing of boron is accomplished using a solid- state, low-energy boron ion source. %%% Nickel aluminide, Ni3Al, belongs to a class of aluminum- based intermetallics that are potentially valuable for high temperature structural applications. This work assists in understanding factors affecting the ductility of these alloys. ***