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

小行星9713052 过去的研究表明，硼在金属间化合物合金（如Ni 3Al）塑性化中的主要作用是抑制湿致脆化。 人们普遍认为，脆化是由于氢原子向裂纹尖端的扩散引起的，而氢原子是由水蒸气和新暴露的金属间表面之间的化学反应产生的。 硼抑制脆化过程的机制尚未详细探讨。 本项目研究了硼对单晶Ni 3（Al，Ti）表面对水蒸气的表面反应性的三种物理和化学效应，即：（1）物理位阻;（2）在氧存在下形成表面羟基;（3）氢解吸和表面扩散的相对速率的改变。 为了理解最近的一个令人惊讶的发现，即硼掺杂的Ni 3Al在干氢中比无硼的Ni 3Al延展性差，研究了氢在Ni 3（Al，Ti）表面上的解离。 这些研究是在原位进行的，作为硼表面浓度和温度的函数，主要使用X射线光电子能谱和热解吸单晶Ni 3（Al，Ti）（100）和（111）表面。 使用固态、低能硼离子源实现硼的受控剂量。 %镍铝化物Ni 3Al属于一类铝基金属间化合物，其对于高温结构应用具有潜在价值。 这项工作有助于了解影响这些合金的延展性的因素。 ***

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. ***