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

9713052 Chung Past的研究表明，硼在金属间合金（如Ni3Al）的延展性中的主要作用是抑制水分引起的脆化。人们普遍认为，脆性是由氢原子向裂纹尖端扩散引起的，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理尚未详细探讨。本项目研究了硼对单晶Ni3（Al,Ti）表面水蒸气反应性的三种物理和化学影响，即(1)物理位点阻断；(2)在氧气存在下形成表面羟基；(3)改变氢的相对解吸速率和表面扩散速率。为了理解最近令人惊讶的发现，掺硼Ni3Al在干氢中的延展性不如无硼Ni3Al，研究了氢在Ni3（Al,Ti）表面的解离。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射线光电发射和单晶Ni3(Al,Ti)（100）和（111）表面的热解吸。硼的控制剂量是使用固态、低能硼离子源来完成的。镍铝化物，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。这项工作有助于了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. ***