Role of Competitive Surface Chemical Reactions in Ductility of Intermetallics

小行星9410701 尽管金属间化合物具有许多有趣的特性，使它们成为有吸引力的高温材料，但它们有一个共同的缺点：在室温空气中的脆性行为。 在过去的十年里，有许多研究探索了不同的方法来使这类材料延展。 最近的研究表明，金属间化合物合金，如镍-镍（Ni 3Al）， 本质上是延展性的，并且它们的脆性是氢脆（在变形期间暴露的新鲜金属间表面与空气中的水蒸气之间的反应产生的氢）的结果。 然而，其他金属间化合物，如镍-铁（Ni 3Fe）似乎没有表现出任何水分诱导脆化。 据推测，如果水分解反应产生氢以某种方式被氧的存在淬灭，将不会有氢脆，材料将显示出改善的延展性。 为了验证这一假设，详细的表面科学调查之间的相互作用的水蒸气和金属间合金的清洁表面，有和没有氧的存在下，计划。 使用金属间合金镍-铝-钛，Ni 3（Al，Ti）的单晶，测量水蒸气在各种晶面上的室温粘附概率，并与氧进行比较。 振动光谱和热脱附用于确定在哪个晶面上吸附的水解离形成氢，以及这种解离是否受共吸附氧的影响。 为了理解为什么Ni 3Fe多晶体不遭受湿气诱导的脆化，进行类似的研究以确定氧气是否与水蒸气竞争可用的吸附位点，以及水蒸气和氧气的共吸附是否导致形成表面羟基而不是氢。 这些研究为理解和控制这类重要材料的湿致艾德脆化提供了基本框架。 ***

9410701 Chung In spite of the many interesting properties of intermetallics that make them attractive as high temperature materials, they share one common disadvantage: brittle behavior in room temperature air. There have been numerous studies in the last ten years to explore different ways to ductilize this class of materials. Recent studies suggest that intermetallic alloys such as Nickel-Aluminide (Ni3Al) are intrinsically ductile, and their brittleness is a result of hydrogen embrittlement (the hydrogen produced by reactions between water vapor in air and fresh intermetallic surfaces exposed during deformation). However, other intermetallics such as Nickel-Iron (Ni3Fe) do not appear to exhibit any moisture-induced embrittlement. It is hypothesized that if the water dissociation reaction to produce hydrogen is quenched in some way by the presence of oxygen, there will be no hydrogen embrittlement, and the material will show improved ductility. To test this hypothesis, detailed surface science investigations of the interactions between water vapor and clean surfaces of intermetallic alloys, with and without the presence of oxygen, are planned. Using single crystals of the intermetallic alloy nickel-aluminum-titanium, Ni3(Al,Ti), the room temperature sticking probability of water vapor on various crystal faces will be measured and compared with oxygen. Vibration spectroscopy and thermal desorption are used to determine on which crystal face the adsorbed water dissociates to form hydrogen and if such dissociation is affected by co-adsorbed oxygen. To understand why Ni3Fe polycrystals do not suffer moisture-induced embrittlement, similar studies are performed to determine if oxygen out competes water vapor for available adsorption sites and if co-adsorption of water vapor and oxygen results in the formation of surface hydroxyls instead of hydrogen. %%% These studies provide the basic framework to understand and possibly control moisture-induc ed embrittlement in this important class of materials. ***