Role of Competitive Surface Chemical Reactions in Ductility of Intermetallics

[9410701]尽管金属间化合物有许多有趣的特性，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：在室温空气中易碎。在过去的十年中，有许多研究探索了不同的方法来延展性这类材料。最近的研究表明，金属间合金，如镍铝（Ni3Al）本质上是延展性的，它们的脆性是氢脆的结果（空气中的水蒸气和变形过程中暴露的新鲜金属间表面之间的反应产生的氢）。然而，其他金属间化合物如镍铁（Ni3Fe）似乎没有表现出任何湿致脆。据推测，如果产生氢的水解离反应被氧以某种方式淬灭，则不会有氢脆，材料将显示出改善的延展性。为了验证这一假设，计划对水蒸气和金属间合金清洁表面之间的相互作用进行详细的表面科学研究，无论是否存在氧气。利用镍铝钛，Ni3（Al,Ti）金属间合金单晶，测量了水蒸气在不同晶面上的室温粘附概率，并与氧进行了比较。振动光谱和热解吸被用来确定吸附的水在哪个晶面解离形成氢，以及这种解离是否受到共吸附的氧的影响。为了理解为什么Ni3Fe多晶不会遭受水分引起的脆化，进行了类似的研究，以确定氧气是否在可用的吸附位点上与水蒸气竞争，以及水蒸气和氧气的共同吸附是否导致表面羟基而不是氢的形成。这些研究为理解和可能控制这类重要材料的湿致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. ***