Role of Competitive Surface Chemical Reactions in Ductility of Intermetallics

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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. ***