AVS 52nd International Symposium
    Surface Science Wednesday Sessions
       Session SS1-WeM

Paper SS1-WeM3
Bulk/Surface Mass Exchange and the Special Role of Bulk Dislocations in the Growth of NiAl Alloy

Wednesday, November 2, 2005, 9:00 am, Room 200

Session: Growth and Alloying of Surfaces
Presenter: J.P. Pierce, Sandia National Laboratories
Authors: J.P. Pierce, Sandia National Laboratories
K.F. McCarty, Sandia National Laboratories
N.C. Bartelt, Sandia National Laboratories
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When aluminum atoms are deposited on the surface of an Al-depleted NiAl crystal at high temperatures (> 900 K), new layers of NiAl alloy grow on the surface. We directly observe the appearance of new alloy by watching atomic steps advance across the surface using low-energy electron microscopy (LEEM). This behavior means that while Al atoms are delivered to the surface by our evaporator, Ni atoms are supplied to the surface by the bulk of the crystal. The competition in the arrival rates of these species determines how the surface evolves. At low temperatures (< 750 K), Ni atoms are unable to diffuse to the surface fast enough to accommodate the incoming Al atoms, so the Al atoms form a film on top of the alloy. At intermediate temperatures (750 to 900 K), we find that dislocations play a special role in transporting mass between the surface and the bulk. Al deposition causes the composition (i.e., the relative concentration of Al and Ni) at the surface to reach a critical level at which the points where bulk dislocations terminate on the (110) surface begin to move linearly. The dislocations provide a channel for mass exchange between the surface and the bulk; as the dislocations move, new crystal is left in their wake. As predicted by the one-dimensional diffusion equation, we find that the duration of Al exposure required bring the surface to the critical composition for dislocation motion grows as the inverse square of the Al flux. This confirms that the behavior on the surface is indeed limited by the diffusion of atoms through the bulk. We use our observations to identify the point defects that allow the Ni and Al atoms to diffuse and use the one-dimensional diffusion model to determine how the depth profile of the relative concentration of Ni and Al atoms evolves with time as the crystal tries to equilibrate. This work was supported by the Office of Basic Energy Sciences, Division of Materials Sciences of the U.S. DOE under Contract No. DE-AC04-94AL85000.