The modified embedded atom method (MWM), an empirical extension of the embedded atom method (w) that includes angular forces. has been used to examine the interface between a silicon substrate and a thin overlayer of nickel. A brief review of the MEAM is given and parameters are determined for the Si-Ni system. As verification of the reliability of the model, the geometry, energy and elastic constants of a number of ideal Si-Ni wmpounds are calculated and are found Lo agee reasonably well With experiment and first-principles calculations. Planar defect energies are also presented. Calculations of the relaxed energy and geometry of a coherent IO A overlayer of Ni on Si(00t) yield two similar Ftlllctms, both of which were typified by a slightly rippled Ni structnre relative to psfect FCC Ni. The lower-energy interface also contained rows of slightly shifted Ni atoms. It is found that significant differences occur between the energetics of a rigid or relaxed separation of the overlayer. Separation of the overlayer with a monolayer of Si atoms attached to the Ni yields a significantly lowerenergy StIUchue than separatjon exactly at the interface. The relaxed brittle fracture energy of this interface is found to be 1.5 J m-l, which is significantly lower than the unrelaxed fracture energy of 4.8 1 m-2,
The particle flux and angular distribution of 3.5 MeV alpha particles impinging on the first wall from uncontained banana orbits in an axisymmetric tokamak reactor have been calculated. The resulting helium concentration profiles in the first wall can give rise to surface exfoliation under specified conditions. The major mitigating factor is the simultaneous surface recession due to sputtering by the D-T charge-exchange neutral flux. For the parameters used in these calculations blistering in high-sputtering-rate materials such as beryllium is unlikely, whereas in low-sputtering-rate materials such as niobium helium-induced surface deformation is quite probable.
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