Modification of alloy surface composition by segregation processes as a means of impurity control in fusion devices

Abstract

The bulk properties required of materials to be used in fusion devices: tensile strength, swelling resistance, thermal conductivity, low induced activity, and high melting point, are not generally compatible with the required surface properties of low Z and low chemical reactivity with hydrogen. Coatings have been suggested as a means of providing the necessary surface properties, while preserving the requisite substrate properties. The coatings, however, introduce new problems in terms of maintenance, thermal conductivity and mechanical integrity. We are investigating the use of surface segregation in alloys as a means of producing low Z coatings which are self-sustaining in a reactor environment, present no thermal barrier to the substrate, and avoid mechanical problems associated with the interface region. Several candidate materials have emerged from our calculations, including alloys of copper, vanadium and tungsten. The segregation calculations, light and heavy ion sputtering properties and thermodynamic properties of these materials are presented. Calculations indicate that as little as one atomic layer of low Z material significantly reduces the substrate erosion for both light and heavy ion sputtering. Experimental data on the degree of surface segregation and the rate at which the low Z component migrates to the surface are presented using dilute alloys of lithium in copper as a reference system. The long term stability of the overlayer is limited primarily by the rate at which radiation-enhanced diffusion can replace the eroded surface material. The radiation-enhanced process proceeds much more quickly than the purely thermodynamic process and depends on the damage profile, which in turn depends on the mass and energy spectrum of the incident radiation.