Kinetic processes during ion bombardment

A number of processes change the structure and the distribution of alloying elements in near-surface regions of alloys during ion bombardment. Displacements and displacement cascades lead to a randomization of atom locations and alloying elements: ordered alloys become disordered; concentration gradients are smoothed; crystalline materials become highly defective or even amorphous. Displacement processes dominate at low temperatures where thermal rearrangements of atoms are minimal. At higher temperatures, defects become mobile, and the resulting enhanced counteracts the randomization resulting from the displacement processes and aids in the approach to thermodynamic equilibrium. With increasing temperature and mobility of point defects, annihilation of defects at sinks, such as dislocations, grain boundaries and the surface, become dominant, causing persistent defect fluxes. Preferential coupling of certain alloying elements to defect fluxes may cause redistribution of alloying elements between regions around sinks and the matrix, a phenomenon thermal radiation-induced segregation (RIS). The induced concentration changes frequently are large enough to precipitate a second phase in solid solution alloys or to dissolve precipitates in two-phase alloys. Complex redistribution of alloying elements and phases via RIS can occur throughout and well beyond the damage range. In addition, preferential loss of elements by sputtering affects the alloy composition in the near-surface region. Such loss can occur directly by preferential sputtering, or indirectly by Gibbsian surface adsorption or RIS which modify the composition at the surface, and hence, the composition of the sputtered material. (orig.)