The repetitive flaking of Inconel 625 by 100 keV helium bombardment

Repetitive flaking of Inconel 625 occurs with ion bombardment doses of > than 1018 100 keV helium ions cm-2, with up to 39 exfoliations being observed after bombardment with 3 x 1019 ions cm-2. The thickness of the flakes, measured by scanning electron microscopy, is some 30% greater than when measured by Rutherford backscattering (RBS) of 1.8 MeV helium ions. These RBS measurements compare well with the thickness of the remaining layers in the resultant craters and to the most probable range of the 100 keV helium. The area of the flakes is dictated by the grain boundaries, and when one flake is ejected, the adjacent grains are prevented from doing so since there now exists an escape route for the injected helium. A strong dose rate dependence is observed; decreasing the beam current from 640 μA cm-2 to 64 μA cm-2 results in a factor 20 fewer flakes being exfoliated (for the same total dose of 3 x 1019 ions cm-2). Successive flakes decrease in area, suggesting that eventually a cratered, but stable, surface will result with the only erosion being by the much less effective mechanism of sputtering. (orig.)