Ion-induced selective grain growth

The microstructure of thin fcc metal films can be controlled by high energy ion bombardment under an angle of 35.24° to the surface normal. The untreated sample shows a strong (111) fiber texture as indicated by the peak in the center (corresponding to the (111) axes parallel to the surface normal) and the continuous ring at 70.54° (corresponding to the additional randomly distributed (111) axes) of the (111) polefigure (cf. figure). After cooling the sample to LN2 temperature and bombardment with 2.5 MeV Ar+, the (111) out-of-plane texture still exists (indicated by the peak in the center of the (111) polefigure) while the in-plane orientation has changed from random to preferred orientation (indicated by the sharp C3 symmetry instead of the continuous ring).

Fast ions show channeling effects in single crystalline materials if their velocity vectors are parallel to an open crystal direction (channeling axis). The repulsive potential in the channeling axes decreases the cross section for defect generation to approximately 10% of the cross section in the non-channeling case. In fcc materials the (110) axis are the most open directions. Since the incident angle of 35.24° to the surface normal corresponds to the angle between the (111) and (110) direction in fcc materials, few favorable oriented grains of the thin film show channeling while the majority of the grains do not show channeling. Due to the difference in defect generation, the defect density in the thin film becomes locally inhomogeneous during ion exposure.

Ion bombardment enhanced grain growth between uniformly damaged grains is driven by the minimization of the grain-boundary energy, where the larger grains grow at the expense of the smaller grains. Elevated temperatures promote this process due to a higher mobility of the vacancies. Since the grain size is independent of their orientation, this grain growth does not change the texture.

Ion irradiation induced grain growth between non-uniformly damaged grains is driven by the minimization of the free volume energy F. Grain boundary shifting into the more damaged grain releases energy since the grain boundaries absorb successively thin grain boundary near and highly damaged regions (high F) and rearrange the atoms in a highly ordered state on the opposite but less damaged side (low F). The grains with low defect density (grains permitting ion channeling!) grow at the expense of the more damaged grains and consequently the grain growth is directional and changes the texture. Since elevated temperatures promote the recovery of damaged specimens, the effect is more pronounced at lower temperatures where the vacancies are immobile.

Usually both type of grain growth (directional as well as non-directional grain growth) are superposed. But cooling the sample below the threshold temperature for vacancy mobility suspends the non-directional grain growth and only directional (selective!) grain growth occurs. For sufficient high fluences, a textured thin film becomes converted into a “single crystal like” thin film.

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