It is shown that UNSM induced nanoskin prolongs fatigue life of all three alloys. In many instances UNSM treated specimens lasted well beyond 109 cycles or run out. S-N curve of UNSM treated Al6061T6 is shifted approximately 25 – 30 % towards higher stress level compared with untreated specimens. Meanwhile, UNSM induced nanoskin improved fatigue strength of TC4 and TAMZ by 11 % and 13 % respectively. Untreated Al6061-T6 specimens show various fracture surfaces. At high stresses (Figure 8a) cracks are initiated at inclusions or other defects and rapidly spread, forming a “canyon” shaped damage. At medium stresses (Figure 8b) fatigue failure occurs at the surface followed by propagation into the interior, as indicated by cross-section lines. At lower stresses (Figure 8c) slip bands occur at crack starting point. Figure 8d shows a typical high cycle fatigue fracture of UNSM treated specimens. a b c d Figure 8. Untreated (a-c) and UNSM treated (d) Al6061-T6 specimens fracture surfaces Although fatigue strength of TC4 is higher than that of TAMZ alloy, their fracture behavior is very similar. Fractographic analysis of typical untreated Ti-alloy gigacycle (> 108 cycles) specimens displays that cracks always initiate at the surface, as shown on Figure 9. The fracture surface can be divided in to four areas: (I) an initial relatively flat area; (II) a distinctly rough area with propagation traces like radiate wave lines; (III) a wider fracture surface with radial streaks along propagation direction and (IV) overload fracture area. For specimens with UNSM induced nanoskin, the fracture surface can be divided in four areas, much the same way as untreated specimens. The important difference, however, is that the crack initiation always occurs in the subsurface, about 50 μm below, at the internal inclusion and form a fish-eye, as shown on Figure 10.
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