Cyclic deformation and fracture of pure aluminium polycrystals

Cyclic deformation and fatigue failure at room temperature of pure aluminium polycrystals, with grain size in the range 60–500 μm, have been investigated and compared with results obtained from aluminium single crystals, as well as with results from copper single and polycrystals. The cycling was carried at constant plastic strain with amplitudes in the range 3.2 × 10−5-4.3 × 10−4. A hardening-softening-secondary hardening was observed at all levels of strains. The maxima and minima in the cyclic stress response (csr)-curve were displaced to higher levels with increasing plastic strain amplitudes. The csr-curves were also shifted to higher levels with reduced grain sizes. The slip patterns consisted of slip bands clustered in macrobands, but also the deformation patterns characteristic of cycled [001] aluminium single crystal were frequently observed. Fracture occurred at all plastic strain levels and at all grain sizes. A Coffin-Manson relationship was established. The first fatigue cracks were formed along grain boundaries at a very early stage of the cyclic deformation. Crack propagation occurred mostly along the grain boundaries.