SECONDARY CYCLIC HARDENING AND GRAIN BOUNDARY CRACKING IN HIGH PURITY Al

Symmetric push-pull fatigue was performed on well annealed 99.999% purity Al specimens with grain sizes up to 10 mm under constant strain amplitude in a Schenck machine. The applied strain amplitudes ranged from ±2.82×10~(-4) to ±6.1×10~(-3). Peak stresses in both tensile and compressive half-cycle after various cycles were measured. A secondary cyclic hardening was observed at all strain amplitudes used. A maximum of the hardening is presented at a strain amplitude of about 3×10~(-3).The metaUograph and SEM observations indicated that at high strain amplitude the various plastic deformations occur in the region of grain boundary which cause the grain boundary glide and migration with intergranular cracking. It was also found that different types of fatigue striations coexist on the same fracture surface.A brief discussion was made of the secondary cyclic hardening which is assumed to result from the local hardening due to the secondary slip and grain boundary sliding.