Dislocation Crack Interactions and Lattice Rotation Under Uniaxial and Cyclic Loading
NAVAL RESEARCH LAB WASHINGTON DC WASHINGTON United States
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We investigate here the configuration of dislocations emitted by a sharp crack tip in structural alloys, and the characteristics of plastic zone as a result of the accumulation of fatigue damage, which is still unknown and is crucial for understanding the underlying physics of the deformation process ahead of a crack tip. We report a significant lattice rotation in the plastic zone ahead of a fatigue crack at room temperature in Al 1100 H14 and Al 7075 T7, observed by employing X-ray diffraction XRD and transmission electron microscopy TEM. A series of high resolution XRD scans measured at different locations in the vicinity of crack tip showed variations in the relative intensities of 111, 200 and 220 Al peaks. The intensity of 111 peaks was observed to gradually increase as compared to that of the 200 peak as we approach the crack, suggesting lattice rotation as a result of fatigue crack growth at room temperature. We ascribe such rotation to glide of large number of dislocations along 111 planes across grain boundaries, which results in increase in the misorientation during crack growth. In addition, the cyclic deformation causes 200 increase in residual stress in Al 7075, but decrease in residual stress by 80 in commercially pure Al. Such a change in residual stress cannot be explained by the difference in dislocation density alone in the plastic zone. We demonstrate that the deformation associated with the lattice rotation is a major factor controlling the stress. These findings provide new insights on the role of lattice rotation on the deformation process under fatigue loading. This suggests a new the energy dissipation mechanism with implications for predicting material life under cyclic load.