The correlation between stress corrosion resistance and positions in extruded Al-Zn-Mg alloys with heavy rectangular section was experimentally studied and the relation between stress corrosion resistance and grain shape was discussed.
(1) Along the geometrical ST-direction, stress corrosion resistance was the lowest at the upper surface, decreased towards the center and was the highest at the side surface. On the other hand, along the geometrical LT-direction, the stress corrosion resistance was the highest at the upper surface, decreased towards the center and was the lowest at the side surface.
(2) In materials with elongated or flat grain shape, stress corrosion resistance was higher when stress was applied parallel to elongated or flat grains while it was lower when stress was applied perpendicularly to elongated or flat grains. However, when materials had equiaxed grains, the directionality of stress corrosion resistance was weak and stress corrosion resistance showed a medium value along any direction.
(3) When applied stress was constant, time to failure decreased as the anisotropy factor K increased and became constant when K exceeded about 4. The anisotoropy factor K was expressed as n
1/n
2, where n
1 was the average grain diameter in the perpendicular direction to the stress axis and n
2 was that in the parallel direction to the stress axis, respectively.
(4) When applied stress, σ, was constant, log{σK
2/(1+K
2)}, which was stress perpendicular to the grain boundary, was linearly related to logt, where t was time to failure. This suggests that grain shape dependence of stress corrosion resistance was equivalent to dependence of stress corrosion resistance on stress perpendicular to grain boundaries.
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