The crack propagation resistance for alloy 8090 sheet decreases with the increase of content ofNa and K impurities.An improvement over the fracture toughness and crack initiation resist-ance for the alloy containing Na and K within certain limit may be made by adding 0.05%Ce.This beneficial modification would not be manifested if the alloy contained more Na andK.One of the reasons why Na and K injure the toughness of the alloy might be due to thatthey cause the precipitation of T1 phase along grain and subgrain boundaries and acceleratethe abnormal growth of recystallized grains.
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