Please provide an clearly and right explanation as to why ov

Please provide an clearly and right explanation as to why overaged aluminium alloys might be less tough than under-aged ones.

Solution

The ageing precipitates inside grains have an obvious effect on the fatigue crack growth resistance of Al alloy. The crack growth resistance is higher for the under-aged and treated samples in which the precipitates have a high coherency with Al matrix lattice. The crack growth rate resistance of the peak-aged and over-aged samples in which the main precipitates exhibits a lower coherency with Al matrix lattice is lower.

2) When the precipitate is coherent with Al matrix lattice, the larger the precipitate is, the slower the fatigue crack grows.

3) The effect of grain boundary precipitates and PFZ adjacent to the grain boundary on the crack growth resistance is not significant as compared with that of the precipitates inside grains.