The following graph shows yield strength versus time & temperature during a prec

Question

The following graph shows yield strength versus time & temperature during a precipitation strengthening heat treatment for an aluminum alloy. Why is the strength-versus-time behavior of the sample treated at 260 degree C so different than the sample treated at 149 degree C? 260 degree C will cause creep to occur, thereby lowering the strength of the material Precipitates formed at higher temperatures will grow more rapidly, thereby increasing strength faster at first but lowering strength soon thereafter because of precipitates growing too large to remain effective at strengthening At 260 degree C, the precipitates will dissolve back into the matrix after a short time so the strengthening effect is reversed 149 degree C results in a heterogeneous precipitation structure that is more beneficial for strength - but it takes more time to develop than the homogeneous structure that forms at 260 degree C The grain size of the material treated at 149 degree C is larger which provides Hall-Petch strengthening Both A and D None of the above

Explanation / Answer

The rates and amounts of the changes in strength and hardness can be increased by holding the alloys at moderately elevated temperatures for alloys of all types. This treatment is called precipitation heat treating or artificial aging.

Precipitates formed at higher temperatures will grow more rapidly, thereby increasing strength faster at first but lowering strength soon thereafter becuase of precipitates growing too large to remain effective at strengthening.

So option b) holds good for the given question.

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