Entropically favored ordering: The metallurgy of Al2Cu revisited

The famous sequence of precipitates which form upon heat treating Al-Cu is part of neatly every metallurgical textbook. Numerous precipitation land ot her) experiments have led to a long-standing belief that the energetic grou nd start: of Al2Cu is the theta phase. Modern first-principles calculations at T = 0 K surprisingly predict the energy of the observed Al2Cu-theta pha se to be higher than that of its metastable counterpart, theta '. We show t hat vibrational entropy reverses this energetic preference at T similar to 150-200 degreesC, resolves the apparent discrepancy between theory and expe riment, and hence plays a critical (but previously unsuspected) role in the precipitation sequence.