Entropically favored ordering: the metallurgy of Al(2)Cu revisited

The famous sequence of precipitates which form upon heat treating Al-Cu is part of nearly every metallurgical textbook. Numerous precipitation (and other) experiments have led to a long-standing belief that the energetic ground state of Al(2)Cu is the theta phase. Modern first-principles calculations at T = 0 K surprisingly predict the energy of the observed Al(2)Cu-theta phase to be higher than that of its metastable counterpart, theta('). We show that vibrational entropy reverses this energetic preference at T approximately 150--200 degrees C, resolves the apparent discrepancy between theory and experiment, and hence plays a critical (but previously unsuspected) role in the precipitation sequence.