Evolution of martensitic transformation in Cu–Al–Ni shape memory alloys during low-temperature aging

The effect of thermal aging at 473 K on martensitic transformation characteristics have been studied in the Cu–Al–Ni shape memory alloys. An increase of martensitic transformation temperatures as a result of aging has been observed. Furthermore, the martensitic transformation shows an evolution with aging time from a single β₃ ⇒ β′₃ structural transformation to a β₃ ⇒ γ′₃ + β′₃ transformation. Using internal friction and thermoelectric power measurements, as well as specific thermal treatments, it has been determined that the mechanism responsible for this evolution is an increase in the degree of order, thus rejecting the participation of a γ₁ precipitation process. Finally, the increase of order at the next nearest neighbors has been established as the main factor responsible of the evolution of the martensitic transformation characteristics.