On the fcc-B2 transformation in CuPd alloys

At high temperatures the Cu0.50Pd0.50 alloy forms a fcc solid solution phase that at 770 K undergoes a bainitic transformation into a B2 phase, bcc based and chemically ordered. On the basis of first-principles calculations performed along the appropriate Bain paths, both for the ordered and disordered alloys, we find that this phase transition is due to the competition between the band electronic energy and the electrostatic interactions, the former favouring fcc, the latter bcc geometrical arrangements. Below the transition temperature the chemical ordering, driven by the conventional Fermi surface mechanism, causes an increase of the charge transfer and changes the shape of the energy curve along the Bain path. As a result, a plastic deformation of the geometrical lattice occurs. In this scenario, the coupling between concentration and uniaxial strain fluctuations plays a fundamental role and the key physical quantity is the charge transfer.