Atomic Ordering and Martensitic Transformation in Cu-Zn-Al and Cu-Al-Ni Industrial Alloys

Abstract
The isothermal relaxation process near room temperature in b Cu-Zn-Al and Cu-Zn-Al-Ni industrial alloys which follows a step quenching or an up quenching thermal treatment has been studied by isothermal calorimetric technique. The transition temperature Ms and its evolution have been obtained by differential scanning calorimetry and by detecting the acoustic emission. The dissipated energy of the sample measured during the relaxation process present a complex behaviour, characterized by a continuous slowing down. On the other hand, the evolution of Ms at the beginning of the phenomenon obeys a different law. We show that all measured evolutions are much more slow in the case of Cu-Zn-Al-Ni alloy, comparing to Cu-Zn-Al alloy. The presence of nickel, which slows diffusion processes can explain this behaviour, influenced by quenching rate and annealing time. We interpret the relaxation processes and the evolutions of Ms in term of atomic reordering (B2 and L21). These phenomena are so slow in Cu-Zn-Al-Ni alloys near room temperature that the equilibrium state of order cannot be reached.