Kinetics of Reordering in Quenched Ni 2 Mn 0.8 Cu 0.2 Ga Ferromagnetic Shape Memory Alloys.

Quenched Ni₂Mn_1−xCu_xGa ferromagnetic shape memory alloys undergo two consecutive post-quench ordering processes. The kinetics of order recovery has been analysed in detail for Ni₂Mn_0.8Cu_0.2Ga, based on the calorimetric curves obtained during post-quench heating at constant rates. Isoconversional methods have been used to determine the activation energy, the pre-exponential factor, and the reaction model that best fits the two reordering processes. The kinetic analysis has been extended to samples quenched from different temperatures. The kinetic study shows that order improvement processes in quenched Ni₂Mn_0.8Cu_0.2Ga alloys can be described by a first order reaction model, consistent with site-saturation nucleation and homogeneous diffusion-controlled growth, with apparent activation energies around 1.1 eV. The pre-exponential factors, especially those obtained for samples quenched from different temperatures, highlight the crucial role of the vacancies retained by high temperature quenching on the atomic reordering underlying the observed processes. [ABSTRACT FROM AUTHOR]