MODIFICATIONS OF THE MICROSTRUCTURE AND DISORDERING OF THE GAMMA' PHASE IN NI-AL-TI ALLOYS UNDER ELECTRON-IRRADIATION

The microstructural evolution of two-phase gamma/gamma' Ni-Al-Ti alloys and the kinetics of disordering of the gamma' phase were investigated during 1 MeV electron irradiation. At low temperatures, a high density of dislocations was nucleated during irradiation. Dark-field observations showed a progressive disappearance of the image of the gamma' precipitates, due to their disordering, followed in certain cases by a reprecipitation of fine gamma'-type particles. Order-disorder kinetics were studied by determining the intensities of superlattice spots on the diffraction patterns. The long-range order parameter decreased to eventually reach a steady-state value. Except at very low temperatures, this value was finite and resulted from a competition between disordering by irradiation and thermal reordering enhanced by irradiation. The experimentally observed kinetics were found to agree with previously proposed theoretical expressions. The effects of alloy composition and of the irradiation parameters on the steady-state degree of order are clearly apparent.