Disordering kinetics of Ni₃Al under ion irradiation

The stability of the long-range-ordered (LRO) γ′ phase was investigated in the temperature interval 100 K ≤ T ≤ 773 K under 300 keV Ni⁺ irradiation. The radiation-induced intensity changes of the L1₂ superlattice reflections were monitored by TEM technique using a computer assisted, quantitative evaluation of the amplitudes of diffraction patterns obtained by image plates. The dynamical diffraction theory where the thickness of the investigated foils was taken into account was employed to determine the degree of order from the diffraction patterns. The fluences were varied up to 10 dpa. The results are compared with recent measurements of disordered zone sizes after low dose ion irradiation in Ni₃Al. The measured temperature dependence below room temperature is consistent with the predictions of the thermal spike model for disordering, but for higher temperatures the results hint to a more complicated re-ordering process in disordered areas. A Ni₃Al specimen which was completely disordered after 10 dpa at 200 K was used for a thermal annealing experiment. The annealing behavior confirms the importance of the heterogeneous state of the order for the re-ordering process under ion irradiation.