The effects of third elements on the kinetics of order change in Ni 3Al under H.V.E.M. irradiation

Abstract

The effects of boron, cobalt or titanium additions on the kinetics of order changes in Ni 3Al under irradiation were investigated in a HVEM. The long-range order (LRO) parameter was determined by measuring the intensities of superlattice spots on the diffraction patterns. Order-variation kinetics was investigated by using a previously proposed theoretical model. The number of replacements per displacement ɛ and the steady-state LRO parameter S ∞ were directly deduced by fitting a kinetic law to experimental data ; a good agreement was generally found. A weak effects of the above third elements on the disordering efficiency was observed whereas the effect of stoichiometry appeared to be more important. Reordering was found to be controlled by long-range vacancy migration with a migration enthalpy of 0.9 to 1.0 eV ; this was deduced from a comparison between the experimental temperature dependence of the steady — state LRO parameter with calculated curves. The effects of third elements on the reordering kinetics is clearly revealed : compared to binary alloys, a decrease of the effective reordering rate is generally observed.