The effect of boron additions on irradiation-induced order changes in Ni 3Al intermetallic compounds

Abstract

The effects of boron additions (0.1 wt%) on the kinetics of atomic order changes in a Ni{sub 76}Al{sub 24} intermetallic compound, under 1 MeV electron irradiation, were investigated at temperatures of 293 K and 410 K and displacement rates of 0.09 {times} 10{sup {minus}3} to 4.7 {times} 10{sup {minus}3} dpa.s{sup {minus}1}. In these irradiation conditions, a state of residual order was obtained for long irradiation times, characterized by a steady state order parameter S{infinity}; it corresponds to a competition between two opposite features: irradiation disordering and thermal reordering enhanced by irradiation. Boron additions did not affect the efficiency of irradiation-induced disordering: the disordering cross-section (or, equivalently, the number of replacements per displacement {var_epsilon}) were comparable with and without a boron addition. By contrast, the S{infinity} values at 293 K were much lower in the alloy containing boron. Since boron does not change the disordering rate, the large difference between the values obtained in undoped and in boron-doped alloys shows that the reordering rate is strongly reduced by the presence of boron. Thus, boron modifies the mobility of the defects responsible for the irradiation-enhanced diffusion. The data on dislocation-loop size and the reordering kinetics suggest that vacancies are trapped by boronmore » at low temperatures and immobilized, probably by the formation of a boron-vacancy complex. The effect becomes weaker at higher displacement rates and higher temperatures, probably due to the boron-vacancy complexes becoming unstable. It is proposed that two different reordering mechanisms may be operative at 293 K, according to the presence of boron: reordering is promoted by vacancy migration in the Ni{sub 76}Al{sub 24} alloy, whereas in the Ni{sub 76}Al{sub 24} (0.1 wt%B) alloy, it is promoted by the migration of split-interstitials or/and of low-mobility vacancy-boron complexes.« less