“Order–order” kinetics in Ni50.5Al49.5 single crystal

“Order–order” kinetics in a single crystal of B2-type ordered β′-Ni50.5Al49.5 was studied by means of resistometry at temperatures between 961 and 1105 K. Reversible isothermal relaxations of the electrical resistivity yielded the activation energy EA=2.3±0.1 eV, the same for ordering and disordering processes and markedly lower than the activation energy for Ni diffusion in NiAl. In view of the recently proposed mechanism for Ni-tracer diffusion in NiAl the result is discussed in terms of the formation and elimination of triple defects. The mechanism means a strict correlation between chemical ordering and vacancy formation and provides a possible explanation for the relationship between the activation energies for Ni-diffusion and ordering in β′-NiAl, as well as for the dramatic difference between the rates of ordering/disordering in β′-NiAl and the previously investigated γ′-Ni3Al.