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

"Order-order" kinetics in a single crystal of B2-type ordered *b'- 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 plus/minus 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 *b'-NiAI, as well as for the dramatic difference between the rates of ordering/disordering in *b'-NiAl and the previously investigated *g'-Ni3Al.