Order-order kinetics in Ni50.5Al49.5 single crystal

Order-order kinetics in a single crystal of B2-type ordered beta'-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 E-A = 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 mechanisin 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 beta'-NiAl, as well as for the dramatic difference between the rates of ordering/disordering in beta'-NiAl and the previously investigated gamma '-Ni-3 Al. (C) 2003 Elsevier Ltd. All rights reserved.