Equilibrium and Kinetics of Thermal Ordering or Disordering in Ni₃Al

Abstract

The high sensitivity of residual electrical resistivity to structural modifications in metallic materials was used for studying the composition and temperature dependence of the equilibrium and kinetics of atomic order, in binary Ni₃Al intermetallic compounds with 24.6 to 26.5 at% Al and in ternary materials with 4 at% Ti or Ta. In the binary alloys, the resistivity characterizing the equilibrium long-range order exhibits a positive temperature dependence, which is quantitatively consistent with changes of the LRO parameter calculated by the Cluster Variation Method. The analysis of the isothermal ordering (or disordering) kinetics shows that two first order processes are involved. The order change-rates are dependent on composition; they are minimum at stoichiometry and increase with off-stoichiometry. The presence of a third element (Ti or Ta) results in a slower diffusion.