Abstract
Thermodynamic activity of Ga in the L12-ordered intermetallic phase Ni3Ga has been determined by measuring the electromotive force of a galvanic cell with zirconia solid-electrolyte, Ni–Ga, Ga_2O_3|ZrO_2 (+ Y_2O_3) | Ga, Ga_2O_3in the temperature range between 1050 and 1300 K for six compositions. The activity of Ga varies with composition most steeply at the stoichiometric composition (25 mol%Ga), the dependence being more pronounced at low temperatures. By applying a thermodynamic model based on the Bragg-Williams approximation (Ikeda et al.: Acta Mater. 46 (1998) 6605–6613), the effective pair interaction energy between Ni and Ga atoms has been determined to be 9.75±0.19 kJ/mol. The thermodynamic factor in chemical diffusion in the L12 phase has been derived from the composition dependence of the thermodynamic activity.
