Order-disorder transformations and diffusion in ternary bcc alloys: The role of second-order boundaries

Abstract

Isopotential curves of the thermodynamic forces for ternary diffusion have been calculated as a function of the composition for the bcc Fe-Cr-Al system at T = 1073 K and T = 1273 K, using the cluster variation method in the irregular tetrahedron approximation with interaction parameters fitted to experimental thermochemical data. At both temperatures, the system presents extended second-order lines for the B2/A2 equilibrium. The results show that the isopotential lines present a discontinuous change of slope coinciding with the position of the second-order line in the phase diagram. This justifies experimental observations published in the literature of anomalies in the concentration plots of diffusion couples corresponding to the position of the corresponding second-order boundary. The results of the calculation suggest that the magnitude of the discontinuity should decrease with temperature.