Ordered domain characterization in α-Cu-Al alloys from dissolution kinetics studies

Abstract

Through microcalorimetric experiments, disperse order (DO) dissolution kinetics in preannealed e-Cu-AI alloys containing 1 9, 13 and 6.5 at% aluminium were adequately described by the integrated kinetic model function arising from the steady-state part of the diffusion field: f(y) = 1 -(1 -y)2/3. Domain sizes after the annealing treatment, and also critical radii, were determined from differential scanning calorimetry data analysis at different heating rates. The existence of critical radii indicates that the disperse-order dissolution process is a firstorder transition. Nevertheless, it was inferred that for very dilute alloys, only short-range order is present. After pre-annealing the alloys at different temperatures, volume fractions and domain concentrations were computed by employing the above kinetic model under high heating-rate conditions. On the basis of appropriate time constant and diffusion time calculations, the range of such temperatures compatible with equilibrium attainment was established. Prolonged pre-anneals alter the particle distribution, but do not influence either volume fractions or domain sizes. A semi-quantitative particle radius-particle concentration-temperature diagram was proposed for e-Cu-AI alloys.

Modelling the kinetics of solute segregation to partial dislocations for isothermal microcalorimetric evaluations

“…where the activation energy for pipe diffusion Qp was taken similar to that of grain boundary diffusion [ 11] which in turn is a little more than half of that for volume diffusion Ob ~ 162.8 kJ mo1-1 [28]. On the other hand, the pre-exponential factor of Dp, can be taken from lattice diffusion Dop=Dob=6.68xl 0 -s m2s -1 [29]. For instance, at 530 K, which is the higher temperature employed in isothermal previous work, one obtains in principle D/Du=l.57xl 0 thus making very hkely that most of the diffusion takes place along dislocations.…”