Anthony set up a thermal diffusion experiment involving vacancies as a driving force18-22,25,63 in aluminum alloys. The gradient of vacancy concentra­tion was produced by a slow decrease of temperature. At the beginning of the experiment, the ratio between solute flux and vacancy flux is the following:

JB _ _ LBB + LAB r-ml

г V V V [20]

LAA ‘ LBB ‘ 2Lab

The volume of the cavity and the amount of solute segregation nearby yield a value for the flux ratio. Note, that secondary fluxes induced by the formation of a segregation profile are neglected in the present analysis.

This experiment, combined with an interdiffusion annealing, could be a way to estimate the complete Onsager matrix. Unfortunately, the same experiment does not seem to be feasible in most alloys, especially in steels. In general, vacancies do not form cavities, and solute segregation induced by quenched vacan­cies is not visible when the vacancy elimination is not concentrated on cavities.

1.18.3.3.1 Diffusion during irradiation

In the 1970s, some diffusion experiments were per­formed under irradiation.64 The main objective was to enhance diffusion by increasing point defect concentrations and thus facilitate diffusion experi­ments at lower temperatures. Another motive was to measure diffusion coefficients of the interstitials cre­ated by irradiation. In general, the point defects reach steady-state concentrations that can be several orders of magnitude higher than the thermal values. In pure metals, some experiments were reliable enough to provide diffusion coefficient values at temperatures that were not accessible in thermal conditions.64

The analysis of the same kind of experiments in alloys happened to be very difficult. A few attempts were made in dilute alloys that led to unrealistic values of solute-interstitial binding energies.65 How­ever, a direct simulation of those experiments using an RIS diffusion model could contribute to a better knowledge of the alloy diffusion properties.

Another technique is to use irradiation to implant point defects at very low temperatures. A slow annealing of the irradiated samples combined with electrical resistivity recovery measurement high­lights several regimes of diffusion; at low tempera­ture, interstitials with low migration energies diffuse alone, while at higher temperatures, vacancies and point defect clusters also diffuse. Temperatures at which a change of slope is observed yield effective migration energies of interstitials, vacancies, and point defect clusters.66 In situ TEM observation of the growth kinetics of interstitial loops in a sample under electron irradiation is another method of determining the effective migration of interstitials.67