Small-angle X-ray scattering study of the early stages of precipitation in a Mg-Nd-Gd (EV31) alloy

A quantitative description of the Mg-rare earth EV31 alloy during the first stages of the precipitation sequence using in situ small-angle X-ray scattering (SAXS) is presented. In situ evolutions of the size, volume fraction and number density of precipitates formed at 150 ◦C and 200 ◦C were obtained. A kinetic mechanism suggests that the precursor nanoparticles are nucleated at the beginning of the artificial ageing and, at 200 ◦C, these particles grow mainly by accretion of the solute from the matrix without further nucleation. The particles grow within two regimes: (i) at the beginning of ageing, the growth is associated with solute diffusion with an apparent activation energies of 0.78 eV (diffusion assisted by vacancies); (ii) further growth is associated with solute diffusion with an apparent activation energies of 1.16 eV (bare solute diffusion). After about 2 h at 200 ◦C, corresponding to the condition of maximum hardness for this alloy, the present results indicate a volume fraction of about 1.5% occupied by particles with an average Guinier radius of 2 nm. The evolution of the volume fraction at 150 ◦C, studied for a similar time interval, is weaker than the one found at 200 ◦C.