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 degrees C and 200 degrees C were obtained A kinetic mechanism suggests that the precursor nanoparticles are nucleated at the beginning of the artificial ageing and, at 200 degrees C. these particles grow mainly by accretion of the solute from the matrix without further nucleation The particles grow within two regimes: (1) 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 degrees 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 degrees C, studied for a similar time Interval, is weaker than the one found at 200 degrees C. (C) 2009 Elsevier B.V. All rights reserved.