In-situ SANS for the kinetic analysis of β″ precipitation in an Al-Mg-Si alloy following deep cryogenic treatment

Deep cryogenic treatment (DCT) can modify the mechanical properties of Al-Mg-Si alloys, but the early stages of precipitate growth are difficult to study due to natural aging. Here, small-angle neutron scattering is used to measure precipitate growth in solution-treated and deep-cryogenically treated EN AW-6026-T6 in-situ during artificial aging. The two-dimensional patterns are modelled using spherical Guinier-Porod scatters (representing solute clusters and Guinier-Preston zones) and prolate ellipsoids (representing needle-shaped Mg-Si precipitates). As aging proceeds, the volume fraction and length-diameter ratio of the ellipsoids grow in both samples. However, the transformation from solute cluster to precipitate is more rapid in the DCT sample than in the sample without cryogenic treatment. This supports ex-situ studies on DCT Al alloys, which suggest greater dislocation density increases the precipitate growth rate due to more rapid diffusion of solutes, thus increasing the opportunity for precipitates to absorb solute from the matrix. • Small-angle neutron scattering used to study precipitation in Al-Mg-Si alloy. • Natural aging avoided by analysing immediately after homogenisation or removal from liquid nitrogen. • Cryogenically-treated sample has more rapid precipitation kinetics, consistent with shrinkage-pressure-induced dislocation formation.