Effect of nanostructure on phase transformations during heat treatment of 2024 aluminum alloy

In this work, we studied the transformations involving the coherent (theta ''/S ''), semicoherent (theta'/S'), and incoherent (theta/S) precipitates (theta = Al2Cu, S = Al2CuMg) formed during the heat treatment of an annealed nanostructured powder of Al 2024 produced by cryogenic milling. These precipitates form over a wide temperature range (300-500 degrees C) depending on the chemical composition. They influence grain growth during sintering and may facilitate the formation of a fully dense nanostructured material. Mechanical milling was performed for 20 h at a cryogenic temperature to obtain nanostructured particles. The structural evolution and morphology of the particles during the heat treatment were investigated for three particle size ranges (<25, 25-45, and 45-90 mu m). Heat treatment of the milled powder was performed at three different temperatures (475, 500, and 525 degrees C) for 5, 10, 15, and 20 min. Morphological analysis of the as-milled particles using scanning electron microscopy indicated that the milling process was not completed. X-ray diffraction analysis yielded the q and S volume fractions for the heat-treated and as-milled powders. DSC analysis indicated the precipitation and dissolution of GP (Guinier-Preston zone), coherent, and incoherent precipitates during heating of the as-atomized and milled powders. Accumulation of the deformation energy during milling is suggested to lead to an early transformation of the S and q phases in the finer powder. (C) 2021 The Authors. Published by Elsevier B.V.