The microstructure evolution of U1 and U2 nanowires constrained in Al matrix

SD Xiao and Y Kong and Y Qiu and Y Du, COMPUTATIONAL MATERIALS SCIENCE, 117, 180-187 (2016).

DOI: 10.1016/j.commatsci.2016.01.028

Needle-shaped U1-Al2MgSi2 and U2-Al4Mg4Si4 precipitates are formed in the late stage of the age hardening of Al-Mg-Si alloys. In this work, atomic simulations are used to investigate the microstructure evolution of the metastable U1 and U2 phases constrained in Al matrix, identify the crystal structures of the newly formed phases after relax, and simulate the tensile loading of these metastable phases. The obtained results imply that, Al atoms in U1 nanowire are tend to segregate when approaching equilibrium, quite different from Al atoms in U2 nanowire, where they are tend to be extruded from the nanowire. These different behaviors will apply different stress fields on the Al matrix, and thus affect the local strain concentrations. Furthermore, through comparing the radial distribution functions, the newly formed phases after relaxation are identified to be beta-Mg2Si or beta'-Mg9Si5 (a precursor phase to beta) for U1 and U2 phases. From the calculated tensile stress- strain curves, the elastic modulus and ultimate strength of U1 phase constrained in Al matrix are both higher than that of U2. Knowledge about the evolution and mechanical properties of these metastable phases is important for a complete understanding of the age hardening, and for optimizing the material properties of Al alloys. (C) 2016 Elsevier B.V. All rights reserved.

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