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Research Letter

Dislocation dynamics study of precipitate hardening in Al–Mg–Si alloys with input from experimental characterization Inga Ringdalen, SINTEF Materials and Chemistry, NO-7491 Trondheim, Norway Sigurd Wenner, SINTEF Materials and Chemistry, NO-7491 Trondheim, Norway; Department of Physics, Norwegian University of Science and Technology (NTNU), NO-7491 Trondheim, Norway Jesper Friis, SINTEF Materials and Chemistry, NO-7491 Trondheim, Norway Jaime Marian, Department of Materials Science and Engineering, University of California Los Angeles, Los Angeles, CA 90095, USA; Department of Mechanical and Aerospace Engineering, University of California Los Angeles, Los Angeles, CA 90095, USA Address all correspondence to Inga Ringdalen [email protected] (Received 17 June 2017; accepted 17 August 2017)

Abstract Partial aging of AA6060 aluminum alloys is known to result in a microstructure characterized by needle-shaped Si/Mg-rich precipitates. These precipitates belong to the non-equilibrium β′′ phase and are coherent with the face-centered cubic Al lattice, despite of which they can cause considerable hardening. We have investigated the interaction between these β′′ precipitates and dislocations using a unique combination of modeling and experimental observations. Dislocation-precipitate interactions are simulated using dislocation dynamics (DD) parameterized with atomistic simulations. The elastic fields due to the precipitates are described by a decay law fitted to high-resolution transmission electron microscopy measurements. These fields are subsequently used in DD to study the strength of individual precipitates as a function of size and dislocation character. Our results can be used to parameterize crystal plasticity models to calculate the strength of AA6060 at the macroscopic level.

Introduction Precipitation strengthening is the most important strengthening mechanisms in age-hardenable aluminum alloys. Precipitation of the β-Mg2Si secondary phase and its precursors from solid solution has been examined in great detail,[1–4] because they are responsible for the AA6XXX class of alloys being the strongest aluminum alloys in relation to the amount of solute elements added. The initial stage of the formation of Mg2Si includes the formation of atomic clusters and Guinier–Preston zones. Subsequently, β′′ precipitates form as coherent needles in the 〈001〉Al crystallographic directions, typically giving Al–Mg–Si alloys their maximum strength.[5] The β′′ needles eventually transform into semi-coherent β′ rods, and ultimately, these rods transform into β-Mg2Si platelets,[5] which represent the true equilibrium phase. The most likely composition for β′′ particles appears to be Mg5Al2Si4,[6,7] whose unit-cell crosssection in the [001] plane is shown in Fig. 1, and which will be the structure considered in this work. The observed shape of these precipitates is acicular, with quadrilateral cross-sections on the face-centered cubic (fcc) crystallographic plane.[8] Although coherent with the surrounding matrix on