Prediction of Precipitation Strengthening in the Commercial Mg Alloy AZ91 Using Dislocation Dynamics
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INTRODUCTION
REDUCING the weight of structural components is a major focus of current efforts to improve fuel efficiency in transportation applications. Because of magnesium’s low density, Mg-based alloys are increasingly being used in structural components. Alloys from the Mg-Al system have been widely used for cast components. The commercial alloy AZ91, containing 9 wt pct Al and 1 wt pct Zn, is among the best known alloys for casting applications due to its excellent castability, corrosion resistance, and low cost. In spite of the advantages of Mg-based alloys, their use in transportation applications has been limited by their low strength compared with aluminum alloys and steels. A promising approach to improve the mechanical properties of Mg-based alloys is the use of Integrated Computational Materials Engineering (ICME), in which a set of computational tools is
L.K. AAGESEN is with the Idaho National Laboratory, P. O. Box 1625, Idaho Falls, ID 83415. Contact e-mail: [email protected] J. MIAO and J.E. ALLISON, are with the Department of Materials Science and Engineering, 2300 Hayward St., University of Michigan, Ann Arbor, MI 48109. S. AUBRY and A. ARSENLIS, are with the Lawrence Livermore National Laboratory, P. O. Box 808, Livermore, CA, 94551. Manuscript submitted October 19, 2017. Article published online March 5, 2018 1908—VOLUME 49A, MAY 2018
used to predict the microstructure of a given alloy with particular processing conditions, and to understand how that microstructure affects material properties. Important microstructural features that impact the mechanical properties of AZ91 include grain size, the presence of solute atoms in the Mg matrix, and the formation of precipitates.[1] AZ91 can be age hardened, which results in the formation of bcc b-Mg17Al12 precipitates.[1–4] These precipitates block the motion of gliding dislocations and thus contribute to strengthening. Two predominant morphologies of the b-Mg17Al12 precipitates are observed.[1,3,4] Precipitates formed by the so-called continuous precipitation mode are lathshaped and have the habit plane of the precipitate parallel to the basal (0001) plane of the hcp matrix. The other precipitation mode, referred to as discontinuous, results in a lamellar microstructure consisting of the b phase and the hcp matrix, and nucleates at grain boundaries. It has been observed that the discontinuous precipitation mode accounts for a maximum of approximately 20 vol pct of the precipitate phase.[5] Additionally, the discontinuous precipitate morphology is predominantly found near grain boundaries, and therefore these regions have a lower influence on freely gliding dislocations in the interiors of grains. For these reasons, in this work we focus on the lath-shaped precipitates formed by the continuous mode. When a gliding dislocation comes into contact with a precipitate particle in the glide plane, the dislocation may either shear through the precipitate or bypass the METALLURGICAL AND MATERIALS TRANSACTIONS A
precipitate by forming an Orowan loop arou
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