Interaction Between Basal Slip and a Mg 17 Al 12 Precipitate in Magnesium
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INTRODUCTION
MAGNESIUM (Mg) and Mg alloys have a low density (1740 kg/m3 for pure Mg) that is very attractive for structural applications where fuel efficiency is critical, but their tensile strength is relatively low (135 to 285 MPa for most Mg alloys). To improve strength of Mg alloys, precipitation hardening has been applied but with limited success. In stark contrast to aluminum (Al) alloys, Mg alloys are strengthened ineffectively via precipitation. The nature of precipitate strengthening lies in the interactions between dislocations and precipitates, i.e., either looping (the Orowan mechanism for non-deformable precipitates) or shearing (for deformable precipitates).[1] For deformable precipitates, six interaction modes may contribute to strengthening,[2] i.e., chemical hardening,[3] modulus hardening,[3] order-strengthening,[4,5] stacking fault (SF) hardening,[6] Peierls-Nabarro stress hardening,[7] and coherency strain hardening.[8] Generally, precipitate strengthening may be a combination of these hardening modes. For Mg-Al alloys, Mg17Al12 is a common type of equilibrium precipitate phase. The maximum solid solubility of Al in Mg alloys is about 12.5 wt pct,[9,10] and Mg17Al12 precipitates are the major secondary phase that strengthens Mg alloys. To achieve good hardening effects in Mg alloys, extensive research has been performed.[11–14] After processing, the equilibrium Mg17Al12 phase assumes two types of morphology: continuous (plate-shape) and discontinuous (cellular).[11,12] The predominant orientation relationship MIN LIAO, Associate Professor, is with the School of Mechanical Engineering and Automation, Xihua University, Cheng-du, 610039 Sichuan, P.R. China. Contact e-mail: [email protected], [email protected] B. LI, Associate Research Professor, and M.F. HORSTEMEYER, Professor, are with the Center for Advanced Vehicular Systems, Mississippi State University, Starkville, MS 39759. Manuscript submitted August 12, 2013. Article published online April 2, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A
(OR) between the Mg17Al12 precipitates and Mg matrix is Burgers OR which satisfies (0001)Mg||(011)P, ½2110Mg jj½111P .[9,12] When solution treated, basal slip and f1012g twinning are the dominant deformation modes.[13] As the size and number of Mg17Al12 precipitates increase, prismatic slip is activated and the amount of f1012g twinning decreases.[13] However, it was found that even if the volume fraction of Mg17Al12 precipitates in Mg alloys is higher than that in Al alloys, the strengthening effect is still limited and this is ascribed to that the precipitates are not oriented to effectively block the basal slip.[12,13] The inter-precipitate spacing is relatively large (about 200 nm);[13] so, it is hard for dislocations to shear the equilibrium Mg17Al12 plates. When the number of precipitate per unit volume increases, the inter-precipitate spacing decreases and the hardening contribution is enhanced.[11,12,14] On the other hand, the precipitate size also affects the strengthening. An increased precipitate
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