Modeling the precipitation processes and strengthening mechanisms in a Mg-Al-(Zn) AZ91 alloy
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I. INTRODUCTION
THE total hardening response of a metal or alloy arises from the sum of contributions of different hardening modes. These contributions arise from the interactions between the defects accomplishing deformation and the characteristics of the microstructure. In a single-phase, polycrystalline alloy these modes include contributions from the interactions between dislocations and the lattice itself (Peierls–Nabarro stress), solute atoms (solid solution strengthening), grain boundaries (Hall–Petch hardening), or other defects such as dislocations or twins (forest hardening). In alloys containing a distribution of second-phase precipitates, additional contributions may arise from the interactions between defects and the particles themselves. For some materials, under particular conditions, one mode of hardening may dominate the total hardening response, such as the contribution arising from dislocation-precipitate interaction to the yield stress of 2xxx and 7xxx series Al alloys. Accordingly, these alloys are referred to as “precipitation hardened,” even though other modes of hardening do contribute to the overall mechanical response. In other alloys, the dominance of a particular hardening mode is not as obvious. In an effort to design microstructures, it is desirable to understand the relative magnitudes of each of the different contributions to hardening so that some guidance can be given on the most effective means of modifying a particular mechanical response through tailored microstructural changes. In the field of cast magnesium alloys, those based on the Mg-Al system are the best known and most thoroughly studied. Magnesium alloy research is currently receiving considerable attention because of the potential weight savings offered by replacing heavier steel and Al alloy components in automobiles with Mg-based counterparts. The driving force for this development is the pressure to reduce the fuel emissions of automobiles and one approach is a reduction in their C.R. HUTCHINSON, University Fellow, and J.F. NIE, Reader, are with the School of Physics and Materials Engineering, Monash University, Clayton, 3168, Victoria, Australia. Contact e-mail:[email protected] S. GORSSE, Maˆitre de Conference, is with the Institut de Chimie de la Matière Condensée de Bordeaux et Ecole, Nationale Supérieure de Chimie et de Physique de Bordeaux, 33608 Pessac, Cedex, France. Manuscript submitted November 1, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
weight. The best known of the two-phase Mg-Al alloys is that based on the composition Mg-9Al-1Zn (wt pct), known as AZ91. At temperatures around 200 °C, the equilibrium volume fraction of second-phase particles in this alloy approaches 15 pct, but the room-temperature 0.2 pct proof strength typically reaches only 160 MPa. This represents 15 pct of the theoretical shear strength (G/15). As a comparison, consider a high-strength Al alloy such as 7075 (nominal composition: Al-5.6Zn-2.5Mg-1.5Cu (wt pct)), which similarly treated may result in 5 pct precipita
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