Effect of Stress Triaxiality on the Flow and Fracture of Mg Alloy AZ31
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TRODUCTION
MAGNESIUM has the lowest density of all structural metals (1.74 g/cm3). Mg alloys are endowed with superior specific stiffness and strength, and these characteristics make them ideal material candidates for lightweight structural applications, notably in the transportation industry.[1] One challenge facing their implementation as wrought products in load-bearing components is their relatively low ductility, which limits their formability at room temperature. During the last decade, most experimental and modeling efforts have been devoted to understanding the plastic flow and strengthening of Mg alloys.[2–8] On the other hand, little is known about the damage and fracture behavior of these materials. It is well established that stress state triaxiality plays an important role in the ductile fracture of metallic alloys.[9,10] However, published studies on fracture in Mg alloys either have been restricted to uniaxial loading[11–13] or consist of exploratory experimental studies.[14–17]
BABAK KONDORI, Graduate Research Assistant, is with the Department of Materials Science and Engineering, Texas A&M University, College Station, TX 77843-3141. A. AMINE BENZERGA, Associate Professor, is with the Department of Materials Science and Engineering and Department of Aerospace Engineering, Texas A&M University. Contact e-mail: [email protected] Manuscript submitted January 9, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
Although some differences between single- and polycrystals are noted,[4] it is widely believed that the low ductility of pure or alloyed Mg polycrystals stems from their plastic anisotropy associated with the limited number of deformation systems, as a result of their hexagonal-closed-packed (hcp) crystalline structure. However, the literature remains elusive on the issue of how plastic anisotropy affects ductility for triaxial loading conditions. A commonly accepted understanding of fracture under uniaxial tensile loading perpendicular to the c-axis is as follows.[4] Subsequent to basal slip, anisotropic plastic flow leads to stress concentrations, for example, at grain boundaries (GBs), which are then accommodated by f1012g extension twinning.[18] Concomitant with prismatic hai slip, the latter produces a strain transverse to the loading direction but normal to the c-axis. While some details pertain to the hardening behavior that ensues, it is clearly evident that a transverse strain along the c-axis can only be produced by the so-called contraction twins and, to some extent, hc þ ai dislocations. The former concentrate large shears which lead to failure by strain incompatibility at the twin boundaries or inside the twins. Clear evidence of twin-sized microcracks parallel to f1011g-f10 12g contraction double twins has recently been documented in different alloys.[11,19] It is emphasized that the above mechanisms pertain to uniaxial loading conditions. How the plastic anisotropy plays out under more complex triaxial loading states, which are encountered during processing or in service,
remains unex
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