The Effect of Indium Additions on Mg-Li and Mg-Li-Al Alloys
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NTRODUCTION
MAGNESIUM (Mg) alloys are of great interest due to their low density and their potential to become a light-weight alternative to conventional aluminum (Al) and steel alloys for structural applications.[1,2] It is commonly understood that the widespread adoption of Mg is hindered because of the limited number of slip systems and thus low room-temperature formability, owing largely to its hexagonal close-packed (HCP) crystal structure.[3,4] HCP systems suffer from a lack of independent slip systems, which results in an inability to accommodate plastic strain during deformation.[5] It has been shown that the addition of lithium (Li) can reduce the c/a ratio of the Mg crystal structure and effectively reduce the critical resolved shear stress (CRSS) required to activate additional slip systems thus improving the ductility.[6] The addition of greater than 5 wt pct Li to Mg results in the formation of a two phase bodycentered cubic (BCC) + HCP microstructure, while greater than 11 wt pct results in the whole structure crystallizing as BCC upon solidification.[7] The formation of the BCC phase in Mg-Li alloys demonstrates greater
RYAN JOHN-MICHAEL HOOPER and ZACHARY LEE BRYAN, Graduate Students, and MICHELE VIOLA MANUEL, Assistant Professor, are with the Department of Materials Science Engineering, University of Florida, Gainesville, FL. Contact e-mail: [email protected]fl.edu Manuscript submitted March 22, 2013. Article published online June 29, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
ductility than their solely HCP counterparts;[8–10] however, these alloys do not intrinsically display the necessary strength for structural applications.[8] It has been found that Al is one of the most potent strengtheners of Mg-Li alloys due to the formation of a coherent, metastable phase MgAlLi2, commonly referred to as h-phase.[11,12] This phase rapidly transitions to the equilibrium, incoherent AlLi phase, even at room temperature.[11–13] AlLi has the same crystal structure and orientation relationship with the matrix as the h-phase but is not a potent strengthener due to its lack of coherency with the matrix. This is due to a contraction of the lattice parameter from 6.7 to 6.36 A˚, resulting in a misfit of 9 pct.[11] The AlLi phase is known to segregate to grain boundaries resulting in a loss of tensile ductility.[14] For this reason, only one fully BCC Mg-Li-Al alloy, Mg-36.3 at. pct Li-0.7 at. pct Al (LA141), was ever commercialized.[15] LA141 exhibits good ductility but displays lower strength than its HCP Mg alloy complement, limiting its use in high-performance applications.[16,17] It is commonly known that lattice misfit can alter the driving force for segregation and nucleation.[18] Thus, limiting the lattice contraction of the AlLi phase may reduce the nucleation and growth of this phase at grain boundaries and concurrent reduction in its strengthening potency. Indium (In) forms an InLi phase with Li that has the same crystal structure as AlLi and h-phase but with a larger lattice parameter.[19] However, currently
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