The Recrystallization Behavior of Unalloyed Mg and a Mg-Al Alloy
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MAGNESIUM alloys are important materials due to their attractive properties including low density and an excellent strength-to-weight ratio.[1–3] One study found that the fuel efficiency of vehicles can be increased by 6 to 8 pct for each 10 pct in weight reduction.[4] Despite the advantages Mg alloys offer, they can have poor ductility and can be difficult to form at room temperature due in part to the limited number of independent slip systems available in hcp Mg and a tendency to form a strong deformation texture.[1,2,5–9] Grain refinement achieved via recrystallization is a technique for improving the strength and ductility of Mg alloys and to weaken the strong basal texture formed during deformation, and thus should be studied more extensively.[7–10] Deformation during processing leads to a buildup of dislocations and this increases the amount of strain energy stored in the material. During subsequent annealing reducing this stored energy drives the nucleation and growth of new, strain-free grains.[11] The formation of twins plays an important role in static recrystallization (SRX) of Mg alloys. Under certain conditions, Mg and Mg alloys have a tendency to form AERIEL D. MURPHY and JOHN E. ALLISON are with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109. Contact e-mail: [email protected] Manuscript submitted September 5, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
twins during deformation.[12–16] Several studies have shown that twins and twin boundaries serve as dominant nucleation sites during recrystallization of Mg alloys.[9,12–19] Dislocations cannot penetrate through twin boundaries leading to a buildup of stored energy common in their vicinity.[13,17,20–22] There are three twinning modes in Mg alloys: the primary 10 12 h10 11i tension twins withan 86 deg lattice rotation about the h1120i axis, 1011 h1012i compression twins that have a 56 deg rotation around the h1120i axis, and double twins that form when compression twins undergo tension twinning.[12,14] Guan et al. found that double twins and double twin-grain boundary intersections were preferential nucleation sites during static recrystallization of the rare-earth magnesium alloy WE43. They also reported that tension twins did not recrystallize during annealing and were instead consumed by neighboring grains during grain growth.[15] Other studies have found that shear bands serve as nucleation sites in addition to twins and grain boundaries.[23–25] Shear band boundaries have a high angular misorientation which can accommodate dislocation cells and entanglements, increasing their stored energy.[13] A study by Su et al. in AZ31 found that during the initial stages of recrystallization, small, recrystallized grains were found only in the vicinity of twins and shear bands and complete recrystallization of these areas occurred before large, deformed grains began to recrystallize.[14] Inhomogeneous nucleation of recrystallized grains due to the addition of twins has a profound effect on recr
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