Microstructural features of rolled Mg-3Al-1Zn
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THE microstructure of rolled magnesium alloys, as in many other metals, exerts a significant influence over mechanical behavior both during processing and in service. As early as 1939, Menzen[1] interpreted the favorable mechanical properties seen in certain wrought alloys to be a consequence of microstructure (grain size) rather than a direct effect of alloy composition. In that work, the improved rolling performance of extruded slab over cast slab was also ascribed to microstructural changes. Couling et al.,[2] along with Reed-Hill (in a comment attached to Couling et al.’s work) have rationalized the superior cold rollability of Mg0.5Th and Mg-0.4Zr-0.2 (Misch metal) alloys in terms of the appearance of banded microstructural features (“compression bands” in Roberts’ terminology[3]). Deformation twinning also plays a significant role in the development of rolled structures,[4,5,6] and a double-twinning mechanism in which {101 2} twinning occurs within the interior of {101 1} twins has been observed in pure Mg.[2,3] The present work examines the microstructural phenomena evident in hot- and cold-rolled commercial quality Mg-3Al-1Zn (AZ31), the most common wrought magnesium alloy. After hot rolling, the grain size of AZ31 is generally of the order of 10 to 40 m,[7,8,9] with sizes closer to 5 m reported in some instances.[10,11] If the grain size of the AZ31 billet received for use in the present work (380 m) is typical (and our experience suggests that it is), grain refinement by a factor of 10 to 80 times commonly occurs during hot rolling. The hot rolling process involves multiple cycles of rolling followed by reheating, and in a number of Mg alloys it has been reported that even finer structures can be achieved if a small number of high strain rolling passes are used.[12,13] This approach is obviously restricted by the capacity of the rolling mill, but it is also limited by the homoM.R. BARNETT, QEII Research Fellow, Z. KESHAVARZ, Ph.D. Student, and M.D. NAVE, Postdoctoral Research Fellow, are with the School of Engineering, Deakin University, Geetong VIC 3217, Australia. Contact e-mail: [email protected] This article is based on a presentation made in the symposium entitled “Phase Transformations and Deformation in Magnesium Alloys,” which occurred during the Spring TMS meeting, March 14–17, 2004, in Charlotte, NC, under the auspices of ASM-MSCTS Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
geneity of the structure.[13] With fewer rolling passes, there are fewer cycles of annealing and this is expected to accentuate the effect of the inhomogeneity of the deformed structure on the final recrystallized structure. Optimized hot rolling microstructures can thus be considered to be a trade-off between grain refinement and microstructure homogeneity. Steps toward determining optimal hot rolling parameters are taken in the present work. Turning to cold rolling, early studies showed ubiquitous diagonally inclined banded features containing basal planes closely align
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