Microstructural Development during Hot Working of Mg-3Al-1Zn
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INTRODUCTION
REDUCING the weight of vehicles, for increased fuel efficiency, is a high priority for the automotive industry. Due to its low density, magnesium is a potential material for a range of automotive components. Most of the consumption of magnesium alloys by the automotive industry has been in the form of die castings.[1] However, wrought magnesium products have the advantage over castings of higher strength and ductility, and so are more suited to structural applications. Wrought magnesium alloys can be deformed at elevated temperatures using primary fabrication methods such as rolling, extrusion, and forging, but as the workability of the material is limited, production rates are slow and hence the final product is comparatively expensive. Deforming magnesium and its alloys at elevated temperatures is of great metallurgical importance because not only is the workability improved but also the final grain size and, to a great extent, the final properties of the material are altered. The operation of dynamic recrystallization (DRX) during hot deformation of magnesium is of particular importance, because it reduces the flow stress during deformation and controls the final grain size.[2] Therefore, to be able to successfully control the microstructural evolution during bulk working operations, an understanding of hot deformation behavior of magnesium, particularly the operation of DRX, is essential. A.G. BEER, Research Academic, and M.R. BARNETT, QEII Research Fellow, are with the CRC for Cast Metals Manufacturing (CAST), School of Engineering and Technology, Deakin University, Geelong VIC 3217, Australia. Contact e-mail: [email protected] Manuscript submitted: June 30, 2006. Article published online July 13, 2007. 1856—VOLUME 38A, AUGUST 2007
Magnesium has a high stacking-fault energy and it might thus be expected to dynamically soften by dynamic recovery (DRV) instead of DRX. However, the early work by Humphreys and co-workers[2,3] showed that DRX was in fact an important mechanism during the high-temperature deformation of magnesium. The presence of DRX was attributed to the constraints imposed by the lack of easily activated slip systems of magnesium, rather than its stacking-fault energy. The operation of DRX in magnesium may also be linked to its high grain boundary diffusion rate.[4] The literature presents a complex picture of the mechanisms by which DRX operates in magnesium. It appears that different types of DRX take place under different deformation conditions, e.g., References 5, 6, and 7. Twinning also seems to play a significant role in the nucleation of dynamically recrystallized grains, e.g., References 5, 8, and 9. The mechanism of conventional discontinuous DRX (DDRX) has been identified in magnesium alloys by a number of workers.[3,5–7,10–12] This mechanism involves the development of high-angle grain boundaries via the nucleation and growth of new grains. This typically initiates at high-angle boundaries: original grain boundaries, the boundaries of dynamically recrystallized grains, or boun
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