Effects of Temperature and Strain Rate on the High-Temperature Workability of Strip-Cast Mg-3Al-1Zn Alloy
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I.
INTRODUCTION
MAGNESIUM alloys are receiving wide interest in automobile and electronic industries due to their low density, excellent recyclability, and good damping characteristics. However, its poor formability at room temperature makes it difficult to produce the alloy in the form of sheet or plate.[1] Recently, thin plates of a commercial AZ31 alloy have been produced via a stripcasting process.[2] In this process, a thin strip (4 to 6 mm) can be directly cast from the hot melt through a water cooling twin roll without a subsequent hot rolling process. As compared to the conventional slab casting and hot rolling process, the strip-casting process exhibits many advantages saving time, energy, and costs for producing thin strips owing to its single operation. The strip-casting process is also known to have a beneficial effect on microstructure, suppressing compositional segregation and refining microstructure.[2–4] Nevertheless, a further high-temperature deformation process after strip casting is still needed to obtain the desired mechanical properties (high strength, good formability, and homogeneous grain size) and shapes. Considering the industrial application of strip-cast AZ31 alloys, it is important to understand their high-temperature deformation behavior. In general, poor formability of Mg alloys at room temperature results from a limited number of active slip systems.[5] The critical resolved shear stress (CRSS) of basal slip at room temperature is much lower than that of nonbasal slip and twinning.[6] Therefore, plastic B.H. LEE, Graduate Student, and C.S. LEE, Professor, are with the Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Korea. Contact e-mail: [email protected] W. BANG, Senior Staff Research Engineer, and S. AHN, Principal Research Engineer, are with the Materials and Processes Research Center, Magnesium Project Team, Research Institute of Industrial Science and Technology, Pohang 790-784, Korea. Manuscript submitted October 17, 2007. Article published online April 16, 2008 1426—VOLUME 39A, JUNE 2008
deformation of Mg alloys appears to occur entirely by the basal slip modes that provide only two independent slip systems. With the increase of temperature, the CRSS values of other slip systems are reduced to a comparable level with that of the basal slip, and these systems become activated during deformation. However, an incorporation of nonbasal slip of prismatic and pyramidal planes offers only two more independent slip systems.[7] For this reason, the operation of the additional twinning modes is important to improve deformation homogeneity by offering other independent modes of deformation for the necessary five independent slip systems. In this study, the effects of processing variables such as temperature and strain rate on twin formation and the related workability of the AZ31 alloy produced by a strip-casting method were investigated.
II.
THEORETICAL BACKGROUNDS
A. Processing Map Prasad et al. proposed the concept of the proc
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