Microstructural Aspects of Grain Boundary Bulge in a Dynamically Recrystallized Mg-Al-Zn Alloy
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INTRODUCTION
IN recent years, due to the demand of light weight materials, structural application of magnesium alloys has become an important issue. Good workability is essential for structural application of magnesium alloys. Unfortunately, the room temperature workability of magnesium alloys is quite poor due to their limited slip systems; therefore, hot working is often used to deform magnesium alloys. During hot working, dynamic recrystallization (DRX) occurs in magnesium alloys, so that the DRX behavior of magnesium alloys has been studied by many authors (e.g., References 1 through 8). To understand DRX, one important issue is to know the nucleation mechanism. Reviews on the nucleation mechanisms can be found in several articles (e.g., References 9 and 10) Grain boundary bulging has often been found to be an important nucleation mechanism in magnesium alloys.[4–7,9,11] Grain boundary bulging, often called strain-induced boundary migration, was originally proposed as a nucleation mechanism of static recrystallization.[12–14] It was suggested that when there is a stored energy difference across a high-angle boundary, local boundary migration occurs.[14] When the migrating boundary is anchored by pre-existing cell/subgrain boundaries, a grain boundary bulge develops.[14–16] Most researchers believed that a boundary migrates toward the grain having a smaller cell/subgrain size, which possesses higher stored energy.[14,16,17] In some cases, a grain D.K. SUN, Research Student, C.P. CHANG and P.W. KAO, Professors, are with the Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung, 80424, Taiwan, Republic of China. Contact e-mail: [email protected] Manuscript submitted September 28, 2009. Article published online April 21, 2010 1864—VOLUME 41A, JULY 2010
boundary segment was found to bulge in an oscillating way, that is, adjacent regions of the same grain boundary moving in opposite senses into two grains.[13,18] Since cell/subgrain size does not normally change rapidly within such a short distance, Doherty and Cahn[19] proposed two possibilities to account for the two-way bulging. The first one is that during annealing, subgrain coalescence may occur, which then acts as a recrystallization nucleus and starts to grow. When two coalesced subgrains are formed a short distance away on both sides of a grain boundary, the growth of these two enlarged subgrains will cause the grain boundary to form a two-way bulge. The second possibility they proposed is that after a large strain rolling or compression, grains are deformed to an elongated shape, and during annealing, the growth of recrystallized grains will tend to restore their equilibrium shape; therefore, from a geometrical consideration, a two-way bulge may be formed (refer to Figure 7 in Reference 19). Apart from the stored energy theory, there are several theories that have been proposed to explain the occurrence of grain boundary bulging. After deformation, cells/subgrains form on both sides of a grain boundary. Hurley
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