**1. Introduction**

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138 Recent Developments in the Study of Recrystallization

438-440 (2006) 870.

464.

301.

Magnesium alloys have a great potential for application to lightweight structural components due to their low density, high specific strength and stiffness. In particular, wrought Mg alloys have attracted much attention due to their more advantageous mechanical properties compared to cast Mg alloys. The strong preferred orientations, mechanical planar anisotropy, and thus the poor formability of wrought magnesium alloys at room temperature, however, prevent their wider use in areas such as automotive and aerospace parts, electronic devices, and consumable products. These disadvantages are mainly attributed to their hcp (hexagonal close packed) structure and the associated insufficient independent slip systems.

Various studies of wrought magnesium alloys have focused on a microstructure modification of the fine grains and off-basal texturing to improve their formability by means of alloy designs and appropriate forms of thermo-mechanical processing. Alloying designs, which mainly incorporate rare-earth metals, have been used to obtain better off-basal texturing [1–4]. Deviation from the strong basal texture by extrusion [5, 6] and equal-channel angular extrusion (ECAE) processes [7] has been shown to enhance the formability. The weakening of the basal textures was also observed during an asymmetric rolling process [8–13], which resulted in better elongation.

Table 1 shows the frequently-found deformation modes in magnesium alloys, including both slip and twinning systems. The critical resolved shear stresses (CRSS) of non-basal slip systems such as prismatic and pyramidal slip systems show much higher values at room temperature than those of basal slip systems [14, 15]. These hinder the strong activation of the deformation modes enough to accommodate external loading or plastic deformation at room temperature, finally resulting in poor formability. The basal a system, which is mainly activated at room temperature, does not fully accommodate

Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Cho and Kang; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

©2012 Cho and Kang, licensee InTech. This is an open access chapter distributed under the terms of the

external elongation, although tensile twinning offers lattice reorientation and further adjusts the degree of deformation. The CRSS of magnesium alloys commonly varies with the temperature. Particularly, non-basal slip systems are more sensitive to temperature, and the CRSS in those cases quickly decreases with the temperature. The activation of non-basal slip systems at elevated temperatures improves the formability of magnesium alloys. The various misorientation relationships between the matrix and the twins commonly observed during the microstructural evolution of Mg alloys are summarized in Table 2.

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Deformation and Recrystallization Behaviors in Magnesium Alloys

For magnesium alloys, the most dominant deformation mode is the basal �a� slip system regardless of the temperature. The basal �a� slip system is usually aligned with the deformation direction. It aligns the c-axis to the plane normal direction (ND) of the sheet during the rolling process [13]. Similarly, the c-axis vertically aligns to the extrusion direction of the billets during extrusion [6, 16]. The extrusion process contains axisymmetric deformation along the extrusion direction. Most thermo-mechanical processing methods of Mg alloys are carried out at elevated temperatures due to the limited slip system at room temperature. The warm- or hot-working processes frequently cause complex microstructural changes due to dynamic recrystallization in addition to deformation. The rolled sheets or extruded billets should also be annealed to relax the stored energy and to improve the degree

Here, we present the textural and microstructural evolution during the deformation and recrystallization of various magnesium alloys. Mechanical responses corresponding to the microstructure are also discussed. Uniaxial compression and rolling processes followed by

Four different magnesium alloys of extruded AZ31B (Mg-Al-Zn system), twin-roll casted AZ31B, ingot-casted ZK60 (Mg-Zn-Zr system) and ingot-casted AM31 (Mg-Al-Mn system) alloys were studied. The overall chemical compositions of the materials are presented in

For uni-axial compression tests, cylindrical extruded AZ31B billets and ingot-casted ZK60 alloys were used. Through the compression tests, the deformation and mechanical responses were discussed. The extruded AZ31B billets were commercially fabricated and had an initial diameter of 9 mm. They were simply cut into compression samples with a length of 12 mm. The other compression samples were prepared using ingot-casted ZK60 alloys, which were originally fabricated by conventional direct chill casting (DC) in a laboratory scale. Its initial thickness was approximately 20 mm and it was warm-rolled down to 15 mm, and then solution heat-treated at 673 K (400 ◦C) for 15 hours (T4). ZK60 billets with a thickness of 15 mm were machined into cylindrical samples for uniaxial compression. Figure 2 depicts the sample geometry obtained from the initial 15 mm-thick ingot. The character *V* referes to the vertical direction and *H*, denotes the horizontal direction. The V0, V45 and V90 samples were machined from the rolling direction (RD) to the ND in each case. The numbers 0, 45, and 90 refer to the angles between the RD and the particular sample used. The ZK60 alloys are typical magnesium alloys with aging (precipitation) hardening, where their strength levels also change with aging [17–21]. Here, we focus on the solid solution state (T4) of the ZK60 alloys. A more detailed discussion of ZK60 alloys including both solid-solution (T4) and

The AZ31B strips fabricated by twin-roll casting and the ingot-casted AM31 alloys were used for warm-rolling and subsequent annealing. These were also fabricated at a laboratory scale. The initial thickness of the AZ31B strips was about 5 mm, and it was warm-rolled down to about 2 mm, as suggested in the literature [22]. The initial thickness of the ingot-casted

of microstructural inhomogeneity for the next forming process.

annealing were used as case studies.

artificial-aging (T6) states will be given later.

**2. Experiments**

**2.1. Materials**

Table 3.


**Table 1.** Various deformation modes in magnesium alloys.


**Table 2.** Twinning misorientations commonly observed in magnesium alloys

**Figure 1.** Lattice planes and directions of the hexagonal crystal structure: (a) basal and prismatic planes, and (b) pyramidal planes

For magnesium alloys, the most dominant deformation mode is the basal �a� slip system regardless of the temperature. The basal �a� slip system is usually aligned with the deformation direction. It aligns the c-axis to the plane normal direction (ND) of the sheet during the rolling process [13]. Similarly, the c-axis vertically aligns to the extrusion direction of the billets during extrusion [6, 16]. The extrusion process contains axisymmetric deformation along the extrusion direction. Most thermo-mechanical processing methods of Mg alloys are carried out at elevated temperatures due to the limited slip system at room temperature. The warm- or hot-working processes frequently cause complex microstructural changes due to dynamic recrystallization in addition to deformation. The rolled sheets or extruded billets should also be annealed to relax the stored energy and to improve the degree of microstructural inhomogeneity for the next forming process.

Here, we present the textural and microstructural evolution during the deformation and recrystallization of various magnesium alloys. Mechanical responses corresponding to the microstructure are also discussed. Uniaxial compression and rolling processes followed by annealing were used as case studies.
