**2.1. Materials**

2 Recent Developments in the Study of Recrystallization

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

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

> **Deformation mode** {hk*i*l}uv*t*w Basal *<sup>a</sup>* {0002}1120¯ Prismatic *<sup>a</sup>* {<sup>1100</sup> ¯ }1120¯ Pyramidal *<sup>c</sup>* <sup>+</sup> *<sup>a</sup>* {112¯2¯}1123¯ Tensile twin {1012¯ }101¯1¯ Compressive twin {1011¯ }101¯2¯

{1011¯ } <sup>56</sup>◦<sup>1210</sup> ¯ compressive twin {1012¯ } <sup>86</sup>◦<sup>1210</sup> ¯ tensile twin

1011

Pyr:{1012}

(b)

PyrII : {1122}

Tensile twin *<sup>c</sup> <sup>a</sup>*

{1011¯ }−{1012¯ } <sup>38</sup>◦<sup>1210</sup> ¯ double twin

during the microstructural evolution of Mg alloys are summarized in Table 2.

**Twin type Misorientation angle/axis**

Prs : {1 100}

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

*a*

{1013¯ } <sup>64</sup>◦<sup>1210</sup> ¯

{1013¯ }−{1012¯ } <sup>22</sup>◦<sup>1210</sup> ¯

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

Basal : {0002}

(a)

planes

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 Table 3.

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

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 AM31 alloys was 20 mm. The discussion pertaining to the recrystallization behaviors was based on the warm-rolling and annealing processes of the AM31 and AZ31B samples.

10.5772/55597

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http://dx.doi.org/10.5772/55597

Deformation and Recrystallization Behaviors in Magnesium Alloys

(a) (b)

**Figure 3.** Sample geometry for EBSD measurements (a), and orientation color code of the inverse pole figure maps (IPFs) (b)

**Element Al Zn Mn Si Cu Ca Fe Ni Zr Mg** AZ31B 2.5-3.5 0.6-1.4 0.2-1.0 < 0.1 < 0.05 < 0.04 < 0.005 < 0.005 - Bal. ZK60 0.01 5.47 0.009 0.022 0.003 0.005 0.003 0.007 0.58 Bal. AM31 3.3 - 0.8 - - - - - - Bal.

Ingot-casted AM31 alloys were hot-rolled at 673 K (400 ◦C) for a reduction in area of about 5% and the microstructure of the as-rolled state was measured using EBSD. AZ31B sheets with an initial thickness of 2 mm were warm-rolled down to 1 mm at a temperature of 498 K (225 ◦C). The average reduction in area per rolling pass was approximately 10%, and the total reduction in area was 50%. Static annealing was carried out on the warm-rolled AZ31B sheets for 10 min at 573 K (300 ◦C). The warm-rolling was carried out using a rolling mill with a diameter of 280 mm. The intermediate annealing time between each pass was about

Characterization of the microstructure and texture was mainly carried out using EBSD (electron backscatter diffraction). An automated HR-EBSD (JEOL7001F) with a HKL Channel-5 and the generalized EBSD data analysis code REDS [23] were both used. EBSD samples were mechanically polished and then electropolished using a solution of butyl cellosolve (50 ml), ethanol (10 ml) and perchloric acid (5 ml) at a voltage of 10 V and at

Textural and microstructural evolution and associated mechanical responses were investigated during the uniaxial compression of the Mg alloys at various temperatures and strain rates. Two different Mg alloys of extruded AZ31B billets and ingot-casted ZK60 alloys

**Table 3.** Chemical composition of AZ31B, ZK60 and AM31 alloys used[wt%].

temperatures ranging from 253 K (−20 ◦C) to 258 K (−15 ◦C).

5 min. No lubrication was applied.

**3. Deformation of Mg alloys**

were prepared for the mechanical tests.

**2.3. Microstructure characterization**

**Figure 2.** Uniaxial samples taken from the ZK60 billet along various directions with regard to the rolling direction (RD) and normal direction (ND). V0, V45 and V90 represent the samples taken from the RD to the ND, while H0, H45 and H90 represent the samples taken from the RD to the transverse direction (TD).

## **2.2. Thermo-mechanical processing**

Uniaxial compression tests were carried out using the Thermecmastor-Z (Fuji Electronic Industrial Co.). Cylindrical AZ31B billets with a diameter of 9 mm and a length of 12 mm were used for the compression tests. The other alloys, ZK60 alloys, were machined into compression samples 12 mm in length and 8 mm in diameter, as shown in Fig. 2.

The deformation temperatures of the AZ31B billets were 473 K (200 ◦C), 523 K (250 ◦C), and 573 K (300 ◦C), while the strain rates were 0.00139/s and 0.139/s. Ex-situ experiments to determine the microstructural evolution were also carried out at a temperature of 523 K (250 ◦C) and a strain rate of 0.32/s. For the ex-situ compression, the sample was marked with micro-indentations and compression and EBSD measurements were then alternately repeated.

Uniaxial compression tests for the ZK60 alloys were also carried out at various temperatures and strain rates. Wider deformation temperatures were imposed on the ZK60 alloys than on the AZ31B samples, and the temperatures were 298 K (25 ◦C), 398 K (125 ◦C), 448 K (175 ◦C), 498 K (225 ◦C), 548 K (275 ◦C), 598 K (325 ◦C), and 698 K (425 ◦C). Two different strain rates were used, 0.0069/s and 0.139/s. At total strain values of 3% and 7% at a strain rate of 0.139/s, microstructural mapping was carried out using EBSD. Each cylindrical sample was prepared according to the specimen preparation sequence for EBSD mapping before compression, after which the uniaxial compression and EBSD mapping were repeated alternately to determine the microstructural evolution during compression.

Figure 3 shows a schematic diagram of the cylindrical sample and the EBSD measurement region. The extrusion direction is parallel to the compression direction. The orientation color code of the EBSD inverse pole figure maps is also shown.

**Figure 3.** Sample geometry for EBSD measurements (a), and orientation color code of the inverse pole figure maps (IPFs) (b)


**Table 3.** Chemical composition of AZ31B, ZK60 and AM31 alloys used[wt%].

Ingot-casted AM31 alloys were hot-rolled at 673 K (400 ◦C) for a reduction in area of about 5% and the microstructure of the as-rolled state was measured using EBSD. AZ31B sheets with an initial thickness of 2 mm were warm-rolled down to 1 mm at a temperature of 498 K (225 ◦C). The average reduction in area per rolling pass was approximately 10%, and the total reduction in area was 50%. Static annealing was carried out on the warm-rolled AZ31B sheets for 10 min at 573 K (300 ◦C). The warm-rolling was carried out using a rolling mill with a diameter of 280 mm. The intermediate annealing time between each pass was about 5 min. No lubrication was applied.
