Weakened anisotropy of mechanical properties in rolled ZK60 magnesium alloy sheets with elevated deformation temperature

doi:10.1016/j.jmst.2018.02.019

Abstract

Abstract:

The rolling direction (RD) and the transverse direction (TD) samples were obtained from an as-rolled ZK60 magnesium alloy sheet with strong anisotropy of initial texture and their mechanical properties were tested at various deformation temperatures. Meanwhile, the microstructure and texture of these samples after fracture were investigated. Results revealed that a higher flow stress along the RD than that along the TD at room temperature were ascribed to the strong anisotropy of transitional texture, and this texture effect was remarkably weakened with the increase of deformation temperature. Deformation structure was dominant at 100 °C, and was replaced by dynamic recrystallization structure when the deformation temperature increased to 200 °C and 300 °C. The texture presented a strong texture (transitional texture in the RD sample and basal texture in the TD sample) at 100 °C, but its intensity visibly decreased and texture components became more disperse at 200 °C and 300 °C. These microstructure and texture results were employed in conjunction with calculated results to argue that raising deformation temperature could increase the activity of non-basal slip by tailoring the relative critical resolved shear stress of each deformation mode and finally result in low texture effect on mechanical anisotropy.

Key words: ZK60 magnesium alloy, Deformation temperature, Texture anisotropy, Dynamic recrystallization, Non-basal slip

Wenke Wang, Wenzhen Chen, Wencong Zhang, Guorong Cui, Erde Wang. Weakened anisotropy of mechanical properties in rolled ZK60 magnesium alloy sheets with elevated deformation temperature[J]. J. Mater. Sci. Technol., 2018, 34(11): 2042-2050.

Figures/Tables 14

Fig. 1. Schematic of samples used for tension test and microstructural observations.

Fig. 2. Microstructural characteristics and texture state of as-rolled ZK60 magnesium alloy sheets in RD-TD plane: (a) inverse pole figure map; (b) grain size distribution; (c) frequency vs misorientation map; (d) (0002), (10$\bar{1}$0) and (11$\bar{2}$0) pole figures; (e) distribution of (0002) pole density along the RD and the TD.

Fig. 3. Mechanical behaviors for ZK60 magnesium alloy sheets along RD and TD at various deformation temperatures: (a) true stress-strain curves; (b) variations of yield stress at various deformation temperatures.

Table 1 Summarized mechanical properties of ZK60 magnesium alloy sheets at various deformation temperatures (σs: yield stress; σp: peak tension stress; εp: elongation determined by the σp).

Temperature (°C)	σ_s (MPa)		σ_p (MPa)		ε_p (%)
Temperature (°C)	RD	TD	RD	TD	RD	TD
As-rolled	226.0 ± 5.0	178.0 ± 5.0	315.0 ± 3.0	323.0 ± 3.0	14.0 ± 1.0	20.0 ± 1.0
100 °C	127.0 ± 5.0	117.0 ± 4.0	170.0 ± 3.0	155.0 ± 3.0	2.9 ± 0.2	2.8 ± 0.3
200 °C	34.0 ± 4.0	35.0 ± 2.0	44.0 ± 2.0	43.0 ± 1.0	1.5 ± 0.1	1.4 ± 0.1
300 °C	9.1 ± 0.8	9.4 ± 1.1	10.5 ± 1.1	10.3 ± 1.2	0.5 ± 0	0.4 ± 0

Fig. 4. Mechanical behaviors for ZK60 magnesium alloy sheets along RD and TD at room temperature.

Fig. 5. Microstructural characteristics using IQ maps for RD (a, c, e) and TD (b, d, f) samples at 100 °C (a, b), 200 °C (c, d) and 300 °C (e, f). (IQ map: mapping the IQ obtained for each point in an OIM scan onto a gray scale; the grains are defined as the sets of connected and similarly oriented points within the tolerance angle of 5°; SGB: sub-grain boundary with misorientation less than 5° shown in red; LAGBs: low angle grain boundaries between 5° and 15° shown in yellow; HAGBs: high angle grain boundaries higher than 15° shown in blue).

Fig. 6. Grain size distribution in RD and TD samples at various deformation temperatures (a) and variations of average grain size at different deformation temperatures (b).

Fig. 7. Grain average misorientation (GAM) maps for distributions of dynamic recrystallization structure and deformation structure for RD (a, c, e) and TD (b, d, f) samples at deformation temperatures of 100 °C (a, b), 200 °C (c, d) and 300 °C (e, f).

Fig. 8. (0002) and (10$\bar{1}$0) pole figures after tension deformation for RD (a, c, e) and TD (b, d, f) samples at temperatures of 100 °C (a, b), 200 °C (c, d) and 300 °C (e, f).

Fig. 9. (0002) pole density distributions along LD in RD (a) and TD (b) samples at various deformation temperatures.

Fig. 10. Dependence of critical resolved shear stress on deformation temperature [22,30].

Fig. 11. Calculated results of friction stress σ0 (a) and fraction (b) of participating for each deformation mode at each angle λ between tension loading direction and normal direction of slip plane.

Fig. 12. Calculated results about friction stress in RD and TD samples at various deformation temperatures based on actual crystal orientation.

Fig. 13. TEM micrographs of tension samples for ZK60 magnesium alloys at deformation temperatures of 100 °C (a), 200 °C (b) and 300 °C (c).

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