In-situ investigation on the microstructure evolution of Mg-2Gd alloys during the V-bending tests

doi:10.1016/j.jmst.2022.05.039

Abstract

Abstract:

In this work, the in-situ observation of the microstructure in the Mg-2Gd (wt%) alloys during V-bending tests was operated. The microstructure and texture evolutions were characterized by the in-situ electron backscatter diffraction (EBSD), scanning electron microscopy (SEM) and the high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) technique. The results revealed a unique microcracks nucleation mechanism in the Mg-Gd alloys compared with the traditional Mg alloy (AZ31 alloy). The microcrack was nucleated at the grain boundary for the Mg-2Gd alloy and in the intragranular for the AZ31 alloy during the bending process. This difference mode between the Mg-Gd and the AZ31 alloys was mainly attributed to the more random topology of the grain boundary network, the lower grain boundary cohesion and the enhanced hinder ability for the dislocations due to the segregated Gd atoms in the Mg-Gd alloy. Furthermore, the microcracks that had a large angle θ with the extruded direction (ED) in the Mg-Gd alloys were preferred nucleated since grain boundaries had the larger normal stress during the bending process.

Key words: Magnesium, In-situ V-bending, Microstructure, Texture, Microcracks

Chao He, Jianxin Zhou, Yan Yang, Bin Jiang, Ming Yuan, Zhihua Dong, Yanfu Chai, Weijun He, Guangsheng Huang, Dingfei Zhang, Fusheng Pan. In-situ investigation on the microstructure evolution of Mg-2Gd alloys during the V-bending tests[J]. J. Mater. Sci. Technol., 2022, 131: 167-176.

Figures/Tables 14

Fig. 1. Microstructure and texture of the as-extruded Mg-2Gd (wt%) alloys: (a) {0001} PF; (b) IPF coloring maps; (c) the bending load-punch stroke (PS) curves and (d) the slopes of the loads-PS curve.

Fig. 2. (a) FE-SEM image, (b) electron backscatter diffraction results of inverse pole figure (IPF) coloring map and (c) the correspond {0001} PF and the {10-10} PF measured on the ND surface of as-received and deformed specimens at different PSs under the in-situ bending test of Mg-2Gd alloys.

Fig. 3. High magnification FE-SEM surface image, IPF map and SF distribution map measured on the surface (ND plane) of the deformed specimens at PSs = 8 mm in the ex-situ bending test of Mg-2Gd alloys: (a) grains 1-5, (b) grains 6-10 and (c) grains 11-14.

Fig. 4. Angle θ between the crack direction and the ED of 32 microcracks in the 8 mm-bent sample.

Fig. 5. In-situ observation of the twinning behavior when the bending sample at different PSs under the in-situ bending test of Mg-2Gd alloys: (a) M1, (b) M2 and (c) M3.

Fig. 6. Texture evolution in the cross-sectional of the 2 mm-bent Mg-Gd samples: (a, d) the IPF coloring map, (b, e) special grain boundaries map and (c, f) the {0001} PF (a—c) in the outer tensile region and (d-f) in the inner compress region. The locations of the EBSD observation are highlighted in the SEM images.

Fig. 7. Microstructure and the texture of the as-extruded AZ31 alloys: (a) {0001} PF; (b) IPF maps; (c) the bending load-punch stroke (PS) curves and (d) the slopes of the loads-PS curve of the Mg-Gd alloy under the condition that V-shaped die angle of 60°, the die span of 20 mm.

Fig. 8. (a) FE-SEM surface image and (b) the IPF measured on the ND surface of as-received and deformed specimens at different PSs under the in-situ bending test of AZ31 alloys.

Fig. 9. High magnification FE-SEM surface image measured on the ND surface of the deformed specimens at different PSs under the in-situ bending test of (a) AZ31 and (b) Mg-2Gd alloys.

Fig. 10. HAADF-STEM image and corresponding EDX maps showing solute distribution near the grain boundaries in (a-d) AZ31 alloy and the (e, f) Mg-Gd alloy; (g) the Gd atoms distribution along the green line in (f).

Fig. 11. Schematics of the normal stress at grain boundaries.

Table. 1. Orientation messages and corresponding SF basal values of the selected grains in Fig. 3.

Grain	Euler angle (°)			SF
Grain	φ1	φ	φ2	SF
1	165.4	11.6	14.4	0.04
2	32.6	140.4	52.9	0.32
3	106.9	169.6	12.5	0.17
4	177.1	160.6	25.7	0.02
5	176.5	147.2	8.7	0.03
6	62.7	124.3	34.0	0.48
7	119.8	172.4	4.6	0.10
8	130.1	173.3	21.4	0.08
9	35.6	165.0	35.9	0.13
10	143.6	162.6	33.8	0.16
11	102.9	170.4	21.9	0.36
12	93.2	153.0	56.2	0.4
13	145.3	164.3	34.4	0.14
14	127.3	156.9	9.5	0.26

Fig. 12. Origin-to-point misorientation line profile along the direction indicated as an arrow in Fig. 3.

Fig. 13. Misorientation angle distributions of the AE (a) AZ31 alloys and (b) the Mg-2Gd alloys.

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