Unveiling annealing texture formation and static recrystallization kinetics of hot-rolled Mg-Al-Zn-Mn-Ca alloy

doi:10.1016/j.jmst.2020.01.018

Abstract

Abstract:

The development of Mg-Al-Zn-Mn-Ca series alloys provides a potential prospect to achieve high strength and formability at room temperature (RT). The formation of elliptical annular texture is treated as a crucial factor for the enhanced RT formability. However, the origin of such an elliptical annular texture formation has been rarely reported. Herein, we unveiled the formation and evolution of elliptical annular texture in the hot-rolled Mg-1.6Al-0.8Zn-0.4Mn-0.5Ca (AZMX1100, wt.%) alloy after annealing at different temperatures for 1 h, and its static recrystallization (SRX) kinetics in given annealing temperature for different time. The results revealed that the formation of elliptical annular texture in the hot-rolled AZMX1100 alloy after annealing was derived from nucleation-oriented SRX mechanism, which took place in 200-300 °C, induced by cracked chain-shaped Al₂Ca phases, contraction twins, intersections of double twins, intersections of double twins and grain boundaries and non-basal slips. On further annealing from 300-450 °C, the grains with 45°-70° transverse direction (TD) preferentially grew, which made elliptical annular texture extended along the TD. Based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model, Avrami exponent n value was estimated to be 0.68-1.02, attributed to non-random SRX nucleation, giving rise to the lower activation energy Q_R of nucleation of ～74.24 kJ/mol. Since the co-segregation of Al, Zn and Ca atoms in grain boundaries created a strong interaction of solutes and grain boundaries, the hot-rolled AZMX1100 alloy exhibited the higher activation energy Q_g (～115.48 kJ/mol) of grain growth.

Key words: Mg-Al-Zn-Mn-Ca alloy, Elliptical annular texture, Nucleation, Grain growth, Static recrystallization kinetics

Qinghang Wang, Bin Jiang, Aitao Tang, Jie Fu, Zhongtao Jiang, Haoran Sheng, Dingfei Zhang, Guangsheng Huang, Fusheng Pan. Unveiling annealing texture formation and static recrystallization kinetics of hot-rolled Mg-Al-Zn-Mn-Ca alloy[J]. J. Mater. Sci. Technol., 2020, 43: 104-118.

Figures/Tables 23

Table 1 Detailed chemical composition of as-cast AZMX1100 alloy (wt.%).

Alloy	Mg	Al	Zn	Mn	Ca
AZMX1100	Bal.	1.64	0.84	0.38	0.49

Fig. 1. OM images and (0002) pole figures obtained by X-ray texture analysis of (a) hot-rolled AZMX1100 alloy and (b) fully annealed AZMX1100 alloy at 450 °C for 1 h.

Fig. 2. Two-beam bright-field TEM images showing the microstructure of hot-rolled sample, where these images are taken under two beam conditions using diffraction vectors of (a) g = 0002, (b) g = 01 $\bar{1}$ 0.

Fig. 3. (a) EBSD map of hot-rolled sample; (b) line profiles of point-to-point along white arrow AB in (a).

Fig. 4. Microstructures, grain size distributions of SRXed grains and (0002) pole figures of hot-rolled samples subjected to annealing at different temperatures: (a-f) 200, 250, 300, 350, 400 and 450 °C for 1 h, respectively.

Fig. 5. (a) Texture intensity values of RD and TD texture components as a function of annealing temperature; (b) Tilted angles of RD and TD texture components as a function of annealing temperature; (c) Schematic diagram of elliptical annular texture evolution.

Fig. 6. SRX nucleation of hot-rolled sample during annealing at 200 °C: (a) image quality including multiple twin types; (b) SEM image, where chain-shaped phase is marked by yellow dotted lines; (c) extracted SRXed grains from EBSD map in Fig. 3(a); (d) (0002) pole figure of extracted SRXed grains corresponding to (c).

Fig. 7. (a, b) SEM images of as-cast sample via homogenization at 450 °C for 12 h; (c) local magnification view and EDS results corresponding to (b); (d, e) SEM images of hot-rolled sample; (f) local magnification view and EDS results corresponding to (e).

Fig. 8. (a) SEM image of hot-rolled sample after annealing at 200 °C; (b) local magnification view corresponding to (a). Yellow dotted line area is SRX region at the around of cracked chain-shaped Al2Ca phases; (c) SEM image of hot-rolled sample after annealing at 250 °C; (d) local magnification view corresponding to (c). Yellow dotted line area is SRX region at the around of cracked chain-shaped Al2Ca phases. Green dotted line area is SRX region insides twins. Red area is SRX region insides deformed grain; (e-h) SEM images of hot-rolled samples after annealing at 300, 350, 400 and 450 °C, respectively.

Fig. 9. SRX nucleation induced compression twins in hot-rolled sample during annealing at 250 °C: (a) EBSD map extracted from Fig. 3(b); (b) KAM map corresponding to (a); (c) line profiles of point-to-point along the white line AB in (a); (d) orientation distributions of SRXed grains G1-G4 and matrix in (0002) pole figure.

Fig. 10. SRX nucleation induced double twin intersection in hot-rolled sample during annealing at 250 °C: (a) EBSD map extracted from Fig. 3(b); (b) KAM map corresponding to (a); (c) image quality including multiple twin types; (d) orientation distributions of SRXed grains G1-G14, double twins D1-D5 and matrix in (0002) pole figure.

Fig. 11. SRX nucleation induced intersections of double twins and grain boundaries in hot-rolled sample during annealing at 250 °C: (a) EBSD map extracted from Fig. 3(b); (b) KAM map corresponding to (a); (c) image quality including multiple twin types; (d) orientation distributions of SRXed grains G1- G2, double twin D1 and matrix in (0002) pole figure.

Fig. 12. Two-beam dark-field TEM images showing microstructure of hot-rolled sample, where these images were taken under two beam conditions using diffraction vectors of (a) g = 0002, (b) g = 01 $\bar{1}$ 0, (c) view of high magnification of red rectangular frame in (b).

Fig. 13. EBSD maps and (0002) pole figures of four groups of grain orientation components 0°-20° (TCA), 20°-45° (TCB), 45°-70° (TCC) and 70°-90° (TCD) tilted away from the ND in the hot-rolled sample after annealing at 400 °C (a-d) and hot-rolled sample after annealing 450 °C (e-h); (i, j) volume fraction and average grain size of each group of grain orientation component in hot-rolled samples after annealing at 400 °C and 450 °C.

Table 2 Summary of volume fractions and average grain sizes of TCA, TCB, TCC and TCD texture components in hot-rolled samples after annealing at 400 °C and 450 °C.

Texture component	400 °C		450 °C
Texture component	Volume fraction (%)	Ave. grain size (μm)	Volume fraction (%)	Ave. grain size (μm)
TCA	8.5	4.8	7.8	6.8
TCB	52.2	5.1	38.0	7.1
TCC	30.1	5.3	44.3	7.9
TCD	9.8	5.3	9.9	7.0

Table 3 Summary of eight possible low-energy grain boundaries in HCP metals and corresponding angle range [35].

Low-energy grain boundary	Angle range (°)
31.99° [1$\bar{2}$10]	32±2
43.11° [1$\bar{2}$10]	45±2
61.91° [1$\bar{2}$10]	62±2
75.06° [1$\bar{2}$10]	75±2
28.41° [01$\bar{1}$0]	28±2
39.06° [01$\bar{1}$0]	39±2
58.36° [01$\bar{1}$0]	58±2
72.88° [01$\bar{1}$0]	73±2

Fig. 14. Rotation axis distribution of 8 special grain boundaries shown in Table 3 in hot-rolled samples after annealing at 400 °C and 450 °C.

Fig. 15. Schematic diagram of SRX process: stage ①① represents the initial homogenized state at 450 °C; stages ②-④ show the SRX nucleation process at low temperature annealing (200-250 °C); stages ⑤-⑥ demonstrates the grain growth of SRXed grains (300-450 °C).

Fig. 16. (a) Hardness of annealing samples at 200, 300 and 450 °C as a function of annealing time; (b) fractional softening (XH) of annealing samples at 200, 300 and 450 °C as a function of annealing time; (c) lnln(1/(1-XH) of annealing samples at 200, 300 and 450 °C vs. ln t; (d) linear fitting of Eq. (5): ln(1/tR) versus 1/T.

Table 4 Hardness of hot-rolled sample after annealing at 200 °C for different time.

	Annealing time (min)
	10	30	60	120	600	960	2280
Hardness (HV)	70.1	68.0	64.9	60.8	58.2	57.1	57.0

Table 5 Hardness of hot-rolled sample after annealing at 300 °C for different time.

	Annealing time (min)
	0.5	1	5	10	20	30	40	50	60	120	300
Hardness (HV)	69.0	68.1	64.3	61.5	59.6	58.1	57.6	57.1	56.9	55.9	54.3

Table 6 Hardness of hot-rolled sample after annealing at 450 °C for different time.

	Annealing time (min)
	0.1	0.5	1	3	5	10	20	30	40	50	60	120
Hardness (HV)	69.9	68.1	65.1	62.2	61.3	55.1	53.7	53.7	53.2	53.1	53.1	52.3

Fig. 17. (a) Average grain size, D, of hot-rolled samples after annealing at 300 °C and 450 °C as a function of annealing time; (b) linear fitting of Eq. (11): ln(dD/dt) as a function of lnD; (c) linear fitting of Eq. (13): lnk versus (1/T).

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