Microstructure and mechanical properties of large-scale Mg-Gd-Y-Zn-Mn alloys prepared through semi-continuous casting

doi:10.1016/j.jmst.2020.04.013

Abstract

Abstract:

Large-scale Mg-8Gd-4Y-1Zn-Mn (wt.%) alloy ingot with a diameter of 315 mm and a length of 2410 mm was prepared through semi-continuous casting. Chemical composition, microstructure and mechanical properties at different locations of the samples with as-cast, T4 and T6 heat-treated states, respectively, were investigated. No obvious macro segregation has been detected in the high-quality alloy ingot. The main eutectic structures at all different locations are composed of α-Mg, Mg3RE-type, Mg5RE-type and LPSO phases. At the edge of ingot, the unusual casting twins including $\text{ }\!\!\{\!\!\text{ 10}\bar{1}\text{2 }\!\!\}\!\!\text{ }$ extension twins and $\text{ }\!\!\{\!\!\text{ 10}\bar{1}\text{1 }\!\!\}\!\!\text{ }$ compression twins were observed due to the intensive internal stress. In T4 heat-treated alloy, the micro segregation was eliminated. The remained phases were α-Mg and LPSO phase. Combined with the remarkable age-hardening response, T6 samples exhibits improved mechanical properties at ambient temperature, which derives from the dense prismatic β' precipitates and profuse basal γ' precipitates.

Key words: Mg-Gd-Y-Zn-Mn alloy, Semi-continuous casting, Thermal treatment, Microstructure, Mechanical properties

Kui Wang, Jingfeng Wang, Xiaoxu Dou, Yuanding Huang, Norbert Hort, Sarkis Gavras, Shijie Liu, Yanwu Cai, Jinxing Wang, Fusheng Pan. Microstructure and mechanical properties of large-scale Mg-Gd-Y-Zn-Mn alloys prepared through semi-continuous casting[J]. J. Mater. Sci. Technol., 2020, 52: 72-82.

Figures/Tables 18

Fig. 1. Schematic illustration of the semi-continuous casting process.

Fig. 2. (a) Large-scale alloy ingot fabricated by semi-continuous casting; (b) Macroscopic casting defects observed in the head of the ingot.

Table 1 Heat treatments of the large-scale alloy ingot.

Designation	Heat treatment process
As-cast	Semi-continuous casting
T4	at 350 °C for 5 h + at 500 °C for 8 h + at 525 °C for 6 h
T6	T4 + at 200 °C for 60 h

Table 2 Actual chemical compositions of the as-cast samples at different locations.

Analyzed locations	Content (wt.%)
Analyzed locations	Mg	Gd	Y	Zn	Mn	Fe	Cu	Ni
Center	Bal.	8.83	4.09	1.32	0.82	0.0093	0.018	0.0015
1/2 radius	Bal.	8.41	4.26	1.35	0.84	0.0092	0.015	0.0007
Edge	Bal.	8.86	4.09	1.32	0.82	0.0089	0.017	0.0011

Fig. 3. XRD patterns of the as-cast samples at different locations.

Fig. 4. Optical microstructures of the as-cast samples at different locations: (a) schematic illustration of the observed locations: center (b, e), 1/2 radius (c, f) and edge (d, g).

Fig. 5. SEM-BSE micrographs of the as-cast samples at different locations: center (a, d), 1/2 radius (b, e) and edge (c, f). Note, the arrows indicate regions with high concentrations of solute in solid solution.

Fig. 6. Microstructure of the as-cast samples: (a) Local magni?ed area of Fig. 4(b); representative EDS spectrum collected from the Mg3RE-type (b), Mg5RE-type (c) and LPSO phase (d), respectively.

Fig. 7. EBSD analysis of the as-cast sample near the edge: (a) IPF map, (b) local misorientation map, (c) band contrast map with highlighted twin boundaries; (d) the 0001, $\text{ }\!\!\{\!\!\text{ 11}\bar{2}\text{0 }\!\!\}\!\!\text{ }$ and $\text{ }\!\!\{\!\!\text{ 10}\bar{1}\text{0 }\!\!\}\!\!\text{ }$ pole ?gures.

Fig. 8. DSC curves of the as-cast samples at different locations: (a) center; (b) 1/2 radius; (c) edge.

Fig. 9. XRD patterns of the T4 samples at different locations.

Fig. 10. DSC curves of the T4 samples at different positions: (a) center; (b) 1/2 radius; (c) edge.

Fig. 11. Optical microstructures of the T4 samples at different locations: (a, d) center; (b, e) 1/2 radius; (c, f) edge. Note, the red and yellow arrows indicate lamellar LPSO and block-shape LPSO phases, respectively.

Fig. 12. SEM-BSE images of the T4 samples at different positions: (a, d) center; (b, e) 1/2 radius; (c, f) edge. Note, the red and yellow arrows indicate the lamellar and block-shape LPSO phase, respectively.

Fig. 13. Age-hardening curves of T4 samples at 1/2 radius location aged at 180 °C and 200 °C.

Fig. 14. Combined plot of UTS, TYS and EL of as-cast samples, T4 samples and T6 samples at different locations.

Table 3 Ambient tensile properties of samples at different locations in different thermal conditions.

Analyzed locations	Thermal Condition	Tensile mechanical properties
Analyzed locations	Thermal Condition	UTS (MPa)	TYS (MPa)	EL (%)
Center	As-cast	179 ± 2	141 ± 4	2.2 ± 0.2
	T4	189 ± 2	175 ± 2	1.7 ± 0.2
	T6	213 ± 5	184 ± 2	1.3 ± 0.2
1/2 radius	As-cast	155 ± 7	136 ± 2	1.3 ± 0.2
	T4	208 ± 2	185 ± 2	1.9 ± 0.2
	T6	234 ± 5	188 ± 7	1.5 ± 0.2
Edge	As-cast	210 ± 2	164 ± 3	3.8 ± 0.2
	T4	240 ± 2	194 ± 3	2.9 ± 0.2
	T6	268 ± 3	214 ± 4	1.8 ± 0.2

Fig. 15. TEM BF images and corresponding SAED patterns of the peak-aged alloy: (a, b) block-shape LPSO and β' phases; (c, d) lamellar γ' and β' phases.

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