Microstructures and mechanical properties of a newly developed high-pressure die casting Mg-Zn-RE alloy

doi:10.1016/j.jmst.2020.04.030

Abstract

Abstract:

A newly developed Mg-4Zn-2La-3Y alloy with high strength was fabricated by high-pressure die casting method, and its microstructures were thoroughly studied using transmission electron microscopy. The results demonstrate that it owns fine grains and approximately highly interconnected intermetallic phase skeletons, and exhibits ultra-high strength at both room and high temperatures. Interestingly, the eutectic intermetallic skeleton of this alloys is consisted of numerous fine particles, which are mainly consisted of two intermetallic phases, namely W and Mg₁₂RE. Multiple {101} twins and SFs were found in the Mg₁₂RE phase while a few of SFs in the W phase. Additionally, minor long-period stacking ordered phase was observed in the eutectoid phase, and it probably nucleated on the Mg₁₂RE phase following a certain OR as (0002)_14H//(110)_Mg12RE and [$11\bar{2}0$]_14H//[$1\bar{1}\bar{1}$]_Mg12RE, or (0002)_14H//(211)_Mg12RE and [$11\bar{2}0$]_14H//[$1\bar{1}\bar{1}$]_Mg12RE. This special intermetallic skeleton with many interfaces and planar faults can efficiently transfer dislocations across grain boundaries, and this is the key factor for the outstanding mechanical properties of the studied alloy.

Key words: Magnesium alloys, High-pressure die casting (HPDC), Transmission electron microscopy (TEM), Intermetallic phase, Mechanical properties

Xiru Hua, Qiang Yang, Dongdong Zhang, Fanzhi Meng, Chong Chen, Zihao You, Jinghuai Zhang, Shuhui Lv, Jian Meng. Microstructures and mechanical properties of a newly developed high-pressure die casting Mg-Zn-RE alloy[J]. J. Mater. Sci. Technol., 2020, 53: 174-184.

Figures/Tables 14

Fig. 1. SEM images of the HPDC ZLaW423 alloy.

Fig. 2. XRD pattern of the HPDC ZLaW423 alloy.

Fig. 3. BF-TEM image (a) and HAADF STEM image (b) of the eutectoid phase, (c) the enlarged TEM image, (d, f) the SAED patterns from the dark phases, and (e, g, h) the corresponding HRTEM images along with FFT patterns.

Fig. 4. (a) HAADF STEM image of chained eutectoid phase, (b) BF-TEM image of the dark particle marked by the yellow arrow in (a), the corresponding SAED pattern (c), (d) EDS spectrum along with the analysis result, and (e, f) HRTEM images of the dark phase.

Fig. 5. BF-TEM images (a, b) along with the corresponding SAED patterns (c, d), (e) EDS spectrum along with the analysis result, and (f) HRTEM image for the bright phase. The inserts in (a, b) are the corresponding HAADF STEM images.

Fig. 6. HAADF STEM image (a) and BF-TEM image (b) of the plates at grain boundaries, and (c, d) the corresponding SAED patterns. (e) the EDS spectrum along with the analysis result for the 14H LPSO phase, and (f) the HRTEM image of the fine 14H LPSO phase in (a).

Fig. 7. (a) HAADF STEM image and the corresponding EDS mappings for the eutectoid particle simultaneously containing three intermetallic phases, whose corresponding SAED patterns were shown in (c, d, e).

Fig. 8. (a) SAED pattern from Mg12RE and 14H LPSO phases, (b, c) the corresponding HRTEM images of Mg12RE/14H interfaces along with the FFT patterns from local regions marked by 1-4.

Fig. 9. HAADF-STEM image (a) and the corresponding EDS mappings (b) for the fine intermetallic particle in the α-Mg grain, the corresponding SAED patterns of the bright (c) and the dark phases (d), respectively.

Fig. 10. Representative engineering compression curves of the HPDC ZLaW423 alloy at both room and high temperatures.

Table 1 Mechanical properties of the studied alloy.

Temperature (°C)	CYS (MPa)	UCS (MPa)	ε_C (%)
Room temperature	202 ± 5	436 ± 3	7.5 ± 1.5
150	184 ± 4	436 ± 2	12.1 ± 1.4
200	165 ± 5	440 ± 3	14.5 ± 1.7
250	153 ± 4	448 ± 4	16.9 ± 1.2

Fig. 11. Yield strength versus total RE loading for Mg-RE and traditional Mg-Al-RE based alloys.

Fig. 12. BF-TEM images of microstructures in α-Mg grains of the samples with fracture, (a) twins in α-Mg grain, (b) bands with high density dislocations, (c) dislocation bands interacting with intermetallic skeleton, (d, e) dislocation substructures in the region of dislocation bands interacting with intermetallic skeleton, (f) dislocations around fine particles in α-Mg grain. (a-c, f) B// $\bar{1}2\bar{1}0$Mg, (d, e) B close to $\bar{1}2\bar{1}0$Mg and g = $\bar{1}0\bar{1}0$Mg and [0002]Mg, respectively.

Fig. 13. (a) BF-TEM image along with the collected SAED patterns from regions B (b) and C (c), respectively, under the same tilting angles.

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