Effect of Zn Addition on the Microstructure and Mechanical Properties of Cast Mg-10Gd-3.5Er-xZn-0.5Zr Alloys

doi:10.1007/s40195-020-01106-z

Abstract

Abstract:

In this work, the effects of Zn content (0-2 wt%) on microstructural evolution and mechanical properties of cast Mg-10Gd-3.5Er-0.5Zr alloys are studied. The results show that the as-cast Mg-10Gd-3.5Er-xZn-0.5Zr alloys are mainly composed of Mg matrix and secondary (Mg, Zn)₃(Gd, Er) phases distributed along grain boundaries. With the increase in Zn content, the volume fraction of secondary (Mg, Zn)₃(Gd, Er) phases increases and the grains get refined. In the process of solid solution treatment, Zn addition can lead to the formation of long-period stacking ordered (LPSO) structures and the volume fraction of LPSO structures increases with Zn content. In addition, the Zn addition can reduce the vacancy formation energy and accelerate the diffusion rate of RE elements in Mg matrix. Because of the comprehensive effect of secondary phases and the accelerated diffusion rate, the base alloy and 2Zn alloy have less grain growth after solid solution treatment than that of the 0.5Zn alloy and 1Zn alloy. The precipitation process is also accelerated by enhanced diffusion rate. At room temperature (RT), the strengthening effect of β^'+ β₁ precipitates is more effective than that of LPSO structures, so the peak-aged 0.5Zn alloy exhibits the most excellent mechanical performance at RT, with yield strength of 219 MPa, ultimate tensile strength 296 MPa and elongation of 6.4%. While LPSO structures have stronger strengthening effect at elevated temperature than that of β^'+ β₁ precipitates, so the 1Zn alloy and 2Zn alloy have more stable mechanical performance than that of the base alloy and 0.5Zn alloy with the increase in tensile temperature.

Key words: Mg alloy, Mg-Gd-Er-Zn, Microstructure, Mechanical properties, Strengthening mechanism

Qiu Zhang, Wencai Liu, Guohua Wu, Liang Zhang, Wenjiang Ding. Effect of Zn Addition on the Microstructure and Mechanical Properties of Cast Mg-10Gd-3.5Er-xZn-0.5Zr Alloys[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(11): 1505-1517.

Figures/Tables 13

Table 1 Actual chemical compositions and solution regimes of the studied alloys

Alloy	Gd	Er	Zn	Zr	Mg	Solution regimes
Base	9.80	3.38	-	0.57	Bal.	500 °C × 6 h
0.5Zn	10.20	3.66	0.60	0.54	Bal.	500 °C × 6 h
1Zn	9.82	3.76	1.12	0.58	Bal.	520 °C × 10 h
2Zn	9.89	3.67	2.10	0.64	Bal.	520 °C × 10 h

Fig. 1 Optical and SEM-BSE images of the as-cast Mg-10Gd-3.5Er-xZn-0.5Zr alloys: a base alloy; b 0.5Zn alloy; c 1Zn alloy; d 2Zn alloy

Fig. 2 Line scanning results of the as-cast Mg-10Gd-3.5Er-0.5Zn-0.5Zr alloy

Fig. 3 XRD patterns of the as-cast Mg-10Gd-3.5Er-xZn-0.5Zr alloys

Fig. 4 DSC curves of the as-cast Mg-10Gd-3.5Er-xZn-0.5Zr alloys

Fig. 5 XRD patterns of the as-quenched Mg-10Gd-3.5Er-xZn-0.5Zr alloys

Fig. 6 OM images and corresponding BSE images (insets) of the as-quenched Mg-10Gd-3.5Er-xZn-0.5Zr alloys: a base alloy; b 0.5Zn alloy; c 1Zn alloy; d 2Zn alloy

Fig. 7 SEM, and point scanning results of as-quenched Mg-10Gd-3.5Er-0.5Zn-0.5Zr alloy: a SEM with point scanning location labelled; b-d point scanning results of cubic phases; e point scanning results of LPSO structures

Fig. 8 Vickers hardness curve of Mg-10Gd-3.5Er-xZn-0.5Zr alloys aged at 225 °C

Fig. 9 TEM bright-field images and corresponding SAED patterns of peak-aged Mg-10Gd-3.5Er-xZn-0.5Zr alloys: a, b peak-aged base alloy with electron beam parallel to [0001] and $ [21\bar{1}0] $; c, d peak-aged 0.5Zn alloy with electron beam parallel to [0001] and $ [2\bar{1}\bar{1}0] $; e, f peak-aged 1Zn alloys with electron beam parallel to [0001] and the precipitates in higher magnification

Fig. 10 Mechanical properties of Mg-10Gd-3.5Er-xZn-0.5Zr alloys in various conditions: a as-quenched at RT; b peak-aged at RT; c peak-aged at 150 °C; d peak-aged at 300 °C

Fig. 11 SEM-SE images of fracture surfaces of the peak-aged alloys in different thermal conditions

Fig. 12 A part of the ageing sequence for Mg-Gd-Er alloy: a lath-shaped early-stage β? phase distributed on $ \{ 11\bar{2}0\} $ planes; b β? phase with necks; c β1 phase forming on $ \{ 10\bar{1}0\} $ planes in the necking area of β? phase; d β? phase between two β1 phases will break after a long time ageing treatment

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