Development of high mechanical properties and moderate thermal conductivity cast Mg alloy with multiple RE via heat treatment

doi:10.1016/j.jmst.2017.12.011

Abstract

Abstract:

A new cast Mg-2Gd-2Nd-2Y-1Ho-1Er-0.5Zn-0.4Zr (wt%) alloy was prepared by direct-chill semi-continuous casting technology. The microstructure, mechanical properties and thermal conductivity of the alloy in as-cast, solid-solution treated and especially peak-aged conditions were investigated. The as-cast alloy mainly consists of α-Mg matrix, (Mg, Zn)₃ RE phase and basal plane stacking faults. After proper solid-solution treatment, the microstructure becomes almost Mg-based single phase solid solution except just very few RE-riched particles. The as-cast and solid-solution treated alloys exhibit moderate tensile properties and thermal conductivity. It is noteworthy that the Mg alloy with 8 wt% multiple RE exhibits remarkable age-hardening response (ΔHV = 35.7), which demonstrates that the multiple RE (RE = Gd, Nd, Y, Ho, Er) alloying instead of single Gd can effectively improve the age-hardening response. The peak-aged alloy has a relatively good combination of high strength/hardness (UTS (ultimate tensile strength) > 300 MPa; TYS (tensile yield strength) > 210 MPa; 115.3 HV), proper ductility (ε ≈ 6%) and moderate thermal conductivity (52.5 W/(m K)). The relative mechanisms mainly involving aging precipitation of β￠ and β′′ phases were discussed. The results provide a basis for development of high performance cast Mg alloys.

Key words: Mg alloys, Age hardening behavior, Microstructure, Mechanical properties, Thermal conductivity

Guoqiang Li, Jinghuai Zhang, Ruizhi Wu, Yan Feng, Shujuan Liu, Xiaojun Wang, Yufeng Jiao, Qiang Yang, Jian Meng. Development of high mechanical properties and moderate thermal conductivity cast Mg alloy with multiple RE via heat treatment[J]. J. Mater. Sci. Technol., 2018, 34(7): 1076-1084.

Figures/Tables 14

Table 1 Chemical composition of the investigated alloy (wt%).

Alloy	Gd	Nd	Y	Ho	Er	Zn	Zr	Mg
Mg-2Gd-2Nd-2Y-1Ho-1Er-0.5Zn-0.4Zr	2.1	1.9	2.0	0.9	1.0	0.5	0.4	Bal.

Fig 1. Microstructure of as-cast alloy: (a) OM image and (b) SEM image.

Table 2 EDS results of micro-constitution in Fig. 2(b) (wt%).

	MgK	NdL	GdL	HoL	ErL	YK	ZnK	ZrK
Location A	84.29	4.57	2.20	1.75	1.44	2.08	3.67	-
Location B	86.78	3.84	1.93	1.47	1.46	1.98	2.54	-
Location C	28.25	0.40	0.19	0.31	0.62	9.55	0.84	59.85

Fig. 2. XRD patterns of the as-cast and solid-solution treated alloys.

Fig. 3. TEM analyses of the as-cast alloy: (a, b) Mg3Gd-type phase; (c, d) basal plane SFs.

Fig. 4. DSC heating curve of the studied alloy.

Fig. 5. Microstructure of the solid-solution treated alloy: (a) OM image; (b) TEM result of the cuboid-shaped phase.

Fig. 6. Age hardening curve of the alloy aged at 200 °C.

Fig. 7. TEM analyses of precipitates in the peak-aged alloy: (a, f) BF image; (b, d) SAED pattern and (c, e) HRTEM image.

Fig. 8. Tensile stress-strain curves of as-cast, solution-treated and peak-aged alloys.

Table 3 Mechanical properties of the studied alloys.

Alloy states	UTS (MPa)	TYS (MPa)	ε (%)
As-cast	183 ± 4	111 ± 2	8.1 ± 0.5
Solid-solution treated	208 ± 3	132 ± 3	10.3 ± 0.5
Peak-aged	306 ± 3	215 ± 2	5.7 ± 0.4

Fig. 9. OM and SEM images of fracture side and fracture surface after tensile testing at room temperature: (a, d) as-cast alloy; (b, e) solid-solution treated alloy; (c, f) peak-aged alloy. Note: the specimen for (a) was etched by 4% HNO3/C2H5OH solution; the specimen for (b, c) was etched by mixed solution of 2 ml deionized water, 2 ml acetic acid, 14 ml alcohol and 0.84 g picric acid.

Fig. 10. Thermal diffusivity (a) and thermal conductivity (b) of the as-cast, solution-treated and peak-aged alloys.

Fig. 11. Plot of UTS vs elongation of the current studied alloy and published Mg alloys.

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