Heat-treatable Mg-9Al-6Sn-3Zn extrusion alloy

doi:10.1016/j.jmst.2017.11.012

Abstract

Abstract:

Mg-9Al-6Sn-3Zn (wt%) alloy was extruded and heat treated in T5 and T6 conditions, and its mechanical properties and microstructures were investigated. The extruded product can be slightly strengthened by the T5 treatment as a result of sparse and heterogeneous precipitation. Significant increase in strength is achieved by the T6 treatment, and this is mostly attributed to the formation of lamellar discontinuous Mg₁₇Al₁₂ precipitates. The segregation of Al and Zn at grain boundaries is responsible for the discontinuous Mg₁₇Al₁₂ nucleation. The T6-treated alloy exhibits a tensile yield strength of 341 MPa and an ultimate tensile strength of 409 MPa, together with an elongation to fracture of 4%.

Key words: Magnesium alloys, Extrusion, Mechanical property, Precipitation, Electron microscopy

Chaoqiang Liu, Chenglong Liu, Houwen Chen, Jian-Feng Nie. Heat-treatable Mg-9Al-6Sn-3Zn extrusion alloy[J]. J. Mater. Sci. Technol., 2018, 34(2): 284-290.

Figures/Tables 9

Fig. 1. (a) Age hardening curves of E and E + S samples during isothermal ageing at 200 °C, (b) tensile and compressive stress-stain curves of E, E + S, T5 and T6 samples at room temperature, T represents tension and C represents compression.

Table 1 Mechanical properties of E, E + S, T5 and T6 samples.

Samples	TYS (MPa)	UTS (MPa)	EL (%)	CYS (MPa)	YR
E	288	364	7.8	284	0.99
E + S	256	354	9.7	205	0.80
T5	301	366	5.4	292	0.97
T6	341	409	4.0	313	0.92

Fig. 2. (a, b) IPF maps and (c, d) grain size distribution histograms of (a, c) E and (b, d) E + S samples on the section parallel to ED. (e, f) (0002) and (101ˉ0) pole figures of (e) E and (f) E + S samples on the cross section perpendicular to ED.

Fig. 3. (a, c) Low-magnification and (b, d) high-magnification BSE images of (a, b) E and (c, d) E + S samples.

Fig. 4. (a) HAADF-STEM image of E sample, (b) and (c) SAED patterns of Mg17Al12 and Mg2Sn phases. (d-f) EDS maps of E sample, (d) Al map, (e) Sn map, (f) Zn map.

Fig. 5. (a) HAADF-STEM image of a grain boundary in the E + S sample, (b) enlarged HAADF-STEM image of the grain boundary, (c, d) EDS maps of the grain boundary, (c) Al map, (c) Zn map.

Fig. 6. (a, c) ECC and (b, d) HAADF-STEM images of (a, b) T5 and (c, d) T6 samples. Corresponding selected area electron diffraction patterns are inserted in (b) and (d), respectively. Electron beam direction is parallel to [21ˉ1ˉ0]Mg in (b, d).

Fig. 7. HRTEM image of the (a) basal and (b) no-basal Mg17Al12 precipitates viewed along [21ˉ1ˉ0]Mg. Corresponding FFT patterns are inserted in the images.

Table 2 Comparison of tensile/compressive properties and processing conditions of Sn-containing wrought magnesium alloys.

Samples	Process	Grain size (μm)	TYS (MPa)	UTS (MPa)	EL (%)	CYS	YR
Mg-9Al-6Sn-3Zn (this work)	E + T6	13.9	341	409	4.0	313	0.92
Mg-7.89Al-1.94Zn-3.96Sn-0.15 Mn [16]	E (Indirect)	1.2	280	370	10.1	304	1.09
Mg-9.3Sn-2.5Zn-0.2 Mn [11]	E + T6	54	190	260	12	-	-
Mg-9.8Sn-1.2Zn-1.0Al [9]	E	1.9	308	354	12	280	0.91
Mg-6.6Sn-5.9Zn-2.0Al-0.2 Mn [10]	E + T6^a	7.8	370	399	14	307	0.83

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