A simultaneous improvement of both strength and ductility by Sn addition in as-extruded Mg-6Al-4Zn alloy

doi:10.1016/j.jmst.2019.04.048

Abstract

Abstract:

Commercial wrought Mg alloys normally contain low alloying contents to ensure good formability. In the present work, high-alloyed Mg-6Al-4Zn-xSn (x = 1, 2 and 3 wt.%, respectively) alloys were fabricated by extrusion. Hereinto, Sn was proven to play an effective contribution to simultaneous improvement in strength and ductility that are traditional trade-off features of synthetic materials. It was found that the average grain size of those alloys decreases significantly from ~11 to ~4 μm as a function of Sn contents increasing from 0 to 3 wt.%, while the amounts of Mg₂Sn and Mg₁₇Al₁₂ particles continuously increase. More importantly, the addition of Sn leads to the transformation of dominated deformation modes from {10$\bar{1}$2} extension twinning (1 wt.%) to pyramidal <c+a> slip (3 wt.%) during tensile tests along the extrusion direction at room temperature. The advantageous combination of ultimate tensile strength (~366 MPa) and elongation (~19 %) in Mg-6Al-4Zn-3Sn alloy is mainly attributed to the strong strain hardening ability induced by the enhanced activity of non-basal <c+a> slip. This work could provide new opportunities for the development of high-alloyed wrought Mg alloys with promising mechanical properties.

Key words: Magnesium alloys, Microstructure, Deformation modes, Ductility

Xiao-Yuan Wang, Yu-Fei Wang, Cheng Wang, Shun Xu, Jian Rong, Zhi-Zheng Yang, Jin-Guo Wang, Hui-Yuan Wang. A simultaneous improvement of both strength and ductility by Sn addition in as-extruded Mg-6Al-4Zn alloy[J]. J. Mater. Sci. Technol., 2020, 49: 117-125.

Figures/Tables 15

Table 1 Nominal and measured compositions of the studied alloys.

Notation	Nominal composition	Measured composition (wt.%)
		Al	Zn	Sn	Mg
AZ64	Mg-6Al-4Zn	5.80	4.13	-	Bal.
AZT641	Mg-6Al-4Zn-1Sn	5.98	4.27	0.99	Bal.
AZT642	Mg-6Al-4Zn-2Sn	6.11	4.63	1.89	Bal.
AZT643	Mg-6Al-4Zn-3Sn	6.18	4.52	2.95	Bal.

Fig. 1. Engineering stress-strain curves of as-extruded AZ64-xSn alloys under tension along extrusion direction at RT.

Table 2 Tensile properties of as-extruded AZ64-xSn alloys with different Sn contents tested along extrusion direction at RT.

Nominal composition	Yield strength σ_0.2 (MPa)	Ultimate tensile strength σ_b (MPa)	Elongation ε (%)	Fracture strain ε_b (%)
AZ64	$163_{-1}^{+2}$	$283_{-2}^{+3}$	$11.9_{-0.2}^{+0.8}$	$12.1_{-0.2}^{+0.8}$
AZT641	$171_{-6}^{+5}$	$328_{-7}^{+6}$	$14.8_{-1.7}^{+1.4}$	$16.3_{-1.7}^{+1.4}$
AZT642	$197_{-1}^{+2}$	$349_{-2}^{+3}$	$17.8_{-0.4}^{+0.9}$	$17.9_{-1.2}^{+1.9}$
AZT643	$207_{-3}^{+3}$	$366_{-1}^{+2}$	$19.1_{-0.6}^{+0.9}$	$20.3_{-0.6}^{+0.9}$

Fig. 2. Elongation and ultimate tensile strength of various Mg based as-extruded alloys with extrusion ratio varied from 25:1 to 40:1 reported in literatures [12,14,[20], [21], [22], [23], [24], [25], [26], [27], [28]].

Fig. 3. Optical micrographs (inset: corresponding grain size distribution, dave represents the average grain size) of as-extruded AZ64-xSn alloys: (a) AZ64; (b) AZT641; (c) AZT642; (d) AZT643. (ED and TD stand for extrusion direction and transverse direction, respectively).

Fig. 4. X-ray diffraction patterns of as-extruded AZ64-xSn alloys: (a) AZ64; (b) AZT641; (c) AZT642; (d) AZT643.

Fig. 5. BSE-SEM and corresponding SE-SEM micrographs of as-extruded AZ64-xSn alloys: (a, e and i) AZ64; (b, f and j) AZT641; (c, g and k) AZT642; (d, h and l) AZT643. Af Mg2Sn and Af represent the area fraction of Mg2Sn and total precipitates, respectively.

Fig. 6. (a) BSE-SEM micrograph of as-extruded AZT643 sample; (b) EDS line scan for Mg, Al, Zn and Sn of the particle in (a).

Fig. 7. EBSD inverse pole figure (IPF) maps of as-extruded AZ64-xSn alloys and the different types of grains: (a,e) AZ64; (b,f) AZT641; (c,g) AZT642; (d,h) AZT643: blue-recrystallized, yellow-substructured, red-deformed.

Table 3 Corresponding area fraction of different types of grains in as-extruded AZ64-xSn alloys.

Nominal composition	Recrystallized (%)	Substructured (%)	Deformed (%)
AZ64	98.20	1.51	0.28
AZT641	95.08	3.57	1.35
AZT642	94.20	2.32	3.47
AZT643	89.99	3.85	6.15

Fig. 8. Pole figures of as-extruded AZ64-xSn alloys sheets: (a) AZ64; (b) AZT641; (c) AZT642; (d) AZT643.

Fig. 9. Hardening curves for AZ64 and AZT643 alloys under tension at RT.

Table 4 Average Schmid factors calculated from the inverse pole figures of AZ64 and AZT643 samples at different tensile stages along ED.

	AZ64			AZT643
	Non-deformed	8% deformed	Variation (%)	Non-deformed	8% deformed	Variation (%)
Basal <a>	0.3	0.312	4	0.339	0.236	-30.4
Prismatic <a>	0.138	0.146	5.8	0.161	0.082	-49
Pyramidal <a>	0.276	0.274	-0.7	0.301	0.203	-32.6
Pyramidal <c+a>	0.396	0.386	-2.5	0.369	0.442	19.8

Fig. 10. EBSD band contrast maps of (a-c) AZ64 and (d-f) AZT643 samples superimposed by specific twin boundaries at different stages of tensile strain: 10$\bar{1}$2 extension twins identified by red lines reorient the basal planes by 86°, 10$\bar{1}$1 contraction twins marked by blue lines reorient the basal planes by 56° and 10$\bar{1}$1-10$\bar{1}$2 double twins in green lines reorient the basal planes by 38°.

Fig. 11. Kernel average misorientation map and misorientation histograms of (a-f) AZ64 and (g-l) AZT643 samples at different stages of tensile strain.

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