Atomic-scale roles of Zn element in age-hardened AlMgSiZn alloys

doi:10.1016/j.jmst.2020.09.009

Abstract

Abstract:

Adding alloying elements to improve the performances or the manufacturing processes of Al-Mg-Si alloys has long been a serious issue in developing advanced automotive aluminum materials. The Zn element, among those promising ones, has demonstrated positive alloying effects on Al-Mg-Si alloys. However, the atomic-scale roles of Zn in an age-hardened Al-Mg-Si-Zn alloy have not been adequately understood. Using atomic-resolution electron microscopy, here we report the precise locations of Zn elements in all hardening precipitates involved and their alloying mechanism at the atomic scale when alloying the alloy. Our results show that Zn atoms enter all the major hardening phases to occupy specific featured atomic sites of the original elements, e.g. the Si1 and Mg2 sites in the β'-2 phase, and modify their crystal structures, interfacial structures and morphologies in characteristic manners. It is revealed that for the β'-phase, Zn atoms occupy unique atomic sites, whereas for other phases, they demonstrate similar behaviors as other additive alloying elements such as Ag and Cu do.

Key words: Al-Mg-Si alloy, Zn addition, Precipitate, Electron microscopy

N.N. Jiao, Y.X. Lai, S.L. Chen, P. Gao, J.H. Chen. Atomic-scale roles of Zn element in age-hardened AlMgSiZn alloys[J]. J. Mater. Sci. Technol., 2021, 70: 105-112.

Figures/Tables 10

Fig. 1. Age-hardening curves of the two investigated alloys aged at 180 °C for different time.

Fig. 2. TEM bright-field images of the two alloys peak-aged (a, b) and over-aged (c, d) at 180 °C. (a, c) Zn-free alloy, (b, d) Zn-containing alloy.

Fig. 3. HRTEM images of the typical precipitates formed in the Zn-free alloy after peak-aging (a) and over-aging (b) at 180 °C. The inset in each image is the corresponding FFT pattern.

Fig. 4. Legend representing the structure models in Fig. 5, Fig. 6 and schematic drawing of the unit cell of β", U2, Q' and β'-2. The characteristic sub-unit(s) of the four phases are marked by dashed-line circles, solid-line hexagons, double-solid-line triangles and dashed-line triangle-like hexagon, respectively.

Fig. 5. Atomic-resolution HAADF-STEM images of β"Zn-precipitate (a) and β"Zn/U2Zn/β'Zn/Q'Zn composite precipitate (c) formed in the Zn-containing alloy peak-aged at 180 °C. (b) and (d) are suggested structure models for the images (a) and (c), respectively. See legend in Fig. 4.

Fig. 6. Atomic-resolution HAADF-STEM images of β"Zn/U2Zn/β'Zn (a) and U2Zn/β'Zn/Q'Zn (c) composite precipitates formed in the Zn-containing alloy over-aged at 180 °C for 36 h. (b) and (d) are suggested structure models for the images (a) and (c), respectively. See legend in Fig. 4.

Fig. 7. The formation enthalpies of β'-2 unit cells with different atomic sites replaced by Zn.

Fig. 8. Schematic drawing of the atomic structures of β'Zn and β'Ag phases.

Table 1 Comparison between the crystal structure of β'Zn and β'Ag unit cells.

	β'_Zn					β'_Ag [28]
Lattice parameters	a = b =0.690 nm, c =0.405 nm					a = b =0.690 nm, c =0.405 nm
Space group	P-62 m (hexagonal)					P-62 m (hexagonal)
Composition	Mg₃Si₂Zn₄					Mg₃Al₃Si₂Ag
Atomic position	Atom	x	y	z	Occ.	Atom	x	y	z	Occ.
	Zn1	0	0	0	1	Ag	0	0	0	1
	Si	1/3	2/3	1/2	1	Si	1/3	2/3	1/2	1
	Mg	0.4	0.4	0	1	Mg	0.4	0.4	0	1
	Zn2	0.74	0.74	1/2	1	Al	0.74	0.74	1/2	1

Fig. 9. Formation enthalpies of modified β'-2 unit cells plotted as a function of composition.

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