Effect of the Addition of High Li Concentration on the Microstructure and Mechanical Properties of Al-Mg-Si Alloys with Different Mg Contents

doi:10.1007/s40195-021-01210-8

Abstract

Abstract:

In the present work, the microstructural characteristics and mechanical properties of Al-1.5 Mg-0.6Si and Al-3.0 Mg-0.6Si alloy containing 3 wt% Li were investigated by optical microscopy (OM), X-ray diffraction analysis (XRD), scanning electron microscopy (SEM), time of flight-secondary ion mass spectrometry (SIMS), transmission electron microscopy (TEM), and mechanical performance testing. The addition of Li reduces the density of the base alloy by up to 8.4%. The residual second phases contain Mg and Si in the hot-rolled condition, but the Mg/Si atomic ratio decreases after quenching, which means that Li substitute some of the Mg and convert Mg₂Si into a (Mg, Li)₂Si phase during solution treatment. The results of SIMS observations confirm this. The high Mg-containing alloy has a more rapid hardening response compared to the low Mg-containing alloy. TEM observation reveals that the δ′-Al₃Li + β′′-Mg₂Si dual phases can be observed in the high Mg-containing alloy after aging for 100 h at 170 °C. The higher Mg content enhances the precipitation of the δ′ phase, which results in the high Mg-containing alloy having a larger average diameter size of δ′ particles and wider δ′-precipitate-free zones (δ′-PFZs). The mechanical properties are significantly improved with the elastic modulus increasing by more than 16.5%. However, the existence of large second phases and wide δ′-PFZs in Li-containing alloys is detrimental to their ductility; as a result, their elongation is much lower than that of the base alloy.

Key words: Al-Mg-Si alloy, Li addition, Microstructure, Aging behavior, Mechanical properties, Secondary ion mass spectrometer

Xiao-Kun Yang, Bai-Qing Xiong, Xi-Wu Li, Li-Zhen Yan, Zhi-Hui Li, Yong-An Zhang, Ya-Nan Li, Kai Wen. Effect of the Addition of High Li Concentration on the Microstructure and Mechanical Properties of Al-Mg-Si Alloys with Different Mg Contents[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(12): 1721-1733.

Figures/Tables 18

Table 1 Chemical compositions of the investigated alloys (wt%)

Alloys	Li	Mg	Si	Al
Alloy-1	0.00	1.50	0.66	Bal
Alloy-2	3.00	1.40	0.65	Bal
Alloy-3	2.98	2.93	0.64	Bal

Fig. 1 Schematic diagram of the thermo-mechanical treatment process

Fig. 2 Density of the experimental alloys under hot-rolled condition

Fig. 3 Optical micrographs a-c and EBSD IPF maps d-f (the embedded graphs reveal the grain size distribution diagrams and average grain size) for a, d alloy-1, b, e alloy-2, and c, f alloy-3 in hot-rolled condition

Fig. 4 XRD patterns of the experimental alloys under hot-rolled and water-quenched conditions

Fig. 5 EDS spots scan results of the second phases in a, d alloy-1, b, e alloy-2, and c, f alloy-3 in a-c hot-rolled, d-f water-quenched condition, respectively

Table 2 Chemical composition of the second phases analyzed by EDS spot scan in Fig. 5 (at.%)

Elements	A	B	C	D	E	F
Al	54.20	39.56	92.86	83.18	53.32	28.11
Mg	30.57	40.04	4.71	11.05	11.79	33.25
Si	15.23	20.40	2.44	5.77	34.88	38.65
Mg/Si	2.01	1.96	1.93	1.92	0.34	0.86

Fig. 6 SIMS mapping results of alloy-2 a-c, alloy-3 d-f in the water-quenched state. The color scale indicates the concentration

Fig. 7 Hardness evolution at the aging time at 170 °C isothermal aging for three experimental alloys

Fig. 8 TEM images and corresponding SAED patterns showing the precipitates in the [100]Al direction of a, d alloy-1, b, e alloy-2, and c, f alloy-3 aged at 170 °C for 8 h a-c and 48 h b-f

Fig. 9 TEM images of alloy-2 a-c and alloy-3 d-f aged at 170 °C for 100 h a, d, 150 h b, e and 200 h c, f

Fig. 10 HRTEM images of the precipitates in alloy-3 aged at 170 °C for 150 h: a δ′-Al3Li phase and b β′′-Mg2Si phase

Table 3 Element-vacancy bonding energy of Li, Mg, Si in aluminum [30]

Element	Li	Mg	Si
Bonding energy (eV)	0.26	< 0.20	0.30

Fig. 11 Precipitate size distributions of the δ′-Al3Li phase in alloy-2 a-c and alloy-3 d-f at different aging time at 170 °C: a, d 8 h, b, e 48 h and c, f 100 h

Fig. 12 TEM images of the δ′-precipitation-free zones (δ′-PFZs) half width as a function of the aging time in alloy-2 a-c and alloy-3 d-f aged at 170 °C for a, d 8 h, b, e 48 h, and c, f 100 h

Fig. 13 Mechanical properties and corresponding stress-stain curves of three experimental alloys aged at 170 °C for: a, d 8 h, b, e 48 h, c, f 100 h

Fig. 14 Dynamic elasticity modulus of the three experimental alloys aged at 170 °C for 8 h

Fig. 15 SEM fractographs of the three experimental alloys aged at 170 °C for a-c 8 h and b-f 48 h: a, d alloy-1, b, e alloy-2, and c, f alloy-3

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