Microstructure and strengthening mechanism of hot-extruded ultralight Mg-Li-Al-Sn alloys with high strength

doi:10.1016/j.jmst.2021.07.009

Abstract

Abstract:

Duplex-structured Mg-7Li-2Al-1.5Sn alloys with high strength were fabricated and their strengthening mechanism was investigated. The Mg-7Li-2Al-1.5Sn alloys were prepared by casting and extruded at the temperature of 533 K with an extrusion ratio of 25:1. The microstructure and mechanical properties of Mg-7Li-2Al-1.5Sn alloys were systematically investigated by OM, XRD, SEM, TEM, and tensile tests. The results show that Mg-7Li-2Al-1.5Sn alloys are mainly composed of α-Mg, β-Li, LiMgAl₂, Mg₂Sn and Li₂MgSn phases. The yield strength (YS), ultimate tensile strength (UTS) and elongation (EL) of the extruded alloy at room temperature reach 250 MPa, 324 MPa and 11.9%, respectively. A lot of Sn-rich precipitates (Mg₂Sn and Li₂MgSn) are precipitated during extrusion with an average size of ∼14 nm, which is beneficial to the grain refinement. Dynamic recrystallization occurs during hot deformation and the nano-precipitates effectively refine the dynamic recrystallized (DRXed) grains. Besides, the residual dislocations existed in DRXed and un-DRXed grains result in the dislocation strengthening in the extruded alloy. Mg-7Li-2Al-1.5Sn alloys possess excellent high-temperature mechanical properties with the YS, UTS and EL of 200 MPa, 237 MPa and 26.7% at 423 K, respectively. Sn-rich precipitates with good thermal stability can effectively prevent grain growth, which is good for the improvement of the high-temperature performance of Mg-Li-Al-Sn alloy.

Key words: Ultralight Mg-Li alloy, Microstructure, Mechanical properties, Precipitation, Strengthening mechanism

Gang Zhou, Yan Yang, Hanzhu Zhang, Faping Hu, Xueping Zhang, Chen Wen, Weidong Xie, Bin Jiang, Xiaodong Peng, Fusheng Pan. Microstructure and strengthening mechanism of hot-extruded ultralight Mg-Li-Al-Sn alloys with high strength[J]. J. Mater. Sci. Technol., 2022, 103: 186-196.

Figures/Tables 17

Fig. 1. Schematic representation of (a) extrusion die, and (b) tensile test specimen.

Table 1. Chemical compositions of Mg-7Li-2Al-1.5Sn alloy (wt.%).

Alloy	Li	Al	Sn	Mg
Mg-7Li-2Al-1.5Sn	7.17±0.21	1.96±0.02	1.38±0.01	Bal.

Fig. 2. Optical microstructure of Mg-7Li-2Al-1.5Sn alloys, (a) as-cast alloy; (b) as-extruded alloy (vertical to ED); (c) and (d) as-extruded alloy (parallel to ED); (e) grain size distribution of the DRXed grains.

Fig. 3. Pole figure maps for α-Mg of the as-extruded Mg-7Li-2Al-1.5Sn.

Fig. 4. XRD patterns of as-cast and as-extruded Mg-7Li-2Al-1.5Sn alloys.

Fig. 5. SEM morphologies (a, b) and EDS results (c-h) for precipitates of the Mg-7Li-2Al-1.5Sn alloy: (a) as-cast, (b) as-extruded, (c-e) points A-C of as-cast alloy, (f-h) points D-F of as-extruded alloy.

Fig. 6. (a) TEM morphology showing the second phase after homogenization at 250 °C for 4 h (before extrusion); (b) Precipitate morphology after extrusion; (c) TEM image of the DRXed grains; (d) TEM image of the un-DRXed region.

Fig. 7. Microstructures of the Mg-7Li-2Al-1.5Sn alloy: (a) HAADF-STEM image of the as-cast alloy after homogenization at 250 °C for 4 h; (e) HAADF-STEM of α-Mg grains in the as-extruded alloy; (i) HAADF-STEM of β-Li grains in the extruded alloy, and elemental mappings of related areas: (b), (f) and (j) Mg; (c), (g) and (k) Al; (d), (h) and (l) Sn.

Fig. 8. (a) HRTEM micrograph of Sn-rich precipitate; (b) Partially enlarged image of Sn-rich precipitate; (c) FFT pattern of HRTEM image.

Fig. 9. Dislocation analysis in as-extruded alloy under two-beam diffraction conditions near $ [11\bar{2}0] $. (a) two-beam bright-field (TBBF) image of un-DRXed grain under g = (0001); (b) TBBF image of un-DRXed grain under g = $ (0\bar{1}11) $; (c) TBBF image of DRXed grain under g = (0001); (d) TBBF image of DRXed grain under g = $ (0\bar{1}11) $.

Fig. 10. Tensile tensile curves of Mg-7Li-2Al-1.5Sn alloys: (a) as-cast, (b) as-extruded.

Table 2. Mechanical properties of Mg-7Li-2Al-1.5Sn test alloys at room temperature.

State	Yield strength (MPa)	Ultimate tensilestrength (MPa)	Elongation (%)
As-cast	126±7	164±9	5.4 ± 2.1
As-extruded	250±10	324±2	11.9 ± 0.4

Fig. 11. High-temperature tensile curve of as-extruded Mg-7Li-2Al-1.5Sn alloy (423 K).

Fig. 12. Fracture morphologies of Mg-7Li-2Al-1.5Sn alloys: (a) as-cast, (b) as-extruded, (c) extruded alloys at 423 K.

Fig. 13. Plot of tensile strength and elongation of the reported extruded dual-phase Mg-Li alloys [13], [15], [16], [17], [23], [29], [32], [40], [41], [42], [43], [44].

Table 3. The meaning and value of parameters used in the strengthening mechanism calculations.

Symbol	Meaning	Value	Unit
R	the average radius of pining particles	14	nm
Φ	the volume fraction of the pining particles	0.0018
K	material-related constants	245 [53]	μm^-1/2
d	the average grain size	2	μm
G	the shear modulus of α-Mg	17 [52]	GPa
b	the value of Burgers vector	0.32 [33]	nm
λ_p	the effective inter-particle spacing	227	nm
v	the Poisson ratio	0.3
d_p	the average size of particles	14	nm
r₀	the core radius of the dislocation equivalent to b	0.32 [33]	nm
M	the Taylor factor	4.2 [53]
α	constant	0.2
ρ	the dislocation density estimated form the alloy	∼6.0 × 10¹³	m ^{- 2}

Table 4. The strengthening contributions of as-extruded Mg-7Li-2Al-1.5Sn alloy from different strengthening mechanisms.

Alloy	Grain boundary strengthening (MPa)	Precipitation strengthening (MPa)	Dislocation strengthening (MPa)
Mg-7Li-2Al-1.5Sn	∼173	∼73	∼35

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