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. 2017 Jan 5:7:40078.
doi: 10.1038/srep40078.

Natural ageing responses of duplex structured Mg-Li based alloys

C Q Li  1   2 , D K Xu  1 , B J Wang  1 , L Y Sheng  3 , Y X Qiao  4 , E H Han  1
Affiliations

Natural ageing responses of duplex structured Mg-Li based alloys

C Q Li et al. Sci Rep. .

Abstract

Natural ageing responses of duplex structured Mg-6%Li and Mg-6%Li-6%Zn-1.2%Y alloys have been investigated. Microstructural analyses revealed that the precipitation and coarsening process of α-Mg particles could occur in β-Li phases of both two alloys during ageing process. Since a certain amount of Mg atoms in β-Li phases were consumed for the precipitation of abundant tiny MgLiZn particles, the size of α-Mg precipitates in Mg-6%Li-6%Zn-1.2%Y alloy was relatively smaller than that in Mg-6%Li alloy. Micro hardness measurements demonstrated that with the ageing time increasing, the α-Mg phases in Mg-6%Li alloy could have a constant hardness value of 41 HV, but the contained β-Li phases exhibited a slight age-softening response. Compared with the Mg-6%Li alloy, the age-softening response of β-Li phases in Mg-6%Li-6%Zn-1.2%Y alloy was much more profound. Meanwhile, a normal age-hardening response of α-Mg phases was maintained. Tensile results indicated that obvious ageing-softening phenomenon in terms of macro tensile strength occurred in both two alloys. Failure analysis demonstrated that for the Mg-6%Li alloy, cracks were preferentially initiated at α-Mg/β-Li interfaces. For the Mg-6%Li-6%Zn-1.2%Y alloy, cracks occurred at both α-Mg/β-Li interfaces and slip bands in α-Mg and β-Li phases.

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Figures

Figure 1
Figure 1
Optical observations to the etched surfaces of: (a) as-extruded and (b) solid solution treated samples of Mg-6%Li alloy, (c) as-extruded and (d) solid solution treated samples of Mg-6%Li-6%Zn-1.2%Y alloy.
Figure 2
Figure 2
(a) Secondary electron and (b) backscattered electron images of the as-extruded Mg-6%Li-6%Zn-1.2%Y alloy; Images (c) and (d) are the SEM-EDS results of the marked areas of “C” and “D” in image (b).
Figure 3
Figure 3
Microstructural evolution of the Mg-6%Li alloy subjected to natural ageing for (a) 0 days, (b) 7 days, (c) 21 days, (d) 35 days, (e) 42 days and (f) 180 days; High magnification observations to the squared area are inserted.
Figure 4
Figure 4
Microstructural evolution of the Mg-6%Li-6%Zn-1.2%Y alloy subjected to natural ageing for (a) 0 days, (b) 7 days, (c) 21 days, (d) 35 days, (e) 42 days and (f) 180 days; High magnification observations to the squared area are inserted.
Figure 5
Figure 5
TEM morphologies and TEM-EDS elemental mappings of Mg-6%Li-6%Zn-1.2%Y alloy subjected to natural ageing for 7 days: (a) and (b) bright field images; The selected area diffraction patterns (SADPs) to the α-Mg precipitates is inserted in image (a); TEM-EDS elemental mappings of: (c) Mg, (d) Zn and (e) Y.
Figure 6
Figure 6
Micro hardness of β-Li and α-Mg phases versus natural ageing time curves for: (a) Mg-6%Li and (b) Mg-6%Li-6%Zn-1.2%Y alloys.
Figure 7
Figure 7
Tensile stress-strain curves of: (a) Mg-6%Li and (b) Mg-6%Li-6%Zn-1.2%Y alloys before and after natural ageing for 35 days.
Figure 8
Figure 8
Observations to side surfaces near the fracture sites of: (a) and (c) Mg-6%Li alloy before and after natural ageing for 35 days, (e) and (g) Mg-6%Li-6%Zn-1.2%Y alloy before and after natural ageing for 35 days. Images (b,d,f and h) are high magnification observations to the squared areas in images (a,c,e and g), respectively.
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