Effect of Icosahedral Phase on Crystallographic Texture and Mechanical Anisotropy of Mg-4%Li Based Alloys

doi:10.1016/j.jmst.2016.10.003

Abstract

Abstract:

Through investigating and comparing the microstructure and mechanical properties of the as-extruded Mg alloys Mg-4%Li and Mg-4%Li-6%Zn-1.2%Y (in wt%), it demonstrates that although the formation of I-phase (Mg₃Zn₆Y, icosahedral structure) could weaken the crystallographic texture and improve the mechanical strength, the mechanical anisotropy in terms of strength remains in Mg-4%Li-6%Zn-1.2%Y alloy. Failure analysis indicates that for the Mg-4%Li alloy, the fracture surfaces of the tensile samples tested along transverse direction (TD) contain a large number of plastic dimples, whereas the fracture surface exhibits quasi-cleavage characteristic when tensile samples were tested along extrusion direction (ED). For the Mg-4%Li-6%Zn-1.2%Y alloy, typical ductile fracture surfaces can be observed in both “TD” and “ED” samples. Moreover, due to the zonal distribution of broken I-phase particles, the fracture surface of “TD” samples is characterized by the typical “woody fracture”.

Key words: Mg-Li alloy, Texture, Mechanical anisotropy, Fracture

Li C.Q., Xu D.K., Yu S., Sheng L.Y., Han E.H.. Effect of Icosahedral Phase on Crystallographic Texture and Mechanical Anisotropy of Mg-4%Li Based Alloys[J]. J. Mater. Sci. Technol., 2017, 33(5): 475-480.

Figures/Tables 11

Table 1. Chemical composition of the Mg-4%Li and Mg-4%Li-6%Zn-1.2%Y alloys

Alloys	Li	Zn	Y	Mg
Mg-4%Li	4.2	—	—	Bal
Mg-4%Li-6%Zn-1.2%Y	4.3	5.8	1.2	Bal

Fig. 1. Schematic diagram of tensile samples.

Fig. 2. Microstructures of as-extruded Mg-4%Li-6%Zn-1.2%Y alloy: (a) SEM image, (b) high magnification observation to the squared area of image (a), (c) TEM bright field image and (d) 5-fold SADP of I-phase, (e) TEM bright field image of β1′, β2′, (f) SADP of α-Mg and β2′.

Fig. 3. Optical observations on the etched (a) Mg-4%Li and (b) Mg-4%Li-6%Zn-1.2%Y samples.

Fig. 4. EBSD results of the as-extruded alloys: (a) and (b) orientation maps, (c) and (d) recrystallized maps, (e) and (f) distribution of recrystallized fraction for Mg-4%Li and Mg-4%Li-6%Zn-1.2%Y samples, respectively.

Fig. 5. (0002) pole figures of the as-extruded alloys: (a) Mg-4%Li and (b) Mg-4%Li-6%Zn-1.2%Y.

Fig. 6. Engineering stress-strain curves of as-extruded alloys: (a) Mg-4%Li and (b) Mg-4%Li-6%Zn-1.2%Y.

Table 2. Tensile properties of as-extruded Mg-4%Li and Mg-4%Li-6%Zn-1.2%Y alloys

Alloys	Orientation	YS (MPa)	UTS (MPa)	EL (%)
Mg-4%Li	TD sample	80 ± 5	141 ± 3	25.7 ± 2
Mg-4%Li	ED sample	105 ± 4	163 ± 4	16.5 ± 3
Mg-4%Li-6%Zn-1.2%Y	TD sample	103 ± 3	226 ± 5	22.8 ± 2
Mg-4%Li-6%Zn-1.2%Y	ED sample	141 ± 5	240 ± 5	21.6 ± 3

Fig. 7. Observations on the side surfaces of differently oriented samples: (a) TD and (b) ED samples cut from Mg-4%Li alloy, (c) TD and (d) ED samples cut from Mg-4%Li-6%Zn-1.2%Y alloy.

Fig. 8. Observations on the etched side surfaces of TD (a) and ED (b) samples cut from Mg-4%Li alloy, TD (c) and ED (d) samples cut from Mg-4%Li-6%Zn-1.2%Y alloy.

Fig. 9. Observations on the fracture surfaces of TD (a) and ED (b) samples of Mg-4%Li alloy; TD (c) and ED (d) samples of Mg-4%Li-6%Zn-1.2%Y alloy; images (e) and (f) are backscatter secondary images in-situ of images (c) and (d).

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