Ultrahigh strength Mg-Y-Ni alloys obtained by regulating second phases

doi:10.1016/j.jmst.2019.11.026

Abstract

Abstract:

Mg-Y-Ni alloys with different second phases were designed by changing Y/Ni atomic ratio from 1.5 to 0.5. The microstructure and mechanical properties of as-cast and as-extruded alloys were investigated. The as-cast Mg-Y-Ni alloy with Y/Ni ratio of 1.5 is composed of α-Mg and long period stacking ordered (LPSO) phase. When Y/Ni ratio is equal to 1, nanoscale lamellar γ' phase and eutectic Mg₂Ni phase are formed in addition to LPSO phase. As Y/Ni ratio decreases further, the amount of eutectic Mg₂Ni phase increases, while the amount of LPSO phase decreases. After extrusion, the LPSO and γ' phases are distributed along the extrusion direction, while eutectic Mg₂Ni phase is broken and dispersed in the as-extruded alloys. LPSO phase and Mg₂Ni phase in the alloys promote dynamic recrystallization (DRX) during extrusion, while γ' phase inhibits DRX. Consequently, the Mg96Y2Ni2 (at.%) alloy with LPSO phase and γ' phase as the main second phases shows the strongest basal texture after extrusion. The tensile yield strength of the as-extruded Mg-Y-Ni alloys increases first and then decreases with decreasing Y/Ni ratio. The as-extruded Mg96Y2Ni2 (at.%) alloy with Y/Ni = 1 exhibits excellent mechanical properties with tensile yield strength of 465 MPa, ultimate tensile strength of 510 MPa and elongation to failure of 7.2%, which is attributed to the synergistic effect of bulk LPSO phase and nanoscale γ' phase.

Key words: Mg-Y-Ni alloys, LPSO, γ' Phase, Mg₂Ni, Mechanical properties

S.Z. Wu, X.G. Qiao, M.Y. Zheng. Ultrahigh strength Mg-Y-Ni alloys obtained by regulating second phases[J]. J. Mater. Sci. Technol., 2020, 45(0): 117-124.

Figures/Tables 12

Table 1 Chemical compositions of Mg-Y-Ni alloys.

Alloys	Composition (at.%)	Composition (wt%)	Y/Ni ratio
Mg95Y3Ni2	Mg95.1Y3.0Ni1.9	Mg-9.8Y-4.2Ni	1.5
Mg96Y2Ni2	Mg96.1Y2.0Ni1.9	Mg-6.7Y-4.3Ni	1
Mg97Y1Ni2	Mg96.9Y1.1Ni2.0	Mg-3.7Y-4.6Ni	0.5

Fig. 1. XRD patterns of the as-cast Mg95Y3Ni2 alloy, Mg96Y2Ni2 alloy and Mg97Y1Ni2 alloy.

Fig. 2. SEM images of the as-cast Mg-Y-Ni alloys with different Y/Ni atomic ratios: (a) Mg95Y3Ni2 alloy; (b) Mg96Y2Ni2 alloy; (c) Mg97Y1Ni2 alloy.

Fig. 3. (a) TEM bright field image and selected area electron diffraction pattern and (b) corresponding composition of 18R-LPSO in Mg95Y3Ni2 alloy; (c) TEM bright field image and selected area electron diffraction pattern and (d) corresponding composition of Mg2Ni phase in Mg97Y1Ni2 alloy; (e) TEM bright field image, (f) HADDF-STEM image and (g) elemental mappings of γ' phase in Mg96Y2Ni2 alloy.

Table 2 Volume fraction of the main phases in the as-cast Mg-Y-Ni alloys.

Alloys	α-Mg	LPSO	Mg₂Ni
Mg95Y3Ni2	40.6	59.4	—
Mg96Y2Ni2	47.8	50.3	1.9
Mg97Y1Ni2	65.9	30.0	4.1

Table 3 EDS analysis of the matrix by SEM mapping in the as-cast Mg-Y-Ni alloys (at.%).

Alloys	Mg	Y	Ni
Mg95Y3Ni2	98.4 ± 0.3	1.2 ± 0.3	0.4 ± 0.1
Mg96Y2Ni2	98.5 ± 0.2	0.8 ± 0.1	0.7 ± 0.3
Mg97Y1Ni2	99.1 ± 0.4	0.4 ± 0.1	0.5 ± 0.2

Fig. 4. SEM images of (a) Mg95Y3Ni2 alloy, (b) Mg96Y2Ni2 alloy and (c) Mg97Y1Ni2 alloy after preheating at 420 °C for 15 min; (d) TEM bright field image and the HADDF-STEM images of (e) γ' and (f) 18R-LPSO in Mg96Y2Ni2 alloy after preheating.

Fig. 5. SEM images of the as-extruded Mg-Y-Ni alloys with different Y/Ni atomic ratios along extrusion direction: (a) Mg95Y3Ni2 alloy; (b) Mg96Y2Ni2 alloy; (c) Mg97Y1Ni2 alloy.

Fig. 6. (a-c) IPF images and (d-i) inverse pole ?gures of as-extruded alloys along extrusion direction: (a, d, g) Mg95Y3Ni2 alloy; (b, e, h) Mg96Y2Ni2 alloy; (c, f, i) Mg97Y1Ni2 alloy; (d-f) non-DRXed + DRXed regions and (g-i) DRXed regions.

Table 4 DRX ratio and DRXed grain size of as-extruded Mg-Y-Ni alloys.

Alloys	α-Mg (%)	Non-DRXed (%)	DRXed (%)	DRX ratio (%)	Grain diameter (μm)
Mg95Y3Ni2	40.6	10.1	30.5	75.2	3.70 ± 1.36
Mg96Y2Ni2	47.8	23.1	24.7	51.7	0.67 ± 0.20
Mg97Y1Ni2	65.9	10.6	55.3	83.9	0.83 ± 0.46

Fig. 7. SEM images observed from the position of (a-c) 10 mm and (d) 0 mm below the die-entrance of partially extruded alloys: (a) Mg95Y3Ni2 alloy; (b) Mg96Y2Ni2 alloy; (c, d) Mg97Y1Ni2 alloy.

Fig. 8. (a) Tensile stress-strain curves of the as-extruded Mg-Y-Ni alloys, (b) mechanical properties of the as-extruded Mg-Y-Ni alloys with decreasing Y/Ni atomic ratio and (c) comparison of tensile properties of the as-extruded Mg-Y-Ni alloys and other high strength Mg alloys containing RE.

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