Phases equilibrated with long-period stacking ordered phases in the Mg-rich corner of the Mg-Y-Zn system

doi:10.1016/j.jmst.2020.08.019

Abstract

Abstract:

Phase equilibria involving the long-period stacking ordered phases including 14H, 18R and 10H in the Mg-rich corner of the Mg-Y-Zn system at 400 and 500°C have been experimentally investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The coexistence of 14H and 18R as well as 18R and 10H was confirmed from atomic scales. The phases 14H, 18R and 10H were all stable phases from 400 to 500°C.The experimentally proved three-phase equilibrium of 14H, 18R and α-Mg instead of 14H, 18R and Mg₂₄Y₅ were presented in the modified isothermal sections. The latter three-phase equilibrium was reported in the available literature. The modified isothermal sections are conducive to guide the composition design to obtain the alloys with favorable microstructure constituents and mechanical properties.

Key words: Mg-Y-Zn system, LPSO phases, solubility, thermodynamic stability, isothermal sections

Yongxin Ruan, Changrong Li, Yuping Ren, Xiaopan Wu, R. Schmid-Fetzer, Cuiping Guo, Zhenmin Du. Phases equilibrated with long-period stacking ordered phases in the Mg-rich corner of the Mg-Y-Zn system[J]. J. Mater. Sci. Technol., 2021, 68: 147-159.

Figures/Tables 16

Fig. 1. The nominal and measured compositions of the designed typical alloys marked in the Mg-rich corner. The composition points were designed guided by the straight line with the molar ratio of Y/Zn = 4/3.

Fig. 2. XRD patterns of the alloys equilibrated at 500°C in comparison with the data from Powder Diffraction File released in 2002. Meanwhile, the high magnification of characteristic peaks of the (14H/18R) and 10H are attached in each subgraph: (a) Mg88Y8Zn4(#1); (b) Mg86Y8Zn6(#2); (c) Mg95Y3Zn2(#3); (d) Mg95Y2Zn3(#4); (e) Mg80Y15Zn5(#5); (f) Mg74Y14Zn12(#6).

Table 1 The determined compositions of the Mg-Y-Zn alloys annealed at 500°C as well as the constituent phases and phase compositions (the compositions of the alloys and the corresponding constituent phases were determined by EDS, the phase identifications were performed by SEM / XRD and TEM / SAED).

Sample number	Alloy compositions determined by EDS (± 0.02 at.%)			Constituent phases determined by SEM / XRD	Constituent phases determined by TEM / SAED	Phase compositions determined by EDS (± 0.02 at.%)
Sample number	Mg	Y	Zn	Constituent phases determined by SEM / XRD	Constituent phases determined by TEM / SAED	Mg	Y	Zn
#1	89.01	7.73	3.26	Mg₂₄Y₅	Mg₂₄Y₅	82.60	14.12	3.28
				α-Mg	α-Mg	96.08	3.46	0.46
				14H/18R	18R	86.60	7.81	5.59
#2	86.25	7.96	5.79	α-Mg	α-Mg	98.46	0.79	0.75
				14H/18R	14H	87.38	6.85	5.77
				14H/18R	18R	84.55	8.73	6.72
#3	95.26	2.55	2.16	α-Mg	α-Mg	98.72	0.86	0.42
				14H/18R	14H	87.30	7.60	5.10
				14H/18R	18R	85.68	8.31	6.01
#4	94.62	2.17	3.21	α-Mg	α-Mg	98.64	0.59	0.77
				14H/18R	14H	88.02	6.31	5.67
				W	W	38.87	22.92	38.21
#5	80.60	13.36	5.04	Mg₂₄Y₅	Mg₂₄Y₅	83.40	13.96	2.64
				14H/18R	18R	79.50	12.23	8.27
				10H	10H	77.04	13.44	9.52
#6	77.31	12.65	10.04	W	W	33.23	25.82	40.95
				14H/18R	18R	79.63	11.85	8.52
				10H	10H	77.67	12.79	9.54

Table 2 The determined compositions of the Mg-Y-Zn alloys annealed at 400°C as well as the constituent phases and phase compositions (the compositions of the alloys and the corresponding constituent phases were determined by EDS, the phase identifications were performed by SEM / XRD and TEM / SAED).

Sample number	Alloy compositions determined by EDS (± 0.02 at.%)			Constituent phases determined by SEM / XRD	Constituent phases determined by TEM / SAED	Phase compositions determined by EDS (± 0.02 at.%)
Sample number	Mg	Y	Zn	Constituent phases determined by SEM / XRD	Constituent phases determined by TEM / SAED	Mg	Y	Zn
#1	88.74	7.18	4.08	Mg₂₄Y₅	Mg₂₄Y₅	85.60	12.94	1.46
				α-Mg	α-Mg	97.14	2.25	0.61
				14H/18R	18R	87.36	7.51	5.13
#2	86.24	7.73	6.03	α-Mg	α-Mg	98.79	0.48	0.73
				14H/18R	14H	88.55	6.86	4.59
				14H/18R	18R	86.80	7.44	5.75
#3	95.05	2.78	2.17	α-Mg	α-Mg	98.83	0.32	0.85
				14H/18R	14H	91.49	4.96	4.95
				14H/18R	18R	89.48	6.24	4.28
#4	96.43	2.04	3.33	α-Mg	α-Mg	97.82	0.83	1.35
				14H/18R	14H	88.81	6.07	5.13
				W	W	32.23	25.16	42.61
#5	81.47	13.86	4.67	Mg₂₄Y₅	Mg₂₄Y₅	85.28	13.78	0.94
				14H/18R	18R	80.67	11.40	7.93
				10H	10H	78.55	12.90	8.54
#6	76.20	13.77	10.03	W	W	35.79	25.45	38.76
				14H/18R	18R	81.40	10.66	7.94
				10H	10H	80.14	11.47	8.39

Fig. 3. Backscattered SEM images and SAED pattern of the Mg88Y8Zn4(#1) alloy: (a) microstructure after equilibration at 500°C; (b) microstructure after equilibration at 400°C; (c) SAED pattern of [-1-120]hcp zone axis taken from the strip-shaped phase with dark-gray contrast in the alloy equilibrated at 500°C.

Fig. 4. Backscattered SEM images of the Mg86Y8Zn6(#2) alloy equilibrated at different temperatures: (a) 500°C; (b) 400°C.

Table 3 The experimentally determined compositions of 14H and 18R of the Mg-Y-Zn system, together with the relevant ideal compositions summarized from the references with direct proofs.

LPSO	Composition of Alloy	Composition of Phase (at.%)	Ideal Composition	Refs.
14H	Mg₈₃Y_8.5Zn_8.5 (at.%)	Mg₈₈Y_5.8Zn_6.2	Mg₁₂Y₁Zn₁	[34]
	Mg₉₇Y₂Zn₁ (at.%)	Mg₈₇Y₆Zn₇	-	[18]
	Mg₉₇Y₂Zn₁ (at.%)	Mg₉₄Zn_2.0±1.0Y_4.0±2.0	Mg₃₅Y₄Zn₃	[24]
	Mg₈₆Y₈Zn₆ (at.%)	Mg_87.56Y_7.10Zn_5.32	Mg₁₂Y₁Zn₁	[31]
	-	Mg_85.9Y_7.9Zn_6.1	-	[41]
	Mg_89.4Y₈Zn₂Zr_0.6 (wt.%)	Mg₈₇Y_7±2Zn_6±2	Mg₁₂Y₁Zn₁	[32]
18R	Mg₉₇Y₂Zn₁ (at.%)	Mg₉₀Y₆Zn₄	-	[18]
	Mg_89.4Y₈Zn₂Zr_0.6 (wt.%)	Mg₈₄Y_8±2Zn_8±2	Mg₁₀Y₁Zn₁	[32]
	Mg₉₇Y₂Zn₁ (at.%)	Mg₉₄Y_4±2Zn_2±1	-	[42]
	Mg₈₅Y₉Zn₆ (at.%)	Mg₈₅Y₉Zn₆	Mg₂₉Y₄Zn₃	[24]
	Mg₉₇Y₂Zn₁ (at.%)	Mg_92.3Y_6.2Zn_1.5	-	[43]
	Mg₉₇Y_1.7Zn_1.3 (at.%)	Mg_89.7Y_5.6Zn_4.6	Mg₁₂Y₁Zn₁	[44]

Fig. 5. Backscattered SEM images of the Mg95Y3Zn2(#3) alloy equilibrated at different temperatures: (a) 500°C; (b) 400°C.

Fig. 6. TEM images, SAED patterns and HAADF-STEM image of the Mg86Y8Zn6(#2) / Mg95Y3Zn2(#3) alloys equilibrated at 500°C: (a) and (b) bright-field TEM images of the Mg86Y8Zn6(#2) alloy at different scales; (c) and (d) bright-field TEM images of the Mg95Y3Zn2(#3) alloy at different scales; (e) SAED pattern of [11-20]hcp zone axis taken from area A in (a); (f) SAED pattern of [1-100]hcp zone axis taken from area B in (a); (g) Fourier-filtered HAADF-STEM image of the LPSO phases in Mg95Y3Zn2 (#3) alloy, with the original HAADF-STEM image at the top part. The electron beam is parallel to [11-20]hcp.

Fig. 7. (a) and (b) backscattered SEM images of the Mg95Y2Zn3(#4) alloy equilibrated at 500 and 400°C, respectively; (c) Fourier-filtered HAADF-STEM image of the LPSO phases in the Mg95Y2Zn3(#4) alloy, with the original HAADF-STEM image at the top part. The electron beam is parallel to [11-20]hcp; (d) SAED pattern of [-1-120]hcp zone axis taken from the gray phase in (a).

Fig. 8. Backscattered SEM images of the Mg80Y15Zn5(#5) alloy equilibrated at different temperatures: (a) 500°C; (b) 400°C.

Fig. 9. Bright-field TEM images of the Mg80Y15Zn5(#5) alloy equilibrated at 500°C: (a) and (b) micromorphology images of the Mg80Y15Zn5(#5) alloy at different scales; (c) and (d) SAED patterns of [1-100]hcp zone axis.

Fig. 10. Backscattered SEM images showing the microstructures of the Mg74Y14Zn12(#6) alloy equilibrated at different temperatures: (a) 500°C; (b) 400°C.

Fig. 11. TEM images and SAED patterns of the Mg74Y14Zn12(#6) alloy equilibrated at 500°C: (a) and (b) bright-field TEM images at different scales; (c) SAED pattern of [11-20]hcp zone axis taken from area B in (a); (d) SAED pattern of [-12-10]hcp zone axis taken from area C in (a); (e) Fourier-filtered HAADF-STEM image of the LPSO phases. The left part is the original HAADF-STEM image. The electron beam is parallel to [11-20]hcp.

Fig. 12. Atomic models viewed along [-12-10]hcp showing the ideal crystal structure of LPSO phases and the relationship between the ideal crystal structures of the LPSO phases and the Fourier-filtered HAADF-STEM images: (a) 18R; (b) 14H; (c) 10H.

Fig. 13. Isothermal sections of the Mg-rich corner in the Mg-Y-Zn system: (a) 500°C; (b) 400°C.

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