Phase equilibria of long-period stacking ordered phase in the ternary Mg-Y-Al alloys

doi:10.1016/j.jmst.2022.02.028

Abstract

Abstract:

Phase equilibria focusing on the long-period stacking ordered (LPSO) phases have been experimentally investigated in the Mg-Y-Al alloys. The microstructures of the eleven representative alloys annealed at 450, 500 and 550 °C were systematically investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Two thermodynamically stable LPSO polytypes were identified, including 18R and 10H structures. Meanwhile, the transformation of 18R → 10H was detected in Mg-9.0Y-1.7Al (at.%) alloy, while the coexistence of 18R and 10H was observed in Mg-14.2Y-12.8Al (at.%) alloy. It was found that the LPSO phases were non-stoichiometric compounds with ternary solubilities. The 18R can dissolve up to 7.21 at.% - 8.37 at.% Al and 10.75 at.% -12.01 at.% Y, while the 10H can dissolve up to 9.35 at.%-10.37 at.% Al and 14.50 at.%-14.94 at.% Y. All of the experimental results were used to construct the isothermal sections, which is conducive to the design of Mg-Y-Al alloys.

Key words: Mg-Y-Al alloys, LPSO phases, Phase relationships, Isothermal sections, Atomic structure

Yiwen Chen, Quan Li, Yangxin Li, Weisen Zheng, Jingya Wang, Xiaoqin Zeng. Phase equilibria of long-period stacking ordered phase in the ternary Mg-Y-Al alloys[J]. J. Mater. Sci. Technol., 2022, 126: 80-92.

Figures/Tables 17

Fig. 1. Measured compositions of the designed typical Mg-Y-Al alloys. For sake of clear description, these alloys are divided into two categories: LAY with low Al:Y ratio (< 1, LAY1# - 8#) marked with black solid triangles, and HAY with high Al:Y ratio (≥ 1, HAY1# - 3#) represented by red solid hexagons.

Table 1. Eleven Mg-Y-Al samples compositions and related annealing process.

Samples no.	Measured compositions of Mg-Y-Al alloy (at.%) by ICP method			Annealed process
Samples no.	Mg	Y	Al	Annealed process
Empty Cell
LAY1#	96.4	3.0	0.6	450 °C/30 days	500 °C/30 days	550 °C/30 days
LAY2#	95.0	3.8	1.2	450 °C/30 days	500 °C/30 days	550 °C/30 days
LAY3#	95.7	3.8	0.5	450 °C/30 days	500 °C/30 days	550 °C/30 days
LAY4#	80.3	9.0	1.7	450 °C/60 days	500 °C/30 days	550 °C/30 days
LAY5#	82.0	14.0	4.0	450 °C/60 days	500 °C/30 days	550 °C/30 days
LAY6#	81.0	14.5	4.5	450 °C/60 days	500 °C/30 days	550 °C/30 days
LAY7#	73.9	18.1	8.0	450 °C/60 days	450 °C/30 days	550 °C/30 days
LAY8#	73.0	14.2	12.8	450 °C/60 days	500 °C/30 days	550 °C/30 days
HAY1#	96.8	1.6	1.6	450 °C/30 days	500 °C/30 days	550 °C/30 days
HAY2#	75.3	10.3	14.4	450 °C/60 days	500 °C/30 days	550 °C/30 days
HAY3#	90.2	2.9	6.9	450 °C/60 days	500 °C/30 days	550 °C/30 days

Table 2. Equilibrium phase constituents and phase compositions of Mg-Y-Al alloys annealed at 450 °C for 30/60 days, 500 °C for 30 days and 550 °C for 30 days.

Sample No.	Equilibrium phases constituent and compositions (at.%)
	Phase constitutions at 450 °C	Compositions		Phase constitutions at 500 °C	Compositions		Phase constitutions at 550 °C	Compositions
	Phase constitutions at 450 °C	Al	Y	Phase constitutions at 500 °C	Al	Y	Phase constitutions at 550 °C	Al	Y
LAY1#	α-Mg	0.03	1.95	α-Mg	0.17	2.05	α-Mg	0.23	2.64
LAY1#	18R	7.62	10.64	18R	7.27	10.73	18R	7.24	10.90
LAY2#	α-Mg	0.06	2.58	α-Mg	0.15	2.95	α-Mg	0.24	2.80
LAY2#	18R	7.74	11.59	18R	7.59	11.11	18R	7.21	10.75
LAY3#	α-Mg	0.01	2.30	α-Mg	017	3.31	α-Mg	0.28	3.45
	18R	7.92	11.96	18R	7.95	11.63	18R	7.38	10.97
	Mg₂₄Y₅	0.24	12.88
LAY4#	α-Mg	0.1	2.77	α-Mg	0.18	3.31	α-Mg	0.25	4.23
	18R	8.03	12.01	18R	7.99	11.80	10H	9.69	14.39
	Mg₂₄Y₅	0.24	12.65	Mg₂₄Y₅	0.38	12.99	Mg₂₄Y₅	0.74	13.22
LAY5#	10H	9.40	13.82	10H	9.54	14.24	10H	9.80	14.50
LAY5#	Mg₂₄Y₅	0.53	14.28	Mg₂₄Y₅	0.54	14.06	Mg₂₄Y₅	0.79	13.65
LAY6#	10H	9.46	13.91	10H	9.54	14.34	10H	9.96	14.72
LAY6#	Mg₂₄Y₅	0.46	14.76	Mg₂₄Y₅	0.54	14.48	Mg₂₄Y₅	0.97	14.56
LAY7#	10H	9.35	13.95	10H	9.75	14.76	10H	10.16	14.94
	Mg₂₄Y₅	0.61	15.95	Mg₂₄Y₅	0.69	15.10	Mg₂₄Y₅	0.95	14.95
	Mg₂Y	20.51	27.66	Mg₂Y	22.88	29.89	Mg₂Y	28.02	30.26
LAY8#	18R	8.37	11.72	18R	8.23	11.80	18R	7.71	11.22
	10H	10.37	13.74	10H	9.73	14.31	10H	9.75	14.41
	Al₂Y	58.65	33.86	Al₂Y	56.39	33.78	Al₂Y	59.20	33.71
HAY1#	α-Mg	0.01	0.62	α-Mg	0.24	0.96	α-Mg	0.23	0.65
	Al₂Y	63.12	34.41	Al₂Y	63.18	34.35	Al₂Y	64.38	33.41
	18R	7.26	9.99
HAY2#	α-Mg	0.01	0.72	α-Mg	0.18	1.61	α-Mg	0.24	2.45
	18R	7.43	10.19	18R	7.16	10.39	18R	7.21	10.65
	Al₂Y	62.9	33.7	Al₂Y	57.74	34.02	Al₂Y	64.18	33.44
HAY3#	α-Mg	1.77	0.024	α-Mg	1.99	0	α-Mg	1.66	0
HAY3#	Al₂Y	64.38	32.90	Al₂Y	64.58	32.12	Al₂Y	65.17	32.44

Fig. 2. BSE images of LAY1# alloy annealed at (a) 450 °C; (b) 500 °C; (c) 550 °C and (d) corresponding XRD patterns of the alloys annealed at the three temperatures.

Fig. 3. The BF-TEM, SAED pattern and HR-TEM images of LAY1# alloy annealed at (a-c): 450 °C and (d-f) 550 °C. All the SAED pattern and HR-TEM images were taken along the [112¯0] zone axis.

Fig. 4. BSE images of LAY4# alloy annealed at: (a) 450 °C; (b) 500 °C; (c) 550 °C and (d) the corresponding XRD patterns of the alloys annealed at the three temperatures.

Fig. 5. BF-TEM, SAED pattern and HAADF-STEM images of LAY4# alloy annealed at (a-c) 500 °C and (d-f) 550 °C. The SAED patterns and HAADF-STEM images were taken along the [112¯0] zone axis.

Fig. 6. BSE images of LAY6# alloy annealed at (a) 450 °C; (b) 500 °C; (c) 550 °C and (d) corresponding XRD patterns of the alloys annealed at the three temperatures.

Fig. 7. BSE images of LAY7# alloy annealed at (a) 450 °C; (b) 500 °C; (c) 550 °C and (d) corresponding XRD patterns of the alloys annealed at the three temperatures.

Fig. 8. BSE images of LAY8# alloy annealed at (a) 450 °C; (b) 500 °C; (c) 550 °C and (d) corresponding XRD patterns of the alloys annealed at the three temperatures.

Fig. 9. (a) The BF-TEM image of LAY8# alloy annealed at 550 °C; (b-c) the SAED patterns images taken from areas A and B in (a); (d-e) the HAADF-STEM images taken from the areas A and B in (a). All the electron beam is parallel to [112¯0] axis.

Fig. 10. BSE images of the HAY1# alloy annealed at (a) 450 °C; (b) 500 °C; (c) 550 °C and (d) the corresponding XRD patterns annealed at the three temperatures.

Fig. 11. (a) BF-TEM image of the HAY1# alloy annealed at 550 °C; (b) the corresponding SAED patterns taken form the area A in (a) along the [001] zone axis.

Fig. 12. BSE images of HAY2# alloy annealed at (a) 450 °C; (b) 500 °C; (c) 550 °C and (d) corresponding XRD patterns annealed at the three temperatures.

Fig. 13. BSE images of HAY3# alloy annealed at (a) 450 °C; (b) 500 °C; (c) 550 °C and (d) corresponding XRD patterns annealed at the three temperatures.

Fig. 14. (a) Chemical compositions of the 18R and 10H phases in Mg-Y-Al alloys annealed at 450, 500 and 550 °C in the present work; (b) and (c) atomic structure models of ideal crystal structure of the LPSOs viewed along [12¯10]direction of 18R and 10H structure, respectively.

Fig. 15. Isothermal sections of the Mg-rich corner in Mg-Y-Al alloys at: (a) 450 °C, (c) 500 °C and (e) 550 °C, respectively, constructed by the experimental data in this work. The blue filled triangles and red circles represent the Mg-Y-Al alloys in two-phase and three-phase regions, respectively. Meanwhile, the black filled squares represent the phase composition measured by the EDS analysis. Besides, the extrapolated lines are plotted as dotted lines. Local magnifications of the isothermal sections around the phase regions focusing on the LPSO phases at (b) 450 °C; (d) 500 °C and (f) 550 °C, respectively.

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