Effect of Partially Substituting Ca with Mischmetal on the Microstructure and Mechanical Properties of Extruded Mg-Al-Ca-Mn-Based Alloys

doi:10.1007/s40195-018-0820-7

Abstract

Abstract:

Mg-2Al-1.2Ca-0.2Mn (at%)-based alloys with Ce-rich mischmetal (MM) substitution of 0-0.6 at% for Ca were hot extruded at 400 °C. The effect of MM substitution on the microstructure and mechanical properties of the extruded alloys was investigated. The as-cast Mg-2Al-1.2Ca-0.2Mn alloy is mainly composed of α-Mg, Mg₂Ca and (Mg,Al)₂Ca phases and Al₈Mn₅ precipitates, whereas the substitution of MM brings about the formation of Al₁₁MM₃, Al₂MM phases and Al₁₀MM₂Mn₇ particles with the absence of (Mg,Al)₂Ca phase. The volume fraction of MM-containing phases increases with increasing MM contents. All of the extruded alloys exhibit bimodal microstructure comprising fine dynamically recrystallized grains with almost random orientation and coarse deformed grains with strong basal texture. Dense nanosized planar Al₂Ca and spherical Al-Mn phases precipitate inside the deformed grains. High tensile yield strengths of ~350 MPa and moderate elongations to failure of >12% are obtained in all extruded alloys; the MM substitution induces negligible difference in the tensile properties at ambient temperature, while the highest MM substitution improves the strength at 180 °C due to the better thermal stability of the fragmented MM-containing phases.

Key words: Magnesium alloys, Extrusion, Microstructures, Precipitation, Mechanical properties

Chao Xu, Taiki Nakata, Guo-Hua Fan, Kosuke Yamanaka, Guang-Ze Tang, Lin Geng, Shigeharu Kamado. Effect of Partially Substituting Ca with Mischmetal on the Microstructure and Mechanical Properties of Extruded Mg-Al-Ca-Mn-Based Alloys[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(2): 205-217.

Figures/Tables 11

Table 1 Chemical compositions in at% and wt%

Alloy	at%					wt%
Alloy	Al	Ca	RE	Mn	Mg	Al	Ca	RE	Mn	Mg
AX212	1.75	0.94	-	0.17	Bal.	1.92	1.54	-	0.37	Bal.
AXE2102	1.58	0.75	0.20	0.14	Bal.	1.72	1.21	1.09	0.30	Bal.
AXE20804	1.8	0.7	0.37	0.16	Bal.	1.93	1.06	2.04	0.35	Bal.
AXE20606	1.77	0.47	0.64	0.15	Bal.	1.89	0.74	3.62	0.33	Bal.

Fig. 1 Optical micrographs of the as-cast alloys: a AX212, b AXE2102, c AXE20804, d AXE20606

Fig. 2 EDX elemental mapping analysis of Mg, Al, Ca, Mn, and MM in the as-cast alloys. a AX212 and b AXE20606

Fig. 3 TEM BF images and corresponding SAED patterns taken from the eutectic compounds in the as-cast AX212 and AXE20606 alloys: a Mg2Ca with C14 structure and b (Mg,Al)2Ca with C36 structure in AX212, c Mg2Ca with C14 structure and d Al11MM and Al2MM with bco and fcc structures, respectively, in AXE20606

Fig. 4 IPF maps and corresponding pole figures of the as-cast alloys: a AX212, b AXE2102, c AXE20804, d AXE20606

Fig. 5 SEM micrographs of the as-extruded alloys observed on the longitudinal sections: a, b AX212; c, d AXE20606

Fig. 6 Texture analysis of the as-extruded AX212 and AXE20606 alloys: a, b IPF maps, c, d (0001) and (10$\bar{1}$0) pole figures, e (0001) <11$\bar{2}$0> Schmid factor distribution

Fig. 7 TEM observations of the precipitates in the deformed region of the as-extruded AX212 and AXE20606 alloys: a, c BF images and SAED patterns, b, d HREM images taken from [10$\bar{1}$0]α-Mg and corresponding FFT of the Al2Ca precipitates with C15 structure

Fig. 8 TEM observations of the compounds distributed in the DRXed regions of the as-extruded alloys: a BF image of the AX212 alloy, b, c SAED patterns of (Mg,Al)2Ca and Mg2Ca, respectively, marked in a, d BF image of the AXE20606 alloy, e SAED pattern of Al2MM f magnified region marked in (d)

Fig. 9 Nominal tensile stress-strain curves of the as-extruded alloys tested at a RT and b 180 °C

Table 2 Tensile properties of the as-extruded alloys at room temperature and 180 °C

Alloy	RT			180 °C
Alloy	TYS (MPa)	UTS (MPa)	Elongation (%)	TYS (MPa)	UTS (MPa)	Elongation (%)
AX212	353±4	353±3	12.9±0.5	182±8	199±13	41.8±4.5
AXE2102	345±3	345±4	15.7±0.5	173±5	189±9	48.2±2.7
AXE20804	353±6	353±1	12.2±0.2	178±7	198±11	36.2±3.1
AXE20606	352±6	352±3	12.8±0.3	185±9	210±14	33.7±2.2

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