Microstructure and mechanical properties of sand-cast Mg-6Gd-3Y-0.5Zr alloy subject to thermal cycling treatment

doi:10.1016/j.jmst.2020.01.013

Abstract

Abstract:

This work was undertaken to investigate the microstructural evolution, mechanical properties and fracture behavior of sand-cast Mg-6Gd-3Y-0.5 Zr (GW63) alloy subject to thermal cycling treatment. In order to simulate the thermal cycling under extreme service conditions (space or moon environments), the sand-cast and T6 treated GW63 alloys were subjected to thermal cycling treatment which consists of deep cryogenic-elevated temperature cycling treatment (DCET) and deep cryogenic cycling treatment (DCT). Results indicate that there are significant gains in yield strength (YS) and ultimate tensile strength (UTS) of the sand-cast GW63 alloy after DCET, whereas the T6 state alloy undergoes a different variation in mechanical properties. However, no appreciable influence is revealed on the mechanical properties of the tested GW63 alloys after DCT. Meanwhile, the DCT and DCET have no obvious effects on the fracture morphology. The DCT enhances the precipitation kinetics via providing favorable nucleation sites for the precipitation of second phases. The elevated temperature process of DCET plays a crucial role in improving the aging-hardening responses and releasing the stress concentration brought by DCT to a great extent, leading to overcome the obstacle of essential phase transformation. The changes in mechanical properties are primarily attributed to the phase transformation of the studied alloys during DCET.

Key words: Mg-Gd-Y-Zr, Deep cryogenic-elevated temperature cycling, Deep cryogenic cycling, Mechanical properties, Microstructure evolution

Beiping Zhou, Wencai Liu, Guohua Wu, Liang Zhang, Xiaolong Zhang, HaoJi Wen, jiang Ding. Microstructure and mechanical properties of sand-cast Mg-6Gd-3Y-0.5Zr alloy subject to thermal cycling treatment[J]. J. Mater. Sci. Technol., 2020, 43: 208-219.

Figures/Tables 14

Fig. 1. Schematic illustration of one-cycle process of thermal cycling: (a) deep cryogenic-elevated temperature cycling treatment (DCET); (b) deep cryogenic cycling treatment (DCT).

Table 1 Heat treatment conditions for the tested alloys.

Conditions	3 cycles of DCET	5 cycles of DCET	5 cycles of DCT
Sand-cast	Cast-DCET-3C	Cast-DCET-5C	Cast-DCT-5C
T6	T6-DCET-3C	T6-DCET-5C	T6-DCT-5C

Fig. 2. Optical and SEM backscattered micrographs of non-thermal cycling treated GW63 alloy in different states: (a) optical micrograph of sand-cast alloy; (b) optical micrograph of T6 state alloy; (c) SEM backscattered micrograph of sand-cast alloy; (d) SEM backscattered micrograph of T6 state alloy.

Fig. 3. X-ray diffraction patterns of GW63 alloys subject to different treatments: (a) sand-cast; (b) Cast-DCT-5C; (c) Cast-DCET-5C; (d) T6; (e) T6-DCT-5C; (f) T6-DCET-3C; (g) T6-DCET-5C.

Table 2 EDS results of micro-constitution in Fig. 2(c).

	Mg (at.%)	Gd (at.%)	Y (at.%)
Location A	60.41	14.7	24.2
Location B	90.5	5.75	3.74

Fig. 4. Optical microstructures of tested GW63 alloys after subject to different thermal cycling treatments: (a) Cast-DCET-3C; (b) T6-DCET-3C; (c) Cast-DCET-5C; (d) T6-DCET-5C; (e) Cast-DCT-5C; (f) T6-DCT-5C.

Fig. 5. TEM images and corresponding SAED patterns of GW63 alloys under different conditions: (a) TEM image of T6 state; (b) SAED pattern of T6 state (B=[0001]); (c) TEM image of T6-DCET-3C; (d) SAED pattern of T6-DCET-3C state (B=[0001]); (e) TEM image of T6-DCET-5C; (f) SAED pattern of T6-DCET-5C state (B=[0001]).

Fig. 6. HRTEM images of the T6-DCET-5C alloy.

Fig. 7. Hardness curves of GW63 alloys treated by different cycle numbers of thermal cycling.

Fig. 8. Tensile properties of sand-cast GW63 alloys treated by different cycle numbers of thermal cycling: (a) DCET; (b) DCT.

Fig. 9. Tensile properties of T6 treated GW63 alloys treated by different cycle numbers of thermal cycling: (a) DCET; (b) DCT.

Fig. 10. Optical images of longitudinal section in fracture surfaces of GW63 alloys tested under different conditions: (a) sand-cast; (b) T6; (c) Cast-DCET-3C; (d) T6-DCET-3C; (e) Cast-DCET-5C; (f) T6-DCET-5C; (g) Cast-DCT-5C; (h) T6-DCT-5C.

Fig. 11. SEM images of tensile fracture surfaces of sand-cast GW63 alloys subject to different treatments: (a) sand-cast; (b) magnified view of the marked region in (a); (c) Cast-DCET-3C; (d) magnified view of the marked region in (c); (e) Cast-DCET-5C; (f) magnified view of the marked region in (e); (g) Cast-DCT-5C; (h) magnified view of the marked region in (g).

Fig. 12. SEM images of tensile fracture surfaces of T6 treated GW63 alloys under different conditions: (a) T6; (b) magnified view of the marked region in (a); (c) T6-DCET-3C; (d) magnified view of marked region in (c); (e) T6-DCET-5C; (f) magnified view of marked region in (e); (g) T6-DCT-5C; (h) magnified view of marked region in (g).

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