Microstructural evolution and mechanical properties of Mg-9.8Gd-2.7Y-0.4Zr alloy produced by repetitive upsetting

doi:10.1016/j.jmst.2018.01.009

Abstract

Abstract:

A newly developed severe plastic deformation (SPD) technique, i.e. repetitive upsetting (RU), is employed to improve the strength and ductility of a Mg-Gd-Y-Zr alloy. During the RU processing, dynamic recrystallization occurs in the Mg alloy, which leads to a significant grain refinement from 11.2 μm to 2.8 μm. The yield strength (YS), ultimate tensile strength (UTS) and elongation increase simultaneously with increasing RU passes. The microstructural evolution is affected by processing temperatures. Dynamic recrystallization prevails at low temperatures, while dynamic recovery is the main effect factor at high temperatures. Texture characteristics gradually become random during multiple passes of RU processing, which reduces the tension-compression asymmetry of the Mg-Gd-Y-Zr alloy.

Key words: Severe plastic deformation (SPD), Repetitive upsetting (RU), Mg-RE alloy, Mechanical properties

H. Zhou, H.Y. Ning, X.L. Ma, D.D. Yin, L.R. Xiao, X.C. Sha, Y.D. Yu, Q.D. Wang, Y.S. Li. Microstructural evolution and mechanical properties of Mg-9.8Gd-2.7Y-0.4Zr alloy produced by repetitive upsetting[J]. J. Mater. Sci. Technol., 2018, 34(7): 1067-1075.

Figures/Tables 13

Fig 1. Process diagram of RU: (a) schematic diagram of each pass, (b) die schematic of the cross-section perpendicular to the x axis, and (c) schematic of Route A.

Fig. 2. Microstructure of Mg-9.8Gd-2.7Y-0.4Zr alloy: (a) as-cast and (b) as-extruded.

Fig. 3. Microstructure of the Mg-Gd-Y-Zr alloy: (a) RU-1pass, (b) RU-2passes, (c) RU-3passes, (d) RU-4 passes, and TEM images of Mg-Gd-Y-Zr alloy: (e) RU- 1pass and (f) RU-4 passes.

Fig. 4. Microstructure of the Mg-Gd-Y-Zr alloy processed by four RU passes: (a) at 350 °C, (b) at 400 °C, (c) at 450 °C and (d) TEM image of the sample processed at 400 °C.

Fig. 5. EBSD images of grain boundary structures of RU at 350 °C: (a) 1 pass, (b) 2 passes, (c) 3 passes and (d) 4 passes. The red, green and blue lines represent three groups of grain boundaries with misorientation angles of 2°-5°, 5°-15°, and larger than 15°, respectively.

Table 1 Misorientation of grain boundaries of different temperatures and passes of RU.

Angle Deformation	2°-5°	5°-15°	15°-180°
As-extruded	24.9%	6.4%	68.7%
RU-1pass, 350 °C	52.4%	16.2%	31.4%
RU-2passes, 350 °C	36.4%	6.2%	57.4%
RU-3passes, 350 °C	43.8%	12.2%	44.0%
RU-4passes, 350 °C	9.4%	3.4%	87.2%
RU-4passes, 400 °C	25.4%	8.4%	66.2%
RU-4passes, 450 °C	7.9%	7.4%	84.7%

Fig. 6. EBSD images of grain boundary structures of: (a) as-extruded, (b) RU-4passes at 350 °C, (c) RU-4passes at 400 °C and (d) RU-4passes at 450 °C. The red, green and blue lines represent three groups of grain boundaries with misorientation angles of 2°-5°, 5°-15°, and larger than 15°, respectively.

Fig. 7. Inverse pole figures of as-extruded Mg-Gd-Y-Zr alloy.

Fig. 8. Inverse pole figures of the Mg-Gd-Y-Zr alloy produced at 350 °C: (a) 1 pass, (b) 3 passes and (c) 4passes.

Fig. 9. Inverse pole figures of the Mg-Gd-Y-Zr alloy processed by 4 passes of RU at: (a) 350 °C, (b) 400 °C and (c) 450 °C.

Fig. 10. Engineering stress-strain curves of tensile testing Mg-Gd-Y-Zr alloy: (a) as-cast, as-extruded, RU-1pass and RU-4 passes samples and (b) RU-4 passes sample processed at 350 °C, 400 °C and 450 °C.

Fig. 11. Linear fitting of Hall-petch equation of Mg-Gd-Y-Zr alloy.

Fig. 12. Tensile and compression engineering strain-stress curves of the Mg-Gd-Y-Zr alloy: (a) as-extruded and (b) RU-4 passes at 350 °C.

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