Balancing the strength and ductility of Mg-6Zn-0.2Ca alloy via sub-rapid solidification combined with hard-plate rolling

doi:10.1016/j.jmst.2020.11.069

Abstract

Abstract:

In this study, we successfully prepared a Mg-6Zn-0.2Ca alloy by utilizing sub-rapid solidification (SRS) combined with hard-plate rolling (HPR), whose elongation-to-failure increases from ~17 % to ~23 % without sacrificing tensile strength (~290 MPa) compared with its counterpart processed via conventional solidification (CS) followed by HPR. Notably, both samples feature a similar refined grain structure with an average grain size of ~2.1 and ~2.5 μm, respectively. However, the high cooling rate of ~150 K/s introduced by SRS modified both the size and morphology of Ca₂Mg₆Zn₃ eutectic phase in comparison to those coarse ones under CS condition. By subsequent HPR, the Ca₂Mg₆Zn₃ phase was further refined and dispersed uniformly by severe fragmentation. Specially, the achieved supersaturation containing excessive Ca solute atoms due to high cooling rate was maintained in the SRS-HPR condition. The mechanisms that govern the high ductility of the SRS-HPR sample could be ascribed to following reasons. First, refined Ca ₂Mg₆Zn₃ eutectic phase could effectively alleviate or avoid the crack initiation. Furthermore, excessive Ca solute atoms in α-Mg matrix result in the yield point phenomenon and enhanced strain-hardening ability during tension. The findings proposed a short-processed strategy towards superior performance of Mg-6Zn-0.2Ca alloy for industrial applications.

Key words: Magnesium alloys, Ductility, Strength, Microstructure, Texture

Zhong-Zheng Jin, Min Zha, Hai-Long Jia, Pin-Kui Ma, Si-Qing Wang, Jia-Wei Liang, Hui-Yuan Wang. Balancing the strength and ductility of Mg-6Zn-0.2Ca alloy via sub-rapid solidification combined with hard-plate rolling[J]. J. Mater. Sci. Technol., 2021, 81: 219-228.

Figures/Tables 14

Fig. 1. (a) The sketch map of sub-rapid solidified (SRS) setups and (b) the cooling curves of CS and SRS samples.

Fig. 2. EBSD maps of as-cast (a) CS and (b) SRS sample and (c) the corresponding distribution of grain size. The unindexed black regions are second phase particles.

Fig. 3. SEM micrographs of as-cast (a) CS and (b) SRS sample, (c) the corresponding EDS line scan and (d) distribution of particle size.

Fig. 4. (a) XRD patterns of CS and SRS samples and (b) the diffraction peaks of (100), (002) and (101) crystal planes.

Fig. 5. EPMA analysis of as-cast (a) CS and (b) SRS sample and the corresponding solute profiles of (c) Ca and (d) Zn element.

Table 1 Average partition coefficients (k) of Ca and Zn elements under SRS and CS processes.

Sample	k_Ca	k_Zn
SRS	0.07	0.43
CS	0.03	0.40

Fig. 6. SEM micrographs of (a) CS-HPR and (b) SRS-HPR sample and the corresponding distribution of (c) particle size and (d) grain size.

Fig. 7. (0002) EBSD maps of the as-rolled (a) CS and (b) SRS sample with thickness reduction of ~50 % and the corresponding elemental mappings of Zn and Ca. The unindexed black regions are Ca2Mg6Zn3 particles as marked by white arrows.

Fig. 8. (a) XRD patterns of CS-HPR and SRS-HPR samples and (b) the diffraction peaks of (100), (002) and (101) crystal planes.

Fig. 9. HAADF-STEM images of (a) CS-HPR sample and (b) SRS-HPR sample, and (c, d) the corresponding STEM-EDS maps (Zn-blue and Ca-green). The typical fragmented Ca2Mg6Zn3 spherical particles are indicated by yellow arrows.

Fig. 10. Bright-field TEM image of (a) SRS-HPR and (b) CS-HPR sample, (c) HRTEM image of interface between α-Mg matrix and nano-precipitate (marked by dashed lines), FFT images of (d) α-Mg and (e) MgZn 2 phase.

Fig. 11. EBSD maps of (a) CS-HPR and (b) SRS-HPR sample and (c) the corresponding deviation of c-axis of grains from ND. Macro-texture of (d) CS-HPR and (e) SRS-HPR sample.

Fig. 12. Hard oriented grains with c-axis parallel to ND identified by red regions with a tolerance of 10° in the (a) CS-HPR sample and (b) SRS-HPR sample. Inserted images depict the three-dimensional crystallographic orientation of grains.

Fig. 13. Room-temperature tensile tests of CS-HPR and SRS-HPR samples: (a) engineering stress-strain curves, (b) the corresponding true stress-strain curves and (c) work hardening curves. (d) Relationships between UTS and elongation, including representative rolled [8,24,[56], [57], [58], [59], [60]] Mg-Zn-Ca alloys.

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