Superplastic behavior of an ultrafine-grained Mg-13Zn-1.55Y alloy with a high volume fraction of icosahedral phases prepared by high-ratio differential speed rolling

doi:10.1016/j.jmst.2017.05.003

Abstract

Abstract:

An ultrafine-grained (UFG) Mg-13Zn-1.55Y alloy (ZW132) with a high volume fraction (7.4%) of icosahedral phase (I-phase, Mg₃Zn₆Y) particles was prepared by applying high-ratio differential speed rolling (HRDSR) on the cast microstructure following homogenization. The alloy exhibited excellent superplasticity at low temperatures (tensile elongations of 455% and 1021% 473 K- 10^-3 s^-1 and 523 K- 10^-3 s^-1, respectively). Compared with UFG Mg-9.25Zn-1.66Y alloy (ZW92) with a lower volume fraction of I-phase particles (4.1%), which was prepared using the same processing routes, the UFG ZW132 alloy exhibited a higher thermal stability of grain size. Rapid grain coarsening, however, occurred at temperatures beyond 523 K, leading to a loss of superplasticity. The high-temperature deformation behavior of the HRDSR-processed ZW132 alloy could be well described assuming that the mechanisms of grain boundary sliding and dislocation climb creep competed with each other and considering that the grain-size was largely increased by accelerated grain growth at the temperatures beyond 523 K.

Key words: Magnesium alloy, Superplastic deformation, Ultrafine grain, Icosahedral quasicrystal, Grain growth

Kwak T.Y., Kim W.J.. Superplastic behavior of an ultrafine-grained Mg-13Zn-1.55Y alloy with a high volume fraction of icosahedral phases prepared by high-ratio differential speed rolling[J]. J. Mater. Sci. Technol., 2017, 33(9): 919-925.

Figures/Tables 11

Fig. 1. SEM micrographs of ZW132 in (a) homogenized cast state and (b) after HRDSR. High magnification SEM micrographs of HRDSRed ZW132 showing (c) elongated I-phase eutectic pockets and (d) finely fragmented eutectic pockets and I-phase particles dispersed over the matrix composed of ultra fine grains. The EDS analysis result on the phase in (a) is presented in Table 1.

Table 1 EDS analysis on particles in Fig. 1(a) and Fig. 9(a).

Position	Mg (at.%)	Zn (at.%)	Y (at.%)
1	60.55	32.71	6.74
2	87.21	11.19	1.60
3	87.56	10.44	2.01
4	83.08	14.32	2.60

Fig. 2. TEM micrographs of HRDSRed ZW132 alloy at high (a) and low (b) magnifications and the SADPs of the particles.

Fig. 3. Strain rate-stress relationship of the HRDSR ZW132 alloy constructed in double logarithm plots at (a) 443 K, (b) 473 K, (c) 523 K, (d) 573 K and (e) 623 K.

Fig. 4. Strain rate-stress (in log-log format) curves for the HRDSR ZW132 alloy at various temperatures constructed using the first SRC cycle curves. The dotted curves represent the prediction by Eq. (1).

Fig. 5. True stress-true strain curves for the HRDSR ZW132 alloy obtained (a) from 443 K to 623 K at a strain rate of 10-3 s-1 and (b) at different strain rates at 523 K.

Fig. 6. Hart criterion for stable flow for HRDSR ZW132 alloy at different temperatures at a strain rate of 10-3 s-1.

Fig. 7. Optical microstructures at the gauge sections of HRDSR ZW132 alloy after SRC tests at (a) 523 K and (b) 573 K.

Fig. 8. Low (a, c) and high (b, d) magnification SEM micrographs of HRDSR ZW132 alloy at the gauge (a, b) and grip (c, d) regions after tensile elongation of 1021% at 523 K and 1 × 10-3 s-1.

Fig. 9. (a) Secondary electron image and (b) Mg, (c) Zn and (d) Y element mappings in gauge region of HRDSR ZW132 alloy after tensile elongation of 1021% at 523 K and 1 × 10-3 s-1. The EDS analysis results on the particles in (a) are presented in Table 1.

Fig. 10. Optical micrographs of the HRDSR ZW132 alloy in the gauge (a) and grip regions (b) after the tensile elongation of 1021% at 523 K and 1 × 10-3 s-1.

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