Effect of refinement of grains and icosahedral phase on hot compressive deformation and processing maps of Mg-Zn-Y magnesium alloys with different volume fractions of icosahedral phase

doi:10.1016/j.jmst.2018.06.019

Abstract

Abstract:

The effect of the volume fraction of I-phase on the hot compressive behavior and processing maps of the extruded Mg-Zn-Y alloys was examined, and the obtained results were compared with those of the cast alloys in a previous work. The average grain sizes, fractions of dynamically recrystallized (DRXed) grains, and sizes of DRXed grains of the extruded alloys after compressive deformation were significantly smaller, higher and smaller, respectively, than those of the cast alloys after compressive deformation under the same experimental conditions. This was because the microstructures of the extruded alloys, having much more grain boundaries and more refined I-phase particles than the cast alloys, provided a larger number of nucleation sites for dynamic recrystallization than those of the cast alloys. The constitutive equations for high-temperature deformation of the extruded and cast alloys could be derived using the same activation energy for plastic flow, which was close to the activation energy for lattice diffusion in magnesium. Compared with the cast alloys, the onset of the power law breakdown (PLB) occurred at larger Zener-Holloman (Z) parameter values in the extruded alloys. This was because the extruded alloys had finer initial grain sizes and higher fractions of finer DRXed grains compared to the cast alloys, such that the onset of PLB caused by creation of excessive concentrations of deformation-induced vacancies was delayed to a higher strain rate and a lower temperature. The flow-stress difference between the extruded alloys and the cast alloys could be attributed to the difference in the fraction of DRXed grains. According to the processing maps, the extruded alloys exhibited higher power dissipation efficiency and flow stability than the cast alloys. This agreed with the microstructural observations.

Key words: Hot compression, Magnesium alloy, Processing map, Grain refinement

T.Y. Kwak, W.J. Kim. Effect of refinement of grains and icosahedral phase on hot compressive deformation and processing maps of Mg-Zn-Y magnesium alloys with different volume fractions of icosahedral phase[J]. J. Mater. Sci. Technol., 2019, 35(1): 181-191.

Figures/Tables 13

Fig. 1. Grain boundary (GB) (a-c) and inverse pole figure (IPF) (d-f) maps for extruded ZW20 (a, d), ZW51 (b, e) and ZW133 (c, f) samples, observed on a longitudinal section along the extrusion direction (ED). The black areas, where the EBSD patterns could not be indexed, represent I-phase. In the GB maps, low angle boundaries (2°-5°) are in green, intermediate angle boundaries (5°-15°) are in blue and high angle boundaries (>15°) are in red. The insets in (a)-(c) show the SEM images of the extruded ZW20, ZW51 and ZW133 samples and the insets in (d)-(f) show the IPFs of the extruded ZW20, ZW51 and ZW133 samples.

Fig. 2. True stress-true strain curves for the extruded ZW20 obtained from compression tests at different temperatures in strain rate range of 10-3 to 10 s-1: (a) 473?K; (b) 523?K; (c) 573?K; (d) 623?K; (e) 673?K.

Fig. 3. True stress-true strain curves for the extruded ZW51 obtained from compression tests at different temperatures in strain rate range of 10-3 to 10 s-1: (a) 473?K; (b) 523?K; (c) 573?K; (d) 623?K; (e) 673?K.

Fig. 4. True stress-true strain curves for the extruded ZW133 obtained from compression tests at different temperatures in strain rate range of 10-3 to 10 s-1: (a) 473?K; (b) 523?K; (c) 573?K; (d) 623?K; (e) 673?K.

Fig. 5. Plots of lnε?-lnσ(a-c) and lnε?-lnσ(d-f) for extruded ZW20 (a, d), ZW51 (b, e) and ZW133 (c, f) at ε?=?0.2 (σ0.2: flow stress at ε?=?0.2).

Fig. 6. Plots of lnsinh(ασ)-1000/T at different strain rates for extruded ZW20 (a), ZW51 (b) and ZW133 (c) at ε?=?0.2.

Fig. 7. Plots of lnZ-lnσ0.2 and lnZ-lnσ0.8 for extruded ZW20, ZW51, ZW92 and ZW133 at ε?=?0.2 (a) and ε?=?0.8 (b).

Fig. 8. Plot of lnZ-lnsinh(ασ0.8) for the extruded and cast ZW20, ZW51 and ZW133 constructed with Qc?=?132?kJ/mol and α?=?0.0162?MPa-1 at ε?=?0.8.

Fig. 9. GB and IPF maps for the extruded ZW20 (a, d), ZW51 (b, e) and ZW133 (c, f) after compression tests at 623?K at strain rates of 10-3 s-1 (a-c) and 10?s-1 (d-f). The black areas, where the EBSD patterns could not be indexed, represent I-phase. In the GB maps, low angle boundaries (2°-5°) are in green, intermediate angle boundaries (5°-15°) are in blue and high angle boundaries (>15°) are in red. The insets in (a)-(f) show the IPFs of the extruded samples on the longitudinal planes.

Fig. 10. Processing maps composed of power dissipation maps and flow instability maps for ZW20 (a, d), ZW51 (b, e) and ZW133 (c, f) at ε?=?0.2 (a-c) and ε?=?0.8 (d-f).

Fig. 11. (a) Average grain sizes, (b) fraction and (c) size of DRXed grains, and (d) maximum intensities of (0002) pole figures for the cast and extruded alloys.

Fig. 12. Flow stresses of the cast and extruded alloys at 623?K with strain rates of 10-3 and 10?s-1 plotted as a function of fraction of DRXed grains (a) and average grain size (b).

Fig. 13. η values of ZW20, ZW51 and ZW133 plotted as a function of strain rate at 523?K (a), 623?K (b) and 673?K (b) at ε?=?0.2.

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