Drag Effects of Solute and Second Phase Distributions on the Grain Growth Kinetics of Pre-Extruded Mg-6Zn Alloy

doi:10.1016/j.jmst.2016.06.025

Abstract

Abstract:

In order to control the grain size during hot forming, grain growth behavior of a pre-extruded Mg-6Zn magnesium alloy and its correlation with solute and second phase distribution were investigated. Isothermal annealing was conducted on a Gleeble-1500 thermo-mechanical simulator. The mean grain size D_g of each annealed specimen was measured by the quantitative metallography technique. The grain growth kinetics of the Mg-6Zn alloy annealed at 473-623 K was obtained as D_g⁴-D_g0⁴=2.25×10¹¹exp(-95450/RT)t by the least square linear regression method. The deviation of grain growth exponent n = 4 from the theoretical value of 2 may be attributed to the presence of solute zinc and second phases which will retard the boundary migration. Microscopic observations show that the non-uniform distribution of grain size for samples pre-extruded or annealed at low temperatures is closely related to the non-uniform distribution of fine and dispersed second phases but not to the non-uniform distribution of solute zinc. This indicates that second phase pinning effect plays an important role in microstructure refinement.

Key words: Grain growth kinetics, Second phase distribution, Solute dragging, Magnesium alloys

Jin Zhaoyang,Yu Donghua,Wu Xintong,Yin Kai,Yan Kai. Drag Effects of Solute and Second Phase Distributions on the Grain Growth Kinetics of Pre-Extruded Mg-6Zn Alloy[J]. J. Mater. Sci. Technol., 2016, 32(12): 1260-1266.

Figures/Tables 9

Fig. 1. Optical microstructures of Mg-6Zn alloy under different conditions: (a) initial as-extruded, (b) heated at 523 K for 600 s, (c) heated at 573 K for 600 s, (d) heated at 623 K for 600 s, (e) heated at 523 K for 60 s, (f) heated at 523 K for 360 s.

Fig. 2. Variation of mean grain size with annealing condition.

Fig. 3. Variation of grain growth rate with annealing condition.

Fig. 4. Plot of ln(dDg/dt)ln(dDg/dt) versus ln(Dg)ln(Dg).

Fig. 5. Plot of lnB against 1/T.

Fig. 6. Comparison of the measured mean grain size with the calculated one.

Fig. 7. Effects of second phase distribution on grain growth behavior by optical metallography for different samples: (a) as-cast and then annealed under 593 K for 2 h, (b) as-extruded, (c) annealed at 473 K for 600 s, (d) annealed at 573 K for 600 s. The extrusion direction is parallel to the horizontal line in (b) to (d).

Fig. 8. SEM morphology of second phase distribution for samples: (a) as-extruded, (b) annealed at 473 K for 600 s, (c) annealed at 573 K for 600 s.

Fig. 9. Distribution of solute zinc by EDS at, near, and far away from the coarse eutectic phase for the sample annealed at 573 K.

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