Solidification of Mg-Zn-Zr Alloys: Grain Growth Restriction, Dendrite Coherency and Grain Size

doi:10.1007/s40195-020-01069-1

摘要/Abstract

Abstract:

The solidification characterization of Mg-xZn-0.5Zr (x = 0, 1, 3, 4, 5 wt%) alloys has been extensively investigated through thermal analysis, microstructure characterization and thermodynamic calculations. The impact of Zn content on the grain growth restriction, dendrite coherency and thus the final grain size has been investigated and discussed. Increasing Zn content, the grain size of Mg-xZn-0.5Zr alloy was firstly refined and then coarsened with the finest grain size of ~ 50 μm for the Mg-3Zn-0.5Zr (ZK31) alloy. Significant effects of the grain size on the mechanical properties were observed in the investigated alloys. The combination of growth restriction factor theory and dendrite coherency point provides a reasonable explanation of the grain size results. It helps to further understand the mechanisms of grain refinement and grain coarsening related to solute content, providing reference for alloy design and grain size prediction.

Key words: Magnesium alloy, Grain size, Solute content, Growth, restriction, Dendrite coherency, Solidification

. [J]. 金属学报英文版, 2020, 33(11): 1477-1486.

Pei Li, Danhui Hou, En-Hou Han, Rongshi Chen, Zhiwei Shan. Solidification of Mg-Zn-Zr Alloys: Grain Growth Restriction, Dendrite Coherency and Grain Size[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(11): 1477-1486.

图/表 14

Table 1 Chemical compositions of Mg-xZn-0.5Zr (x = 0, 1, 3, 4, 5 wt%) alloys

Alloys	Zn	Zr	Mg
K1	0	0.38	Bal.
ZK11	0.97	0.40	Bal.
ZK31	2.82	0.50	Bal.
ZK41	3.72	0.56	Bal.
ZK51	4.60	0.47	Bal.

Fig. 1 a Device, b result of thermal analysis for determining the dendrite coherency temperature (TDCP) (Tc and Te refer to the temperatures in the center and near the edge of ingots, respectively)

Fig. 2 Optical microstructures of a K1, b ZK11, c ZK31, d ZK41, e ZK51 alloys

Fig. 3 EBSD observations of a K1, b ZK11, c ZK31, d ZK41 e ZK51 alloys, f the detailed grain sizes variation

Fig. 4 BSE observations of a K1, b ZK11, c, d ZK31, e, f ZK41 g, h ZK51 alloys (the black and white arrows in the enlarged views indicate the Zr-rich areas and Zr particles, respectively)

Fig. 5 Thermodynamic calculated phase diagram of Mg-xZn-0.5Zr alloys

Fig. 6 Phase fraction evolution of ZK31 alloy during solidification with Scheil simulation

Fig. 7 a Zn content, b Zr content in the remaining liquid phase during Scheil simulation

Fig. 8 a Plots of the constitutional undercooling against solid-phase fraction of Mg-xZn-0.5Zr (x = 0, 1, 3, 4, 5 wt%) alloys at the initial stage of solidification ,b the calculated Q values variation against Zn content

Fig. 9 Plot of the measured grain size against 1/Q of Mg-xZn-0.5Zr (x = 0, 1, 3, 4, 5 wt%) alloys

Table 2 Dendrite coherency temperatures of Mg-xZn-0.5Zr (x = 0, 1, 3, 4, 5 wt%) alloys

Alloys	K1	ZK11	ZK31	ZK41	ZK51
T_DCP (°C)	650	645	636	626	630

Fig. 10 a Plots of temperature against solid fraction of Mg-xZn-0.5Zr (x = 0, 1, 3, 4, 5 wt%) alloys calculated by the Scheil simulation, b dendrite coherency solid fractions (fDCPs ) of the alloys obtained from (a)

Fig. 11 a Stress-strain curves, b detailed properties of as-cast Mg-xZn-0.5Zr (x = 0, 1, 3, 4, 5) alloys

Fig. 12 Tensile yield strength experimentally measured and the yield strength contributed by grain size based on the Hall-Petch equation

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