Elsevier

Calphad

Volume 35, Issue 3, September 2011, Pages 411-415
Calphad

Isothermal section of Mg–Zn–Zr ternary system at 345 ° C

https://doi.org/10.1016/j.calphad.2011.05.009Get rights and content

Abstract

Isothermal section of Mg–Zn–Zr ternary system at 345 C has been determined by X-ray diffraction, differential scanning calorimetry and scanning electron microscopy assisted with energy dispersive X-ray spectroscopy using a series of Mg–Zn–Zr alloys. The results show that there exist three intermetallic compounds ZnZr, Zn2Zr3, (Mg,Zn)2Zr, and a liquid phase in equilibrium with the α-Mg phase. The presence of two other three-phase regions in equilibrium, Liquid+MgZn+(Mg,Zn)2Zr and MgZn+Mg2Zn3+(Mg,Zr)Zn2, has also been confirmed. The addition of Zn can significantly increase the solubility of Zr and vice-versa in the α-Mg matrix.

Graphical abstract

Highlights

► Isothermal section of Mg–Zn–Zr ternary system at 345 °C has been determined by the XRD, DSC and SEM-EDS analysis using a series of Mg–Zn–Zr alloys. ► There exist three intermetallic compounds ZnZr, Zn2Zr3, (Mg,Zn)2Zr, and a liquid phase in equilibrium with the α-Mg phase. ► The addition of Zn can significantly increase the solubility of Zr and vice-versa in the α-Mg matrix.

Introduction

The Mg–Zn–Zr series Mg alloys (ZK) have been paid much attention due to their good comprehensive mechanical properties resulting from a remarkable refining efficiency of Zr and subsequent solid-state forming process or precipitation hardening [1], [2], [3], [4], [5], [6], [7], [8], [9]. Recently, Arroyave and co-workers [10] thermodynamically explained the effect of Zn on grain refining, i.e. the solubility of Zr in the liquid phase decreased remarkably with temperature, and thus large supercooling caused high density of Zr or Zr-rich nucleus. However, it remains an open question what nuclei play a governing role in the grain refinement. Moreover, as one of the as-cast or wrought Mg-based alloys, the effect of the second phase on mechanical property, such as ZnZr, Zn2Zr3, etc., was not well understood in the Mg–Zn–Zr alloys. Morozova et al. [6] pointed out that the phase constituents were composed of the Zn2Zr3+Zn2Zr, Zn2Zr3+Zn2Zr+Mg2Zn3, and Zn2Zr3+Zn2Zr+Mg2Zn3+ZnZr intermetallic compounds as well as the α-Mg matrix in the as-cast Mg94.78Zn4.5Zr0.72, Mg92.07Zn7.1Zr0.83, Mg90.5Zn8.5Zr1.0 (wt%) alloys, respectively. Lashko et al. [11] believed that there existed MgZn, Mg7Zn3, Zn2Zr3, ZnZr, (Zn,Mg)2Zr, and (Mg,Zr)Zn2 intermetallic compounds in foundry cast Mg–Zn–Zr alloys. Unfortunately, the role of these phases was little clarified in the thermo-mechanically deformed ZK series Mg alloys [1], [2], [3], [4], [5], making it hard to correlate their mechanical properties to the microstructure. Thus, it becomes necessary to experimentally determine the phase equilibria of the Mg–Zn–Zr system in an intermediate temperature range of 300–400 °C, where the ZK series Mg alloys are thermally deformed. Seifert [12] summarized recent progress on the phase diagram study of the Mg–Zn–Zr system, but only one isothermal section at 300 °C was investigated, where three phases including Zr, Zn2Zr3, and MgZn were considered to be in equilibrium with the α-Mg matrix. This conclusion is contradictory to the microstructure observations of ZK series Mg alloys mentioned above. In this study, therefore, the isothermal section of the Mg–Zn–Zr at 345 °C is determined by means of the equilibrated alloy method. Based on this information, the relationship between microstructure, processing and mechanical properties of the ZK Mg alloys can be better understood.

Section snippets

Experimental procedure

Nine desired alloys with nominal compositions, Mg90Zn6Zr4, Mg90Zn4Zr6, Mg90Zn2Zr8, Mg80Zn12Zr8, Mg70Zn25Zr5, Mg60Zn33Zr7, Mg60Zn37Zr3, Mg50Zn45Zr5, Mg40Zn57Zr3 (all in at.%), were prepared by using high purity Mg (99.99%), Zn (99.999%), and Mg-31 wt% Zr master alloy supplied by the Hunan Institute of Rare-Metal Materials. The Mg–Zn–Zr alloy ingots were melted in a high purity graphite crucible in an argon atmosphere using an induction furnace. Samples were cut from the ingots and sealed in a

Results and discussion

Backscattering electron (BSE) images of the alloys treated at 345 °C for 360 h are shown in Fig. 1. In the case of treated Mg90Zn4Zr6 alloys, the microstructure is composed of dark and bright phases (Fig. 1(a)), but the phase constituent consists of α-Mg, ZnZr, and Zn2Zr3 phases as determined by X-ray diffraction analysis (Fig. 2(a)). This result is also confirmed by means of the energy dispersive X-ray spectroscopy (Table 1). SEM-EDS analysis confirms that the bright phase with a regular shape

Conclusions

The phase equilibria of the Mg–Zn–Zr ternary system at 345  °C were determined by means of the equilibrated alloy method. The results show that there exist four three-phase regions consisting of α-Mg+Liquid+(Mg,Zn)2Zr, α-Mg+(Mg,Zn)2Zr+ZnZr, α-Mg+ZnZr+Zn2Zr3, and α-Mg+Zr+Zn2Zr3 phases in the Mg-rich region, as well as other three-phase regions composed of Liquid+MgZn+(Mg,Zn)2Zr, and MgZn+Mg2Zn3+(Mg,Zr)Zn2 phases. The solubility of Zn in the α-Mg matrix drastically increases with trace additions

Acknowledgments

This work was supported by the Fundamental Research Funds of the Central Universities (No. N090402006 and No. N090502002), Projects of National Natural Science Foundation of China (No. 50731002 and No. 50901017) and “125” National Key Technology R&D Program (No. 2011BAE22B01-5).

References (23)

  • A. Das et al.

    Mater. Sci. Eng. A

    (2006)
  • M. Shahzad et al.

    Scripta Mater.

    (2009)
  • M. Shahzad et al.

    J. Alloys Compd.

    (2009)
  • D.K. Xu et al.

    Scripta Mater.

    (2008)
  • S. Spigarelli et al.

    Scripta Mater.

    (2010)
  • Y.C. Guo et al.

    J. Alloys Compd.

    (2008)
  • C.P. Guo et al.

    CALPHAD

    (2007)
  • Y.P. Ren et al.

    J. Alloys Compd.

    (2009)
  • Y.P. Ren et al.

    J. Alloys Compd.

    (2009)
  • H.D. Zhao et al.

    J. Alloys Compd.

    (2009)
  • H. Somekawa et al.

    J. Mater. Res.

    (2007)
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