Elsevier

Materials Chemistry and Physics

Volume 204, 15 January 2018, Pages 105-131
Materials Chemistry and Physics

Microstructure characterization of a directionally solidified Mg-12wt.%Zn alloy: Equiaxed dendrites, eutectic mixture and type/ morphology of intermetallics

https://doi.org/10.1016/j.matchemphys.2017.10.032Get rights and content

Highlights

  • A dendritic αMg matrix and a eutectic αMg/Mg21Zn25 typify the microstructure of the Mg-12 wt.%Zn alloy.

  • The eutectic mixture has two morphologies: lamellae and rods, the latter increasing with decreasing cooling rates.

  • Eutectic spacing (λ) and the growth rate (V) are related by the Jackson-Hunt equation: λ2V = constant.

  • Non-equilibrium MgZn2 and Mg4Zn7 nanoparticles are distributed throughout the αMg matrix.

Abstract

Over the last years, magnesium and its alloys have deserved special attention due to their good mechanical properties and promising biomedical applications. In the present investigation the Mg-12 wt.%Zn alloy has been directionally solidified (DS) under an extensive range of cooling rates. Based on the scarceness of characterization works related to both the morphology of the eutectic mixture and the development of equilibrium and non-equilibrium intermetallic particles (IMCs) in Mg-Zn alloys, a number of analyses are performed emphasizing and discussing such aspects. Rod and lamellar eutectic spacings are experimentally correlated with the growth rate and the cooling rate. The volume fraction of rod-like eutectic is shown to increase with the decrease in cooling rate during solidification. The characterization on the DS Mg-12 wt.%Zn samples is carried out using X-ray diffraction (XRD), scanning (SEM) and transmission (TEM) electron microscopies, which allowed the identification of two different non-equilibrium IMCs, MgZn2 and Mg4Zn7, which are distributed as nanoparticles throughout the α-Mg matrix. The Mg-Zn eutectic morphologies (lamellar and rod-like) are found to be constituted by α-Mg/Mg21Zn25 phases, the latter being formed by the decomposition of the eutectic IMC Mg51Zn20 through a eutectoid reaction.

Introduction

Mg-Zn alloys belong to the most important class of magnesium-based materials due to characteristics such as remarkable precipitation hardening effect [1] and significant mechanical strength. These alloys have important applications in components of the automotive [2], [3] and aerospace industries [4], in portable electronic devices [2] and in biomedical engineering [5], [6], [7], [8], [9]. Mg–Zn alloys are also being studied with regard to their hydrogen-storage properties [10], [11].

Mg is the eighth most abundant element in Earth's crust, and has relatively low yield stress, elongation, modulus of elasticity and density. Also, Mg is a very active chemical element, being easily ignited. Considering that, adding alloying elements into pure Mg is a very important way to improve and stabilize its mechanical, physical and chemical properties [12], [13]. There are several Mg alloys that are currently being developed based on elements such as: Zn, Mn, Al, Ca, Li, Zr, Y, and rare earth ones [6], [14], [15]. Mg is a biocompatible and biodegradable element. The use of traditional Mg alloys for biomedical applications is not advisable because most alloying elements used (for example, Co and Al) are toxic to the human body. In contrast, compared with several other metals, Zn is relatively harmless, vital for biological functions and plays metabolic activities [16], [17], [18].

In addition, there is a considerable potential for the use of Mg–Zn based alloys, due to the formation of intermetallic compounds beyond the solubility limit of Zn in Mg. There are five possible intermetallic phases according to the binary phase diagram of the Mg-Zn system, namely Mg51Zn20 (also known as Mg7Zn3), Mg21Zn25 (also known as MgZn), Mg4Zn7 (also known as Mg2Zn3), MgZn2 and Mg2Zn11 [19], [20], [21], as shown in Fig. 1. The literature presents some studies on the characterization of intermetallics of Mg-Zn alloys after precipitation-hardening treatments [8], [19], but, to date, none has been reported after non-equilibrium solidification conditions, which is the prior stage to heat treatments.

In the experimental work by Gao et al. [19] and Blanco-Rodriguez et al. [21], the Mg21Zn25 phase has been identified as a β-phase, which should form under equilibrium with the α-Mg matrix during solidification. The Mg51Zn20 phase is metastable at room temperature and should pass through a phase transformation by a eutectoid reaction (Eq. (2)), decomposing into Mg21Zn25 and α-Mg at 598 K (325 °C). The initial phase Mg51Zn20 transforms subsequently into the intermetallic Mg21Zn25 phase upon passing by the transition phase Mg4Zn7 (Eqs. (3), (4)) [19], [21], [22].

For experimental conditions associated with kinetics constraints, the expected phases can be hindered, e.g. for high cooling rates the metastable Mg51Zn20 phase has no enough time to transform into the phases α-Mg and Mg21Zn25 [19], [21].

The eutectic reaction (Eq. (1)) and a eutectoid reaction (Eq. (2)) are reported to occur at 613 K (340 °C) and 598 K (325 °C) or below [19], [21], respectively, that is:613 K (340 °C): L → α-Mg + Mg51Zn20598 K (325 °C): Mg51Zn20 → α-Mg + Mg21Zn25

Besides, generally the formation of the MgZn2 intermetallics is energetically more favorable compared with the Mg4Zn7 IMC, however, when the alloy Zn concentration is below 65.5 at.%, the Mg4Zn7 phase is reported to be more stable than MgZn2 [23]. The formation of Mg4Zn7 during non-equilibrium conditions is possible and can be explained by the occurrence of a eutectoid decomposition (Eq. (3)) for temperatures below 598 K (325 °C) [8], [21]:Mg51Zn20 → α-Mg + Mg4Zn7

The Mg4Zn7 intermetallics has the tendency to form Mg21Zn25, (Eq. (4)) and only under specific circumstances, it can be found in its original composition, as reported by Blanco-Rodrigues et al. [21]. The Mg21Zn25 phase is formed below the transformation temperature. The initial phase Mg51Zn20 transforms into the intermetallic phase Mg21Zn25 upon passing by the transition phase Mg4Zn7. Blanco-Rodriguez et al. [21] reported a similar reaction, which seems to follow the most recent notations of the phases of the Mg-Zn diagram, as revised by Okamoto et al. [20]:L + Mg4Zn7 → Mg21Zn25

The eutectic reaction takes place as the composition of the remaining liquid is at about 51.0 wt.%Zn. Most of the Mg-Zn alloys of interest are expected to form a considerable proportion of eutectic mixture during cooling from the melt. Experimental studies emphasizing factors such as the morphology of the eutectic mixture and the distribution of phases within it are rare in the literature for Mg-Zn alloys. Furthermore, the roles played by solidification cooling and growth rates on these microstructural aspects are of prime importance since the final properties of alloys will be connected to them [24], [25].

In previous studies devoted to cast iron [26], Al-Ni [27], [28] and Al-Si [26] alloys some of the very important aspects of lamellar and rod-like eutectics have been presented. In a number of careful investigations on eutectic microstructures, solidification thermal parameters associated with steady-state growth conditions have been related to microstructural features, which in turn have been shown to be determinant to the resulting mechanical, electrical and thermal properties [29], [30], [31]. Hunt [32] stated that the eutectic in which one of the phases has a very low volume fraction tend to grow in a rod-like manner, whereas those with an almost equi-volume composition grow with a lamellar structure. Some eutectics may produce a lamellar structure at low growth rates and rod-like one at high growth rates. The morphology of the eutectic of Mg-Zn alloys remains an issue to be studied and the applicability of the Jackson-Hunt model [33] seems to be an appropriate starting point with a view to examining the dependences of interlamellar and rod spacing, λ, on the growth rate (V). The referred model is able to describe functional relationship for growth of lamellar eutectics as follows: λ = a (V)−1/2, where V is the growth rate, ½ is the exponent and “a” is a constant.

The aim of the present research work is to perform a detailed characterization of the microstructure of a hypoeutectic Mg-12 wt.%Zn alloy, which was directionally solidified under non-equilibrium conditions, that is the morphologies and representative features of: α-Mg matrix, eutectic mixture and intermetallic phases for a wide range of solidification cooling rates. The experimental data on the length-scale of lamellar and rod eutectic spacings (λ) will be compared with the classic Jackson-Hunt model for eutectics. Finally, advanced SEM-EDS, DRX and TEM techniques will be performed to determine the main characteristics of the intermetallic particles formed not only in the eutectic mixture, but also those that precipitate throughout the α-Mg matrix.

Section snippets

Experimental procedure

With a view to permitting a wide range of solidification cooling rates to be attained in a particular directionally solidified (DS) casting, a hypoeutectic Mg-12 wt.% Zn alloy was directionally solidified from a chill mold by using a solidification apparatus, as detailed in a previous work [34]. Commercially pure metals were used to prepare the alloy with the following individual chemistries: Mg (99.85 wt.%) and Zn (99.97 wt.%). The main impurities related to Zn were Fe (0.015 wt.%), Pb

Microstructure and eutectic characterization

The typical microstructure features of the DS Mg-12 wt.%Zn alloy casting are shown in Fig. 2. The microstructure consists of equiaxed grains formed by a dendritic α-Mg matrix (supersaturated solid solution of Zn in Mg) and the eutectic mixture (dark areas) formed by α-Mg and Mg-Zn intermetallics corresponding to different crystalline phases. The SEM backscattered micrographs in Fig. 2 reveals two different eutectic morphologies: lamellar and rod-like, which envelop the α-Mg dendrites. These

Conclusions

The following conclusions can be derived from the present experimental study:

  • The microstructure of the directionally solidified (DS) Mg-12 wt.%Zn alloy casting was shown to be characterized by equiaxed grains formed by a dendritic α-Mg matrix and a eutectic mixture: α-Mg/Mg21Zn25 (formed by the decomposition of the eutectic IMC Mg51Zn20 through a eutectoid reaction) having two different eutectic morphologies: lamellar and rod-like. The fraction of rod-like eutectic was shown to increase from

Acknowledgements

The authors acknowledge the financial support provided by FAPESP (São Paulo Research Foundation, Brazil: grant 2014/50502-5) and CNPq. The authors would like to thank the Brazilian Nanotechnology National Laboratory — LNNano for the use of X-ray diffractometer and Scanning Electron Microscope.

References (52)

  • B. Langelier et al.

    The effects of microalloying on the precipitate microstructure at grain boundary regions in an Mg-Zn based alloy

    Mater. Des.

    (2017)
  • C.Q. Li et al.

    Effect of volume fraction of LPSO phases on corrosion and mechanical properties of Mg-Zn-Y alloys

    Mater. Des.

    (2017)
  • E.M. Salleh et al.

    Synthesis of biodegradable Mg-Zn alloy by mechanical alloying: effect of milling time

    Proc. Chem.

    (2016)
  • G. Song

    Control of biodegradation of biocompatible magnesium alloys

    Corros. Sci.

    (2007)
  • X. Gao et al.

    Structure and thermal stability of primary intermetallic particles in an Mg–Zn casting alloy

    Scr. Mater

    (2007)
  • P. Blanco-Rodriguez et al.

    Thermophysical characterization of Mg-51%Zn eutectic metal alloy: a phase change material for thermal energy storage in direct steam generation applications

    Energy

    (2014)
  • N.C. Verissimo et al.

    Interconnection of Zn content, macrosegregation, dendritic growth, nature of intermetallics and hardness in directionally solidified Mg–Zn alloys

    J. Alloys Compd.

    (2016)
  • Y.-P. Xie et al.

    The phase stability and elastic properties of MgZn2 and Mg4Zn7 in Mg–Zn alloys

    Scr. Mater

    (2013)
  • P.R. Goulart et al.

    The effects of cell spacing and distribution of intermetallic fibers on the mechanical properties of hypoeutectic Al-Fe alloys

    Mater. Chem. Phys.

    (2010)
  • W. Yu et al.

    Eutectic solidification microstructure of an Al-4Ni-2Mn alloy

    J. Alloys Compd.

    (2016)
  • H. Kaya et al.

    Measurements of the microhardness, electrical and thermal properties of the Al–Ni eutectic alloy

    Mater. Des.

    (2012)
  • F. Bertelli et al.

    Cooling termal parameters, microstructure, segregation and hardness in directionally solidified Al-Sn-(Si;Cu) alloys

    Mater. Des.

    (2015)
  • E. Risueño et al.

    Mg-Zn-Al eutectic alloys as phase change material for latent heat thermal energy storage

    Energy Procedia

    (2015)
  • I. Higashi et al.

    The crystal structure of Mg51Zn20

    J. Solid State Chem.

    (1981)
  • A. Parisi et al.

    Stability of lamellar eutectic growth

    Acta Mater

    (2008)
  • A. Singh et al.

    Structural characteristics of β´precipitates in Mg–Zn-based alloys

    Scr. Mater

    (2007)
  • Cited by (14)

    • Directed energy deposition of Al 5xxx alloy using Laser Engineered Net Shaping (LENS®)

      2020, Materials and Design
      Citation Excerpt :

      The XRD results are also in good agreement with the EBSD analysis with respect to the crystallographic orientation. Fig. 8E,F reveals some differences in the microstructure in two distinct zones in a single solidification cell, which can be explained by variations in the local cooling rates and heat dissipation during the solidification process [67]. The mean value of the hardness of the as-deposited Al alloy (Fig. 11) is substantially lower than the nominal Vickers microhardness of the Al 5083 substrate (83.7 ± 2.6 VHN).

    • Selective laser melting of Mg-Zn binary alloys: Effects of Zn content on densification behavior, microstructure, and mechanical property

      2019, Materials Science and Engineering: A
      Citation Excerpt :

      Among various developed magnesium alloys, the aging hardenable Mg-Zn binary alloys belong to one of the most important classes due to the combination of high strength and moderate corrosion resistance [3–5]. Additionally, because Zn-alloying can significantly enhance the biocompatibility and hydrogen-sorption behavior, Mg-Zn alloys have also been extensively investigated as promising candidates for biomedical and hydrogen-storage applications [6,7]. To date Mg-Zn products are commonly manufactured by using liquid forming technologies like permanent mold casting [8] and directionally solidified (DS) casting [9], or plastic forming methods including extrusion [10], rolling [11], equal channel angular pressing (ECAP) [12], and high-pressure torsion (HPT) [13].

    View all citing articles on Scopus
    View full text