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