Near-eutectic Zn-Mg alloys: Interrelations of solidification thermal parameters, microstructure length scale and tensile/corrosion properties

Highlights

• The Zn-Mg eutectic has bimodal morphology: areas of fine and coarse lamellae and spiral structures.

• Growth laws: dendritic/lamellar spacings vs. growth/cooling rates are proposed.

• Experimental equations relating tensile properties vs. dendritic/lamellar spacings are proposed.

• Best tensile/corrosion properties > alloy with higher eutectic fraction and finer lamellar spacing.

Abstract

Zn-Mg alloys are considered to have potential application in bone implants, since both metals are biocompatible and have biodegradable characteristics. Adding Mg to Zn can boost mechanical and corrosion resistances. However, the literature is very limited on quantifying the interrelation of solidification parameters, microstructural features and mechanical/corrosion properties of Zn-Mg alloys. The present study examines the interrelations of alloy Mg content, macrosegregation effects, morphology and scale of the matrix and eutectic phases, nature of intermetallics and tensile and corrosion properties of near-eutectic Zn-Mg alloys. The alloys samples are obtained by unsteady-state directional solidification resulting in a wide range of solidification thermal parameters and microstructures. We examine microstructural features of both dendritic and complex regular eutectic phases. It is shown that the eutectic exhibits a bimodal pattern with neighboring areas of coarse and fine lamellae. Experimental growth laws relating the primary, secondary and eutectic spacings to the solidification cooling rate and growth rate are proposed. Hall-Petch type equations are derived expressing tensile strength and elongation to dendritic and eutectic spacings. Electrochemical parameters determined by polarization curves during corrosion tests and SEM analyses of corroded areas have shown that the alloy having an essentially eutectic microstructure is associated with better corrosion resistance.

Introduction

Several investigations underlined Zn as a promising bio-absorbable material because of its non-toxicity and inherent benefits to the human body [[1], [2], [3], [4]]. The potential for build-up biocompatible corrosion layers together with superior mechanical strength and castability describes the Zn-Mg family of alloys [5]. This explains why these materials may become so attractive for clinical bone implants. Load-bearing applications with biocompatible Zn-Mg alloys are among those considered as highly innovative [[6], [7], [8]]. Since Mg is nearly exclusively present in the form of Zn-Mg intermetallics (i.e., Mg₂Zn₁₁ and MgZn₂), an increase in mechanical strength does not seem surprising. Therefore, Mg emerges as a natural alloying element that could provide superior mechanical properties [[9], [10], [11], [12], [13], [14]], while preserving the biocompatibility. However, systematic studies correlating microstructural features and mechanical/corrosion properties of Zn-Mg alloys are still scarce in the literature.

The addition of Mg to Zn coatings is reported to improve the corrosion properties [[15], [16], [17]]. The excellent level of protection is also very promising for applications in painted zinc-magnesium coatings on steel sheets. The corrosion resistance of these covered sheets exceeded by far that of the unalloyed zinc coatings, as well as that of many other alloy coatings. These encouraging results open a perspective to reduce coating thickness and maybe even to omit pre-treatments [18]. As stated by Prosek et al. [19], advanced coatings can provide a barrier and galvanic protection to steel parts applied in automotive, building, and other industries, improving durability and aesthetic properties of final products.

It seems unexpected that Mg-containing Zn alloys could display better corrosion properties, as Mg is much less noble than Zn [19]. As a matter of fact, Mg added to the alloys is mostly expended for the growth of Zn-Mg intermetallics during the liquid-to-solid transformation stage. Therefore, the role of the Zn-Mg intermetallic particles on the protection mechanisms defining the corrosion behavior is critical, as outlined in previous investigations [18,20]. The corrosive action starts exactly at the intermetallics in Zn-Mg alloys [20]. Despite the most recent studies of specific mechanisms, the effects of Zn-Mg intermetallics on the corrosion behavior have not yet been fully comprehended [[21], [22], [23], [24]].

According to Yao et al. [1], bulk parts of a specific Mg-Zn alloy may have a very different microstructure as compared to that of surface coatings of the same composition. This is due to the different cooling rates associated with processing of bulk alloys and coatings. As such, the corrosion properties may be affected accordingly. The effect of the imposed cooling rate during casting in the microstructural array and consequently in the final application properties is thoroughly documented [25,26]. Therefore, the solidification cooling rate may have an important impact on the resulting properties of the Zn-Mg alloys, e.g., on mechanical strength and electrochemical behavior.

Many of the experimental researches on Zn-Mg alloys reported a qualitative description of the resulting microstructure [1,22,27,28]. Although it is known that solidification thermal parameters, such as the growth rate and the cooling rate, define the development of the as-cast microstructure, these studies have not focused on the correlations between thermal and microstructure parameters. The literature on quantifying the morphology, size, distribution of microstructural features and mechanical properties related to the solidification thermal parameters of Zn-Mg alloys [29,30] is very limited.

The length scales of the phases forming the microstructure of metallic alloys, that is, cellular, dendritic and interphase spacings, are known to play significant roles in the improvement of mechanical and chemical properties. The solidification thermal parameters influence these microstructural spacings, porosity, segregation patterns, morphology, size and dissemination of second phases, therefore affecting the final properties - physical [31], mechanical and corrosion properties [32,33] - of as-cast alloys. Several studies reported that the microstructural refinement is generally positive to mechanical properties, since a more homogeneous distribution of second/intermetallic phases throughout the alloy matrix makes obstruction to the motion of dislocations more efficient [26,[34], [35], [36]].

Kubasek and Vojtech [37] investigated the effects of Mg on the tensile mechanical properties of Zn-Mg alloys (1.0, 1.5 and 3.0 wt-pct Mg) by pouring the molten alloys into a non-preheated steel mold. A qualitative description of the resulting microstructures was reported. Higher ultimate tensile strengths of 153 MPa and 147 MPa were obtained for the 1.0 and 1.5 wt% Mg-containing alloys, while ductility was quite low in both cases, i.e., 1.5% and 0.4%, respectively. The Zn-3wt.%Mg alloy was characterized by extremely lower strength and ductility - values of 28 MPa and 0.2%, respectively.

As alloys of the Zn-Mg system exhibits a skewed couple zone [38], a variety of microstructures including dendritic arrangements and eutectics, can be formed with an increase in the growth rate. Akdeniz and Wood [38] have described the extent to which a specific microstructure is present in rapidly solidified Zn-Mg alloys by establishing a very complete growth rate-composition map. The coupled zone was shown to shift from a symmetry position. Thus, it skews towards the facet-forming Mg₂Zn₁₁ side of the phase diagram, i.e., towards the hypereutectic side. This means that for eutectic and hypereutectic alloys, there seem to exist critical growth rates beyond which eutectic solidification is not possible for a eutectic composition, with eutectic structures prevailing for hypereutectic chemistries.

The growth of complex regular eutectics involves a group of eutectics where one phase has a high entropy of fusion while the other, a low entropy of fusion. Here, metallic systems such as Pb-Bi [39], Sn-Bi [40], Bi-Ca [39] and Zn-Mg [27] are examples of this kind of growth. Two types of regions generally characterize the complex eutectic structures: zones of a regular repeating pattern and other zones of random orientation (irregular). Some authors reported other type of structures forming the eutectic in Zn-Mg alloys [41]. Fullman and Wood [41], for instance, reported structures in which the two phases composing the eutectic Zn-3wt%Mg alloy appear as intertwined spirals in the examined cross sections. A wide range of cooling rates was obtained by varying the casting conditions. These conditions were associated with either generating a casting onto a large cold brass block (i.e., fast cooling) or imposing slow cooling by controlling the sample in a furnace. The microstructures revealed small spiral segments in the fast-cooled samples, with the presence of various eutectic colonies. These colonies prevented the spirals from reaching a sufficient size of several turns. At the slowest cooling rates, the eutectic phases were distributed in a typical spheroidized shape.

In a previous study, some of the present authors analyzed the microstructural development of dilute hypoeutectic Zn-Mg alloys along a wide range of cooling rates during transient directional solidification [42]. A plate-like cellular matrix has been shown to characterize the microstructure of Zn-0.3 wt%Mg and Zn-0.5 wt%Mg alloys for high cooling rates (higher than 9.5 and 24 K/s, respectively) followed by a granular/dendritic transition with a decrease in the solidification cooling rate. The increase in the alloy Mg content to 1.2 wt% was shown to induce a matrix morphology formed by equiaxed dendritic grains along the entire length of the directionally solidified (DS) casting. A low fraction eutectic mixture was shown to be located along the intercellular regions/grain boundaries. The evolution of the intercellular spacing, λ_c (0.3 and 0.5 wt%Mg alloys), and of the secondary dendritic arm spacings, λ₂ (1.2 wt%Mg alloy) as a function of solidification thermal parameters, such as the growth rate (V_L) and the cooling rate ($\dot{T}$), have been established by experimental growth laws, given by: λ_c = 21V_L^−1/2; λ_c = 39 $\dot{T}$ ^−1/4 and λ₂ = 13V_L^−2/3; λ₂ = 28.5 $\dot{T}$ ^−1/3, where λc,₂ [μm]; V_L [mm/s] and $\dot{T}$ [K/s]. With the increase in the Mg alloy content, the volume fraction of the eutectic mixture also increases, and, therefore, the role of the eutectic phase in determining mechanical and chemical properties becomes more significant. However, the studies on the nature and morphology of the phases forming the eutectic mixture under transient solidification conditions, the length scale of the eutectic and their effects on the resulting properties are still missing in the literature.

The present study aims to investigate the influences of growth rate/cooling rate and segregation effects on the morphology and length scale of both the Zn-rich matrix and eutectic mixture during unsteady-state unidirectional solidification of Zn-Mg near-eutectic compositions (hypoeutectic and hypereutectic). Additionally, experimental correlations between the microstructural spacings and tensile and corrosion properties are investigated.