Elsevier

Corrosion Science

Volume 82, May 2014, Pages 392-403
Corrosion Science

Enhancement of mechanical properties and corrosion resistance of Mg–Ca alloys through microstructural refinement by indirect extrusion

https://doi.org/10.1016/j.corsci.2014.01.041Get rights and content

Highlights

  • Indirect extrusion was used for microstructural refinement in Mg–Ca alloys.

  • Corrosion resistance was tested in the Hank’s solution.

  • Corrosion rates of Mg–Ca alloys (2–3 Ca%) greatly decreased after extrusion.

  • Refining of Mg2Ca phase is important for enhancement of corrosion resistance.

Abstract

The effect of indirect extrusion on microstructural refinement, mechanical properties and corrodibility of the pure Mg and Mg–Ca alloys (up to 3 wt.% Ca) in Hank’s solution has been systematically studied. After extrusion, mechanical properties including tensile strength and ductility, and corrosion resistance of the Mg–Ca alloys were greatly enhanced in the Mg–Ca alloys with Ca  1%. Effective break-up of (semi-)continuous secondary phase (Mg2Ca) existing along grain boundaries or interdendritic regions during extrusion played a critical role in enhancement of the corrosion resistance of the Mg–Ca alloys with Ca  1% compared to the as-cast counterparts.

Introduction

Magnesium (Mg) and its alloys are good candidates for load-bearing degradable biomaterials that are required to have high mechanical strength, a matching degradation rate and tissue healing rate and biocompatibilities. Furthermore, they have low density and low elastic modulus that are close to those of natural bone. Extensive studies have been conducted for the evaluation of the biocorrosion and the biocompatibility of Mg alloys [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Some of the alloy compositions provide proper mechanical properties and biodegrading rates, but the compositions often contain aluminium or rare earth elements, which are hazardous to a person’s health: aluminium is neurotoxic, and rare earth elements are hepatotoxic. Thus, development of new biodegradable Mg alloys that contain nontoxic elements but exhibit high mechanical strength and high bio-corrosion resistance is important. Calcium (Ca) is an important component in the human bone, and the simultaneous release of Mg and Ca ions is known to be beneficial for bone regeneration [11], [12]. For this reason, the binary alloy system Mg–Ca emerges as the type of biodegradable Mg alloys [13], [14], [15], [16]. Adding a high content of Ca increases the strength of Mg alloys but also increases the bio degradation rate [13].

Recent studies on the Mg–Ca alloys focus on improving their mechanical properties and corrosion resistance by the application of thermomechanical processes such as extrusion, forging and rolling. The Mg–1 wt.% Ca alloy has been most widely used for this study. Harandi et al. [17] evaluated the effect of hot forging on mechanical and corrosion properties of the Mg–1Ca alloy. The results showed that the forging process effectively refined the microstructure and improved the mechanical properties but decreased the corrosion resistance in simulated body fluid (SBF). In contrast, Koleini et al. [18] and Li et al. [13] showed that hot rolling or hot extrusion improved the corrosion resistance of the Mg–1Ca alloy in SBF.

The Mg alloys with high amounts of Ca are often difficult to be thermo-mechanically processed because workability decreases and extrusion force increases considerably for higher amounts of calcium [8]. Furthermore, an increased amount of eutectic phase with a low melting temperature in the Mg–Ca alloys with higher amounts of calcium can lead to hot cracking during extrusion. These problems may be overcome by use of indirect extrusion. In this process, unlike in direct extrusion, there is no friction to overcome along with the container walls because the billet and container move together, while the die is stationary. This allows for the extrusion force to be reduced and for the occurrence of cracking caused by excessive heat from friction to be suppressed.

In this work, the effect of indirect extrusion on microstructural refinement, mechanical properties and corrodibility of the Mg–Ca alloys in Hank’s solution has been systematically studied as a function of Ca content (up to 3 wt.%). In addition, we examined the relationship among microstructure, mechanical properties and corrosion resistance. Refinement of secondary phase was important in improving the mechanical and corrosion properties of the Mg–Ca alloys. We discussed how the refinement of second phase influenced the mechanical and corrosion properties of the Mg–Ca alloys and how to obtain optimum microstructures for enhanced mechanical and corrosion properties in the Mg–Ca alloys.

Section snippets

Materials and methods

Pure Mg ingot (99.9%) and Ca powders (99.9%) were used as starting materials. The materials were melted with argon gas in a mild steel crucible at a temperature of 690 °C for 30 min holding time. Following the melting and alloying processes, the molten metals with different contents of calcium (0, 0.4, 1, 2 and 3 wt.%) were poured into a pre-heated mild steel mould to attain ingots. The ingots were then subjected to homogenization treatment at 370 °C for 8 h. With dimensions of 50 mm in diameter and

Microstructures

The chemical compositions of the as-cast pure Mg and as-cast Mg–Ca alloys are presented in Table 2. All of the alloys contain low amounts (within tolerance limits) of iron, copper and nickel. The phase fraction (mole) of Mg2Ca phase in Mg–xCa alloys was calculated using two models available in Pandat package: Scheil model based on the assumption of complete mixing in the liquid but no diffusion in the solid and equilibrium model (using the lever rule) which is based on infinite diffusion in

Discussion

The corrosion potential of Mg2Ca against that of α-Mg is controversial. It was reported that the potential of Mg2Ca is nobler than that of α-Mg, which causes galvanic corrosion between the Mg2Ca phase (cathode) and Mg matrix (anode) [23]. However, recently, a report by Südholz et al. [24], which states Ecorr of Mg2Ca is −1.75 VSCE and that of Mg is −1.65 VSCE in 0.1 M NaCl solution indicates that electrochemically, the Mg2Ca phase is anodic to the α-Mg phase. Fig. 14(a)–(d) show the samples of the

Conclusions

The effect of extrusion on the mechanical properties and the corrosion resistance of the Mg–Ca binary alloys with Ca content up to 3 wt.% was examined, and the following results were obtained.

  • (1)

    The volume fraction of Mg2Ca increased with increasing Ca content, and the phase became continuous at the Ca content ⩾2%. After indirect extrusion, grain refinement of the matrix phase and fragmentation of Mg2Ca occurred. The fragmented Mg2Ca particles were distributed in bands and aligned to the extrusion

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (2013) funded by the Ministry of Education, Science and Technology (2013R1A1A2010637).

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