Enhancement of mechanical properties and corrosion resistance of Mg–Ca alloys through microstructural refinement by indirect extrusion
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).
References (32)
- et al.
Corrosion behavior of an Mg–Y–RE alloy used in biomedical applications studied by electrochemical techniques
CR. Chim.
(2008) - et al.
Enhanced corrosion resistance of high strength Mg–3Al–1Zn alloy sheets with ultrafine grains in a phosphate-buffered saline solution
Corros. Sci.
(2013) - et al.
Relationship between the corrosion behavior and the thermal characteristics and microstructure of Mg-0.5Ca-xZn alloys
Corros. Sci.
(2012) - et al.
Mechanical and bio-corrosion properties of quaternary Mg-Ca-Mn-Zn alloys compared with binary Mg-Ca alloys
Mater. Des.
(2014) - et al.
Degradable behavior and bioactivity of micro-arc oxidized AZ91D Mg alloy with calcium phosphate/chitosan composite coating in m-SBF
Colloid Surface B
(2013) - et al.
Retardation of surface corrosion of biodegradable magnesium-based materials by aluminium ion implantation
Appl. Surf. Sci.
(2012) - et al.
Corrosion behaviour of commercially pure Mg and ZM21 Mg alloy in Ringer’s solution – Long term evaluation by EIS
Corros. Sci.
(2011) - et al.
Microstructure, mechanical properties and bio-corrosion properties of Mg–Zn–Mn–Ca alloy for biomedical application
Mat. Sci. Eng. A-Struct.
(2008) - et al.
In vitro degradation and mechanical integrity of calcium-containing magnesium alloys in modified-simulated body fluid
Biomaterials
(2008) - et al.
The development of binary Mg-Ca alloys for use as biodegradable materials within bone
Biomaterials
(2008)