Preface
5. Low-Density Metallic Glassess
5.3. Magnesium (Mg)-Based Metallic Glasses
Magnesium and its alloys are very attractive as biodegradable implant materials and show excellent biocompatibility, low density, and an elastic modulus close to the human bone [40]. Mg-based BMGs have recently attracted attention as biodegradable implant materials [41]. Mg-Zn-Ca BMGs with varying proportion of each element show unique and attractive attributes for biomedical applications [1,40, 42–44]. This system consists of non-toxic elements [41] and represents a great choice as a biodegradable bone tissue scaffold material [45]. The results of cytotoxicity and cell culture tests confirmed higher cell viability in Mg–Zn–Ca BMGs compared to crystalline Mg alloys [43]. The effect of each of the three components (i.e., Mg, Zn, and Ca) on corrosion behavior of the alloys was analyzed through potentiodynamic polarization experiments. Mg alloys with the lowest Ca content (i.e., 4–6 at. %) had the lowest corrosion current density and good passivation characteristics, while no passivation was observed when Ca content exceeded 50 at. %. The addition of Zn increased the charge transfer resistance of the alloy and improved its corrosion resistance [46]. The Zn-rich Mg60Zn35Ca5 amorphous alloy exhibited a lower corrosion rate compared with Mg66Zn30Ca4 in SBF [41]. Mg66Zn30Ca4 showed more uniform corrosion than Mg70Zn25Ca5[43], indicating that higher Zn content improved the corrosion resistance. A porous crystalline Zn layer was present on the surface of low-Zn containing alloys whereas a dense Zn and oxygen-rich amorphous layer was formed for Zn-rich alloys [42]. The lower corrosion rate of the Mg60Zn35Ca5 alloy was attributed to the presence of CaMg2and CaZn2intermetallic phases. CaZn2
formed CaZn2(PO4)2.2H2O in SBF which is insoluble in the medium and enhanced the corrosion resistance of the alloy. Additionally, CaMg2was able to reduce the corrosion rate in the presence of Zn [41]. However, high Zn content was found to reduce the glass-forming ability of the Mg-Zn-Ca amorphous alloys. In electrochemistry and corrosion studies, Pourbaix diagrams map out possible stable phases for different redox states of all elements in an alloy as a function of pH. These plots can predict the stable redox species formed on the surface as a function of potential and pH in aqueous solutions. The stability regions typically consist of immunity, corrosion, and
passivation, which are generally depicted with solid lines, while the water redox reactions (water stability window) are plotted as dotted lines. The Pourbaix diagram for a specific Mg-Zn-Ca alloy in Figure 5.1 shows the formation of Zn hydroxide at high potentials and pH of ~7.4. ZnO and ZnCO3surface films are formed at higher pH which reduces hydrogen gas evolution [42]. The primary passivation occurs via the formation of Mg(OH)2, Zn(OH)2, Ca(OH)2, and ZnO2-containing films on the surface. The presence of MgO and Mg(OH)2surface films for Mg60Cu30Y10BMG after immersion in NaCl was confirmed by X-ray photoelectron spectroscopy (XPS) analysis, although it showed low stability in the presence of Cl−ions and dissolved gradually with the formation of MgCl2[47].
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corrosion rate of the Mg60Zn35Ca5 alloy was attributed to the presence of CaMg2 and CaZn2 intermetallic phases. CaZn2 formed CaZn2(PO4)2.2H2O in SBF which is insoluble in the medium and enhanced the corrosion resistance of the alloy.
Additionally, CaMg2 was able to reduce the corrosion rate in the presence of Zn [41].
However, high Zn content was found to reduce the glass-forming ability of the Mg- Zn-Ca amorphous alloys. In electrochemistry and corrosion studies, Pourbaix diagrams map out possible stable phases for different redox states of all elements in an alloy as a function of pH. These plots can predict the stable redox species formed on the surface as a function of potential and pH in aqueous solutions. The stability regions typically consist of immunity, corrosion, and passivation, which are generally depicted with solid lines, while the water redox reactions (water stability window) are plotted as dotted lines. The Pourbaix diagram for a specific Mg-Zn-Ca alloy in Figure 5.1 shows the formation of Zn hydroxide at high potentials and pH of ~7.4. ZnO and ZnCO3 surface films are formed at higher pH which reduces hydrogen gas evolution [42]. The primary passivation occurs via the formation of Mg(OH)2, Zn(OH)2, Ca(OH)2, and ZnO2-containing films on the surface. The presence of MgOand Mg(OH)2 surface films for Mg60Cu30Y10 BMG after immersion in NaCl was confirmed by X-ray photoelectron spectroscopy (XPS) analysis, although it showed low stability in the presence of Cl- ions and dissolved gradually with the formation of MgCl2 [47].
Figure 5.1. Potential–pH (Pourbaix) diagram for Mg66Zn30Ca4 alloy (Redrawn using data from reference [41]).
Figure 5.1. Potential–pH (Pourbaix) diagram for Mg66Zn30Ca4alloy (Redrawn using data from reference [41]).
5.3.1. Effect of Alloying Elements
The addition of Ti and Cr to a Mg-based BMG, specifically the (Mg65Cu20Y10Zn5)98M2 (M=Ti, Cr) alloy system, showed lower passive current density and increased corrosion potential in the Cl−-containing solution due to the formation of a more homogenous protective surface layer. However, the addition of Ti and Cr reduced the glass-forming ability of the alloy [48]. The addition of Yttrium (Y) improved the corrosion resistance of the Mg-Cu amorphous alloy system in NaCl [47]. Mg65Cu7.5Ni7.5Ag5Zn5Gd5Y5BMG in alkaline solution (NaOH) formed a passive layer consisting of Mg(OH)2with the presence of other elements such
as silver (Ag), rare-earth elements (RE), and Ni [49]. The addition of ytterbium (Yb) to MgZnCa BMGs not only enhanced their ductility but also improved in vitro biocompatibility [50]. The addition of lithium (Li) to Mg-Zn-Ca BMG, specifically the Mg66- xLixZn30Ca4(x=2, 3, 4, and 5 at. %) alloy system, showed significantly improved corrosion resistance in SBF with an increase in Li content [51]. Immersion in SBF led to the formation of Mg(OH)2, LiOH, and Ca(OH)2 surface passivation layers. Mg(OH)2 is porous and dissolves in chloride-containing solutions [52].
However, hydrolysis of Li increased the local pH value which in turn stabilized the Mg(OH)2film, indicating potential use of this alloy system as a biomedical implant material [51]. Micro-alloying Mg-BMGs with Ni, Sr, Pd, and Ag also enhanced corrosion performance [52–56].
5.3.2. Effect of Structure and Crystallinity
There is limited understanding in terms of the electrochemical properties of Mg-based amorphous alloys compared with their crystalline counterparts.
The corrosion behavior of the Mg65Y10Cu25 amorphous alloy was found to be superior compared to the crystalline alloy with the same composition in borate buffer solution (pH=8.4) and NaOH solution (pH=13) due to heterogeneity and galvanic coupling in the crystalline alloy [57,58]. The same behavior was reported for Mg65Y10Cu15Ag10[55]. On the other hand, partially crystallized Mg–Zn–Ca alloy showed the best corrosion performance in terms of pitting resistance, whereas the fully amorphous alloy showed the highest pitting susceptibility [59]. The Mg60Zn35Ca5 and Mg66Zn30Ca4 alloys showed better corrosion performance in their partially crystallized state compared to a fully crystallized state which was attributed to the formation of a more protective passive film [41].
5.3.3. Mg-Based Metallic Glass Composites
Brittle failure of Mg-based BMGs limits their widespread use. Therefore, Mg-based bulk metallic glass matrix composites (BMGMCs) with crystalline phases distributed in the amorphous matrix have been developed.
(Mg65Cu10Ni10Y10Zn5)91Zr9BMGMC showed slightly lower corrosion resistance than monolithic Mg-based BMGs but significantly better than crystalline magnesium alloys, since it contained large amounts of alloying elements facilitating passive film formation [60]. In another investigation, Mg69Zn27Ca4BMGs reinforced with ductile Fe particles (Mg69Zn27Ca4/Fe) spontaneously passivated with a wide passive region at a low passivation current density in NaCl solution, showing better corrosion resistance compared with AZ31 and pure Mg [61].