Biomedical Engineering Reference
In-Depth Information
Alloy
Yield strength (MPa)
UTS (MPa)
Elongation (%)
Mg-4.0Zn
58±1.0
216.8±15
15.8±5.5
Mg-4.0Zn-0.2Ca
58.1±1.0
225±5
17.5±1.0
Mg-4.0Zn-0.5Ca
70±3.0
180±5
12.3±1.5
Mg-4.0Zn-1.0Ca
83±2.0
175±10
8.7±1.0
Mg-4.0Zn-1.5Ca
83±3.0
167±5
7.1±2.5
Mg-4.0Zn-2.0Ca
90±4.0
143±5
2.1±0.5
Table 4. Mechanical properties of Mg-4.0wt.%Zn-xCa alloys at room temperature
The mechanical properties of magnesium were affected by each alloying constituent. Zinc
was an effective alloying ingredient in magnesium. Because zinc had a relatively high solid
solubility in magnesium at high temperature, a good mechanical properties were achieved
by solid solution strengthen. Binary Mg-Zn alloys like Mg-Al alloys, also respond to age
hardening, and contrary to Mg-Al alloys, coherent GP zone and semeicoherent intermediate
precipitate were formed to have an enhanced effect. However, in the Mg-Zn alloys, the
maximum solubility of zinc in the magnesium drops to 1.6 wt. % (i.e. 0.6 at. %) at room
temperature in the equilibrium state [23]. When the zinc content was more than 4.0 wt. %, in
the solidification process, the melt zinc atoms would be rejected by the growing α-Mg and
enriched in the residual liquid, these rich areas were often prone to formation of
microporosity.
In Mg-Zn alloys, the progressive addition of Ca had been found to substantially increase the
temperature difference between liquid and solid phase lines, which was conducive to the
grain refinement in the solidification process. At the same time, the introduction of Ca to
Mg-Zn alloys result in precipitation of desolventizing phase, Ca2Mg6Zn3 and Ca2Mg5Zn13,
which could enhance the strength and toughness of alloy [24][25][26]. The current work
showed that an addition of small amount of Ca to Mg-4.0 Zn alloys had a marked increase
in the tensile strength, but Ca content was excess of 0.5 wt. % make the tensile strength
prone to decrease. The precipitates in the Mg-4.0 Zn-0.2 Ca and Mg-4.0 Zn-0.5 Ca alloys
were Ca2Mg6Zn3 and Ca2Mg5Zn13 phases, which were small particles in the alloys. Thus,
the tensile property of Mg-4.0 Zn-0.2 Ca and Mg-4.0 Zn-0.5 Ca alloys were improved.
However, the maximum solubility of Ca in the magnesium was only 0.2 wt. % at room
temperature and 1.2 wt. % at high temperature in the equilibrium state, when more than 1.0
wt. % Ca was added, the precipitates in the grain boundary began to continuously
precipitated and the morphogenesis of the precipitates were changed to lamellar structure,
made the tensile properties decline. When the Ca concentration was up to 2.0 wt. %, in the
grain boundary tends to form eutectic structure which caused the tensile property
deteriorate.
3.3 In-vitro degradation tests
3.3.1 The effects of Zn content on in-vitro degradation of the as-cast alloys
The representative potentiodynamic polarization curves of Mg-xZn alloys in Hank's
solution were shown in Fig.8, with pure Mg as contrast. As shown in Fig.8, the corrosion
potential of the Mg-x Zn alloys was higher than that of pure Mg. The corrosion potential of
