| Magnesium and its alloys as a biodegradation materials must overcome two problems:its poor corrosion resistances and mechanical properties which can not meet the medical requirements. High pressure torsion(HPT) process provides an opportunity for achieving exceptional grain refinement, which can enhance corrosion properties and mechanical properties. So HPT process is one of the most effective approaches to solve the application problems of magnesium alloy for biomaterials.In this study, Mg-Zn-Ca alloy was processed by high pressure torsion. The microstructures of HPT treated alloy and as-cast alloy were carried out using OM, SEM, XRD, TEM and SAED measurements. The corrosion behavior of Mg-Zn-Ca alloy after HPT process in simulated body fluid(SBF) was studied using the immersion tests and the electrochemical tests.The results show that, the typical microstructure of as-cast alloy has a-Mg phase and MgZn phase, the grain size was measured as～130μm and the majority of the second phase particles were located at grain boundaries. The microstructure of HPT treated alloy has a-Mg phase and Mg4Zn7 phase, for strain induced phase transformation. A lot of dispersion second phase particles which sizes were in the range of 10-75nm were found in the grain interiors not in grain boundaries.Measurements gave grain sizes of～1μm in the outer region and～1.2μm near the center region where got ultrafine (sub-micrometer) grain structure. The grain boundaries(GBs) in outer region were non-equilibrium GBs and the GBs near the center region were clean.The true logarithmic strain increased gradually from central region to outer region, and the maximum value got 6.5.Nevertheless, local microhardness distribution along the diameter of the HPT treated disk was homogeneous and the mean value was 115Hv0.1 which increased in 3 times than the microhardness (-39Hv0.1)of as cast alloy. It suggests that dynamic recrystallization induced by high pressure may have taken place during HPT processing so that the HPT treated alloy gets UFG structure.The immersion tests results the pH values of HPT treated alloy and as cast alloy in SBF both increased with the increase of immersion time, but the pH values of HPT treated alloy increased more slowly. The surface of as cast alloy was covered with many corrosion pits, where and carbonate layers were deposited as OCP phase and (Ca,Mg)CO3 phase. In contrast, the Mg(OH)2 layer was formed on the surface of HPT treated alloy. With the increase in time, the Mg(OH)2 layer cracked and (Ca,Mg)CO3 was deposited with a little of calcium phosphate on the surface, but corrosion pits did not grow up.The typical potentiodynamic polarization curves show that, the corrosion current density of HPT treated alloy was less than that of as cast alloy by around two orders of magnitude, and the polarization of HPT treated alloy was more than that of as cast alloy by 100 times. It reveals that the corrosion rate of HPT treated alloy in SBF turns slower and the corrosion resistance increases more strongly. The Electrochemical impedance spectroscopy(EIS) plot of as cast alloy consisted of two capacitive loops and one inductance loop, and that of HPT treated alloy consisted of only one capacitive loop. The plot can be explained by the equivalent circuit, the charge transfer resistance of HPT treated alloy more than that of as cast alloy by two more orders of magnitude.It conducts that the corrosion mechanisms of HPT treated alloy and as cast alloy are that:The defects of as cast alloy occur primarily at grain boundaries where the majority of the second phase particles were located.The galvanic corrosion accelerated corrosion pits appearing on the surface. There are clean GBs in HPT treated alloy and the second phase distributed in the grain interiors.Then the corrosion products formed on the surface of the HPT treated samples may act as a barrier to prevent magnesium alloy matrix from further absorbing Ca and P from the SBF, which can stop corrosion deepening into the matrix. |
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