Study On Microstructure Evolution And Corrosion Properties In Simulated Body Fluid Of Biomedical Mg-Zn-Ca Alloy Processed By High Pressure Torsion

While magnesium alloy have attracted most of the researchers’ attention due toits close elastic modulus compared with human bone besides its biocompatible andbiodegradable, the low mechanical properties and rapid corrosion rate have limitedits further clinical application. In this paper, we focused on the microstructure,mechanical and corrosion properties of pure Mg and Mg-Zn-Ca alloy processed byHigh Pressure Torsion (HPT) by using OM, SEM&EDS, TEM&SAED, XRD,Microhardness measurement, Electrochemical analysis, Hydrogen evolution andImmersion test. While analyzing the microstructure evolution and hardeningmechanism of Mg and Mg-Zn-Ca alloy during HPT, we also studied theirdegradation mechanism when immersed into simulated body fluid after HPT.The results indicated that the phase of pure Mg and Mg-Zn-Ca alloy didn’tchange after HPT, the phases of Mg-Zn-Ca alloy before and after HPT were bothconsisted of α-Mg and MgZn phase, but the MgZn particles was redistributedhomogeneously and dispersedly in the substrate. While the grain orientation of bothMg and Mg-Zn-Ca alloy turned to (0002) after HPT, which was good for thedislocation sliding during HPT processing. And the grain size of Mg-Zn-Ca alloywas significantly refined after HPT compared with the grain size of~11μm beforeHPT, but when processed at a lower rotation number the microstructure in the centerwas different from the edge region due to the gradient distribution of strainaccumulated along the diameter of the sample.When the rotation number, N=1turn, there existed many dislocation cells in thecenter compared with the homogeneous sub-grains with an average size of~150nmat the edge. Then with the increasing of rotation number, the difference between thecenter and the edge of the sample was reduced gradually, so the microstructurehomogeneity was improved at the same time. When the rotation number increased to5turns, we could get a homogeneous ultra-fined microstructure with an averagegrain size of~130nm. Because the relative coarser grain size of~5mm, the microstructure of pure Mg was consisted of dislocation cells and sub-grains with aleast grain size of~1.5μm after HPT even rotated to5turns. Based on themicrostructure evolution analysis under different rotation number, we concluded thatthe deformation grain refinement of pure Mg and Mg-Zn-Ca alloy was mostly due tothe interaction of dislocation segmentation and twinning fragmentation produced inthe grain interior under the strain accumulated.Vickers Microhardness measurements showed that when the rotation number,N=1turn, the microhardness values of pure Mg and Mg-Zn-Ca alloy after HPTalong the diameter increased with the increasing of the distance from the center ofthe sample, and decreased slightly to a saturation value after reached to a maximumvalue. And the microhardness value at the edge of pure Mg sample improved from27HV to37HV, while the microhardness value of Mg-Zn-Ca alloy increased from49.8HV to about112HV (increased by124%). Then with the increasing of rotationnumber, the region of lower microhardness values in the center decreased while themicrohardness values became consistent with the microhardness values at the edge,when the rotation number increased to5turns, we could get a homogeneousmicrohardness distribution of pure Mg and Mg-Zn-Ca alloy after HPT.Based on the analysis of strain accumulated during HPT, we concluded that themicrohardness values increased first with the increasing of accumulated strain, andthen decreased slightly to a saturation value after reached to a maximum value underthe interaction of deformation hardening and dynamic recovery softening. And thecorresponding accumulated strain values, εeq, of the maximum microhardness valueand the saturation microhardness value of pure Mg were18and30separately, whilethe corresponding values of Mg-Zn-Ca alloy were27and42separately, thedifference between pure Mg and Mg-Zn-Ca alloy might be due to the second phaseparticles generated by the addition of alloying elements of Zn and Ca, whichinhibited the dislocation sliding during HPT deformation.Both the electrochemical tests and hydrogen evolution tests indicated that thecorrosion potential, Ecorr, of pure Mg in simulated body fluid (SBF) increasedslightly after HPT, while the increasing of corrosion current density, Icorr, andhydrogen evolution rate, and the decreasing of electrochemical impedance value all showed that the corrosion rate of pure Mg in SBF was accelerated after HPT. TheIcorrof pure Mg increased from3.798E-05A cm-2to9.692E-05A cm-2afterprocessed by5turns, and the cracks produced in the loosening accumulatedcorrosion products further verified the worsening of corrosion properties in SBF.As for Mg-Zn-Ca alloy, the Ecorrbecame more positive while the Icorrandhydrogen evolution rate both decreased with the increasing of rotation number afterHPT. When the rotation number increased to5turns, the Icorrof Mg-Zn-Ca alloydecreased from1.719E-04A cm-2to2.187E-05A cm-2, and the increasing of EISvalues also indicated that the corrosion resistance improved with the increasing ofrotation number. When the Mg-Zn-Ca alloy processed by HPT immersed into SBF,the corrosion products accumulated and formed a homogeneous and compactcorrosion products layer with the extending of immersion time which might protectthe Mg substrate from further corrosion, and with the increasing of rotation number,the corrosion products layer became more compact and homogeneous. When therotation number increased to5turns, both of the corrosion interface in the center andedge region of the sample had a smooth profile curve, while the corrosion pitsbefore HPT disappeared, and the Mg-Zn-Ca alloy tended to be degraded in SBFwith a homogeneously corrosion way, which significantly improved the corrosionproperties of Mg-Zn-Ca alloy after HPT.