| Magnesium and its alloys have attracted extensive attention in the field of interventional medicine in recent years due to their good biosafety,biodegradability and non-toxicity.Our previous research results indicated that the Mg-5Gd-4Y-0.5Zn-0.5Zr(wt%)alloy had a good potential in biological implant materials.In order to further reveal the relationship between the processing,microstructure and properties of the alloy,the microstructure and mechanical properties of the as-extruded alloy samples have been investigated after heat treated under different solution and aging conditions,Moreover,in vitro bionic corrosion evalutions were carried out on the samples with good mechanical properties.In the as-cast Mg-5Gd-4Y-0.5Zn-0.5Zr alloy,the net-like second phase at the grain boundary is mainly Mg24Y5.Dynamic recrystallization occurs during extrusion at450 °C with different extrusion ratios(25,79,178).With the increase in extrusion ratio,the grain size decreases.Basal texture appears after extrusion,but its intensity decreases with increasing extrusion ratio.The tensile properties increase with the increasing extrusion ratio also,attributing to the decrease of grain size and the weakening of preferred orientation.When the extrusion ratio is 178,the tensile strength and yield strength of the as-extruded alloy are 350 MPa and 297 MPa,respectively,and its strain exceeds 38%.According to the phase diagram calculation(with Pandat)and DSC results,three solid solution temperatures of the alloy were selected to be 450,510 and 550 °C,and the aging temperature 200 °C.It was found that the strength and plasticity of the alloy are further improved after quenched from 450°C,as compared with the as-extruded samples.The main second phases in the alloy were identified by XRD and TEM to be14H-LPSO,Mg3(Gd,Y),Zn2Zr3 and α-Zr.14H-LPSO and Zn2Zr3 have coherent relationships with the matrix.However,strength and plasticity decreased simultaneously after solid solution at higher temperatures,due the obviously coarsening of grain size.After the aging treated at 200 °C,Mg70Y15Zn15 and Mg YZn phases were formed along the grain boundary.However,due to the decrease in the volume fraction of the lamellar LPSO phase,the strength and plasticity decreased,also.As-extruded and as-solutioned(450,510 and 550 °C)samples were selected for electrochemical and immersion corrosion tests in SBF solution.The electrochemical corrosion results indicated that the corrosion current density icorr was 10-4 A·cm-2 in each state.While after solid solution,the icorr of the alloy was 2~7 times lower than that of the as-extruded alloy.The average immersion corrosion rate of the as-extruded alloy is 9.60 mm/y,which decreased to 6.20 mm/y after solution treatment at 550 °C.At the early stage,the corrosion products on the surface of the specimens are thin and the crack density is large.The protecting effect of the corrosion products is weak leading to a fast corrosion.With the increase of corrosion products on the sample surface,the corrosion process is slowed down gradually.The corrosion of as-extruded alloy is mainly affected by grain size and lattice distortion,while that of solid-solutioned alloy is controlled by the second phase.XRD and XPS results showed that the corrosion products were mainly hydroxyapatite(HA),(Mg,Ca)CO3 and Mg(OH)2.Combined the mechanical properties with bionic corrosion properties,it can be concluded that the best comprehensive properties were achieved on the as-extrued alloy with an extrusion ratio of 79 and bionic corrosion properties,it can achieve the balance comprehensive properties after solution treated at 450 °C. |
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