Research On Microstructure And Mechanical Properties Of Biomedical Mg-Zn-Ca Alloys

Biomedical magnesium alloys for its unique properties get more and moreattention by biological materials workers around the world, and do a lot ofexperiments and clinical research. But so far no breakthrough progress about them.As for the reasons, on the one hand, biomedical magnesium alloys is fabricated in alower standard, at present most biomedical magnesium alloys are all in preparationof industrial system standard, such as raw materials use industrial pure alloy, smeltingprocess adopt covering agent methods or gas protection in the atmosphere casting.These alloys can’t meet the biomedical requirements; on the other hand, the ways ofImproving biomedical magnesium alloy performance are used alone, currently themost commonly used methods are alloying and surface treatment, but heat treatmentand deformation processing method are very good to improve alloy performance.To further searching and developing biodegradable medical magnesium alloysthat have clinical practical value, Zn, Ca elements of having better biocompatibilityare selected as alloying elements. Alloys are fabricated under vacuum inductionmelting. heat treatment and deformation processing method are used improving alloyperformance. And the microstructure，mechanical properties test and the fractographycontrast analysis of as-cast, heat treatment state and extrusion state alloys areresearched. The results show that As-cast Mg-2.5Zn-XCa alloys, with the increase ofthe content of Ca elements, the organization grains have be refined after coarsening.After the solid solution treatment and even homogenizing annealing processing,alloys’ phase composition do not changing. But alloy organization and the secondphase form change significantly, mechanical performance is improved significantly.Study found that the best solid solution process for Mg-2.5Zn-1Ca alloy is410℃+16h, the best uniform annealing process is400℃+16h.And solid solutionprocessing state alloy have higher strength, and even homogenizing annealing statealloy plastic is better. Extrusion also can improve alloy organization and performance,extrusion rate for the Mg-2.5Zn-1Ca alloys’ forming has great influence: If theextrusion rate is too fast, many periodic cracks in alloy surface will be formed, whenreducing extrusion rate to4mm/s, extrusion forming bar has benign formability.When extrusion ratio is5.6, the original as-cast organization of alloy almost bedisappearance, grain become in the "extrusion lines", and the second phase is more diffusion and fine; After alloy is extruded in a7.84of extrusion ratio, grain sizebecome more uniform, eutectic organization and second phase in the grain boundarydisappear due to dynamic recrystallization of the alloy after extrusion. Whenextrusion is done in a49of extrusion ration, alloys happen dynamic recrystallization,but individual grains grew up anomalies, whole grains have grown-up trend. For thefracture analysis, we can conclude that as-cast Mg-2.5Zn-XCa alloy rupture iscomposed of cleavage fracture, quasicleavage fracture and intergranular fracture.After solid solution treatment, the fracture form of Mg-2.5Zn-1Ca alloy do not change,but annealing treatment later, with the extension of time,its shape of fracture have thetrend form transgranular fracture to intergranular fracture. And the fracturemechanism of the alloy changes ductile fracture from brittle fracture, after extrusionprocessing. At the same time, compound Ca₂Mg₆Zn₃is found in toughening nestinternal. Therefore, it can be sure Ca₂Mg₆Zn₃phase is the strengthening phase forextrusion processing of Mg-2.5Zn-1Ca alloy.