Study On Microstructure And Properties Of Biomedical Mg-Zn-Nd-Ca Alloy

Magnesium alloys have broad application prospects in the biomedical field because of their degradable properties,non-toxic degradation products,and similar elastic modulus to human bones.However,the rapid degradation rate of magnesium alloy lead to mechanical properties worse,and poor mechanical properties of magnesium alloy are still present bottleneck problems.And the current research on the mechanism of magnesium alloy degradation is immature.Alloying,heat treatment and deformation treatment are the main methods for improving the corrosion resistance and mechanical properties of magnesium alloys.In addition,rare earth elements have significant effects in melt purification,grain refinement,and solid solution strengthening.Mg-Zn-xNd-xCa alloys were selected and SEM,TEM,XRD,FITR etc.were used and alloying,heat treatment,deformation treatment was adopted to analy the microstructure,corrosion behavior and mechanical properties.Based on the corrosion product test results,the corrosion mechanism of Mg-6Zn-xNd-xCa alloy was analyzed.Research indicates:The grain size of Mg-6Zn-xNd alloy without Ca addition was gradually decreased with the increase of Nd content,the grain size was uniform,the second phase volume fraction gradually increased,and the corrosion rate decreased first and then increased.After T4treatment,the corrosion rate of 0.5wt.%Nd alloy in Simulated Body Fluid?SBF?is 2.75mm/a,and the V/G value is 0.089-0.192.When Ca was added into Mg-6Zn-xNd alloy,in terms of microstructure,the grain of Mg-6Zn-0.5Nd-xCa alloy was refined due to the fine grain of Ca element,and after T4 treatment,the alloy crystal was refined.The grain size becomes larger.In terms of corrosion resistance,the corrosion rate of Mg-6Zn-0.5Nd-xCa alloy in SBF solution first decreased and then increased.When the Ca addition amount was 0.8wt.%,the corrosion rate was 3.93mm/a.After T4 treatment,the corrosion rate was reduced to 1.26mm/a.In terms of mechanical properties,the Mg-6Zn-0.5Nd-xCa alloy exhibited good mechanical properties.The compressive fracture strength of the alloy with 1.3wt.%Ca is 320.6MPa,and the yield strength is 187.3MPa.After T4 treatment,solid solution is obtained.With the strengthening effect,the strength of the alloy is increased and the plasticity is decreased.The compressive strength and yield strength of the 0.3wt.%Ca alloy increase to 319.3MPa and207.5MPa,respectively.After exploring the corrosion mechanism of the alloy,it was found that Cl^-played a catalytic role throughout the corrosion process,but due to the presence of NaOH,MgCl,and Ca?OH?₂,the resulting surface was dense and layered Mg?OH?₂.The corrosion product of Mg?OH?₂ is attached to the surface of the alloy,which can effectively inhibit the migration of Cl^-,which hinders the corrosion reaction and improves the corrosion resistance of Mg-6Zn-0.5Nd-xCa alloy.When the extrusion temperature is 280?and 220?,the surface of the alloy is smooth and crack-free,and the micro-morphology shows that the second phase is broken and the shape is an irregular block shape or a broken line shape.After recrystallization annealing,solid solution and aging treatment?O+T6?of Mg-6Zn-0.5Nd-0.5/0.8Ca alloy,the grain size of the alloy is uniform,and the grain is a fine equiaxed grain,which can prevent the progress of the reaction.As a result,the corrosion performance was greatly improved,and the corrosion rates of the Mg-6Zn-0.5Nd-0.5/0.8Ca alloy were 0.82 mm/a and 2.98 mm/a,respectively;the corrosion resistance was significantly improved compared to the as-cast alloy.The mechanical properties of Mg-6Zn-0.5Nd-0.5/0.8Ca alloys after O+T6 heat treatment were significantly improved.The stress-strain curves of both alloys have a distinct yielding platform.The elastic limit of Mg-6Zn-0.5Nd-0.5Ca alloy is 168.54MPa,the yield strength is173.33MPa,the tensile strength is 222.47MPa and the elongation is 20.16%The elastic limit of Mg-6Zn-0.5Nd-0.8Ca alloy is 229.70MPa,the yield strength is 235.78MPa,the tensile strength is 287.15MPa,which is higher than that of Mg-6Zn-0.5Nd-0.5Ca alloy,however,the elongation is 18.37%.The fractures of the alloy are mostly equiaxed dimples,the dimples have small diameter,and the fracture mechanism is ductile fracture.