| Bio medical magnesium alloy is attent ioned by mater ial industry and medica l communit ies wide ly due to good mechanical compatib ility,biocompat ibilit y and biodegradabilit y,but the difficult control for degradatio n process and the proble m of precis ion forming limit its clinica l application in the medica l fie ld.In recent years,rare earth eleme nts have become a hotspot in the field of alloying of biomedica l magnesium alloy,and WE43 medica l magnesium alloy conta ining rare earth ele ments has good mechanical properties and corrosio n resistance.In order to develop a bio-med ical magnesium a lloy that is more adaptable to human body environme nt,on the basis of WE43 alloy,the replaceme nt of rare earth ele ments,and the non-toxic rare earth Dy eleme nt and the biocompatib le Sr ele ment are selected as the alloying ele ment.Mg-xDy-ySr-2.4Nd-0.5Zr(x=2,6,10;y=0.5,1.5,2)alloy was prepared by means of metal mold casting.The effects of different contents of Dy and Sr on the microstructure,mechanical properties and corrosion resistance of the bio-magnesium alloy were analyzed by optical microscopy,scanni ng e lectron microscopy,room temperature tensile test,electrochemical and weight loss test.A iming at the proble m of plastic ity of bio-ma gnes ium alloy,the Mg-10Dy-0.5Sr-2.4N d-0.5Zr alloy w ith good mechanical properties and corrosion resistance was fabric ated by die casting,and the microstructures and properties of as-cast and die-cast alloy were compared and analyzed.The grain morphology of the as-cast Mg-xDy-ySr-2.4N d-0.5Zr alloy is dendr it ic,and the average grain s ize is significant ly refined wit h the increase of D y content.Sr eleme nt has no obvious effect on the grain size of magnes ium alloy,however,excess Sr increase the tendency o f segregatio n in magnesium allo y.The second phases of the as-cast alloy consist of Mg2 Dy,Mg3 Dy,Mg17 Sr2,Mg12 Nd phases,and these phases distribute along the grain boundary.The increase of Dy eleme nt can improve the mechanical properties of bio medical magnesium alloy by solid solut ion strengt hening and fine gra in strengthe ning,while the mechanical properties of magne sium alloy decrease wit h the increase of Sr content due to the stress concentration caused by the uneve n distr ibut ion of gra in because of excess Sr element.Therefore the as cast Mg-10Dy-0.5Sr-2.4N d-0.5Zr alloy has the best mechanica l properties,and its t ensile strengt h,yield strengt h and elongat ion are 203 MPa,128MPa,7.4%,respective ly.The corrosion resistance of the bio medica l Mg-xDy-ySr-2.4N d-0.5Zr alloy is enhanced by the refine ment o f the grains as the Dy e leme nt increases.Mg17 Sr2 is easy to form ga lvanic corrosion w it h ?-Mg phase,so the corrosio n rate of b io-magnesium alloy increases with the increase of Sr content.The as-cast bio-med ical Mg-10Dy-0.5Sr-2.4N d-0.5Zr alloy has the best corrosio n resistance in PBS simulated body fluids wit h a degradation rate of 0.58mm/a.Die casting does not change the phase composit ion of Mg-10Dy-0.5Sr-2.4N d-0.5Zr alloy,but the second phase precip itated along the grain boundary is distr ibuted in fined lattice.The gra in s ize of d ie-cast ma gnes ium a lloy is obvious ly refined to 18?m,and the grain morphology changes from dendrites to equiaxed crystals.The tensile strengt h of d ie-cast Mg-10Dy-0.5Sr-2.4Nd-0.5Zr allo y is reduced due to the uneven size of t he grains and the pores in t he alloy.However,the elongat ion increases and the plastic ity is enhanced obvious ly,and the fracture type belongs to quasi-cleavage fracture.So the die casting can improve the plastic defor matio n abilit y of the biologica l magnesium alloy.The dissolut io n rate of the corrosion products in the die-cast ma gnes ium alloy is larger than the repair rate,and no stable Mg(O H)2 protective film is for med in the etching layer.The polarizat ion curve of the die-cast magnesium alloy also shows that there is no passivat ion behavior.As a result,the corrosion resistance of Mg-10Dy-0.5Sr-2.4N d-0.5Zr alloy is reduced by die casting process.The corrosion type of die cast alloy is changed from local corrosion of cast alloy to uniform corrosion. |
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