Effect Of Mg-Nd Phase On Degradation Behavior And Mechanical Properties Of Biological Magnesium Alloy

With the increasing demand for biomaterials,magnesium alloys have attracted more and more attention because of their good biocompatibility and biodegradability.Although magnesium alloys have a series of advantages as biomaterials,the poor corrosion resistance and insufficient mechanical properties limit the further development in biomaterials.It has become one of the important research directions to adding rare earth elements to improve the corrosion resistance and mechanical properties of biological magnesium alloy,and how to control the second phase to balance the corrosion resistance and mechanical properties of magnesium alloys is the key problem.In this paper,four different states of Mg-3wt.%Nd alloys were prepared by changing the process conditions,namely,as-cast air cooling,as-cast faster cooling,solution treatment and hot extrusion treatment.OM,SEM,EBSD,XRD and TEM were used to analyze the microstructure and distribution of Mg-Nd phase.The effects of Mg-Nd phase on the degradation behavior and mechanical properties of the alloy were analyzed by immersion experiment,electrochemical workstation and universal testing machine.The microscopic study showed that the Mg-Nd phase in the as-cast air cooling sample was distributed along the grain boundary with network shape,and a small amount was distributed in the grain interior as spherical.In the as-cast faster cooling sample,Mg-Nd phase was randomly distributed on the magnesium matrix in the shape of spherical and short rod.The Mg-Nd phase in the as-cast alloy was both Mg₁₂Nd.After solution treatment,phase transformation occurred in the alloy.It was found that the network Mg₁₂Nd phase along the grain boundary and the spherical Mg₁₂Nd phase in the grain were dissolved and transformed into plate-shaped Mg₃Nd phase with uniform and dense distribution.Some Mg₃Nd phases were combined into bird-like,and a very small amount of cluster Mg₄₁Nd₅ phase precipitated on the grain boundary.After extrusion treatment,the grain size decreased,the grain boundary increased,but the type and distribution of second phase were basically the same as solid solution sample.The immersion experiment showed that Mg₁₂Nd was preferentially dissolved as micro-anode in the as-cast sample,while magnesium matrix as micro-cathode,formed micro-galvanic couples.Howerver in the solid solution and extruded samples,Mg₃Nd was the micro-cathode,while magnesium matrix was dissolved as micro-anode.The corrosion potentials of the four samples and pure magnesium were:-1.7254V,-1.7463V,-1.6278V,-1.6372V and-1.6512V respectively.The analysis of degradation products showed that Mg₁₂Nd phase was unevenly distributed and large in volume,forming an uneven and loose corrosion product layer,which can not effectively hinder the spread of corrosion;The small plate-shaped Mg₃Nd phase,uniform and dense distribution,the degradation product layer was uniform and dense,effectively hinder the spread of corrosion.The hydrogen evolution experiment found that the degradation rate of as-cast samples firstly increased and then keep stable with the increase of time,while the solid solution and extrusion samples firstly increased with the increase of time,then decreased slightly,and finally keep basically stable.The degradation rate of as-cast faster cooling sample was the highest,followed by as-cast air cooling sample,which were much higher than solid solution and extrusion samples.And the degradation rate of extrusion sample was slightly higher than that of solid solution.In the weight loss experiment,the weight loss degradation rate was high in the initial stage,decreased rapidly with the increase of time,and finally remains basically stable.The relationship between the degradation rate of the four alloy samples was consistent with the hydrogen evolution degradation rate.Compared with spherical Mg₁₂Nd phase,the tensile strength of Mg-Nd alloy was significantly improved by continuous network Mg₁₂Nd distributed along the grain boundary,but its elongation was significantly lower than spherical phase;The effect of plate-shaped Mg₃Nd on the tensile strength and elongation of the alloy was significantly higher than that of Mg₁₂Nd phase;The tensile strength and elongation of the sample after extrusion were both the highest.After tensile deformation,the texture type of Mg-Nd alloy sample changed obviously,the deformation texture along the<0001>direction to<1?21?0>direction,dislocation and twin increased,and dislocation mostly gathered at small angle grain boundaries and deformation structures.In the as-cast sample,basal slip and non-basal<c+a>dislocation slip occured in the matrix.The dislocation wsa distributed around the network Mg₁₂Nd phase,and it was difficult to pass through the second phase.There were few twins,and the stress was concentrated around the Mg₁₂Nd phase.In the solid solution sample,the<a>and<c+a>dislocations cut through or bypassed the Mg₃Nd phase,a large number of{101?2}twins appeared,and the dislocation migration and twin strain offset the stress concentration in the tensile deformation.