Microstructure And Deformation Mechanism Of Mg-Zn-Y-Nd Alloy Prepared By Step-Copper Mold Casting

In recent years,biomedical magnesium(Mg)alloys have become the focus of attention due to their good biodegradability,compatibility and comprehensive mechanical properties.Although Mg alloys as biomedical materials have many advantages,they also have a few disadvantages.For instance,due to the excessively fast degradation rates and low plasticity,Mg alloys can not fully meet the performance requirements of biomedical materials.Therefore,improving the plasticity of Mg alloys is still one of the hotspots at present.In this paper,Mg-Zn-Y-Nd alloy bars for vascular stent with different cooling rates were prepared by the stepped copper mold casting,the relationship between the microstructure and mechanical properties at different cooling rates and the microstructure evolution of the alloy samples under different deformations were investigated.In addition,deformation mechanism was elucidated.This study has certain guidance and referenceon the development of Mg alloys with a large plasticity for vascular stent and the research of deformation mechanism.The result of microstructure for the as-cast Mg-Zn-Y-Nd alloy samples showed that the cooling rate of the sample gradually decreases with the increase of the diameter.The average grain sizes of the?2 mm,?4 mm,?6 mm,and?8 mm alloys samples are 10?m,33?m,58?m,and 78?m,respectively.The analysis of the second phases showed that,in addition to?-Mg,the?2 mm sample also includes spherical T-phase,rod-like quasicrystalline phase,strip-like Mg₇Zn₃ phase and framework-like W'-phase.As the cooling rate decreases,the grain size increases and the second phases are coarsened.For the?4 mm sample,the W-phase begins to appear.For the?6 mm and?8 mm samples,the I-phase and W'-phase disappears,and the W phase appeares in large numbers and tends to form a network structure.The room-temperature compression testing for the as-cast Mg-Zn-Y-Nd alloy samples showed that the?2 mm sample exhibits the best mechanical properties with an ultimate compressive strength of about 384 MPa and an elongation of about 30%.The study of the microstructure of the deformed sample shows that the non-basal<c+a>dislocations are activated in the later stage of deformation,making the?2 mm sample has a higher ultimate compressive strength and elongation.As the cooling rate decreases,the ultimate compressive strength and elongation of the alloy decrease significantly.The mechanical properties is relate to the microstructure.It was known that as the cooling rate increases,a large amount of alloying elements are dissolved in the matrix,and the volume fraction of the second phase decreases significantly.Due to the higher cooling rate,the grains have been significantly refined.Thus,the good plasticity of?2 mm sample is attributed to two aspects.First,the higher cooling rate can refine the grains which promotes the dislocation activation and slip.Second,the fewer and dispersively distributed second phases are in favor of improving the plasticity of Mg alloy.The microstructure testing showed that the average grain size of?4 mm Mg-Zn-Y-Nd alloy sample treated by solid solution is about 53.44?m.The analysis of the second phase showed that there are only a few particles of W-phase appear in the matrix.Compression experiments were carried out for the solid-soluted alloy sample,and the microstructure evolution and deformation mechanism under different deformation were investigated.The results showed that the yield strength,ultimate compressive strength and elongation of the solid-soluted sample are 50 MPa,310MPa and 30%,respectively.In the initial stage of deformation(5%),the basal slip and twinning are predominant.While the basal plane dislocations slip,the matrix produces a large number of{1012}twins to relieve the stress concentration.In the later stage of deformation(20%),both of the secondary twins and<c+a>dislocations are predominant.A large number of basal plane dislocations are entangled with each other,and the existence of these dislocations makes the coarsening grains divide into several small-angle subgrains.Conversely,the number of{1012}twinning decreases,and secondary and tertiary twins begin to appear.The appearance of secondary and tertiary twins adjusts the c-axis strain and makes the grains more uniformly deform.In addition,the reduction of the I1 stacking fault energy promotes the nucleation of a large number of<c+a>dislocations.In the later stage of deformation,<c+a>dislocations are activated in a large amount,and even cross-slip phenomenon appears.The activation of<c+a>dislocation provides more slip systems and thus can satisfy the conditions of uniform plastic deformation.