The Effect Of Solution And Aging Treatment On The Microstructure And Proporties Of Medical Mg-Zn-Ca-Zr Alloys

At present,the biomedical materials widely used in clinic are stainless steel,cobalt based alloy,titanium,titanium alloy and Ni Ti alloy,etc.There are obvious disadvantages in the use of these materials,which makes it a research direction of medical and scientific circles to find more suitable metal substitute materials for human body.As a new biomedical material,magnesium and magnesium alloy have the characteristics of similar elastic modulus to human bone,non-toxic and harmless to human body,good biocompatibility and biodegradability,which are widely concerned by medical and scientific circles.Due to the high elastic modulus and low yield strength of magnesium and its alloys,their application in biomedical materials is limited,so improving the mechanical properties of magnesium and its alloys has become a research hotspot.At present,the main ways to improve the mechanical properties of magnesium and its alloys are alloying and heat treatment.In this experiment,four groups of Mg-2.5Zn-0.22Zr-xCa alloys with different Ca contents were prepared under the protection of Ar gas and RJ covering agent,which were treated by solution(T4)and aging(T6).The parameters of T4 heat treatment were determined by DTA and T4 heat treatment at different times.On this basis,the effects of T6 heat treatment at different times on the microstructure and mechanical properties of Mg-2.5Zn-0.22Zr-xCa magnesium alloy were studied.The as cast,T4(410?+24 h)and T6(150?+8 h)were tested by OM,SEM and EDS.The microstructure of Mg-2.5Zn-0.22Zr-xCa magnesium alloy was measured by means of Vickers hardness and compression at room temperature.The grain sizes of as cast,T4 and T6 magnesium alloys were calculated,and the effects of different heat treatment conditions and different Ca contents on the morphology and hardness of the second phase of magnesium alloy were studied,so as to explore the possibility of Mg-2.5Zn-0.22Zr-xCa alloy as medical magnesium alloy.The results show that Ca in as cast Mg-2.5Zn-0.22Zr-xCa alloy tends to segregate at the solid-liquid interface.Because of the large atomic radius and slow diffusion rate of Ca,it is easy to deposit at the grain boundary.Therefore,the second phase of magnesium alloy containing Ca is more distributed in the grain boundary.At the same time,the higher concentration of Ca atom at the interface effectively hinders the growth of solidified grains,and forms component supercooling at the interface,which promotes the solidification nucleation.After T4 heat treatment of as cast Mg-2.5Zn-0.22Zr-xCa magnesium alloy at different times,it was found that the granular MgZn2 phase was solid soluble in ?-Mg matrix,and Ca2Mg6Zn3 phase changed from dendrite to rod distributed on the grain boundary;the rod-like Ca2Mg6Zn3 phase changed to short rod and particle distributed on the grain boundary after prolonging the solution time;the change of the second phase morphology reduced the Gibbs free energy of the system.The Ca2Mg6Zn3 phase "migrates and merges”in the transformation process,which makes the Ca2Mg6Zn3 phase increase in thickness,increase in spacing,decrease in number,decrease in surface energy per unit area and decrease in Gibbs free energy.After T6 heat treatment of Mg-2.5Zn-0.22Zr-xCa magnesium alloy,it was found that the Zn atom which had been dissolved in ?-Mg matrix was re-separated,the embryo of MgZn2 phase began to grow at the early stage of aging,the Zn atom and Mg atom began to nucleate through diffusion,and the granular phase began to precipitate at the grain boundary;after prolonging the aging time,the MgZn2 phase began to grow at the grain boundary,and the granular MgZn2 phase began to grow into short rod.At the same time,the second phase began to grow and coarsen,the second phase began to coarsen,the second phase began to coarsen,and the second phase of the short rod began to coarsen,at the same time,they attached to each other in a lump shape,which had a negative influence on the mechanical properties of magnesium alloy.Therefore,the best heat treatment parameters of Mg-2.5Zn-0.22Zr-xCa alloy are T4(410?+24 h)and T6(150?+8 h)heat treatment.Under these conditions,Mg-2.5Zn-0.22Zr-xCa alloy shows good mechanical properties,among which the compressive modulus of elasticity,yield strength and compressive strength meet the requirements of biomaterials,18.79 GPa,137.26 MPa and 241.97 GPa.