Effects Of Extrusion Process And Heat Treatment On The Microstructures And Corrosion Behaviour Of Mg Alloys And Its Composite

Magnesium?Mg?alloys have been investigated by many researchers as a new absorbable biomaterial owing to their density and elasticity modulo similar to nature bone and their excellent degradability.At present,the major drawback of magnesium alloys serving as implant materials is their low corrosion resistance in electrolytic and poor plastic deformation capacity.In this paper,Mg-3Zn-0.2Ca alloy and Mg-3Zn-0.2Ca-1HA composite were prepared,and the effects of extrusion process and heat treatment on their microstructures and corrosion behavior were researched.The microstructures and corrosion properties of Mg-3Zn-0.8Zr-3HA composite and Mg-3Zn-0.8Zr-5HA composite were studied in order to study the effect of the hydroxyapatite?HA?content on their microstructures and corrosion behavior.The results showed that:1.The main second phases in as-cast Mg-3Zn-0.2Ca alloy were Mg-Zn intermetallic compounds such as MgZn,Mg₇Zn₃,Mg₂Zn₃ and Mg₄Zn₇.In comparison with the as-cast alloy,the average grain size of the as-extruded Mg-3Zn-0.2Ca alloy was significantly decreased 54 times from 119?m to 2.2?m after extrusion ratio 56:1.The yield tensile strength?0.2%YS?,ultimate tensile strength and elongation of the as-extruded alloy were 303 MPa,328MPa and 13.6%,respectively.The electrochemical and the immersion test showed that the corrosion resistance of the as-extruded alloy was obviously higher than that of the as-cast alloy due to the fine grained microstructure.The new designed Mg-3Zn-0.2Ca alloy showed the good combination of mechanical performance and corrosion resistance for biomedical application.2.Compared with the as-cast Mg-3Zn-0.2Ca-1HA composite,the grain size of the as-extruded Mg-3Zn-0.2Ca-1HA composite was refined.Typical fiber texture exhibited along extrusion direction and HA particles distributed along the extrusion direction.The results of electrochemistry and in vitro immersion test showed that the corrosion resistance of as-extruded composite was better than that of as-cast composite with the extension of immersion time.3.Solid solution treatment at 400? for different time was carried out on as-extruded alloy and composite.The results showed that the yield strength of the as-extruded alloy and composite reduced,howere,the plasticity and the corrosion resistance enhanced after solid solution.The best time of solid solution treatment was 3h,when the time exceed 3h,the corrosion resistance decreased due to the grain size grew up.The yield strength of the solid solution treatment as-extruded alloy and composite was increased after aging process?170?,24h?,however,the corrosion resistance was not change significantly.4.The grain size of Mg-3Zn-0.8Zr-5HA composite was smaller than that of Mg-3Zn-0.8Zr-3HA composite,indicating that fine grain can be obtained by increasing addition of HA particles.However,the corrosion resistance of the Mg-3Zn-0.8Zr-3HA composite was better than that of the Mg-3Zn-0.8Zr-5HA composite due to more defects in latter.