Effect Of Al And Mo Microalloying On Microstructure And Properties Of Mg-Zn-Y-Mn Alloys

Magnesium and its alloys have a broad application in biomedical field because of the advantages,such as unique biodegradability,biocompatibility,elasticity modulus close to human bone and so on.However,the rapid corrosion rate often leads to local alkalization and inflammation in the human body,and then damages the mechanical integrity of the implanted materials.These are the main factors for the failing to ensure its normal use during service and greatly limit their application in clinical medicine.At present,Mg-Zn-Y series magnesium alloys with long-period stacking ordered（LPSO）structure have attracted extensive attention due to the excellent mechanical properties and corrosion resistance.A great deal of research has shown that microalloying is one of the most commonly methods to improve the properties of Mg-Zn-Y alloys and obtain high performance alloys.By adjusting the content of alloying elements,the grain size of the alloy can be changed,the second phase or intermetallic compounds can be affected,and the properties can be improved.Recent studies have found that Al and Y can form a highly efficient heteronucleating agent Al₂Y.Al addition can change the composition and structure of the surface film of Mg-Zn-Y-Al alloy prepared by rapid solidification process,forming Al₃Y₄Zn₃ compound and improving the corrosion resistance of the alloy.But it is not sure if a new compound phase which can effectively refine the grain forms with cheap Al addition in the conventional casting Mg-Zn-Y alloy with LPSO phase.The influence of the content of Al on the phase composition and properties of the alloy is unclear.In addition,Mo as necessary microelements participates in the activities of various body metabolism.However,the action mechanism of Mo addition in Mg-Zn-Y-Mn alloy still needs to be further studied.Based on the above research,Mg-Zn-Y-Mn-Al（Mo）alloys containing different content of Al and Mo are prepared by conventional casting in this paper.The mechanical properties and the corrosion resistance of the alloys with different states are explored.The research findings are shown in the following:（1）The Al addition can not only form new（Al,Zn）₂Y phases,which can act as heterogeneous nucleating agent to effectively fine the grains,but also change the number and morphology of the second phases（18R-LPSO and W phases）in as-cast Mg-Zn-Y-Mn alloy.With the increase of Al content,the number of（Al,Zn）₂Y phase increases and the phase morphology changes from point-like,rod-like to Chinese character.The volume fraction of18R-LPSO phase decreased slightly.When Al content is 0.3 wt.%,the grain size is the smallest,and the corrosion resistance,tensile strength and elongation are the best.This is due to the co-action of grain refinement,short-rod-like（Al,Zn）₂Y phase and 18R-LPSO phase with optimal matched volume fraction.（2）The Mo addition can refine the grain and promote the formation of 18R-LPSO phase in as-cast Mg-Zn-Y-Mn alloy.The morphology of W phase changed from fishbone to coral.When the Mo content is 0.3 wt.%,the grain size of the alloy is the smallest,and the volume fraction of 18R-LPSO phase is the largest.The alloy has the optimal corrosion resistance and mechanical properties.Compared with Mg-Zn-Y-Mn-0.3Al alloy,the alloy has better mechanical properties,but the corrosion resistance is slightly poor.（3）The composite addition of Al and Mo can play a synergistic strengthening role.It can further refine the grain and obtain a larger volume fraction of 18R-LPSO phase in Mg-Zn-Y-Mn-0.3Al-x Mo alloy.The morphology and microstructure of precipitated phase are optimized.When the Mo content is 0.3 wt.%,the grain size is the smallest and the volume fraction of18R-LPSO phase is the largest.The corrosion resistance,tensile strength and elongation of the alloy are the most excellent.（4）There is the potential difference between the second phase,（Al,Zn）₂Y compound phase andα-Mg matrix in as-cast alloys based on AFM test.The cathode corrosion effect of W phase is the strongest,followed by（Al,Zn）₂Y phase and 18R-LPSO phase.Less W phase,more uniform and finer（Al,Zn）₂Y phase and more 18R-LPSO phase can be obtained by adjusting the content of Al and Mo,which can further improve the corrosion resistance.（5）After solid solution treatment,the block 18R-LPSO phase is dissolved in varying degrees.The lamellar 14H-LPSO phase is precipitate inα-Mg.W phase is spheroidized and the rod-like（Al,Zn）₂Y phase becomes shorter.The properties of solution-treated alloys are improved.Especially for as-extruded alloys,dynamical recrystallized（DRXed）behavior is occurred and the grains are fined.18R-LPSO phase and 14H-LPSO phase are kinky.The coarse spherical W phase and rod-like（Al,Zn）₂Y phase are broken into fine.For as-extruded alloy,ultimate tensile strength is 385 MPa,the elongation is 20.5%,and the weight loss rate is only 0.39 mm/y.