The Control Of LPSO, W Phases And Biocorrosion Behaviors Of The Mg-Y-Zn-Mn System Alloy

As biodegradable materials Mg-Y-Zn system alloy with long-period stacking ordered(LPSO)structures are outstanding due to the higher mechanical strength and good ductility.However,the poor corrosion resistance of magnesium alloys restricts the application in the field of engineering remarkably.Among many factors,the effect of second phases on corrosion behavior of Mg alloys is particularly prominent.In this work,in view of the Mg-Y-Zn-Mn system alloy with LPSO structures the amounts and distribution of the second phases and precipitates were controlled by adjusting the contents of Y and Zn elements.And the alloys were solution treated in order to control the transformation and precipitation of the phases.The research on the effect of second phases with different composition and distribution on the biocorrosion behaviors of Mg-Y-Zn-Mn alloy was performed by using immersion and electrochemistry tests.Besides,the cell toxicities of the as-cast and solid-solution alloys were investigated by MTT method.The main research contents and conclusions were summarized as follows:(1)The microstructures of the as-cast Mg-Y-Zn-Mn alloy varied with the Zn or Y addition.With increasing Zn content,the 18 R decreased and W phase distributed with continuous net-work structure increased.The microstructure grew uniform and the quasicrystal I phase could precipitate as increasing the content of Zn further.The ?-Mg grains were refined,the volume fraction of W phase shrank evidently but numerous 18 R precipitated by adding Y element.And the block-shape 18 R divorced growth with W phase developed batten shape and grew coupled with W phase with increasing Y/Zn atom ratio.(2)The amount,distribution and the growth pattern of the 18 R and W phase affected the corrosion properties of as-cast Mg-Y-Zn-Mn system alloy obviously.It was studied that a substantial amount of W phase with net-work structure accelerated the corrosion of the alloys while the development of 18 R and decrease of W phase could retard the corrosion of the alloys,which was related to the block-shape 18 R divorced growth with W phase acting as corrosion barrier to prevent the corrosion front.And a stable and dense corrosion product film can be formed on surface of the alloy with numerous 18 R during corroded,which could enhance the corrosion resistance obviously.The dynamic corrosion behavior of the Mg-Y-Zn-Mn alloy was analysed and discussed on the basis of the electrochemical theory,and the kinetic mechanism of the effect of product film on the corrosion behavior of alloy during electrochemical corrosion was illuminated.(3)The microstructure of Mg-Y-Zn-Mn alloy changed obviously after solution treated at 500 oC.The ?-Mg grains grew gradually,some 18 R dissolved and 14 H precipitated within grains as function of solution treated time.Moreover,the W phase was transformed to particle-shaped eutectic W? phase,which was adverse to the corrosion resistance for alloy.Compared with other solid solution samples,the alloy solution treated for 40 h showed the best corrosion resistance,which was related to the minor W? phase and some undissolved 18 R distributed continuously at grainboundary.However the corrosion resistance of solution-treated alloys were not as good as that of the as-cast alloys influenced by the 14 H and W? phase with the intensest cathode effect.The results of the cell toxicity evaluation indicated that both as-cast and solution-treated alloys show desirable cell compatibility.(4)The effect of corrosion product film on the dynamic corrosion behavior of Mg-Y-Zn-Mn alloy was analyzed and discussed by means of electrochemical theory.The Volta potential difference about 18R?14H?W and W? phases compared with ?-Mg phase were determined by AFM method and verified the degree of cathodic effect of these phases to ?-Mg phase.The effect mechanisms of the amount,composition and growth pattern of the second phases and precipitates on the formation of the corrosion product film and electrochemical corrosion characteristics were detected.The corrosion behaviors of Mg-Y-Zn-Mn alloy with different microstructures in the process of biological corrosion in vitro were enucleated.