Microstructure Regulation Of As-cast Mg-Zn-Y Alloy And Its Corrison Behavior

As one of light alloy structural materials,magnesium alloy has broad application and development potential in aerospace,national defense,electronic products and other fields.However,magnesium alloys generally have disadvantages such as high processing cost,poor absolute mechanical properties,and poor corrosion resistance,which limit their extensive use in structural parts.At present,a large number of basic researches have been carried out on magnesium alloys containing long-period stacking ordered structure(LPSO phase)at home and abroad,In particular,the structure,formation conditions and mechanical properties of long-period stacking ordered structure have been deeply and comprehensively understood.However,there are relatively few studies on the corrosion resistance and mechanism of LPSO-containing magnesium alloys.In this paper,four types of Mg-Zn-Y alloys are designed according to the minimum structural unit that forms the LPSO phase,namely Zn/Y=3/4,which are MgZn_0.5Y_0.66,MgZn1Y1.33,MgZn2Y2.66 and MgZn3Y3.99,comprehensively using advanced analysis and characterization techniques such as low-voltage high-resolution scanning electron microscopy,ultra-thin sectioning,and electrochemical mapping,the microstructures and corrosion behavior of four types of Mg-Zn-Y alloys were analyzed,and the LPSO phase volume fraction,Effects of distribution and morphology on corrosion behavior of Mg-Zn-Y alloys.Concluded as follows:(1)18R-LPSO phase is formed in as-cast magnesium alloys MgZn_0.5Y_0.66,MgZn₁Y_1.33,MgZn₂Y_2.66and MgZn₃Y_3.99,and its volume fraction increases with the increase of alloying element content,which is 16.55%,34.45%,54.24%,70.36%,and its shape gradually changed from discrete block to continuous network.After heat treatment,in the Mg-Zn-Y alloy whose LPSO phase volume fraction is greater than or equal to 50%,the morphology will change after 520?,from a block shape to a rod shape,and the grains will grow abnormally above 560?.And with the increase of solution treatment time,the layered LPSO phase will also be precipitated inside the magnesium matrix.(2)The corrosion resistance of as-cast magnesium alloys is the best when the volume fraction of LPSO phase is 50%.When the volume fraction is far less than 50%,about 50%and far greater than 50%,there are different corrosion mechanisms.MgZn₂Y_2.66(volume fraction of 54.24%)alloy has the best corrosion resistance.The research shows that the electrode potential of 18R-LPSO is more positive than that of Mgmatrix,which acts as the cathode of corrosion micro-battery,but in a specific corrosion stage,the magnesium-rich layer inside the LPSO phase will also undergo selective corrosion;the spatial distribution of 18R-LPSO phase and Volume fraction is an important factor affecting the corrosion resistance of alloys.The influence of LPSO phase distribution and morphology on the corrosion resistance of magnesium alloys:when the LPSO phase presents a continuous network,it will play a certain blocking role and slow down the corrosion rate,while the dispersed LPSO phase will lead to the initiation of corrosion points at the interface between Mgand LPSO,The overall corrosion rate:?-Mg>LPSO,resulting in serious corrosion of the magnesium matrix.When there are a large number of rod-like LPSO forms,the overall corrosion resistance can be improved.When the layered LPSO phase appears inside,strong microgalvanic corrosion will occur,resulting in more serious Mgcorrosion.From the point of view of the overall corrosion rate,when two LPSO phases exist at the same time,the corrosion will be faster than that of a single LPSO phase.(3)There are two ways for the LPSO phase to slow down the corrosion rate:one is to form a continuous network of LPSO phase,and the other is to form a large number of rods(dominant).There are also two ways to accelerate the corrosion rate:one is to make the volume fraction of LPSO in the magnesium alloy much larger than 50%to form exfoliation corrosion,and the other is to simultaneously form two or more LPSO forms.