Microstructure,Mechanical Properties and Elastic Modulus of VW75 Magnesium Alloy with Si and Zn Addition
Magnesium alloys are the smallest density metal structural materials in the world today.They have great application pros-pects in the fields of aerospace weapons and equipment.In recent years,the speed of aerospace weapons and equipment is higher and higher and the range is longer and longer.Therefore,higher requirements are put forward for the stability and lightweight of load-bear-ing parts.At present,Mg-RE-Zn(RE:rare earth)magnesium alloy has excellent mechanical properties at room temperature and high temperature,and the tensile strength at room temperature can reach more than 450 MPa,and it is one of the most potential materials for manufacturing missile cabin and rudder wing surface.However,the elastic modulus of Mg-RE-Zn alloy is very low,only 40～45 GPa,far lower than that of aluminium alloy(70 GPa),titanium alloy(110 GPa)and steel(206 GPa),and the problem of"strength modu-lus"mismatch appears in the service state of high stress.In order to improve the anti-elastic deformation ability of missile cabin and rudder wing surface,the commonly used method is to increase the section thickness,but this method increases the overall weight of the equipment and seriously restricts the improvement of flight speed and range of aerospace weapon equipment.Therefore,the de-mand for high elastic modulus magnesium alloy is becoming more and more urgent.This paper took Mg-7Gd-5Y-1Nd-0.5Zr(％,mass fraction;VW75)magnesium alloy as the research object,and studied the microstructure changes,mechanical properties and elastic modulus of three alloys with different contents,revealing the influence of Si and Zn contents of three alloys.The microstructure of as cast,homogenized and extruded phases were observed and analyzed by metallographic microscope(OM)and scanning electron micro-scope(SEM),and the chemical composition of various phases was analyzed by energy dispersive spectrometer(EDS)attached to SEM.Xpert Pro MPD polycrystalline X-ray diffractometer(XRD)was used to analyze the as cast and homogeneous microstructure at the bottom of the ingot.The scanning speed was 2(°)·min-1 and the scanning range was 10°～90°,and the diffraction peak was calibrated by MDI jade 6.0 analysis software.The dynamic elastic modulus of VW75 extruded material,three alloys(VW75+1Si,VW75+2Si and VW75+2Si+2Zn(％,mass fraction))as-cast,extruded and T5 were tested by resonance method.In order to determine T5 peak aging system of the three alloys,the age hardening curve was made.The aging temperature was 200 ℃ and the time was 0～56 h.Vickers hardness of the three alloys was measured,and the tensile tests at room temperature were carried out on extruded and T5 samples of three alloys by sans universal testing machine.After adding Si and Zn to VW75 magnesium alloy,with the formation of RE-Si phase,the content of solid solution rare earth in the matrix decreased,the precipitation strengthening effect decreased,and the strength of the alloy decreased.VW75+2Si+2Zn alloy had the highest elastic modulus in T5 state,which was 53.4 GPa,but it had no age hardening effect,and its tensile strength,yield strength and elongation in extruded state were 232 MPa,145 MPa and 10.5％,respectively.Af-ter adding Si element to VW75 magnesium alloy,in-situ reinforcing phases such as YSi,Gd5Si3 and Zr5Si3 were formed.There were two kinds of reinforcing phase particles in the as-cast structure,one was polygonal particles with large size,the other was ellipsoidal particles with small size,and most polygonal RE-Si reinforcing phase particles contained two regions with large composition differenc-es at the same time,namely,Y-rich area in the heart and Gd rich area at the edge.In the process of melt cooling,YSi phase was formed preferentially,and then Gd5Si3 phase nucleated with YSi as the heterogeneous core to become polygonal particles.If Gd5Si3 phase nucleated independently,it became ellipsoidal particles.The average particle size of reinforced phase was 3.8～5.5 μm interval.Because the specific gravity of RE-Si reinforced phase particles was much heavier than that of melt,in the process of engineering,with the increase of melt amount and the decrease of solidification rate,the settlement problem would be aggravated,which would be solved by semi-solid stirring and field auxiliary technology.
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