Microstructure and corrosion behavior of Mg-Zn-Y-Al alloys with long-period stacking ordered structures

,Jinshn Zhng,,Zilong Zho,Chunxing Xu

aCollege of Materials Science and Engineering,Taiyuan University of Technology,Taiyuan 030024,China

bWeichai Power Co.,LTD.,Weifang 261001,China

cTaiyuan Iron&Steel(Group)Co.,LTD.,Taiyuan 030003,China

Microstructure and corrosion behavior of Mg-Zn-Y-Al alloys with long-period stacking ordered structures

Dan Wanga,Jinshan Zhanga,*,Jidong Xub,Zilong Zhaoc,Weili Chenga,Chunxiang Xua

aCollege of Materials Science and Engineering,Taiyuan University of Technology,Taiyuan 030024,China

bWeichai Power Co.,LTD.,Weifang 261001,China

cTaiyuan Iron&Steel(Group)Co.,LTD.,Taiyuan 030003,China

Mg97-xZn1Y2Alxalloys with long-period stacking ordered(LPSO)structures were prepared by conventional casting method.The optical microscopy(OM),X-ray diffraction(XRD)and the scanning electron microscope(SEM)equipped with energy dispersive X-ray spectroscopy (EDS)were used to analyze the microstructure of the alloys with different compositions.Immersion test and electrochemical measurement were used to evaluate the corrosion behavior of the alloys at room temperature,and the corrosive medium is 3.5%NaCl aqueous solution.The results showed that,with the increasing aluminum(Al)addition,except α-Mg and LPSO phases,new phases also emerged on the grain boundaries.At the same time,the zigzag part of LPSO phases disappeared,and the boundaries between LPSO phases and α-Mg became smooth.Furthermore, the addition of Al to Mg-Zn-Y alloys could hinder the activity of cathodic hydrogen evolution reaction and improve the uniformity and compactness of the protective surface f i lm,thus,enhanced the corrosion resistance of Mg-Zn-Y alloys.

Magnesium alloy;Rare earth element;SEM;XRD;Weight loss;Polarization

1.Introduction

Magnesium(Mg)alloys have become one of the potential engineer materials for automobile and aeronautical industries because of its high strength weight ratio and low density[1,2]. In order to obtain excellent properties and wide applications, many researchers have paid great attention to explore effective strengthening phases in Mg alloys[3,4].During the last decades,the Mg-Zn-RE Mg alloy consists of α-Mg and longperiod stacking ordered(LPSO,X phase)phases have been developed,alloys with this structure own unique microstructure and excellent mechanical properties[5,6].The application of rapidsolidif i cation/powermetallurgyprocessingtoMg97Zn1Y2alloy which can form LPSO phases in the alloy results in excellent yield strength(610 MPa)and elongation(5%), respectively[7,8].The most important is that this long-period stacking ordered structure can also be obtained in conventional copper mold casting[9].Furthermore,it is suggested that,LPSO phases also emerge in Mg-RE-X(X=Ag,Cu, Ni)alloys[10-13].However,the corrosion behavior of LPSO-containing alloys and the effect of fourth element addition on corrosion behavior of Mg-Zn-RE alloys have not been investigated.

Al is one of the common elements in Mg alloys,in general, the addition of Al can improve the stability of protective f i lmformed on the corroded Mg alloy surface,which in turn enhanced corrosion resistance[14,15].Recent years,Yamasaki has investigated the corrosion behavior of LPSO Mg-Zn-Y alloys containing Al,which was fabricated by rapid solidif ication/power metallurgy processing[16].The result showed that,due to the existence of Al which can modify the composition and structure of surface f i lms,corrosion resistance of Mg-Zn-Y-Al alloys was increased with the increase of Al. However,the investigation of Al-containing Mg-Zn-Y LPSO phasealloyswerepreparedbyconventionalcastingmethodwas not reported,therefore,the investigation of effect of Al addition on microstructure and corrosion behavior of Mg-Zn-Y alloy provide a reference for the application of Mg-Zn-Y LPSO phase alloy.

2.Experimental

The alloys used for this investigation were prepared by well type crucible resistance furnace from high purity Mg(99.9%), Y(99.9%),Zn(99.9%)and Al(99.9%)in a shielding gas CH2FCF3+N2atmosphere at 1033 K.In order to reduce the inf l uence of impurity on the corrosion property,the melted alloy was ref i ned at the last step of the melting process.Then it was cast into a preheated iron mold.The chemical compositions of Mg-Zn-Y-Al alloys are listed as follows in Table 1. The prepared Mg97-xZn1Y2Alxingots were cut into cylindershaped specimens of φ30 mm × 3 mm.Specimens for immersion and polarization curve tests were ground to a 2000 grit SiC paper,and subsequently rinsed with absolute alcohol in an ultrasonic bath and dried in warm air.Specimens for metallographic observation were further ground to 3000 grit SiC paper,and then etched by 3%nital.

The immersion test was carried out at room temperature in 3.5%NaCl solution for 40 h.Prior to immersion test,the every surface of specimens was ground with 2000 grit paper,ultrasonically cleaned in acetone and dried with warm air.At the end of the test,they were immersed into 200 g/L CrO3+10 g/ L AgNO3boiling solution to remove the corrosion products attached on the specimen surface,then washed with distilled water and dried with hot air.The weighted mass changes of these specimens were measured on a one over ten-thousand analytical balance,and mass changes per unit of surface area were calculated to evaluate the corrosion resistance.

The polarization curves were measured using Land CS350 electrochemical system in 3.5%NaCl aqueous solution at room temperature.A classical three-electrode cell was used with a platinum as counter electrode,a saturated calomel electrode as reference electrode and the samples sealed by resin with an exposed area of 1 cm2as working electrodes. The working surfaces of the working electrodes for the test were ground using 2000 grit SiC paper and cleaned in acetone before exposed to the solution.After open-circuit potentials (OCP)was measured in 3.5%NaCl aqueous solution for 300 s at room temperature,polarization curves test was conducted at a scan rate of 2 mV/s.

The surface morphologies were observed with JSU-6700F scanning electron microscope (SEM)to determine thedistribution and morphology of the phases on the surfaces of Mg-Zn-Y-Alalloys.The chemicalcompositionsof different phases in Mg-Zn-Y-Al alloys were analyzed by an energy dispersive spectroscopy(EDS).Phase constitution analyses were performed with a Y-2000 X-ray diffractometer, using monochromatic Cu-Kα radiation.

Table 1The chemical compositions of experimental alloys(wt.%).

3.Results and discussion

3.1.Microstructure

The microstructures of the Mg-Zn-Y-Al alloys are shown in Fig.1.Combining with XRD study(as seen in Fig.2),Mg96.9Zn1Y2Al0.1and Mg96.8Zn1Y2Al0.2alloys have similarmicrostructures compared with Mg96.7Zn1Y2Al0.3alloy,therefore,the SEM images of alloys Mg96.7Zn1Y2Al0.3, Mg96.5Zn1Y2Al0.5and Mg96Zn1Y2Al1were selected to show the microstructure of Mg-Zn-Y-Al alloys.It can be seen in Fig.1b,besides α-Mg phase and X phase,a small amount of strip-shaped phases can be observed in Mg96.7Zn1Y2Al0.3alloy,and these strip-shaped phases primarily precipitated on the grain boundaries.When continued to increase the content of Al,the amount of strip-shaped phases increased,as shown in Fig.1c,and had a trend to get together.Then,the amount of these clustered strip-shaped phases increased in the Mg96Zn1Y2Al1alloy,as shown in Fig.1d.Moreover,the morphology of X phase in the alloys changed with the increase of Al,the characteristic of the X phase from zigzag to smooth. It suggested that the formation of X phase based on atom diffusion[17]and the addition of Al could promote the diffusion of Zn and Y into grain boundary which was important for formation of X phase,but Al,simultaneously, consumed the Zn and Y by formed strip-shaped phases,thus, X phase became smooth and f i ne when increased the addition of Al.Fig.1e and f shows the TEM image and SAED patterns of the X phase.In the SAED patterns,the spots were arranged in positions that divided the height between the incident beam and the(0002)Mg6-fold.Based on the XRD peaks in Fig.2 and SAED patterns in Fig.1f,the X phase was determined as Mg12ZnY,and had 18R LPSO structure which had been investigated by the previous study[18].The composition of the strip-shaped phase was determined as Mg4Y2ZnAl3by EDS(Fig.3).And the new peaks compared to the Mg97Zn1Y2alloy in XRD patterns also proved the existence of the new phases which were named as Mg4Y2ZnAl3in this paper.

Fig.1.SEM images of microstructure of(a)Mg97Zn1Y2,(b)Mg96.7Zn1Y2Al0.3,(c)Mg96.5Zn1Y2Al0.5,(d)Mg96Zn1Y2Al1and TEM image of X phase and corresponding SAED patterns of(e)X phase,(f)the SAED patterns of X phase.

Fig.2.XRD analysis of Mg-Y-Zn-Al alloys.

The EDS spectra of α-Mg phase and X phase in the Mg-Zn-Y-Al alloys were shown in Tables 2 and 3, respectively.The distribution of Al in α-Mg and X phase was determined.When the content of Al was 0.3%,trace Al was detected in α-Mg,while no Al was found in the X-phase. However,when the content of Al is 0.5%,Al was found in the X-phase.Similarly,trace element of Al was also found in both α-Mg phase and X phase in alloy Mg96Zn1Y2Al1.In a word, the content of Al in α-Mg and X phase increased with the increase of Al in Mg-Zn-Y-Al alloys.It can be seen that the Al element can dissolve into X phase and α-Mg to some extent [19],and from the view of corrosion,the Al dissolved in the α-Mg phase can enhance the corrosion resistance by improving the stability and compactness of surface layer.From the view of microstructure,the Al dissolved in the X phase may block the diffusion of Zn and Y which participate in the formation of the X phase,thus,weaken the characteristic of the X phase which had a obvious zigzag morphology.

In addition,there were trace zinc(Zn)and yttrium(Y)in α-Mg as can be seen from Table 2.It is well known that the rare earth Y can also improve the compactness of the corroded f i lm,and therefore,the corrosion resistance of Mg-Zn-Y-Al alloys was enhanced[20].

3.2.Corrosion resistance of Mg-Zn-Y-Al alloys

3.2.1.Immersion test

Fig.3.EDS spectra of(a)and(b)Mg96.7Zn1Y2Al0.3,(c)and(d)Mg96.5Zn1Y2Al0.5,(e)and(f)Mg96Zn1Y2Al1.

Theaveragecorrosionrateofalloys derivedfromimmersion test in 3.5%NaCl solution for 40 h at room temperature are shown in Fig.4.The alloy with 1%Al showed the lowest weight loss rate(4.3 mg/cm2day),indicating the best corrosion resistance.But viewed as a whole,Fig.4 shows that the corrosion rate curve had a larger slopewhen the content of Al is lower than 0.3%,comparing to that with higher addition of Al. It implied that 0.3%Al addition was helpful to decrease the corrosion rate to a great extent.And the corrosion rate value of Mg96.7Zn1Y2Al0.3alloy(5.14 mg/cm2day),fell by half compared to the Al-free Mg alloy(10.15 mg/cm2day).In the study of Al-containing Mg alloys corrosion[21,22],Al element had two inf l uences on the corrosion of Mg alloys:on the one hand,it can promote the precipitation of β phase,acting as cathode inthe corrosionmicro-galvanic,whichcanincreasethe corrosion rate and forming continuous,f i ne and uniform netlike phase,which hinder the propagation of the corrosion.On the other hand,the Al dissolved in the α-Mg phase can improve the stability of surface f i lm,therefore,protectingMg alloy from corrosion.The EDS spectra of α-Mg in Mg-Zn-Y-Al alloys indicated that,no Al exist in α-Mg of Mg96.9Zn1Y2Al0.1and Mg96.8Zn1Y2Al0.2alloy,but the corrosion resistance of the two alloys was still enhanced,the reason might be that the content of Al was too small to detect.Furthermore,the percentage of Al dissolved in α-Mg increased with the increase of Al content inMg-Zn-Y alloys,and it could be seen that the percentage of Al in α-Mg of Mg96Zn1Y2Al1alloy reached 0.43%(as seen in Table 2),and this high solid solubility of Al in α-Mg was good for the formation of compact f i lm.It couldbe concluded that,in this study,the stability of corroded f i lm formed on the Alcontaining alloy surface corresponded to the enhancement of corrosion resistance.Theweight loss rate of the Mg96Zn1Y2Al1alloy,having the most content of Al element,is slightly lower than that of Mg96.5Zn1Y2Al0.5alloy,and this result may be due to the coarsening of the grain size of Mg96Zn1Y2Al1alloy and the increase of segregation of Al.3.2.2.Electrochemical measurements

Table 2Elements content in the α-Mg of alloys(wt.%).

Table 3Elements content in the X-phase of alloys(wt.%).

Fig.4.Corrosion rates of Mg-Zn-Y-Al alloys after immersion in 3.5%NaCl solution at room temperature for 40 h.

Fig.5.The polarization curves for Mg-Zn-Y-Al alloys in 3.5%NaCl solution at room temperature.

Fig.5 shows the polarization curves of Mg97Zn1Y2, Mg96.5Zn1Y2Al0.5and Mg96Zn1Y2Al1alloys in 3.5%NaCl solution.The Mg96.5Zn1Y2Al0.5and Mg96Zn1Y2Al1alloys exhibited lower cathodic current densities than Mg97Zn1Y2alloy,and it is well known that the cathodic current densities was followed by the evolution of hydrogen on the specimen surface,thus,it can be deduced that the addition of Al in the Mg-Zn-Y alloy promoted the formation of complete surface f i lm,and hindered the hydrogen evolution reaction on the Alcontaining Mg alloy surfaces,which in turn decreased the cathodic current densities.In the anodic polarization area,the current density of the Mg97Zn1Y2alloy increased sharply.On the other hand,the Al-containing Mg alloys exhibited a passive region.This result was due to the appearance of passivity in the corrosion processing.Table 4 shows the corrosion potential(Ecorr)and corrosion current density(Icorr).The order ofEcorris Mg97Zn1Y2>Mg96.5Zn1Y2Al0.5>Mg96Zn1Y2Al1. The corrosion potential of Al-free Mg alloys is much noble than that of Al-containing Mg alloy.This reveals that the element Al decreased the cathodic activity,thus reduced the potential difference between α-Mg and X phase,which have been reported by Michiaki Yamasaki[16].The order ofIcorris Mg97Zn1Y2>Mg96.5Zn1Y2Al0.5>Mg96Zn1Y2Al1.This can be concluded that the corrosion resistance of Mg96Zn1Y2Al1alloy is higher than Mg97Zn1Y2alloy or Mg96.5Zn1Y2Al0.5alloy.Therefore,the addition of Al can increase the corrosion resistance of the Mg97Zn1Y2alloy with LPSO phase.

3.2.3.Corrosion morphology

The corrosion morphologies of Mg97Zn1Y2, Mg96.7Zn1Y2Al0.3,Mg96.5Zn1Y2Al0.5and Mg96Zn1Y2Al1specimensimmersedfor4hin3.5%NaClsolutionareshownin Fig.6.As can be seen from Fig.6,a lot of corrosion products observed from Fig.6a,corresponding to the occurrence of pittingcorrosion,andthequantityandsizeofcorrosionproducts in Mg97Zn1Y2alloy presented a much larger scale compared with Mg-Zn-Y-Al alloys.It indicated that the addition of Al could enhance corrosion resistance.The corrosion products of the four alloys appeared on the boundaries,and this illustratedthat LPSO phases and/or Mg4Y2ZnAl3phases might act as cathodes while α-Mg around these phases acted as anodes to cause pitting corrosion,which was the primary corrosion on the alloy surface[23].In Mg96Zn1Y2Al1alloy,those few corrosion products mainly distributed in the vicinity of clustered stripshaped phases instead of X phase.This might be attributed to theemergenceofclusteredstrip-shapedphaseswhichincreased cathode-to-anodearearatios.Inaddition,f i liformcorrosionwas alsoobservedonthesurfaceofMg96.7Zn1Y2Al0.3alloyafter4h immersion test.However,there was no obvious f i liform corrosion on the Mg96.5Zn1Y2Al0.5and Mg96Zn1Y2Al1specimen surfaces,except a small number of corrosion products.This can be explained that,as the content of Al was increased,the compactness of the protective surface f i lm was also increased, thus,protected the α-Mg under the f i lm from corrosion.

Table 4The Ecorr and Icorr values of Mg97Zn1Y2, Mg96.5Zn1Y2Al0.5 and Mg96Zn1Y2Al1alloys.

Fig.6.Corrosion morphologies of alloys in 3.5%NaCl solution after 4 h:(a)Mg97Zn1Y2,(b)Mg96.7Zn1Y2Al0.3,(c)Mg96.5Zn1Y2Al0.5,(d)Mg96Zn1Y2Al1.

Fig.7 shows the morphologies of corrosion product of Mg97Zn1Y2and Mg96.7Zn1Y2Al0.3alloys immersed in 3.5% NaCl solution for 6 h.The corrosion occurred on the local surfaces of Mg97Zn1Y2and Mg96.7Zn1Y2Al0.3alloys,and its extending direction was from serious corrosion area to slight areaasindicated bythearrowsinFig.7.Thebrightareawas the serious corrosion area,and there was a boundary between seriousareaandslightareaofcorrosiononthespecimensurface. Itcanbefoundthattheprotectivef i lmdiscoveredonthesurface of Mg97Zn1Y2was loose.On the contrary,as shown in Fig.7b, the protective f i lm discovered on the Mg96.7Zn1Y2Al0.3alloy surface was uniform and compact,which can protect α-Mg underthesurfacelayerfromcorrosion.MichiakiYamasakietal. [16]reported that the f i lm on the rapidly solidif i ed(RS)ribbonconsolidated Mg97.25Zn0.75Y2alloy had a three-layered structurecontainingofaY-freeouterlayer,aY-containinginnerlayer and an underlying alloy layer,and the RS ribbon-consolidated Al-containing Mg96.75Zn0.75Y2Al0.5alloy was found to have a much thicker inner layer than the Mg97.25Zn0.75Y2alloy.As we all know,rare earth(RE)-containing f i lm had a better protective capability than ordinary hydroxide f i lm.From Table 2,the content of Y was increased with the addition of Al,therefore, the Al could promote Y dissolved into α-Mg.Therefore,theaddition of Al in Mg-Zn-Y alloys which were prepared by conventional casting method might also improve the growth of Y-containing layer,increasing the compactness of the surface fi lm,which in turn enhanced corrosion resistance.

Fig.7.Morphologies for corrosion product of alloys after 6 h:(a)Mg97Zn1Y2,(b)Mg96.7Zn1Y2Al0.3.

4.Conclusion

(1).When the content of Al in the Mg97Zn1Y2alloy containing long period stacking ordered(LPSO,X phase) structure was 0.1%,0.2%,0.3%,0.5%and 1%,the stripshaped phases which were named as Mg4Y2ZnAl3phase in this paper precipitated on the grain boundaries of Mg96.7Zn1Y2Al0.3,Mg96.5Zn1Y2Al0.5and Mg96Zn1Y2Al1alloys.In the Mg96Zn1Y2Al1alloy,the interdigitation morphology of X phase disappeared and the boundary between X phase and α-Mg became smooth which was attributed to the addition of Al.

(2).The appropriate addition of Al can improve corrosion resistance of Mg97Zn1Y2alloy containing the long period stacking ordered(LPSO)phases.The Al element dissolved into the α-Mg phase can increase the compactness of the surface f i lm,enhancing the corrosion resistance of the alloys.However,compared to the Mg96.5Zn1Y2Al0.5alloy,the enhancement of the corrosion resistance of Mg96Zn1Y2Al1alloy is not apparent which due to the discontinuous X phase and the increase of Al segregation.

Acknowledgments

This work was supported by the National Natural Science Foundation of China(No.50571073),the Ph.D.Programs Foundation ofMinistry ofEducation ofChina (No. 20111402110004)and the Natural Science Foundation of Shanxi Province,China(No.2009011028-3,2012011022-1).

[1]A.A.Luo,Mater.Sci.Forum 4(2003)57.

[2]M.Bamberger,G.Dehm,Annu.Rev.Mater.Sci.38(2008)505.

[3]B.L.Mordike,T.Ebert,Mater.Sci.Eng.,A 302(2001)37.

[4]Z.Yang,J.P.Li,J.X.Zhang,G.W.Lorimer,J.Robson,Acta Metall.Sin. (Engl.Lett.)5(2008)313.

[5]T.Itoi,T.Seimiya,Y.Kawamura,M.Hirohashi,Scr.Mater.51(2004) 107.

[6]E.Abe,Y.Kawamura,K.Hayashi,A.Inoue,Acta Mater.50(2002) 3845.

[7]A.Inoue,Y.Kawamura,K.Hayashi,A.Inoue,T.Masumoto,Mater. Trans.42(2001)1172.

[8]A.Inoue,Y.Kawamura,M.Matsushita,K.Hayashi,J.Koike,J.Mater. Res.16(2001)1894.

[9]Y.J.Wu,D.L.Lin,X.Q.Zeng,L.M.Peng,W.J.Ding,J.Mater.Sci.44 (2009)1607.

[10]Francois O.Me′ar,Dmitri V.Louzguine-Luzgin,A.Inoue,J.Alloys Compd.496(2010)149.

[11]K.Siarher,R.Lars,K.Bernd,Int.J.Hydrogen Energy 34(2009)7749.

[12]X.H.Shao,Z.Q.Yang,J.H.You,K.Q.Qiu,X.L.Ma,J.Alloys Compd. 509(2001)7221.

[13]M.Eddahbi,P.Perez,M.A.Monge,G.Garces,R.Pareja,P.Adeva,J. Alloys Compd.473(2009)79.

[14]G.M.Abady,N.H.Hilal,M.El-Rabiee,W.A.Badawy,Electrochim.Acta 55(2010)6651.

[15]M.Liu,P.J.Uggowitzer,A.V.Nagasekhar,P.Schmutz,M.Easton, G.L.Song,A.Atrens,Corros.Sci.51(2009)602.

[16]M.Yamasaki,S.Izumi,Y.Kawamura,H.Habazaki,Appl.Surf.Sci.257 (2011)8258.

[17]M.Matsuda,S.Ii,Y.Kawamura,Y.Ikuhara,M.Nishida,Mater.Sci. Eng.,A 386(2004)447.

[18]J.S.Zhang,J.D.Xu,W.L.Cheng,C.J.Chen,J.J.Kang,J.Mater.Sci. Technol.28(2012)1157.

[19]J.S.Zhang,C.G.Chen,Z.P Que,W.L.Cheng,J.D.Xu,J.J.Kang,Mater. Sci.Eng.,A 552(2012)416.

[20]M.Liu,Patrik Schmutz,Peter J.Uggowitzer,G.L.Song,A.Atrens, Corros.Sci.52(2010)3687.

[21]Y.L.Cheng,T.W.Qin,H.M.Wang,Z.Zhang,Trans.Nonferrous Met. Soc.China 19(2009)517.

[22]M.C.Zhao,M.Liu,G.L.Song,A.Atrens,Corros.Sci.50(2008)1939.

[23]Z.M.Shi,Jimmy Xueshan Jia,A.Atrens,Corros.Sci.60(2012)296.

Received 15 November 2013;accepted 6 January 2014 Available online 13 March 2014

*Corresponding author.Tel./fax:+86 351 601 8208.

E-mail address:jinshansx@tom.com(J.Zhang).

Peer review under responsibility of National Engineering Research Center for Magnesium Alloys of China,Chongqing University.

Production and hosting by Elsevier

http://dx.doi.org/10.1016/j.jma.2014.01.008.

2213-9567/Copyright 2014,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.

Copyright 2014,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.