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Materials Science and Engineering A 523 (2009) 145–151

Contents lists available at ScienceDirect

Materials Science and Engineering A

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ffect of Zr on the microstructure, mechanical properties and corrosion resistancef Mg–10Gd–3Y magnesium alloy

ing Sun a,b, Guohua Wu a,b,∗, Wei Wang a,b, Wenjiang Ding a,b

National Engineering Research Center of Light Alloy Net Forming, Shanghai Jiaotong University, Shanghai 200240, ChinaState Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai 200240, China

r t i c l e i n f o

rticle history:eceived 15 December 2008eceived in revised form 21 May 2009ccepted 2 June 2009

a b s t r a c t

The influence of Zr on the microstructure, mechanical properties and corrosion resistance of Mg–10Gd–3Y(wt.%) magnesium alloy was investigated. The grain size of alloys decreased with Zr content from 0% to0.93% (wt.%). The addition of Zr greatly improved the ultimate tensile strength (UTS) and the elongation(EL), while slightly improved the tensile yield strength (TYS). The UTS and the EL of the alloy contain-

eywords:agnesium alloys

ricrostructure

ing 0.93% Zr increased by 125.8 MPa and 6.96% compared with base alloy, respectively. The corrosionresistances were found to decrease with Zr content from 0% to 0.42% and then increase from 0.42% to0.93%. The differences in the sizes and distributions of the Zr-rich particles have significant effects on thecorrosion behaviors. The alloy with 0.42% Zr addition revealed the optimum combination of mechanicalproperties and corrosion resistance.

echanical propertiesorrosion resistance

. Introduction

Magnesium alloys have advantageous properties such asow density, high specific strength, good castability, and excel-ent machinability [1]. It has been reported that the recentlyeveloped Mg–Gd–Y magnesium alloys exhibit higher specifictrength at both room and elevated temperatures and betterreep resistance than conventional Al and Mg alloys, including

E54(Mg–5Y–2Nd–2HRE, wt.%), whose high temperature strengths the top of existing commercial magnesium alloys [2–6].

Many investigations related to the microstructure and mechan-cal properties of the Mg–Gd–Y system have been reported [2,5–9].okhlin et al. investigated the relationship between mechanicalroperties and microstructure of Mg–Gd–Y–Zr alloys [10–12]. He6] observed the precipitation sequence of Mg–Gd–Y–Zr alloy dur-ng isothermal ageing at 250 ◦C for 0–2400 h [12]. Recently, Chang13] investigated the effect of different Gd content on the corro-ion behavior of Mg–Gd–Y–Zr alloys. Wang [14] investigated the

ffect of Y for enhanced age hardening response and mechanicalroperties of Mg–Gd–Y–Zr alloys. Honma [9] investigated effect ofn additions on the remarkable age-hardening and unusual plas-ic elongation behaviors of Mg–Gd–Y–Zr alloys. Given that the

∗ Corresponding author at: National Engineering Research Center of Light Alloyet Forming, Shanghai Jiaotong University, Shanghai 200240, China.el.: +86 21 54742630; fax: +86 21 34203794.

E-mail addresses: ghwu@sjtu.edu.cn, ghwu@sina.com (G. Wu).

921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2009.06.002

© 2009 Elsevier B.V. All rights reserved.

Mg–Gd–Y alloys have superior strength and creep resistance, inter-est in their application is still increasing.

It is known that Zr is a powerful grain refiner for Mg–RE alloysto further improve the mechanical properties. The benefit of Zraddition also includes the improvement of corrosion resistance ofthe magnesium alloys, and the Zr-containing magnesium alloysusually show a higher corrosion resistance than Zr-free magne-sium alloys [15]. Despite the above interesting findings, there isstill a lack of optimization of Zr content for Mg–Gd–Y alloy. Thepurpose of this work is to better understand the effect of Zr onthe microstructure, mechanical property and corrosion behavior ofMg–Gd–Y alloys, and determine optimal chemical composition ofMg–10Gd–3Y–xZr.

2. Experimental method

In this work, Mg–10Gd–3Y–xZr (wt.%) alloys were studied. Theactual chemical compositions of the alloy were determined by aninductively coupled plasma atomic emission spectroscopy (ICP-AES) analyzer and listed in Table 1. Alloy ingots of GWK alloy wereprepared from high purity Mg (99.95 wt.%), the Mg–25Gd (wt.%),Mg–25Y (wt.%) and Mg–30Zr (wt.%) master alloys in an electricresistance furnace under a mixed protective gas of CO2 and SF6 with

the ratio of 100:1. Specimens were etched in a 4 vol.% nital and wereexamined in an optical microscope (OM), a FEI SIRION 200/INCAOXFORD scanning electron microscope (SEM). The average grainsize was measured by linear intercept method in an OM. Phase anal-yses were carried out with Rigaku Dmax-rc X-ray diffractometer

146 M. Sun et al. / Materials Science and En

Table 1Chemical compositions of studied Mg–10Gd–3Y–xZr alloys denoted by GWK (wt.%).

Alloy Gd Y Zr Mg

GW 10.09 2.88 0 Bal.GWK01 10.12 2.70 0.06 Bal.GWK02 10.23 2.84 0.13 Bal.GWK03 10.15 2.77 0.33 Bal.GWK04 10.06 2.92 0.39 Bal.GWK05 10.22 2.90 0.42 Bal.GWK06 10.04 2.76 0.57 Bal.GWK1 10.17 2.96 0.93 Bal.

Fig. 1. Optical microstructures of as-cast GWK alloys with different Zr contents (wt.%): (�-Mg matrix, (2) Mg24(Gd, Y)5 eutectic compound, (3) Zr-rich cores).

gineering A 523 (2009) 145–151

(XRD). Tensile tests were carried out on a Zwick T1-FR020TN.A50electronic universal material testing machine.

The immersion tests were conducted by immersing the speci-mens in 5 wt.% NaCl solution at 25 ◦C for 72 h, which was preparedwith analytical reagent grade NaCl and distilled water. Beforecorrosion immersion tests, the specimens were polished succes-

sively on finer grades of emery papers up to 800 grit. Afterthe immersion, specimens were cleaned by dipping in a solu-tion of 15%CrO3 + 1%AgNO3 in 400 ml water at boiling condition.The extent of corrosion was given in weight loss per surface area

a) 0; (b) 0.06; (c) 0.13; (d) 0.33; (e) 0.39; (f) 0.42; (g) 0.57; and (h) 0.93 (note: (1)

M. Sun et al. / Materials Science and En

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Fig. 2. X-ray diffraction (XRD) patterns of GWK alloys.

nd time (mg cm−2 day−1). The size of immersion specimens was35 mm × 4 mm.

Open circuit potentials (EOC), polarization curves and electro-hemical impedance spectroscopy (EIS) of the specimens wereeasured in 5% NaCl solution saturated with Mg(OH)2 using a

ARSTAT 2273 Advanced Electrochemical System. For all measure-ents, a three-electrode electrochemical cell was used, with a

aturated calomel electrode (SCE) as a reference electrode and aigh-density graphite electrode as the counter.

. Experimental results and discussion

.1. Microstructure

Fig. 1 shows the influence of Zr addition on the microstructuref the GW (Mg–10Gd–3Y) alloys. They all consist of primary �-Mg

Fig. 3. SEM micrograph and EDS of: (a) the second p

gineering A 523 (2009) 145–151 147

matrix and the �-Mg24(Gd, Y)5 eutectic phase distributed alongthe grain boundaries, which can be further confirmed by XRD anal-ysis (Fig. 2) and EDS analysis (Fig. 3). With increasing Zr content,the grain size decrease. Moreover, Zr addition does not lead to theformation of new phase.

Lots of Zr-rich zones with circular or petal-like morphology canbe seen when Zr content is more than 0.13%. There is at least oneZr-rich core in almost each grain. Most of the Zr-rich cores aresmaller than 10 �m, whilst some of them present inside the grains(Fig. 1(d)–(f)), and some congregate near the grain boundaries(Fig. 1(g) and (h)). Earlier study showed that the most character-istic feature of the microstructure of a magnesium alloy containingZr was the Zr-rich cores that exist in most grains. The Zr-rich coreswere believed to be the products of peritectic solidification, andthey played an important role in grain refinement [16]. Ma [17]indicated that fine white-bright particles which were pure Zr par-ticles in Zr-rich cores were observed in the backscattered electron(BSE) mode.

The connection between grain size and Zr addition is shown inFig. 4, showing that the grains become finer with an increase ofZr content, and the grain size of GWK1 alloy with 0.93% Zr is thesmallest (22 �m). Emley [16] investigated the mechanism of grainrefinement by adding Zr into the magnesium alloy, and proposedthat primary Zr particles segregated at temperatures a little abovethe peritectic temperature, and served as nucleation sites owingto the similarity of the lattices and the atomic sizes of magnesiumand Zr. Obviously, in Fig. 1(d)–(h), many Zr-rich cores presents in thealloys with 0.33–0.93% Zr, and the grain refinement can be basedon the peritectic mechanism proposed by Emley. The contradic-tion is that Zr-rich cores cannot be found in the other two alloyswith low Zr content (0.06% and 0.13% as shown in Fig. 1(b) and (c)),although their grains also get refined. In fact, several studies show

that, with a low Zr content (below the peritectic point), the mag-nesium grains are refined significantly. Lee [18] explained that thegrain refinement in magnesium is mostly caused by the growthrestriction effects of Zr solute during solidification. The growthrestriction factor (GRF) values for various alloying elements in mag-

hase and (b) the Zr-rich cores in GWK05 alloy.

148 M. Sun et al. / Materials Science and Engineering A 523 (2009) 145–151

Fig. 4. Relationship between Zr content and average grain size of GWK alloys.

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Table 2Corrosion potential and current of GWK alloys deduced from the potentiodynamicpolarization curves shown in Fig. 6.

Alloy Zr % Ecorr/V Icorr/(×10−5 A/cm2)

GW 0 −1.6135 157.6562GWK01 0.06 −1.5590 122.9739GWK02 0.13 −1.5506 19.1422GWK03 0.33 −1.5287 1.8636GWK04 0.39 −1.4860 1.7468GWK05 0.42 −1.4679 1.4659GWK06 0.57 −1.4882 1.8159GWK1 0.93 −1.5424 2.2057

ig. 5. Effect of Zr on the tensile properties of GWK alloys at room temperature.

esium alloys have been determined, and Zr has the highest growthestriction factor value among the common alloying elements such

s Ca, Si, Zn, Ce, Y, etc. Therefore, the grains of GWK01 and GWK02re refined by low Zr content. More recently, Peng [19] concludeshat the mechanism for Zr refinement in the low Zr alloy is differentrom that in the high Zr alloys: the Zr works mainly by restrict-

Fig. 6. Polarization curves of GWK alloys.

Fig. 7. Electrochemical impend spectra of GWK alloys.

ing grain growth in the former and by generating nucleants in thelatter.

3.2. Mechanical properties

The influence of Zr addition on the mechanical properties isshown in Fig. 5. With Zr addition increases, the UTS and the EL

both gradually increase. The UTS and the EL of alloy with 0.33% Zraddition reach to 204.7 MPa and 3.34%, which are 1.8 times and 3.3times as the base alloy, respectively. The UTS and the EL of alloy with0.57% Zr addition increase by 99% and 593%, respectively. Increasing

Fig. 8. Weight loss corrosion rate of GWK alloys after immersion for 72 h in 5% NaClsolution (pH 8.3, 25 ◦C).

M. Sun et al. / Materials Science and Engineering A 523 (2009) 145–151 149

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ig. 9. Corroded surface photographs of GWK alloys after immersion in the 5% NaCl

he Zr content to 0.93%, the tensile properties reach to the highests the UTS 238.3 MPa and the EL 7.96%. However, the addition of Zras little effect on the tensile yield strength (TYS) of GWK alloys.hen Zr addition is between 0.06% and 0.57%, the TYS keep almost

ame value around 135 MPa. The TYS of alloy with 0.93% Zr addi-ion reaches the highest value as 148.9 MPa, increasing by 30–40%ompared to the base alloy. Apparently, grain refinement causedy Zr addition leads to strengthening effect. And the addition of Zrreatly improves the UTS and the EL, while slightly improves theYS.

.3. Potentiodynamic polarization

Fig. 6 presents the potentiodynamic polarization curves of spec-mens in 5% NaCl solution saturated with Mg(OH)2. Table 2 showshe corrosion potential (Ecorr) and corrosion current density (Icorr),hich are deduced from the potentiodynamic polarization curves.ith Zr addition increases to 0.42%, the Ecorr increases to more pos-

tive and the Icorr decreases. Increasing the Zr addition from 0.42%o 0.93%, the situation is contrary, which can be seen that the Ecorr

ecreases and the Icorr increases. Deduced from the change of cor-osion current density, the corrosion rate is in the following order:W > GWK01 > GWK02 > GWK1 > GWK03 > GWK06 > GWK04 > GWK

ndicating that GWK05 has the best corrosion resistance.

.4. Electrochemical impedance spectroscopy (EIS)

EIS measurement is an effective method to investigate the metalorrosion [20–22]. Fig. 7 discloses the Nyquist plots of studied alloys

fter immersion in 5 wt.% NaCl solution. For all alloys, shapes ofhese EIS spectra are similar, except for the difference in the diam-ter of the loops. This means that the corrosion mechanisms ofhese alloys are the same, but their corrosion rates are different.everal authors [20,23–25] have indicated that the bigger the high-

Fig. 10. SEM morphologies of corrosion surfaces after immersion in the 5% NaCl sol

on for 72 h and removal of the corrosion products: (a) GW; (b) GWK03; (c) GWK05.

frequency (HF) loop was, the better the corrosion resistance was. InFig. 7, the HF semicircle increases continuously with the increase ofZr content from 0% to 0.42%, and then decreases with the Zr contentfrom 0.42% to 0.93%, which indicates that the alloy with 0.42% Zraddition has the lowest corrosion rate. This is in good agreementwith the corrosion resistances of Fig. 8.

3.5. Weight loss corrosion rate

Fig. 8 shows the corrosion rate in 5 wt.% NaCl solution. The cor-rosion rate decreases linearly when Zr addition is from 0% to 0.42%,while it increases when Zr addition is from 0.42% to 0.93%. Thechange of the corrosion rate is similar with the previous reportabout Mg–Zr binary alloy conducted by Ma [26], who disclosed thatthe corrosion rate of Mg–Zr binary alloy increased rapidly when Zrwas more than 0.5%.

3.6. Corrosion morphology

Fig. 9 shows the surface features of the corroded specimens afterthe immersion test in 5% NaCl solution and removal of the corro-sion products. It can be seen specimen with 0.42% Zr addition hasalmost no corroded area, with only some small corrosion pits. Typ-ical corrosion morphologies shown in Fig. 10 indicate that thereis no deep corrosion pit on the GWK05 alloy, compared with therelatively deep corrosion pits on the base alloy.

3.7. Analysis of corrosion damage

Optical micrographs of metallographic cross-section throughspecimens after 72 h immersion are shown in Fig. 11. It is clearthat corrosion has preferentially occurred in the matrix adjacent tothe grain boundary �-phase, and the �-phase is not corroded. Thisaccounts for the deep continuous furrows along the grain bound-

ution for 72 h and removal of the corrosion products: (a) GW and (b) GWK05.

150 M. Sun et al. / Materials Science and Engineering A 523 (2009) 145–151

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Fig. 11. Optical micrographs showing cross-section through corroded sp

ries, as shown in Fig. 12. From Fig. 12, it is clear that the �-phaseould be undercut after the �-Mg matrix adjacent to the �-phasereferentially attacked by the corrosion. Earlier investigations havehown that the �-phase can play a dual role in the corrosion behav-

ig. 12. SEM morphologies of GWK05 sample immersed for 72 h in 5% NaCl solution: (a) thf area A.

ig. 13. Optical micrographs of GWK alloy after different immersion times in 5% NaCl sol

ns after 72 h immersion in 5% NaCl solution: (a) GWK05 and (b) GWK1.

ior of magnesium alloy [27]. It could act as either a galvanic cathodeor a barrier to corrosion. Which role dominated the corrosion pro-cess depended on the amount and distribution of the �-phase.Finely and continuously distributed �-phase could more effectively

e deep continuous furrows area A along the grain boundaries and (b) magnification

ution: (a) GWK05, 30 min; (b) GWK1, 30 min; (c) GWK05, 2 h; and (d) GWK1, 2 h.

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M. Sun et al. / Materials Science a

top the corrosion. Otherwise, if the fraction of the �-phase wasow and the distribution was not continuous, the �-phase accel-rated corrosion. The reason of the difference in corrosion rate ofg–10Gd–3Y–xZr alloy is related to the microstructure, as shown in

ig. 1. The �-phase gets refined with 0.06–0.42% Zr addition, form-ng effective corrosion barrier and resulting in the improvement inorrosion resistance. However, for GWK1 alloys in Fig. 11(b), some ofhe superfluous Zr particles congregate at grain boundary as impu-ities to dissever the continuity of the �-phase, which could notffectively stop corrosion spreading from one grain to another grain.

Optical micrographs taken at different time intervals of immer-ion in 5% NaCl solution are shown in Fig. 13. For GWK05 alloy,fter 30 min immersion (Fig. 13(a)), the �-Mg matrix adjacent tohe �-phase at the grain boundaries is firstly attacked. This formf galvanic corrosion is driven by the potential difference betweenhe �-phase and the �-Mg. After 2 h (Fig. 13(c)), the grain boundaryttack is strengthened, whilst the attack surrounding the Zr-richone initiates. However, the Zr-rich zone is not corroded in GWK05lloy. For GWK1 alloy, after 30 min immersion (Fig. 13(b)), subse-uent to the preferential attack on the site adjacent to the grainoundary �-phase, the Zr-rich cores near the grain boundariesecome the next favorable site for corrosion. After 2 h (Fig. 13(d)),ttacks in some of the Zr-rich zone are severe. Neil [28] found thatevere corrosion occurred in the Zr-rich zone during corrosion ofE41 magnesium alloy, and he related it to the presence of a gal-anic couple between the Zr-rich cores and the surrounding matrix28]. In this paper, the differences in distribution of Zr-rich zoneave substantial effects on the corrosion behaviors. Previous worky Hamu [15] indicates that when Zr particles are concentrated

n grain centers of Mg–2Y–3RE–0.5Zr magnesium alloy, the cor-osion resistance is better in comparison with the homogeneousistribution of small Zr particle.

As can be seen in Fig. 13, after immersion for 2 h, the area thats more protected of GWK05 alloy is larger in comparison with theWK1 alloy. Earlier report shows that Zr produces oxide films (zir-onia), and this oxide film of Zr is much more stable than the oxidelms that magnesium produces in aqueous solutions [29]. So it is

hought that the different protected area is related to different sizesf Zr-rich zone. As shown in Fig. 1(f) and Fig. 1(h), the area of Zr-ich zone in GWK05 alloy (∼10 �m) is greater than that in the GWK1lloy (∼5 �m). More recently, Neil [28] suggests that the Zr-rich par-icles (∼5 �m) in ZE41 magnesium alloy disrupt the formation of arotective film and allowing severe corrosion. These may be rea-ons for the localized corrosion in GWK1 being more severe thanhat in GWK05.

. Conclusion

1. The addition of Zr in GWK alloy refines the grains and does notform new phase. Zr greatly improves the mechanical properties.

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gineering A 523 (2009) 145–151 151

By adding 0.93% Zr, the UTS and the EL increase by 125.8 MPa and6.96%, respectively.

2. Corrosion tests show that the corrosion rate decreases with theincrease of Zr from 0% to 0.42%, and then increases with theincrease of Zr from 0.42% to 0.93%. The differences in the sizesand distributions of the Zr-rich particles have distinct effects onthe corrosion behaviors.

3. The Mg–10Gd–3Y–0.42Zr alloy has the best combination ofstrength and corrosion resistance and this is considered as opti-mal chemical composition.

Acknowledgements

The present study is funded by the National Basic ResearchProgram of China (No. 2007CB613701) and Program of ShanghaiSubject Chief Scientist (No. 08XD14020).

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