*C.Dong

aLaboratory of Materials Modif i cation by Laser,Electron and Ion Beams,School of Materials Science and Engineering,Dalian University of Technology, Dalian 116024,China

bEnvironmental Corrosion Center,Institute of Metal Research,Chinese Academy of Sciences,62 Wencui Road,Shenyang 110016,China

Stress corrosion cracking susceptibility of a high strength Mg-7%Gd-5%Y-1%Nd-0.5%Zr alloy

S.D.Wanga,b,D.K.Xub,*,E.H.Hanb,C.Donga

aLaboratory of Materials Modif i cation by Laser,Electron and Ion Beams,School of Materials Science and Engineering,Dalian University of Technology, Dalian 116024,China

bEnvironmental Corrosion Center,Institute of Metal Research,Chinese Academy of Sciences,62 Wencui Road,Shenyang 110016,China

Through performing the tensile tests with different strain rates in 3.5 wt.%NaCl solution,the stress corrosion cracking(SCC)behavior and the effect of strain rate on the SCC susceptibility of an extruded Mg-7%Gd-5%Y-1%Nd-0.5%Zr(EW75)alloy have been investigated.Results demonstrate that the alloy is susceptible to SCC when the strain rate is lower than 5 × 10-6s-1.At the strain rate of 1 × 10-6s-1,the SCC susceptibility index(ISCC)is 0.96 and the elongation-to-failure(εf)is only 0.11%.Fractography indicates that the brittle quasi-cleavage feature is very obvious and become more pronounced with decreasing the strain rate.Further analysis conf i rms that the cracking mode is predominantly transgranular,but the partial intergranular cracking at some localized area can also occur.Meanwhile,it seems that the crack propagation path is unrelated to the existing phase particles.

Magnesium alloy;Stress corrosion cracking;Strain rate;Pitting corrosion

1.Introduction

Due to the low density and high specif i c strength,magnesium alloys become the potential structural materials for the applications in the f i elds of the automotive,railway and aerospace[1-4].However,the low absolute strength and poor corrosion resistance greatly limited their industrial applications[5].Recently,researchers reported that the newly developed Mg-Gd-Y-Nd-Zr alloys have the superior mechanical property and could meet the mechanical requirements of industrial components [6,7]. After thermal-mechanical processing,the mechanical properties of Mg-Gd-Y-Nd-Zr alloy could be further improved[8-10].Zhang et al.reported that the yield strength and ultimate strength(UTS)of an extruded Mg-8Gd-4Y-Nd-Zr alloy could reach up to 357 and 423 MPa,respectively[11].Although the strength of the Mg-8Gd-4Y-Nd-Zr alloy can be comparable to that of some industrially used Al alloys,their corrosion behavior especially the stress corrosion cracking(SCC)resistance is still unknown and so far few relevant literature papers can be referred. Generally,the SCC is extremely dangerous and complicated in the real industries,which can cause sudden fracture and then lead to catastrophic accidents[12].For Mg alloys,their SCC susceptibility to the service environment is very strong and mainly inf l uenced by various factors such as alloying element, microstructure,environment,mechanical processing and heat treatment[12-16].In Mg-Gd-Y-Nd-Zr alloys,the main existing phases are Mg5Gd,Mg24Y5,Mg41Nd5and the α-Mg matrix[9,10,17,18].Previous work demonstrated that phasecomposition and grain size can greatly inf l uence the mechanical properties and static corrosion performance of the Mg-Gd-Y-Nd-Zr alloys[6,8-10].Li et al.reported that the dissolution of Mg5Gd phases and the ref i nement of grains can be benef i cial for the improvement of the strength and elongation of an as-cast Mg-5Y-5Gd-xNd-0.5Zr alloy[8].However,Zhang et al.reported that the smaller grain size and inhomogeneous grain structure can deteriorate the corrosion resistance of the as-extruded Mg-5Y-7Gd-1Nd-0.5Zr alloy [6,7].Meanwhile,the existing coarse second phase particles acted as strong cathodes and further accelerated the corrosion attack of the surrounding α-Mg matrix[6,7].Following this,it can be predicted that the phase composition and grain structure could affect the interaction between the mechanics and chemistry especially the SCC.

In this work,through investigating the microstructure and SCC susceptibility to the strain rate of an as-extruded Mg-7% Gd-5%Y-1%Nd-0.5%Zr alloy,the underlying failure mechanism of the SCC and their relationship with the existing phases and grain structure will be deeply discussed.

2.Experimental procedures

2.1.Material and microstructural analysis

The material used in the current investigation was an asextrudedMg-7%Gd-5%Y-1%Nd-0.5%Zralloy(inwt.%) with the thickness of 20 mm and deformation ratio of 20, which was prepared in the Magnesium Alloy Research Department of IMR,China.Samples cut from the extruded plate were ground with SiC paper up to 2000 grit,f i nely polished to a 1 μm f i nish with ethanol.Phase analysis was determined with a D/Max 2400 X-ray diffractometer(XRD) using monochromatic Cu Kαradiation (wavelength: 0.154056 nm),a step size of 0.02°and a scan rate for data acquisition of 4°/min.To reveal the start melting points associated with the melting of the existing phases,differential scanning calorimetry(DSC)with the temperature ranging from 200 to 650°C was carried out using a Setaram system at a heating rate of 10°C/min.The specimens for metallographic examination were etched with 4 ml of nitric acid and 96 ml of alcohol,and the average grain size was determined using the mean linear intercept method.Microstructures were observed by optical microscopy(OM)and scanning electron microscopy(SEM;XL30-FEG-ESEM)in conjunction with energy dispersive X-ray spectroscopy(EDS).

2.2.Slow strain rate tensile(SSRT)testing

The SCC behavior of the as-extruded Mg-7%Gd-5%Y-1% Nd-0.5%Zr alloy was investigated using the slow strain rate tensile(SSRT)method.Tensile samples with a gauge length of 25 mm,width of 3 mm and thickness of 3 mm were machined from the extruded sheets.The axial direction of the tensile specimens was parallel to the extrusion direction(ED)of the plate.The surfaces of the gauge sections were polished to a 1 μm f i nish and cleaned up using ethanol immediately before testing.During the SCC tests,samples were stretched at a range of strain rates(1 × 10-6s-1to 5 × 10-5s-1)in 3.5 wt.%NaCl solution or air,as illustrated in Fig.1.The gauge section of the specimen were immersed in 3.5 wt.% NaCl solution at room temperature in an environment cell, while the solution was circulated at a speed of about 167 ml/ min using a circulating pump.Strain was recorded by an axial extensometer attached to the specimen gauge length outside of the environment cell(Fig.1).To ensure the reliability of the measured data,at least three repeated measurements were carried out for each condition.After testing,the fracture surfaces were cleaned in a hot chromic acid bath consisting of 180 g/L CrO3[19],washed in distilled water and rinsed with acetone,and then observed using SEM with secondary electron imaging(SEI)mode.

3.Results

3.1.Microstructural characterization

Fig.2 shows XRD pattern of the as-extruded Mg-7%Gd-5%Y-1%Nd-0.5%Zr alloy,indicating that the main phases in the alloy are Mg5Gd,Mg24Y5,Mg41Nd5,Zr and α-Mg.DSC analysis demonstrates that two endothermic peaks occurred at about 551°C and 631°C can be determined,as shown in Fig.3.It is well-known that the endothermic peaks occurred during the heating process stand for the melting temperatures associated with the existing phases[20].In the previous work, Zhang et al.reported that in an as-cast Mg-7.09%Gd-4.56%Y-1.31%Nd-0.52%Zr alloy(in wt.%),the melting temperature of the Mg5Gd is 549.4°C,whereas the melting temperature of α-Mg matrix is 634.3°C[10].Therefore,it can be determined that the 551°C endothermic peak stands for the melting temperature of Mg5Gd phase and the 631°C endothermic peak is related to melting temperature of α-Mg matrix.No endothermic peaks were found for Mg24Y5,Mg41Nd5and Zr, which may be attributed to the relatively lower content of Mg24Y5and Mg41Nd5and the higher melting point of Zr.

Fig.1.Experimental system for SSRT test with circulated 3.5 wt.%NaCl solution or air at various strain rates.

Fig.2.XRD pattern of the as-extruded Mg-7%Gd-5%Y-1%Nd-0.5%Zr alloy.

Fig.4 shows the backscattered electron image of the asextruded Mg-7%Gd-5%Y-1%Nd-0.5%Zralloy and the existing phases were analyzed by EDS,as listed in Table 1.It reveals that the main particles dispersed in the alloy were Gdrich phases composed of Mg-Gd-Y-Nd(point A),the small amount of cubic-shaped phases were identif i ed as Y-rich phases composed of Mg-Gd-Y-Nd(point B).The etched microstructure of the alloy is shown in Fig.5,which consists of f i ne equiaxed grains with an average grain size of 14.5 μm. However,the grain size was quite inhomogeneous.Moreover, the broken phase particles were distributed parallel to the extrusion direction.

3.2.Mechanical evaluation

Fig.6 shows the tensile curves of the alloy tested at various strain rates in air and 3.5 wt.%NaCl solution.To compare the mechanical properties of the alloy at various strain rates,the 0.2%proof yield stress(σ0.2),ultimate tensile strength(UTS), elongation-to-failure(εf)and time-to-failure(tf)are listed in Table 2.It can be seen that the yield strength and UTS of the alloy at the strain rate of 1 × 10-6s-1in air are 305 and 432 MPa,respectively.At the same strain rate,the UTS is decreased to 274 MPa when immersed in 3.5 wt.%NaCl solution.However,the yield strength of the alloy can hardly be determined because the εfmeasured was only 0.11%in 3.5 wt.%NaCl solution.

Fig.3.DSC curve of the as-extruded Mg-7%Gd-5%Y-1%Nd-0.5%Zr alloy.

Fig.4.Backscattered electron image of the as-extruded Mg-7%Gd-5%Y-1% Nd-0.5%Zr alloy.

Table 1Chemical compositions of secondary phases labeled in Fig.4(wt.%).

Fig.5.The etched microstructure of the as-extruded Mg-7%Gd-5%Y-1%Nd-0.5%Zr alloy.

Additionally,the effect of strain rate on SCC susceptibility was also analyzed.It can be seen that the strength and εfof the alloy were improved,while thetfwas decreased as the strain rates increased in 3.5 wt.%NaCl solution,as shown in Table 2.At the strain rate of 5 × 10-5s-1,the strength of the alloy was even higher while the εfwas increased about 18 times as high as that measured at 1 × 10-6s-1,indicating that the alloy shows remarkable susceptibility to SCC in 3.5 wt.%NaCl solution.

Fig.6.Stress-strain curves of the as-extruded Mg-7%Gd-5%Y-1%Nd-0.5%Zr alloy at various strain rates in air and 3.5 wt.%NaCl solution.

Table 2Mechanical properties of as-extruded Mg-7%Gd-5%Y-1%Nd-0.5%Zr alloy tested at various strain rates in air and 3.5 wt.%NaCl solution.

Fig.7.SEM observations to the fractographs of specimen failed at various strain rates in air and 3.5 wt.%NaCl solution.Image a)is overall fracture surface of specimen in air at strain rate of 1 × 10-6s-1;Images c),e)and g)are overall fracture surfaces of specimens in 3.5 wt.%NaCl at strain rates of 1 × 10-6s-1, 5 × 10-6s-1and 5 × 10-5s-1,respectively;Images b),d),f)and h)are high-magnif i cation observation of the squared area in images a),c),e)and g),respectively.

The fracture surfaces of tensile specimens are shown in Fig.7.The fracture surface of the specimen failed in air at the strain rate of 1 × 10-6s-1exhibited brittle quasi-cleavage feature(Fig.7(a)and(b)).On the other hand,the fracture surface of the specimen tested in 3.5 wt.%NaCl solution at the same strain rate is almost completely composed of brittle cleavage facets(Fig.7(c)and(d)).From the high-magnif i cation observation to the fracture surface,it can be seen that some cracks and local dissolution sites exist on the fracture surface(Fig.7(d)).Moreover,the cleavage features become less obvious as the strain rate increased and even transform into quasi-cleavage feature at the strain rate of 5 × 10-5s-1(Fig.7(c)-(h)).In addition,the cracks and local dissolution sites can hardly be observed in the high strain rate cases in 3.5 wt.%NaCl solution(Fig.7(f)and (h)).

Fig.8 shows the overall longitudinal surfaces of specimens tested at various strain rates in 3.5 wt.%NaCl solution.It reveals that for the specimen tested at the strain rate of 1 × 10-6s-1,the surface is severely corroded with a large amount of localized pits and secondary cracks existing away from the actual fracture surface(Fig.8(a)and(b)).As for the specimen tested at the strain rate of 5 × 10-6s-1,the corrosion attack is relatively weak with shallow localized pits and secondary cracks on the gauge surface(Fig.8(c)and(d)). When the strain rate is increased to 5 × 10-5s-1,the secondary cracks can hardly be observed and the localized pits were only existing at some particular sites on the surface (Fig.8(e)and(f)).

In order to investigate the stress corrosion cracking mechanism,the fracture surface which is perpendicular to the longitudinalaxialofthespecimen atthestrain rateof 1 × 10-6s-1in 3.5 wt.%NaCl is etched for metallographic observation,as shown in Fig.9.It reveals that the cracks mainly initiate from the localized pits on the surface of the gauge section.Meanwhile,the crack propagation mode is dominated by transgranular cracking and partial intergranular cracking.Additionally,the crack propagation path is unrelated to the existing second phase particles.

Fig.8.SEM observations to the corroded surfaces of tensile samples immersed in 3.5 wt.%NaCl.Images a),c)and e)are overall surfaces of specimens at strain rates of 1 × 10-6s-1,5 × 10-6s-1and 5 × 10-5s-1,respectively;Images b),d)and f)are high-magnif i cation observation of the squared area in images a),c)and e),respectively.

Fig.9.The etched microstructure observation to the fracture surface of specimen failed at the strain rate of 1 × 10-6s-1in 3.5 wt.%NaCl.

4.Discussion

4.1.SCC mechanism of the extruded EW75 alloy

To evaluate the SCC susceptibility of the alloy,the SCC susceptibility index(ISCC)was calculated by the loss in elongation[16]:

whereEsolutionandEairare εfin 3.5 wt.%NaCl solution and air,respectively.When the value ofISCCapproaches unity,it is assumed that the alloy is highly susceptible to SCC.Based on the equation(1),the calculatedISCCwas 0.96 at the strain rate of 1 × 10-6s-1,suggesting the investigated alloy was very susceptible to SCC in 3.5 wt.%NaCl solution.The main reason for its high SCC susceptibility will be discussed as follows:

Generally,the existed second phases can cause great impact on the SCC behaviour of magnesium alloy[21-23].In the investigated alloy,the main existing phases are Mg5Gd, Mg24Y5,Mg41Nd5and Zr.Among them,Zr usually reacts with deleterious impurities(i.e.Fe and Ni)to form Zr-rich particles and the Zr-rich particles can exhibit the most positive Volta potential with respect to the matrix[24].Thus,a strong microgalvanic corrosion can occur due to potential difference between the Zr-rich particles and the matrix.Apart from the Zrrich particles,other second phases such as Gd-rich and Y-rich particles can also act as the galvanic cathode to accelerate corrosion because they are much nobler than the Mg matrix [6,7,24].Therefore,localized pits could easily form during the tensile test in 3.5 wt.%NaCl solution(Fig.8(a)and(b)). Kannan et al.reported that hydrogen could diffuse into the Mg matrix through corrosion pits and then cause the hydrogen embrittlement,which signif i cantly decreased the mechanical strength of Mg alloys[15].In fact,the magnesium dissolution/ passivation is an anodic reaction and accompanies with a hydrogen reduction reaction[15,25].Thus,the hydrogen embrittlement due to the hydrogen diffusion can easily occur when Mg alloys are exposed to the aqueous corrosive environment[15].In this study,the micro crack initiation at the pits(Fig.9)further demonstrates that the occurrence of the hydrogen embrittlement.

Stampella et al.reported that grain size could inf l uence the cracking mode of the SCC[26].For the f i ne-grained Mg alloys (～30 μm),the SCC cracking mode is transgranular,whereas the cracking mode is the mixed transgranular and intergranular for the corase-grained Mg alloys(＞60 μm)[23].Moreover,it was demonstrated that for Mg alloys,the propagation of intergranular cracks could be accelerated by the electrochemical corrosion of the continuously distributed grain boundary precipitates[23].In this study,the grain size of the alloy is just 14.5 μm and phase particles are scarcely distributed at grain boundaries(Fig.5).Then,the cracking mode is predominantly transgranular and the partial intergranular cracking can only occur at some localized area(Fig.9). Additionally,the crack propagation route of the alloy seems unrelated to the existing phase particles,which might be ascribed to their homogenous distribution.

4.2.Effect of strain rate on the SCC susceptibility

Previous studies demonstrated that the inf l uence of strain rate on the SCC susceptibility of magnesium alloys can be summarized as follows:1)at a higher strain rate,the propensity for the inert fracture mechanism to overwhelm the SCC fracture mechanism;2)at a lower strain rate,the balance between repassivation and mechanical f i lm rupture at the crack tip[22,27,28].In the current investigation,the SCC susceptibility increases with decreasing the strain rate and the high SCC susceptibility is associated with hydrogen embrittlement. Therefore,it should exist a close relationship between the strain rate and hydrogen embrittlement susceptibility of the alloy.It is well known that the SCC susceptibility is associated with both corrosion and mechanical effect,which is prone to be inf l uenced by strain rates[22,23,29,30].At low strain rates (≤1 × 10-6s-1),the corrosion effect is predominant and thetfvalue is 8.55 h.Then,the time is enough for the diffusion of the generated hydrogen from reduction reaction into the matrix through the localized pits,leading to the occurrence of the hydrogen embrittlement fracture.On the contrary,thetfis only 0.35 h at a high strain rate(5 × 10-5s-1).Then,the time is too short for hydrogen diffusion and thus the mechanical effect was much greater than the corrosion effect,leading to a brittle quasi-cleavage failure much resemble the fracture features that tested in air(Fig.7(b)and(h)).Based on the discussion above, it can be concluded that the as-extruded Mg-7%Gd-5%Y-1% Nd-0.5%Zr alloy is very susceptible to SCC in 3.5 wt.%NaCl solution when the strain rate is below 1 × 10-6s-1.

5.Conclusions

(1)The as-extruded Mg-7%Gd-5%Y-1%Nd-0.5%Zr alloy is very susceptible to SCC in 3.5 wt.%NaCl solution when the strain rate is lower than 1 × 10-6s-1.

(2)The fractograph of the alloy exhibits brittle quasi-cleavage feature,and the cleavage feature becomes more pronounced as the strain rate decreasing.

(3)The crack propagation is dominated by transgranular cracking and partial intergranular cracking,however,itspropagating path is unrelated to the existing second phases.

Acknowledgments

This work was supported by the National Natural Science Foundation of China projects under Grant Nos.51171192, 51271183 and 51301172,the National Basic Research Program of China(973 Program)project under Grant No. 2013CB632205 and the Innovation Fund of Institute of Metal Research(IMR),Chinese Academy of Sciences(CAS).

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Received 19 September 2014;revised 17 November 2014;accepted 18 November 2014 Available online 8 December 2014

*Corresponding author.Environmental Corrosion Center,Institute of Metal Research,Chinese Academy of Sciences,62 Wencui Road,Shenyang 110016, China.Tel.:+86 24 23915897;fax:+86 24 23894149.

E-mail address:dkxu@imr.ac.cn(D.K.Xu).

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

http://dx.doi.org/10.1016/j.jma.2014.11.004.

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.