Yifeng Li,Yushn Zhou,Zhiming Shi,Jeffrey Venezuel,Akif Soltn,Andrej Atrens,?

aSchool of Mechanical and Mining Engineering,The University of Queensland,St Lucia,Queensland 4072,Australia

bUniversity of Science and Technology Beijing,30 Xueyuan Road,Haidian District,Beijing 100083,China

Abstract This paper studied the stress corrosion cracking(SCC)of EV31A in 0.1M Na2SO4saturated with Mg(OH)2using linearly increasing stress tests,compared with pure Mg and WE43B.All three materials were susceptible to SCC.SCC susceptibility increased with decreasing applied stress rate.The threshold stress was 0.3× (yield stress)for pure Mg,0.6× (yield stress)for EV31A,and 0.8×(yield stress)for WE43B.The SCC velocities at an applied stress rate of 7.3 ×10-4MPa s-1were 7.2×10-8m s-1for pure Mg,5.6×10-9m s-1for WE43B,and 1.5×10-9m s-1for EV31A.

Keywords:SCC(stress corrosion cracking);Magnesium alloys;LIST(linearly increasing stress test).

1.Introduction

Magnesium(Mg)alloys are attracting increasing attention because of their good combination of properties:castability,low density,and high strength to weight ratio[1].These properties are important for industries such as automobile,aerospace,and electronics[2].However,Mg is also one of the most reactive industrial metals and has poor corrosion resistance[3-9].Furthermore,stress corrosion cracking(SCC)could be a cause for engineering failures of structures or components[10].

Winzer et al.[3]in their review of the stress corrosion cracking of Mg alloys indicated that(i)Mg SCC could occur in distilled water,so there is no need for a particular aggressive environment,and that(ii)the threshold stress for stress corrosion crack initiation could be as low as 0.5 x yield stress.

The linearly increasing stress test for stress corrosion cracking research was introduced by Atrens et al.[11].A main difference to the constant extension rate test(CERT)is that LIST is load controlled,whereas CERT is displacement controlled.The CERT and LIST are essentially similar up to the threshold stress.Thereafter,the LIST takes less time to the end of the test.In principle both tests can measure the threshold stress and the SCC velocity.

Song et al.[10]found that AZ31 was susceptible to SCC in distilled water,ASTM D1387 solution,0.001M NaCl,and 0.1M NaCl.They also concluded that hydrogen embrittlement was the SCC mechanism;i.e.that hydrogen decreased the cohesive strength between the lattice planes,and that the applied stress increased the hydrogen adsorption from the environment into the alloy.

Winzer et al.[12]found that a decreasing applied stress rate was associated with an increasing SCC susceptibility for Mg-Al alloys AZ91,AZ31,and AM30.AZ91 had a lower SCC threshold than AZ31 and AM30,attributed to(1)H trapping by α-particles at the tip of the primary crack,and(2)the α-particles fractured at a low H concentration.Allthe alloys showed susceptibility to SCC in distilled water.AM30 had a stress corrosion cracking velocity much lower than AZ91 and AZ31.

Fig.1.Schematic of the LIST specimen,with dimensions in mm.

Fig.2.The schematic of the apparatus for the linearly increasing stress tests(LISTs).

Shi et al.[13]found that the threshold stress for SCC of ultra-high-purity Mg5Zn in distilled water was about 70%of the yield stress.

Padekar et al.[14]found that EV31A had a higher SCC threshold than AZ91E in 0.1M NaCl saturated with Mg(OH)2,but AZ91E had a lower stage two stress corrosion crack velocity.Shi et al.[15]found that stress corrosion cracks for AZ31 in distilled water initiated at the specimen surface,and fracture occurred when the crack length became sufficiently long.The threshold stress for SCC was～70%of the yield stress.

Argade et al.[16]found that AZ31 with an ultra fine grain microstructure showed serious SCC susceptibility,which was higher than the SCC susceptibility of AZ31 with a larger grain size.Analysis of the fracture surface indicated that local dissolution was the SCC initiation mechanism for the largegrained AZ31,and that hydrogen embrittlement was the SCC mechanism for the fine-grained AZ31.

Bobby Kannan et al.[17]found that the rare-earth containing Mg alloys ZE41,QE22 and EV31A were susceptible to SCC in 0.5wt.%NaCl solution,with a higher susceptibility than in distilled water.EV31A had the lowest SCC susceptibility.AZ80,ZE41 and QE22 produced transgranular stress corrosion cracks in distilled water,whereas the three rare-earth Mg alloys produced intergranular stress corrosion cracks in 0.5wt.%NaCl.

Fig.3.Potential drop data for pure Mg.

Table 2 Strength and fracture parameters derived from the LISTs for pure Mg and the Mg alloys WE43B and EV31A tested in air and in solution(0.1M Na2SO4 saturated with Mg(OH)2).

Cao et al.[18]studied the SCC of the solution heat-treated as-cast binary Mg alloys:Mg0.1Sr,Mg0.1Zr,Mg5Sn,Mg 0.3Ca,Mg1Mn,and Mg0.3Si.Mg0.3Si and Mg5Sn showed no evidence of SCC,even at the lowest applied stress rate of 7.0×10-4MPa s-1,whereas Mg5Zn and Mg0.3Ca showed the most serious SCC,with transgranular SCC.Hydrogen enhanced decohesion caused the SCC of Mg1Mn,Mg0.3Si,Mg0.1Zr,and Mg0.1Sr,whereas hydrogen enhanced localized plasticity caused the SCC of Mg5Zn,and the SCC of Mg0.3Ca was caused by both.Cao et al.[19]studied the SCC of hot rolled binary Mg alloys:all the above plus Mg0.7La Mg0.9Ce,Mg0.6Nd,Mg6Al,and Mg5Gd.All alloys showed better mechanical properties than the solution heat treated alloys except for Mg1Mn in air.Beside Mg1Mn and Mg0.7La,all other alloys did not show susceptibility to SCC in water.There was no difference in the fracture surfaces of the specimens tested in air and in distilled water.Hot rolling increased the SCC resistance.

Fig.4.Potential drop data for WE43.

Chen et al.[20]found that strontium(Sr)increased the SCC of Mg alloy ZK40xSr.The increased SCC susceptibility was attributed to micro-galvanic corrosion between the precipitates on the grain boundaries and the alpha-Mg.

Thus in summary,many Mg alloys are susceptible to SCC,and SCC can occur at relatively low stresses.Thus,it is important to understand the SCC behaviour for the engineering application of a particular Mg alloy.EV31A is a relatively new Mg alloy,so that there is considerable interest in its corrosion and SCC properties.The prior research has shown that EV31A has some SCC susceptibility in chloride solutions.The current work studied the SCC of EV31 in 0.1M Na2SO4saturated with Mg(OH)2,with pure Mg and WE43 included in the study for comparison.

2.Experimental methods

2.1.Materials,specimens and solution

The specimens of pure Mg and the two Mg alloys WE43 and EV31A were extracted from castings.Pure Mg was evaluated in the as-cast condition whereas WE43 and EV31A had been heat treated to the T6 condition,which is their typical service condition.WE43B was solution treated for 8 hrs at 525°C and precipitation heat treated for 16 hrs at 250°C.EV31A was solution heat treated for 8 hrs at 520°C and precipitation heat treated for 16 hrs at 200°C.The chemical compositions are given in Table 1.Specimens for LISTs were machined to the specifications shown in Fig.1.

All solutions were made with reagent grade chemicals and distilled water.

2.2.Linearly increasing stress tests(LIST)

Fig.2 provides a schematic of the apparatus for the linearly increasing stress test(LIST),(Atrens et al.[11],Winzer et al.[21]).

The lever beam principle is used in the LIST machine.There is a moveable weight on one side of the lever beam.The specimen is connected to the other.The stress on the specimen increases as the weight is moved away from the rest state,at a constant rate,driven by a synchronous motor.Two motor speeds(30rph and 3rph)were used,which gave applied stress rates of 7.3×10-3MPa s-1and 7.3×10-4MPa s-1,respectively.The displacement transducer,and servocontroller keep the lever beam horizontal[11].

Fig.5.Potential drop data for EV31A.

During each experiment,a constant 3 A direct current flowed through the specimen,so that the potential drop of the specimen could be recorded to determine the yield stress(in air),σy,or threshold stress for SCC initiation(in solution),σthand fracture stress,σf.

The size of the stress corrosion cracks was measured on the fracture surface using scanning electron microscopy.The stress corrosion cracking velocity,V,was calculated using

where h represents the depth of the deepest stress corrosion crack on the fracture surface,and t is the time taken for LIST between the threshold stress and fracture stress.

3.Results

Fig.3 shows typical potential drop data for LISTs of pure Mg in air and in the solution(0.1M Na2SO4saturated with Mg(OH)2)at the indicated applied stress rates.The potential drop data for pure Mg in air can be interpreted as indicating an initial slow linear increase in potential drop(i.e.specimen resistance)up to the yield stress,indicated as σy1,where after there was a much greater rate of increase in specimen resistance due to the greater decrease in section due to the plastic deformation.This is as per our prior research(e.g.Shi et al.[15]).For the tests in solution,the potential drop had a similar shape:a slow linear increase up to the threshold stress for stress corrosion crack initiation,indicated by σy2,and σy3,where after the potential drop increased much more rapidly due to the growth of the stress corrosion cracks.The interpretation that σy2and σy3corresponded to the threshold stress for stress corrosion crack initiation,was because both values were much lower than the yield stress in air,σy1,and SCC was verified by the fractography,as described below.Table 2 records the values of the yield stress in air,threshold stress in solution,and the fracture stress values.

Similarly,Fig.4 presents the potential drop data for WE43B,and Fig.5 presents the potential drop data for EV31A.For both Mg alloys,just like for pure Mg,the data indicated the threshold stress for SCC was less than the yield stress,and that the threshold stress for SCC decreased with decreasing applied stress rate.Furthermore,for pure Mg and for the two Mg alloys,the fracture stress in the solution was less than the fracture stress in air,and for the two Mg alloys the fracture stress decreased with decreasing applied stress rate.

The yield strength and fracture stress values of the pure Mg were much lower than those of the two Mg alloys,attributed to the large grain size of the pure Mg,and to the strengthening by heat treatment of the two Mg alloys.

Fig.6.(a)Optical view of the side of the pure Mg LIST specimen tested in air showing fracture at 45°to the tensile axis(horizontal),(b)optical view of the side of the pure Mg LIST specimen tested in solution(0.1M Na2SO4saturated with Mg(OH)2)at an applied stress rate of 7.3×10-4MPa s-1showing stress corrosion cracks perpendicular to the tensile axis(horizontal),(c)and(d)SEM views of the fracture surface after LIST in solution at the slow applied stress rate;the stress corrosion crack is marked in red.(For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)

3.2.Fractography

Fractography was carried out for the specimens subject to LISTs in air and in the solution(0.1M Na2SO4saturated with Mg(OH)2)at an applied stress rate of 7.3×10-4MPa s-1.The fracture features are documented in Figs.6-8.

在語(yǔ)言學(xué)大師索緒爾的眼中，符號(hào)是由 “能指”（signifier）與“所指”（signified）構(gòu)成的二元實(shí)體。[8]商標(biāo)犯罪侵犯的是社會(huì)主義市場(chǎng)經(jīng)濟(jì)秩序。細(xì)言之，以商業(yè)利用的方式而假冒他人的注冊(cè)商標(biāo)應(yīng)用在自己的商品或者服務(wù)上，造成消費(fèi)者對(duì)相關(guān)商品或者服務(wù)的混淆，從而無法清晰分辨各個(gè)商標(biāo)所施指的商品或服務(wù)，致使商標(biāo)指示市場(chǎng)的混亂。

Fig.6(a)shows the surface of the pure Mg LIST specimen after the test in air.The fracture occurred at 45°to the tensile direction which is horizontal in Fig.6(a).In contrast,Fig.6(b)shows that the stress corrosion cracks are horizontal for the LIST in solution.Fig.6(c)and(d)illustrates a top view of the fracture surface.The stress corrosion crack is indicated in the red colour.The crack length was used to calculate the stress corrosion crackling velocity,and the values are presented in Table 2.

Figs.7 and 8 indicate similarly that there was stress corrosion cracking in the LISTs for WE43B and EV31A in the solution(0.1M NaCl saturated with Mg(OH)2)at an applied stress rate of 7.3×10-4MPa s-1.The measured crack sizes were used for the calculations of the stress corrosion crack velocity in Table 2.

4.Discussion

Analysis of the potential drop data provided values of the yield stress,the threshold stress,and the ultimate tensile stress.For all the three materials evaluated in this study,there was stress corrosion cracking in the solution(0.1M Na2SO4saturated with Mg(OH)2)at both applied stress rates.Pure magnesium was the weakest material in this study,and SCC caused the greatest decrease from the yield stress to the initiation stress,and caused large surface cracks as shown in Fig.6(b).The reason for that is attributed to the large grain size and low strength of the as-cast pure Mg.

Fig.7.(a)Optical view of the side of the WE43B LIST specimen tested in air showing fractures at 45°to the tensile axis(horizontal),(b)optical view of the side of the WE43B LIST specimen tested in solution(0.1M Na2SO4saturated with Mg(OH)2)at an applied stress rate of 7.3×10-4MPa s-1showing stress corrosion cracks perpendicular to the tensile axis(horizontal),(c)and(d)SEM views of the fracture surface after LIST in solution at the slow applied stress rate.The stress corrosion cracking is marked in red.(For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)

WE43 showed higher resistance to stress corrosion cracking,in that the threshold stress was much higher and the stress corrosion crack velocity was significantly lower.EV31A had a threshold stress for SCC lower than that of WE43B,and the stress corrosion cracking velocity was considerably lower.

The values of the threshold stress for stress corrosion cracking decreased with decreasing applied stress rate for the pure Mg and the two Mg alloys.

The fractography indicated relatively brittle fracture,consistent with a hydrogen fracture mechanism.

5.Conclusions

1.All three materials(pure Mg,WE43B,EV31A)were susceptible to stress corrosion cracking in 0.1M Na2SO4saturated with Mg(OH)2.

2.SCC susceptibility increased with decreasing applied stress rate.

3.The threshold stress for stress corrosion cracking in 0.1M Na2SO4saturated with Mg(OH)2was 0.3×(yield stress)for pure Mg,0.6×(yield stress)for EV31A,and 0.8×(yield stress)for WE43B.

Fig.8.(a)Optical view of the side of the EV31A LIST specimen tested in air showing fractures at 45°to the tensile axis(horizontal),(b)optical view of the side of the EV31A LIST specimen tested in solution(0.1M Na2SO4saturated with Mg(OH)2)at an applied stress rate of 7.3×10-4MPa s-1showing stress corrosion cracks perpendicular to the tensile axis(horizontal),(c)SEM view of(b)(the tensile direction is vertical),and(d)SEM views of the fracture surface after LIST in solution at the slow applied stress rate,the stress corrosion crack is marked in red.(For interpretation of the references to colour in this figure legend,the reader is referred to the web version of this article.)

4.The stress corrosion cracking velocities in 0.1M Na2SO4saturated with Mg(OH)2at an applied stress rate of 7.3×10-4MPa s-1were 7.2×10-8m s-1for pure Mg,5.6×10-9m s-1for WE43B,and 1.5×10-9m s-1for EV31A.

Acknowledgements

YL wishes to thank The University of Queensland,the University of Science and Technology Beijing,and China Scholarship Council for their support during his study abroad year.

Data statement

The data for this study are all included within this paper.