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16corrosion fatigue

Corrosion fatigue of extruded magnesium alloys

Ya.Unigo v ski,A.Eliezer,E.Abramo v ,Y.Snir,E.M.Gutman *

Department of Materials Engineering,Ben-Gurion Uni v ersity of the Nege v ,P.O.Box 653,Beer-She v a 84105,Israel

Recei v ed 3January 2003;recei v ed in re v ised form 8May 2003

Abstract

Corrosion fatigue tests were carried out on extruded AZ31(3%Al,1%Zn,0.3%Mn,Mg *the rest),AM50(5%Al,0.4%Mn,Mg *the rest)and ZK60(5%Zn,0.5%Zr,Mg *the rest)Mg alloys in air,NaCl-based and borate solutions.N sol /N air ratios (the relati v e fatigue life)were used for the analysis of the corrosion fatigue beha v ior of Mg alloys in v arious en v ironments,where N sol and N air are the numbers of cycles to failure in the solution and in air,respecti v ely.Extruded ZK60alloy re v eals v ery high fatigue and corrosion fatigue properties in comparison with other alloys.Howe v er,it has the lowest relati v e fatigue life (N sol /N air ?10(3á10(2)or the highest sensiti v ity to the action of NaCl-based solutions in comparison with that of AM50and AZ31alloys (N sol /N air ?10(2á10(1).Under the same stress,the corrosion fatigue life of extruded alloys is significantly longer than that of die-cast alloys (N sol for extruded AM50in NaCl is two to three times longer than that of die-cast AM50).#2003Elsevier B.V.All rights reserved.

Keywords:Corrosion fatigue;Magnesium alloys;Fitting equations;Relati v e fatigue life

1.Introduction

Magnesium alloys are among the best lightweight structural materials with a relati v ely high-strength-to-weight ratio and excellent technological properties.Therefore,magnesium attracts special attention of researchers working in automoti v e and aircraft industry.Wrought magnesium alloys show excellent mechanical properties and v ery good surface finish of a profile.For instance,die-cast and extruded AM50alloys ha v e ultimate tensile strength (UTS)of 230and 290MPa,tensile yield strength (TYS 0.2%)of 125and 180MPa and elongation-to-fracture of 12and 18%,respecti v ely (the specimen axis coincides with the extrusion direction).The principal drawback of magnesium as a structural material is its high chemical acti v ity leading in many cases to a low corrosion resistance.Since many mechanically loaded parts are often subjected to pro-longed cyclic stresses in an acti v e medium;it is of significant scientific and practical interest to study corrosion fatigue of extruded Mg alloys,such as ZK60,AM50and AZ31.

The fatigue life of magnesium alloys in such corrosi v e solutions as,for example,NaCl,is always less than that in air [1á6].The degradation in fatigue strength for high-strength extruded AZ80or die-cast AZ91D alloys (8.590.5%A1,1%Zn,Mg *the rest)due to NaCl is more pronounced than that of lower-strength AZ31[1]and AM20-AM40(2á4%Al,0.4%Mn,Mg *the rest)[2]alloys,apparently,due to a higher percentage of the second phase (Mg 17Al 12)in AZ80or AZ91D alloys [1,2,7].

The relati v e fatigue life in corrosi v e solutions can be represented as N sol /N air ratios,which v ary in the same en v ironment within a broad inter v al depending on the processing and test conditions.For example,for mag-nesium alloys containing 8á9%Al,these N sol /N air ratios v ary from 0.01to about 0.9á1in (3.5á5)%NaCl solutions [1á6].

As the corrosion fatigue data for magnesium alloys fall within a wider scatter band,the present paper is de v oted to the correlation analysis of corrosion fatigue beha v ior of extruded Mg áAl áMn,Mg áAl áZn and Mg áZn áZr alloys in NaCl and Na 2B 4O 7-based solu-tions,and to the in v estigation of fatigue fracture mechanisms of these alloys depending on their composi-tion,solution acidity,en v ironment and microstructure.

*Corresponding author.Tel./fax:'972-8-646-1478.

E-mail address:gutman@bgumail.bgu.ac.il (E.M.Gutman).

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www.else v https://www.sodocs.net/doc/4d8817303.html,/locate/msea

2.Experimental

Extruded hour-glass shaped specimens (the minimum gauge diameter and length of 8.0and 49mm,respec-ti v ely,ASTM E466-82)were cut from extruded rods of AZ31,AM50and ZK60alloys by final turnery (rough-ness R c below 3.2m m).Rods 10mm in diameter were produced from raw material 30mm in diameter (Rotem Industries Ltd,Israel).Before the extrusion process,raw pieces were heated during 15min at the working temperatures of 3208C (ZK60),3508C (AZ31)and 3758C (AM50).Extrusion rate was equal to 6mm s (1(ZK60,AM50)and 10mm s (1(AZ31).Chemical composition and standard mechanical properties of these alloys are gi v en in Tables 1and 2.

Fatigue tests were performed on a rotating beam type (R 0(1)fatigue machine (Satec System,Inc.,USA)equipped with a special electrolytic cell at 258C and at the frequency of about 30Hz.Specimens were pre-liminarily rinsed with distilled water and wiped with acetone.The compositions of electrolytes were as follows: 3.5%NaCl (pH :5), 3.5%NaCl saturated with magnesium hydroxide Mg(OH)2(pH 10.5),0.1N Na 2B 4O 7buffer solution (pH 9.3)and 0.1N Na 2B 4O 7saturated with magnesium hydroxide (pH 9.3).The buffer solution was used because of the anodic dissolu-tion of magnesium and instability of pH in other electrolytes.All solutions were prepared from analytical grade chemicals and distilled water.The solutions were carried o v er from the v essel to the gauge of a specimen with a capillary,so that the gauge surface was co v ered with electrolyte layer during the entire test period.The electrolyte thickness on the specimen surface was not controlled,but it was approximately constant and indirectly controlled by the flow rate of the solution.The latter was equal to 591cm 3min (1.All tests were carried out under open circuit conditions,and the authors did not monitor the corrosion potential of the specimens.

Immersion corrosion tests were carried out in a 2000ml glass v essel during 72h both on extruded alloys and on die-cast AM50and AZ91D alloys.For these tests,samples (6mm in diameter and ?20mm long)were cut out from a reduced portion of a tensile specimen.Before tests fresh surfaces of the cutting were co v ered with an epoxy layer.Cleaning after the test implied the remo v al of corrosion products from the surface of the specimen

by dissolution in 20%CrO 3solution at 808C during 0.5á1min.

Microstructure studies were carried out using an optical microscope ‘Nicon’and a scanning electron microscope JEOL JSM-5600with an energy-dispersi v e spectroscope ‘NORON’.

3.Results and discussion

Fatigue life of all the alloys in air was significantly longer than in NaCl-containing solutions (Figs.1á3).In air,the longest fatigue life is obser v ed for ZK60alloy and the shortest one *for extruded AM50and AZ31alloys;extruded AM50has a somewhat lower fatigue life than that of AZ31under low stresses (Fig.1a).An extruded ZK60alloy shows v ery high fatigue properties in comparison with other alloys both in air and in 3.5%NaCl 'Mg (OH)2saturated solution (Fig.1).For example,in air and in this solution,fatigue fracture of ZK60corresponding to N 0105cycles was obser v ed under the applied stresses of 215and 190MPa against 180and 150MPa for AM50and 180and 165MPa for AZ31alloys,respecti v ely.

Under the same applied stresses,extruded AM50shows a significantly longer fatigue life both in air and in NaCl-containing solutions as compared to the earlier data for die-cast alloy [6]due to a significantly lower porosity and higher standard mechanical properties (Table 2).In air,the same fatigue life of N 0105cycles was obser v ed in die-cast and extruded AM50alloys under the applied stresses of 157MPa [6]and 180MPa,respecti v ely.In 3.5%NaCl,these v alues were equal to 137MPa [6]and 150MPa (Fig.1).Under the same stress of 140MPa,the lifetime of die-cast and extruded AM50alloys was equal to 1.8)105and 2.7)106cycles in air,and 9.8)104and 2.6)105cycles in 3.5%NaCl,respecti v ely.

Table 1

Chemical composition (in mass percent)of alloys,Mg *the rest Alloy Al Mn Zn Zr Ni

Cu Fe Be ZK600.040.008 5.300.440.00030.00210.03670.0001B AM50 4.380.360.001á0.00080.00050.01550.0008AZ31

2.84

0.28

0.96

á0.0007

0.0017

0.0111

0.0001B Table 2

Tensile properties of extruded Mg alloys Properties

ZK60AM50AZ31UTS,MPa 336291273TYP 0.2%,MPa

227179155Elongation-to-fracture,%

16.6

18.1

19.4

Ya.Unigo v ski et al./Materials and Engineering A360(2003)132á139133

In 3.5%NaCl solution,AM50and AZ31alloys ha v e practically the same fatigue life.Saturation of 3.5%NaCl solution with magnesium hydroxide leads to a marked increase in the corrosion fatigue life of extruded AZ31,but almost does not affect its v alue for AM50alloy (Figs.1and 2).It is known that the acidity of 3.5%NaCl solution decreases with the addition of magnesium hydroxide (pH increases from 590.5in 3.5%NaCl to 10.5for 3.5%NaCl 'Mg(OH)2saturated solution).In this range of pH,as found earlier [8],the rate of stress-corrosion cracking of AZ61alloy (Mg á6.5%Al á1%Zn)in NaCl áKCrO 4solution remains stable and begins to decrease rapidly only at pH exceeding 12.On the other hand,it is known that in the buffer borate solution comprising 1%NaCl,an increase in pH from 6.5to 9.0leads to a decrease in mass losses of AZ31alloy during 10h (corrosion rate)of immersion tests from 3.0to 0.5g dm (2[9].

An increase in the fatigue life of AZ31with the addition of magnesium hydroxide in 3.5%NaCl (Fig.2b)is connected,probably,with growth of pH from ?5to 10.5and a strengthening of the passi v ating layer on the alloy surface.Saturation of the buffer solution with magnesium hydroxide practically does not affect the corrosion fatigue beha v ior of extruded AZ31alloy due to the same v alues of pH (pH 9.3)of these solutions (Fig.3).Thus,the impro v ement of corrosion fatigue

resistance of AZ31in 3.5%NaCl with such an addition of magnesium hydroxide is understandable,but its absence in AM50alloy under the same conditions calls for further studies.

Under stresses abo v e 160MPa,the fatigue life of AZ31in 3.5%NaCl was shorter than in the

buffer

Fig.1.s áN cur v es for extruded ZK60,AM50and AZ31alloys in air (a)and in 3.5%NaCl solution saturated with Mg(OH)2

(b).

Fig.2.s áN cur v es for extruded AM50(a)and AZ31(b)alloys in 3.5%NaCl solution (k )and in 3.5%NaCl solution saturated with Mg(OH)2(m

Fig.3.s áN cur v es for extruded AZ31alloy in air (1),3.5%NaCl (2),0.1N Na 2B 4O 7(3)and 0.1N Na 2B 4O 7solution saturated with magnesium hydroxide (4)(‘TB’and ‘MH’in the legend are abbre v ia-tions for sodium tetraborate and magnesium hydroxide ).

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134

solution;howe v er,at a lower applied stress,0.1N Na 2B 4O 7buffer solution and the same solution satu-rated with magnesium hydroxide represent more ag-gressi v e corrosion media in comparison with sodium chloride (Fig.3).Thus,under cyclic loading of extruded AZ31,the borate buffer solution represents,like NaCl,a strong corrosi v e en v ironment for this alloy.

It is of interest that for cyclic-stressed die-cast alloys AZ91and AM50,the buffer solution represents a non-corrosi v e medium [6].For instance,the relati v e fatigue life N sol /N air of die-cast AM50alloy in 0.1N Na 2B 4O 7buffer solution was equal to ?1.2.This is connected with predominant cooling and inhibiting actions of the buffer solution in comparison to other factors influen-cing the lifetime in this medium [6].Howe v er,as for extruded alloys,the cooling factor,probably,does not affect so strongly their corrosion fatigue life in the borate solution due to a lower relati v e applied stress (s /TYS )in comparison with cast alloys,and,respecti v ely,a lower energy dissipating during plastic deformation of samples.For example,with an applied stress v arying in the inter v al from 140to 180MPa,s /TYS v alues v ary in the range from 1.1to 1.4for die-cast AM50and from 0.8to ?1for the same extruded alloy.Besides,extruded alloys are more sensiti v e to a corrosion attack under cyclic stresses in comparison with die-cast alloys,probably,due to a higher lattice deformation and higher residual stresses.Therefore,degradation of fatigue properties of extruded AZ31was obser v ed in the buffer solution as compared to that in air.

The corrosion rate of Mg áAl áMn áZn alloys (AZ91D,AZ31and AM50)in 3.5%NaCl is significantly higher in the extruded state than that of die-cast alloys (Fig.4).For example,the corrosion rate of extruded AM50is about four times higher than that of die-cast AM50alloy.This is connected with a relati v ely high plastic deformation of extruded alloys during their production.Such residual deformation leads to strain hardening and to an increase in the chemical potential of metal atoms and their mechanochemical dissolution

[10].It is v ery well known that the corrosion rate of a material can accelerate significantly after the prelimin-ary plastic deformation.For example,mechanoelectro-chemical measurements and immersion tests demonstrate a 7-fold and 1.5-fold increase in the dissolution rate of die-cast AZ91D (9%Al)and AM50(5%Al)alloys,respecti v ely,with the plastic strain growing from 0to 4%[11].An increase in the dissolu-tion rate of pre-strained die-cast alloys with growing Al content obtained earlier [11]is similar to the results obtained for ‘as-extruded’ZK60,AZ31and AM50alloys (Fig.4).The aluminum content growth in extruded alloys subjected to a significant preliminary plastic deformation promotes an increase in the amount of the secondary phases,strain hardening and,thus,increases chemical potential of metal atoms and their mechanochemical dissolution [10,11].

On the other hand,a relati v ely high corrosion resistance of non-deformed die-cast alloys is connected with their protection by the 0.2á0.8mm thick dense surface layer (‘skin’)ha v ing a finer microstructure and a significantly higher amount of b -phase Mg 17Al 12and aluminum in the matrix in comparison with the bulk of a specimen [6,11].Thus,without loading or preliminary plastic deformation,die-cast alloys are more stable to corrosion attack in comparison with extruded alloys.Under cyclic bending loading,the ‘skin’undergoes pitting and cracking and does not protects a more porous bulk from corrosion attack.

Extruded alloys show excellent mechanical properties as compared to die-cast alloys (see Section 1and Table 2)and,in spite of a higher sensiti v ity of extruded alloys to the action of corrosi v e solutions,their corrosion fatigue life is significantly longer than that of die-cast alloys.

Typical microstructure of ZK60,AM50and AZ31alloys is presented in the optical micrographs made in longitudinal and trans v ersal sections of extruded rods (Fig.5).The microstructure is characterized by the presence of extended deformation bands of the solid solutions Mg áAl in AM50and AZ31or Mg áZn in ZK60and second phase stringers (Fig.5).In AM50and AZ31alloys,the second phases are presented by b -phase (Mg 17Al 12),Al m Mn n and Mn 3Mg 2.ZK60alloy contains v ery small Zn 2Zr precipitates,many of them being below 0.5á1m m.

As a result of stress-corrosion attack during fatigue tests,pits and cracks are obser v ed on the machined (Fig.6)and fracture surfaces of the alloys (Fig.7).In Fig.7,the fracture surface of extruded AZ31alloy after a corrosion fatigue test in 3.5%NaCl in the crack origin area and in the area of dimpled fracture is presented.One can see a faceted appearance of fatigue fracture at the fatigue crack origin with pitting.A fatigue crack is initiated from a corrosion pit 1(Fig.7a).In that case,the pit grows to the critical size,at which the

stress

Fig.4.Corrosion rate of die-cast and extruded Mg alloys according to immersion tests in 3.5%NaCl (‘as-cast’and ‘as-extruded’conditions).

Ya.Unigo v ski et al./Materials and Engineering A360(2003)132á139135

intensity factor reaches the threshold v alue for fatigue cracking [12].During fatigue tests,the solution pene-trates into a crack,and this leads to the corrosion pit appearance in the crack origin area (Fig.7a and b).The correlation analysis of the corrosion fatigue beha v ior of extruded alloys at high stresses were carried out using Basquin’s equation Eq.(1)[13]and a simpler Eq.(2):N s p 0C

or lg N 0(p lg s 'C

(1)

lg N 0A s 'B (2)

where N is the number of cycles to fracture;s is the maximum nominal applied stress;p ,C ,A and B are coefficients.In this study,correlation coefficients r in Eq.(2)were equal,as a rule,to 0.80á0.97,except the fatigue data for AZ31and ZK60in air,where r 00.70á0.71(Table 3).Basquin’s equation gi v es approximately the same correlation coefficients [6].Both equations allow us to estimate,for example,N sol /N air

ratio

Fig.5.Optical micrographs of ZK60(a),AM50(b)and AZ31(c)alloys made in longitudinal (on the left)and trans v erse (on the right)sections of the extruded pin.

Ya.Unigo v ski et al./Materials and Engineering A360(2003)132á139

136

depending on the applied stress,if their coefficients are known.Since Eq.(2)gi v es relati v ely high correlation coefficients,we ha v e used it for the analysis of the corrosion fatigue beha v ior of Mg alloys in different en v

ironments.

Fig.6.Pitting and cracking on the machined side surface of extruded AM50.(3.5%NaCl,160

MPa).

Fig.7.Fracture surface in crack origin area with pitting (a,b,c)and in the area of dimpled fracture (d)in a sample of AZ31alloy after a fatigue test in 3.5%NaCl.The direction of the crack propagation represented by the arrows;1and 2(a)are pits on the surface of a specimen and on the fracture surface,respecti v ely;a cubic particle in the center of the micrograph (c)is Mn 3Mg 2.

Table 3

Coef?cients of the ?tting Eq.(2)describing the fatigue beha v ior of extruded magnesium alloys at high stresses;‘r ’is a correlation coef?cient Alloy

En v ironment

AM50Air (0.0378711.730910.97

3.5%NaCl (0.026119.062800.873.5%NaCl 'Mg(OH)2saturated (0.021768.294010.96AZ31

Air (0.0750118.287410.713.5%NaCl (0.0399210.774940.783.5%NaCl 'Mg(OH)2saturated (0.0383811.319240.89ZK60

Air (0.0710920.254120.703.5%NaCl 'Mg(OH)2saturated (0.0324211.162110.95

Ya.Unigo v ski et al./Materials and Engineering A360(2003)132á139137

This analysis was carried out using fitted N sol /N air ratios calculated using coefficients A and B (Table 3,Fig.8).Fig.8also includes our earlier data for die-cast AM50and AZ91D alloys [6].Extruded alloys show a significantly higher sensiti v ity to the action of 3.5%NaCl solution in comparison with die-cast alloys.For instance,N sol /N air ratios corresponding to an applied stress change in the range from 120to 180MPa v ary in the range of ?10(2á10(1for extruded AM50and AZ31alloys and 1.2á1.6for die-cast AM50and AZ91D alloys ([6],Fig.8).An increase in N sol /N air ratios in extruded alloys with the growth of applied stress presented in Fig.8can be explained by a shorter time of the corrosion attack under a higher stress.

In contrast to our data,according to Ref.[1],extruded AZ31and AZ80alloys ha v e almost the same fatigue life both in air and in NaCl solutions (N sol /N air ratio close to 1).Apparently,it is connected with a decrease in the sensiti v ity to the action of a corrosi v e medium of electrolytically polished specimens in com-parison with lathe-formed specimens in our experi-ments.

Among extruded alloys,the highest sensiti v ity to the action of NaCl-based solutions was obser v ed in ZK60(N sol /N air ?10(3á10(2)and the lowest one *in AM50and AZ31alloys (N sol /N air ?10(2á10(1)(Fig.8).Very high stress-corrosion sensiti v ity of ZK60is connected with the presence of a significant content of v ery small precipitates Zn 2Zr (as a rule,below 0.5m m)in Mg áZn solid solution.Such intermetallides promote strain hardening and,thus,increase chemical potential of metal atoms and their mechanochemical dissolution [10].

In spite of a higher sensiti v ity of extruded alloys to the action of NaCl solution,their fatigue life in this medium is significantly longer than that of die-cast alloys.For

instance,fatigue life of extruded AM50in NaCl is two to three times longer than that of die-cast AM50alloy under the same applied stress [6].

4.Conclusions

(1)Extruded alloys show a significantly longer fatigue life both in air and in NaCl-containing solutions in comparison with die-cast alloys.For instance,under the stress of 140MPa,the lifetime of die-cast and extruded AM50alloys was equal to 1.8)105and 2.7)106cycles in air,and 9.8)104and 2.6)105cycles in 3.5%NaCl,respecti v ely.

(2)An extruded ZK60alloy shows v ery high fatigue and corrosion fatigue properties in comparison with other alloys.For example,in air and in 3.5%NaCl saturated with Mg(OH)2,fatigue fracture corresponding to N 0105cycles was obser v ed under the applied stresses of 215and 190MPa in ZK60,180and 150MPa in AM50,180and 165MPa in AZ31alloys,respecti v ely.

(3)Buffer borate solution represents a corrosi v e medium that decreases the fatigue life of extruded AZ31alloy in contrast to die-cast alloys.Saturation of the borate buffer solution with magnesium hydroxide practically does not affect the corrosion fatigue beha v ior of extruded AZ31alloy;howe v er,saturation of 3.5%NaCl solution with magnesium hydroxide leads to a marked increase in the corrosion fatigue life of this alloy.

(4)The dissolution rate of ‘as-extruded’ZK60,AZ31and AM50alloys in 3.5%NaCl increases with growing Al content.This fact agrees with the corrosion beha v ior of pre-strained die-cast alloys AM50and AZ91D.

(5)Extruded alloys show a significantly higher sensiti v ity to the action of 3.5%NaCl solution in comparison with die-cast alloys.Howe v er,the fatigue life of extruded alloys in NaCl-based solutions was significantly longer than that of die-cast alloys.The highest sensiti v ity to the action of NaCl-based solutions was obser v ed in ZK60(N sol /N air ?10(3á10(2),and the lowest one *in AM50and AZ31alloys (N sol /N air ?10(2á10(1).

Acknowledgements

The present work was sponsored by the Consortium of Magnesium Technologies De v elopment (the Israeli Ministry of Industry and Trade).The authors are grateful to B.Edelstein,H.Uziel,G.Backner and G.Ben-Hamu (Ben-Gurion Uni v ersity of the Nege v )for their participation in fatigue and mechanochemical tests,O.Nabuto v ski (Ben-Gurion Uni v ersity of the Nege v )for his kind assistance in SEM,to Dr E.

Aghion

Fig.8.Effect of the applied stress on the relati v e fatigue life (N sol /N air ratios)of die-cast (1á3)[6]and extruded (4á7)Mg alloys in 0.1N Na 2B 4O 7(1,2), 3.5%NaCl (3,6)and in 3.5%NaCl 'Mg(OH)2saturated (4,5,7).(1,3)AZ91D;(2,4)AM50;(5,6)AZ31,(7)ZK60.

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138

and his staff(R&D Department of the Dead Sea Magnesium Ltd)for great help in our experiments,to Z.Koren and H.Rosenson(Israel Institute of Metals, Technion,Haifa)and to A.Ben-Arzty(Rotem Indus-tries Ltd)for supplying us with die-cast and extruded alloys.

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