High cycle fatigue behavior of the forged Mg-7Gd-5Y-1Nd-0.5Zr alloy

,*,E.H.Hn

aThe Group of Magnesium Alloys and Their Applications,Institute of Metal Research,Chinese Academy of Sciences,62 Wencui Road,Shenyang 110016,China

bShenyang Aerospace University,Shenyang 110036,China

High cycle fatigue behavior of the forged Mg-7Gd-5Y-1Nd-0.5Zr alloy

D.Wua,S.Q.Lia,M.Hongb,R.S.Chena,*,E.H.Hana,W.Kea

aThe Group of Magnesium Alloys and Their Applications,Institute of Metal Research,Chinese Academy of Sciences,62 Wencui Road,Shenyang 110016,China

bShenyang Aerospace University,Shenyang 110036,China

This paper investigated the high cycle fatigue behavior of a forged Mg-7Gd-5Y-1Nd-0.5Zr alloy with different stress concentration factor (Kt),under different stress ratio(R),and along different loading direction.The smooth specimen(Kt=1),under R=0.1 and along longitude direction,shows a high fatigue strength of 162 MPa at 107cycles.The fatigue behavior of the forged Mg-7Gd-5Y-1Nd-0.5Zr alloy exhibits a high sensitive to the notch.Moreover,change of stress ratio from 0.1 to-1 may also result in a bad fatigue property.The f l ux inclusions were elongated along longitude direction and/or transverse direction during the forging process of the Mg-7Gd-5Y-1Nd-0.5Zr alloy.The interface between the f l ux inclusion and the matrix may debond and serve as the crack initiation site during the fatigue loading process,leading to the deterioration of the fatigue property along thickness direction and a high anisotropic fatigue behavior between longitude direction and thickness direction.

Magnesium alloy;Forge;High cycle fatigue;Stress ratio;Inclusion

1.Introduction

Low density,high specif i c strength and stiffness make Mg alloys very attractive as structural materials in applications of aircraft,space ship and ground transport,where weight saving is of great importance[1,2].Among them,Mg alloys containing rare earth elements(RE)have received considerable interest in recent years due to their potential for achieving higher strength and better creep resistance at elevated temperatures[3].The most successful commercial Mg-RE alloys have been those based on the Mg-Y-Nd system,such as WE54 and WE43 alloy.Recently,it has been reported that the new-developed Mg-Gd-Y system alloys showed considerable precipitation hardening,and therefore present higher strength at both room and elevated temperatures and better creep resistance than WE54 alloy,and even conventional Al alloys.For instance,the ultimate tensile strength(UTS),yield strength(YS),and elongation of the extruded Mg-7Gd-5Y-1Nd-0.5Zr alloy can reach 415 MPa,340 MPa,and 10%,respectively[4],which is suitable to serve as load bearing parts applied in the future automobiles[5].

As far as structural loading parts are concerned,the fatigue property of the alloys is instrumental in the design of safe application.However,up to now,very few of researches were focused on the fatigue behavior of the wrought Mg-7Gd-5Y-1Nd-0.5Zr alloy.In this paper,the high cycle fatigue behavior of a forged Mg-7Gd-5Y-1Nd-0.5Zr alloy was investigated systematically.The effects of notch,stress ratio,loading direction and inclusion on the fatigue strength,fatigue life and failure mechanism were discussed.

Table 1Analyzed chemical compositions of the investigated alloy(wt.%).

2.Experimental procedures

The forged Mg alloy denoted as GWN751 were examined in the present study,and its nominal composition was Mg-7Gd-5Y-1Nd-0.5Zr(wt.%).Theactualchemical compositions were determined using the inductively coupled plasma(ICP)technique,and the results are listed in Table 1.

For the high cycle fatigue test,two kinds of specimens, notched specimen and smooth specimen,were used,with the stress concentration factor(Kt)of 3 and 1,respectively.The dimensions of the high cycle fatigue specimens are presented in Fig.1,in accordance with the Aviation Industry Standard of China(HB 5287-96)[6].The axial direction of the fatigue specimen is parallel either to the longitude direction(LD)or to the normal direction(ND,i.e.the thickness direction),of the forged GWN751 alloy.Fatigue tests were conducted on the PLG-100C high-frequency fatigue test machine,under load control of pull-pull or pull-push way,with the stress ratio of R=0.1 or R=-1 and a resonance frequency of 120 kHz. These tests were performed at room temperature and in air, cycling up to 107cycles except for failure.

For microstructure observations,samples were cut from the forged GWN751 alloy and etched by a solution of 4 vol.% HNO3in ethanol after mechanical polishing to reveal grain boundaries.The average grain sizes(d)were determined by analyzing the optical micrographs with the mean linear intercept method,whered=1.74L;andLis the linear intercept length.The phases were analyzed by a scanning electron microscope(SEM,Philips XL30 ESEM-FEG/EDAX)equipped with an energy-dispersive X-ray(EDX)spectroscopy analysis system.Texture analysis of the forged sample in LDTD(TD:transverse direction)plane was performed using the Schultz ref l ection method by X-ray diffraction.Calculated pole f i gures were obtained with the DIFFRACplusTEXEVAl software,using the measured incomplete{0002},{10-10} and{10-11}pole f i gures.

Tensile specimens with 25 mm in gauge length and 5 mm in gauge diameter were machined from the forged GWN751 alloy.The tensile tests were carried out along the LD and ND, respectively,at room temperature with an initial strain rate of 1 × 10-3s-1.

3.Results and discussion

3.1.Microstructure and mechanical properties

Typical microstructures of the forged GWN751 alloy are shown in Fig.2.As we can see in Fig.2a,the forged GWN751 alloy has equiaxed grain structures with an average grain size of ～32 μm,which means that complete recrystallization occurred during the forging process.The BSD microstructure, got by SEM in Fig.2b,revealed the morphology and distribution of the second phase in the matrix.Some coarse cuboidshaped phases distribute along the LD,and present the f l ow line.As previously reported[7],the cuboid-shaped phase may be the Mg5(Gd,Y),undissolved and coarsening during the solution treatment.Moreover,a large amount of f i ne particles smaller than 5 μm homogeneously distribute in the matrix. Similar results appeared in the Ref.[8];many f i ne particles dynamic precipitated during the multi-axial forging process of GWN751 alloy,and were identif i ed as b phase with facecentered cubic crystal structure(a=2.22 nm).

Fig.3 presents the(0002)and(10 10)plane pole f i gures of the forged GWN751 alloy on LD-TD plane.It shows quite a weak texture with the maximum intensity of 2.28 m.r.d.(multiples of random distribution),which is much lower than that of the rolled Mg alloy sheet containing mass RE element(usually higher than 10 m.r.d.)[9].Concerning on the(0002)plane pole f i gure,it contains many comparatively high intensity zones with the peak intensity higher than 2 m.r.d..These high intensity zones can be divided into two parts,i.e.the basal texture component with the(0002)planes parallel to the LD-TD plane, and the non-basal texture component with the(0002)planes perpendicular to the LD-TD plane of the forged GWN751alloy.The low texture intensity and the multi-peak distribution type reveal that the forged GWN751 alloy should have a low anisotropy of mechanical properties.

Fig.1.The dimensions and digital picture of the high cycle fatigue specimens:(a,c)notched specimen(Kt=3),(b,d)smooth specimen(Kt=1).

Fig.2.The microstructure of the forged GWN751 alloy on LD-ND plane with LD parallel to the scale bar.

The room-temperature tensile properties of the forged GWN751 alloy were summarized in Table 2.As comparison, the relevant data of the extruded[4]and multi-forged GWN751 alloy[10]were also listed.The LD and ND of the forged GWN751 alloy exhibit the same yield strength(YS)of 239 MPa,corresponding to the texture analysis results.The ultimate tensile strength(UTS)and elongation-to-failure(EL) of the LD are a little higher than that of ND.Due to the weak texture,the strength of the present forged GWN751 alloy is much lower than that of the extruded GWN751 alloy;the YS of the later one can reach 340 MPa.It is worth noting that,the same as forged alloy,the multi-forged GWN751 alloy in Ref.[10]has a similar YS to the present alloy,yet a higher UTS and much higher elongation than the present alloy.The elongation of the present forged GWN751 alloy is only about 1%,indicating a typical brittle fracture.Some inclusions can be observed in the fracture surface,which may lead to the premature failure,the low elongation,and low UTS.

3.2.High cycle fatigue behavior

Fig.3.(0002)and(1010)pole fi gures of the forged GWN751 alloy on LD-TD plane.

Stress-life fatigue data of the forged GWN751 alloy with different stress concentration factor(Kt),under different stress ratio(R)and tensile direction are shown in Fig.4.Fig.4a is the fatigue test results with different Kt,under R=-1,and along LD.As for the notched specimen(Kt=3),fatigue failures can be observed over most of the testing stress amplitudes and the S-N curve appears to have a continuous decreasing trend,with a horizontal asymptote at cycles of higher than 106.The fatigue strength σ-1at 107cycles is calculated using the staircase method[11]to be 56 MPa. Comparatively,the fatigue strength at certain cycles of the smooth specimen(Kt=1)is obviously higher than that of the notched one.It means that the fatigue behavior of the forged GWN751 alloy is very sensitive to the notch.In the case of fatigue with a high number of cycles(107cycles),the fatigue strength can be comparable to the lowest stress to the initiation of a crack.The effect of a notch in a loaded structural element is to intensity the value of the nominal stress near the notch. Localized stress can lead to the local plastic strain.In other words,when the structure is stressed with an imposed load,the regions of localized plastic deformation are subjected to imposed local strains[12].As a consequence,the notched specimen demonstrates a better simulation of the effects of stress concentrations,and exhibits a lower stress to the initiation of a crack and a lower fatigue strength.

Fig.4b shows the S-N curves of the forged GWN751 alloy tested under different R,with Kt=1,and along LD.It is clearly that the specimen cycled under R=0.1 exhibits ahigher fatigue strength than that of the one cycled under R=-1.The fatigue strength σ-1at 107cycles is calculated using the staircase method to be 162 MPa.As the stress ratio R changing from-1 to 0.1,the fatigue loading method changed from tensile-compressive cycle to tensile-tensile cycle,and the stress amplitude during one cycle reduced from 2σ to 0.9σ. According to the physical model of Laird[13],slip occurred at crack tip during the tensile loading process,resulting in its blunting,while the cracking surfaces were compressed together during the succedent unloading or compressive loading process,leading to the crack tip sharpening.Therefore,for the specimen tested under R=0.1,with a weak crack tip sharpening effect,it has a low fatigue crack propagation rate and presents a high fatigue strength.

Table 2Room-temperature tensile properties of the forged GWN751 alloy(YS,yield stress;UTS,ultimate tensile stress;EL,elongation-to-failure).

The fatigue test results along different direction are presented in Fig.4c.Although the forged GWN751 alloy does not show a strong texture,it exhibits an obvious anisotropy of fatigue behavior.As tested along ND,many specimens failed from internal defects.The distribution of the data points is extremely divergent.A normal S-N curve and fatigue strength cannot be obtained.Therefore,the internal defects play a key role on the deterioration of the fatigue property along ND,yet it has a weak impact on the high cycle fatigue behavior along LD.The further analysis will be discussed later.

3.3.Fractography

Fig.5 shows a typical fracture surface of the forged GWN751 alloy fatigue failed with life of 7.69 × 106cycles, under R=0.1,and with Kt=1.It is found that the overall fracture surface can be divided into three regions:crack initiation region(region 1),steady crack propagation region (region 2),and tearing region(region 3).In region 1,the fatigue crack initiated at surface,lots of cleavage planes and tear ridges can be found,and the cleavage planes are in the order of grain size.It indicates that the fatigue failure originated from local shearing near the specimen surfaces.The local shearing can easily occur along the weakest cleavage plane of a grain, and then localized damage from persistent slip bands was formed.In the fatigue crack propagation region,the coarse primary cleavage planes were replaced by the f i ne secondary cleavage planes,and some dimples can be found,which means the crack mode transited from brittle to ductile.In region 3,the fracture surface has an extremely rough appearance with clear evidence of over load failure mechanisms.

Fig.4.Stress-life curves of the forged GWN751 alloy with different stress concentration factor(Kt),under different stress ratio(R)and tensile direction:(a) R=-1,Kt=3 or 1,along LD,(b)R=0.1 or-1,Kt=1,along LD,(c)R=-1,Kt=1,along LD or ND.

Fig.5.(a)Overall fracture surface of the forged GWN751 alloy under R=0.1,with Kt=1,and along LD;and the high magnif i cation SEM images of(b)region 1 and(c)region 2 indicated in the image(a).

General speaking,the castings demonstrate easy fatigue crack initiation due to the presence of casting f l aws such as porosity and inclusion at or below the casting surface[14]. The porosity may be welded together and disappear during the thermal-mechanical process,while the inclusion still exists in the matrix,and continue to affect on the mechanical and fatigue properties of the wrought material.A typical fatigue fracture surface with inclusion is show in Fig.6.EDX results indicate that the inclusion is very likely the f l ux inclusion, containing high content of element Cl.Traditional f l ux ref i ning is always accepted as one of the most effective purifying methods during the smelting process of Mg alloy.However,MgCl2in the f l ux is prone to react with the rare earth elements Gd and Y,producing new f l ux inclusions[15]. Lankford and Kusenberger[16]summarized a series of stages occurring in fatigue crack initiation at inclusions.The initial stage is the debonding of the inclusion from matrix.In the crack propagation region around the f l ux inclusion(region 2 in Fig.6a and d),the fracture surfaces mainly consist of cleavage planes,presenting a brittle cleavage fracture mode.In the steady crack propagation region(region 3 in Fig.6a and e), there appears mixed-rupture characteristics of many tear ridges,some cleavage step,and minute tough dimple.

Fig.6.(a)Overall fracture surface of the forged GWN751 alloy under R=-1,with Kt=1,and along ND;and the high magnif i cation SEM images of(b)region 1, (d)region 2 and(e)region 3 indicated in the image(a);and(c)corresponding energy dispersive X-ray spectra of region A indicated in the image(b).

During the thermal-mechanical process,the inclusion can be elongated along the LD,such as the rolling direction(RD) of rolled sheet,then results in an anisotropy of mechanical and fatigue properties.Temmel et al.[17]reported the fatigue limit in the transverse direction(TD)was roughly 50%lower than that in the RD for 42CrMo4 steel sheet.For the present GWN751 forging,the f l ux inclusions were elongated along LD and/or TD,as indicated in Fig.6a.In comparison with loading along LD,the loading force is mainly applied perpendicular to the interface between the inclusion and the matrix,as loading along ND,leading to a high proportion of debonding of the inclusion from matrix.Therefore,a large amount of specimens tested along ND failed from the f l ux inclusion,and exhibit an extremely divergent distribution of fatigue data and bad fatigue property.

4.Conclusions

In this paper,the high cycle fatigue behavior of the forged GWN751 alloy was investigated,and the notch,stress ratio, loading direction and inclusion show great inf l uence on its fatigue strength and life.The major conclusions are summarized as follows:

(1)The fatigue behavior of the forged GWN751 alloy exhibits a high sensitive to the notch.The fatigue strength σ-1of the notched specimen(Kt=3)at 107cycles is only 56 MPa,while that of the smooth one(Kt=1)may reach～100 MPa.

(2)The specimen cycled under R=0.1 exhibits a much higher fatigue strength than that of the one cycled under R=-1,due to the weak crack tip sharpening effect during the fatigue loading process.

(3)The f l ux inclusions were elongated along LD and/or TD during the forging process of the GWN751 alloy.The interface between the f l ux inclusion and the matrix may debond and serve as the crack initiation site during the fatigue loading process,resulting in the deterioration of the fatigue property along ND and a high anisotropic fatigue behavior between LD and ND.

Acknowledgments

This work was funded by the National Basic Research Program of China(973 Program)through project No. 2013CB632202,and National Natural Science Foundation of China(NSFC)through projects No.51105350 and No. 51301173,respectively.

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Received 18 September 2014;accepted 6 November 2014 Available online 5 December 2014

*Corresponding author.Tel.:+86 24 23926646;fax:+86 24 23894149.

E-mail address:rschen@imr.ac.cn(R.S.Chen).

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

http://dx.doi.org/10.1016/j.jma.2014.11.002.

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.