Materials and Design 59 (2014) 287–295

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Materials and Design

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Low cycle fatigue and mechanical properties of magnesium alloyMg–6Zn–1Y–0.6Ce–0.6Zr at different temperatures

http://dx.doi.org/10.1016/j.matdes.2014.03.0010261-3069/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Address: Faculty of Engineering, Vukovarska 58, 51000Rijeka, Croatia. Tel.: +385 51 651 496; fax: +385 51 651 490.

E-mail address: marko.canadija@riteh.hr (M. Cana -dija).

Marko Cana -dija a,⇑, Xuefeng Guo b, Domagoj Lanc a, Wenpeng Yang b, Josip Brnic a

a Department of Engineering Mechanics, Faculty of Engineering, University of Rijeka, Rijeka, Croatiab School of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo, Henan 454000, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 17 December 2013Accepted 2 March 2014Available online 12 March 2014

Keywords:MagnesiumLow cycle fatigueYoung’s modulusTensile strengthElevated temperatures

The paper deals with low cycle fatigue (LCF) and mechanical properties of Mg–6Zn–1Y–0.6Ce–0.6Zr alloyat both room and elevated temperatures. Fractural microstructures of the test specimens were also ana-lyzed. Based on the experimental results some guidelines about the application of the Mg alloy are pro-vided. A detailed review of existing literature on LCF of other Mg alloys is provided and comparison withappropriate results of other authors was made. Due to scarcity of results, special attention is given to thelow cycle fatigue properties at elevated temperatures. LCF results are assessed relative to the loadingdefined as a fraction of the ultimate tensile stress. With such criterion, it can be said that increase in tem-perature leads to the more favorable environment for low cycle fatigue of Mg alloy at hand.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

During the last decade, magnesium alloys are enjoying theincreasing attention of the research community. Such interest ismotivated with the low density, relatively high specific strength,stiffness and machinability. Beside obvious energy savings due tothe lower weight of structural components, environmental andbiological advantages are further boosted by the favorable recycla-bility procedures and suitability as biomaterials. In particular,main fields of the Mg applications are automotive industry, aero-space, medical, electronics, sports and non-structural applications.Automotive industry was perhaps the first industry where magne-sium alloys was applied as early as 1920s. Initially, the sole appli-cation was racing cars, but very soon magnesium found its wayinto transmission housing and crankshafts of commercial vehicles[1]. Other automotive steel parts also started to be replaced by thelight-weight counterparts made of Mg alloys. Nowadays, wheels(made from AM60B alloy), engine components (AS41A) and steer-ing columns, gearboxes, air bag housings, seat frames and fueltanks are frequently made of Mg alloys, mainly AZ91D. In recentyears, further applications of Mg alloys in automotive industryare also driven by the environmental issues. Lower structuralweight has more benefits beyond obvious fuel savings. Differentmass distribution can provide better maneuverability of the

vehicle, lesser vibrations and noise. Similarly to the automotiveindustry, weight reduction is even more critical in aerospace appli-cations. Magnesium can be exposed to extreme temperaturesoccurring in aerospace and to impact of hard objects and highenergy particles, with mechanical performance surpassing popularplastics applications by far. Typical examples are thrust reversersin airplanes, engines, helicopter transmission casings and inintercontinental ballistic missiles. Of especial interest for suchcomponents is ZE41A alloy due to favorable fatigue and creepproperties. Magnesium is also applied in medicine, dominantly inorthopedics. Although Mg corrosion is the issue in some cases, itcould be also a benefit, since an implant does not need to be surgi-cally removed but rather is dissolved by the natural corrosive pro-cesses. Sports like tennis, golf, archery and cycling, but also someother sports profit due to light-weightiness of magnesium; besideAZ91D alloy, components are frequently made from AM60B alloydue to increased ductility demands. Since vibrations dampingproperties of magnesium are about 10 times better than steel, alu-minum or titanium, bicycle frames are more comfortable to ride.Electronic industry dominantly uses magnesium alloys for portableelectronic housings like in cell phones, cameras, laptops etc. In thecase of creep resistance, AS41A and AS21 are preferred due to goodcreep resistance and strength at elevated temperatures. For appli-cations at elevated temperatures, rare-earth elements, like zirco-nium, thorium and yttrium are usually added. Typical alloys forelevated temperatures applications are EZ33A, HK31A, HK32A,ZE41A, AS21X1, AS41XB, QE22A, QH21A, WE43 and WE54A.Regarding non-structural applications, Mg is frequently used as

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the alloying element in non-ferrous metals, but also in ferrousmetals as a desulfurizer. The Mg–6Zn–1Y–0.6Ce–0.6Zr alloy wasdeveloped based on commercial alloy ZK60 which is widely usedas light structural material. However ZK60 has used rarely at ele-vated temperature conditions and has poor corrosion resistancein compare with AZ 91. With very small amount yttrium and cer-ium alloying, Mg–6Zn–1Y–0.6Ce–0.6Zr alloy has much bettermechanical properties at room and elevated temperatures, andhas very good corrosion resistance. Due to its mechanical and cor-rosion characteristics, the alloy prepared by rapid solidificationwith the same composition of Mg–6Zn–1Y–0.6Ce–0.6Zr has alsobeen used as biomaterial research in Germany recently and provedto be one of the best materials up to now [2].

Since structural components frequently operate under repeat-ing loading and at the elevated temperature, this research sets afocus on precisely these issues: for the Mg–6Zn–1Y–0.6Ce–0.6Zralloy, low cycle fatigue (LCF) features, variations of Young’s modu-lus and stress–strain behavior with respect to temperature are re-ported and discussed.

LCF of magnesium alloys at room temperature is frequently ad-dressed by the research community. For start, see [3–13]. Most ofthese experimental researches are conducted under fully reversedtension–compression cycles with prescribed maximum and mini-mum strain. The triangular wave form defines strain loading. How-ever, since magnesium alloys are especially sensitive to twinningdeformation, a vast amount of literature on the magnesium mate-rial properties deal with the twinning behavior. Due to the fact thatmagnesium single crystal has only two independent slip systems,the dominating mechanism of plastic deformation during com-pressive loading is twinning, while subsequent detwinning influ-ences tensile loading response. It appears that nowadays thetwinning effect is reasonably well understood, at least from thematerial science point of view [14].

If the material was plastically deformed during the manufactur-ing process, like in extrusion and rolling, it can be expected thattwinning effects will be more pronounced. In the case when com-pressive loading precedes tension loading, twinning deformationhas effective influence on the number of cycles till failure in thelow cycle fatigue regime. Shiozawa et al. [11] evaluated low-cyclefatigue of extruded Mg alloys and demonstrated that Coffin-Manson law can be used to successfully describe such behavior.Matsuzuki and Horibe [15] noticed in AZ31 Mg alloy that twinningmechanism dominates if higher plastic strains take place, while atlow levels of plastic strains dislocation slips are more important.Therefore, application of such manufacturing procedure inevitablyintroduces anisotropy in the material structure. An interestingresearch [16] tried to determine influence of the anisotropy onlow cycle fatigue, finding that specimen with the axis normal tothe rolling direction exhibited longer fatigue life than the oneswith rolling direction. Similar findings were obtained in [17] whereextruded magnesium alloys were investigated. In contrast to theseresults, [18] reports better resistance to fatigue in the rolling direc-tion for AZ31B alloy. Unlike wrought Mg alloys [6,19], in [10] thi-xomolded AM60B alloy showed symmetrical hysteresis loops intension and compression; such non-typical behavior for Mg alloyswas attributed to the presence of a large number of Mg17Al12 par-ticles and small grain sizes. LCF of squeeze casted AZ31 alloy gavesymmetrical hysteresis loops [7]. An innovative approach to char-acterizing HCF of Mg alloys was recently proposed in [20]. In short,authors used infrared thermography to predict the fatigue strengthof welded joints. The proposed methodology can be easily appliedin LCF testing.

A strange behavior due to twinning and detwinning is a prob-lem for constitutive modeling in solid mechanics and still needsfurther attention, especially if coupled mechanical and thermal ef-fects are considered. A recent and efficient contribution to the

numerical modeling of isothermal twinning in magnesium is givenin [21] and a successful attempt to simulate thermo-mechanical fa-tigue of Mg alloy is given in [22]. Continuum mechanics back-ground for modeling cyclic thermoplasticity of metals is providedin [23,24], just to name few examples.

However, literature review on the research dealing with LCFbehavior of Mg alloys at elevated temperatures shows surpris-ingly low number of papers. For example, Mayer et al. [25] findthat relative decrease of the cyclic strength closely follows reduc-tion of the ultimate strength with the temperature increase. Acontribution to the better understanding of the growth rate ofsmall fatigue cracks AM50 alloy at different temperatures is pro-vided in [26]. Wöhler curves for the creep resistant alloy AZ91 HPat 20 �C and 130 �C were discussed in [27]. A recent study on theLCF of an Mg alloy showed that addition of gadolinium, yttriumand zirconium gives favorable LCF behavior at elevated tempera-ture. Already mentioned paper [22] sets a focus on both develop-ment of determination of material parameters for Chaboche’s andNagode’s model based on own experimental research on AZ91 al-loy. The same alloy was researched in [28], obtaining results forLCF at room temperature and 130 �C. Cast Mg alloy with rareearth elements at elevated temperatures was investigated understrain-controlled LCF regime, at room temperature and 200 �C[29]. Larger fatigue life was noted in specimens tested at elevatedtemperatures. The paper also addresses thermally driven fatigue,a problem frequently occurring in engine components and trig-gered by start–stop events. Different LCF driving mechanisms atroom temperature and elevated temperatures are noticed in[30]. Consequently, authors’ impression is that research on LCFof Mg alloys at elevated temperatures is far from complete andfurther efforts should be undertaken. This gave impetus for thestudy at hand.

Different to other authors, where usually absolute stress orstrain levels are used to compare LCF performance, in this workupper and lower stress levels relative to the ultimate tensilestrength at corresponding temperature are used. In the design pro-cedure, allowable stresses at the particular temperature are calcu-lated regarding the yield stress or the ultimate tensile stress, sosuch a comparison is quite reasonable. Since the specimens wereloaded in tension during the tests and since the source of twinningis the compressive loading, such approach can be used to evaluatefatigue only due to slip mechanism. For example, stress amplitudevs. number of cycles curve reported in [4] for strain ratio 0.5 atroom temperature has different shape than the curves obtainedwith tension–compression cycles, indicating different underlyingdeformation mechanism in tension-only fatigue.

Mechanical behavior at elevated temperature is also character-ized by stress–strain curves. Such results about Mg alloys are notscarce as LCF behavior at elevated temperatures. Temperatureinfluence on stress–strain curves of AZ31B Mg alloy is given in[31] indicating expected decrease of the ultimate tensile strengthwith the temperature increase. Further researches with similar re-sults were undertaken in [32–39], just to name a few. In all citedpapers, twinning-specific shapes of stress–strain curves were notpresent or barely noticeable, making interpolation of flow curvesmore straightforward. In the particular Mg-alloy at hand, thiswas not the case so no attempt was taken to model stress–straincurve with a particular constitutive model.

Literature about Mg–6Zn–1Y–0.6Ce–0.6Zr alloy is still ratherrare. Guo and Shechtman [40] analyzed influence of rapid solidifi-cation and reciprocating extrusion on the microstructure and themechanical behavior of the alloy. Guo et al. [41] compared differ-ent manufacturing procedures and influence on the microstruc-ture. Superplastic behavior in temperature range 150–250 �C wasinvestigated in [42]. High cycle fatigue was analyzed in [43],reporting favorable fatigue behavior.

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Novel aspects of this paper closely follow motivation for this re-search. In particular, insufficient research data about the LCF alloysbehavior of Mg alloys at elevated temperatures and complete ab-sence of LCF data for the Mg alloy at hand, served as a principalmotivation for this research. In the line with that, the paper set afocus to the LCF behavior of Mg–6Zn–1Y–0.6Ce–0.6Zr alloy at bothroom and elevated temperature. Special effort was invested inmicrostructural analysis of fractured specimens. Contrary to theusual comparison of LCF behavior based on absolute stress levels,more appropriate criteria based on the relative stress levels wasproposed. The experiments carried out in this paper were stresscontrolled, what is not the case in other cited LCF tests at elevatedtemperature. While results of previously published strain con-trolled tests enable estimation of stress softening, the results pre-sented here enable determination of strain ratcheting. Althoughsome structural components operate under stress control andsome under strain control, most frequently met case is a compo-nent with stress concentrations like holes and notches. Such com-ponents are neither in stress nor strain control operation mode,indicating necessity to known both stress softening and strainratcheting. Therefore, the hereby presented results complementresults in strain-controlled tests. In addition, Young’s modulusand stress–strain curves at various temperatures were determinedand the results were analyzed. Interpolation equations for Young’smodulus and maximum tensile stresses were performed. Ratchet-ing strains for each test case were calculated. Consequently, a thor-ough research was carried out on the LCF and mechanical behaviorof Mg–6Zn–1Y–0.6Ce–0.6Zr alloy, carefully documented and dis-cussion about the results is provided for readers’ convenience.

α-Mg

Mg3Y

2Zn

3

(MgZn)12

Ce

/a.u

.

2. Experimental procedure

2.1. Alloy composition and tensile specimen preparation procedure

Alloy composition used in this research was determined by theEnergy-Dispersive X-ray Spectroscopy (EDS). Average compositionwas 6% Zn, 1% Y, 0.6% Ce, 0.6% Zr and Mg (balance). Cast ingotswere prepared by conventional casting from Mg–47%Y, Mg–90%Ce, Mg–30%Zr, and 99.9% pure Mg. Melts were poured into acopper mould with 52 mm diameter and 200 mm length to forma cast ingot. Obtained cast ingot was remelted at 750 �C to producerapidly solidified ribbons in a single-roller melt-spinning machine.Remelting procedure was carried out under low-pressure argonatmosphere. Obtained thickness of ribbons was below 80 lm. Sub-sequently, these ribbons were comminuted into smaller chips ofapproximate dimensions 2 mm � 2 mm � 0.08 mm.

Rapidly solidified chips were heated at 150 �C and then hot-compressed into 32 mm diameter cylindrical billet. The billetswere held in vacuum atmosphere for 24 h in order to remove gasesfrom the billet. Hot extrusion at 350 �C followed and cylindricalbars 10 mm in diameter were produced.

From the cylindrical bars tensile test specimens were manufac-tured, Fig. 1. Longitudinal axis of a specimen corresponds to theextrusion direction, i.e. cylindrical bar longitudinal axis.

Fig. 1. Schematic of dog-bone tensile specimen.

2.2. Experimental apparatus

The phase constitutions were identified by a RigakuD/max-3CX-ray diffraction (XRD) instrument with Cu Ka radiation, using astep size of 0.033�.

The microstructure of the alloy at hand was evaluated on aPhilips CM 30 transmission electron microscopy (TEM). Thespecimens for TEM analysis were twin-jet electron-polished toperforation in a solution of (11.2 g Mg(ClO4)2 + 5.3 g LiCl + 500 mlCH3OH + 100 ml CH2H5OH) at �30 �C. An ion miller was employedto remove the oxide film at an ion accelerating voltage of 4.0 keV.TEM observations were carried out using a JEM-3010 microscopyoperating at 300 kV.

Tensile tests and low cycle fatigue tests were performed atZwick/Roell Z400E universal testing machine equipped with Macroextensometer and temperature chamber BW91250.

The broken fracture and glide and flow between grains on spec-imen’s surface after tested at elevated temperatures were exam-ined by a Philips XL 30 scanning electron microscopy (SEM).

3. Results

3.1. Microstructure and phase constitutions

The XRD spectrum (Fig. 2) reveals that the material was mainlycomposed of a-Mg, Mg3Y2Zn3 (W phase) and (MgZn)12Ce phases.According to TEM study of rapidly solidified Mg–6Zn–1Y–1Ce alloy[44], it is reasonable to conclude that the material at hand also con-tains a small amount of Mg3YZn6 (icosahedron quasicrystal, Iphase) and Mg–Zn binary phase particles. TEM micrograph(Fig. 3) shows that the materials had fine equiaxed-grains withan average grain size of 1.0 lm, and high density strengtheningparticles with average size of 100 nm distributed within matrix.

3.2. Stress–Strain curves

Tensile tests at different temperatures were performed, Figs. 4and 5 and Table 1. Specimens were heated to the test temperatureand then held for 45 min to ensure thermal equilibrium. Duringthat period, strain variable change was monitored and after re-ported heating period strain did not change any more. The testspeed was set to 0.0033 1/s.

As anticipated, the maximum tensile stress decreases with thetemperature rise. An opposite trend is noticeable if elongationsare considered. Upon close inspection of stress–strain curves at

20 30 40 50 60 70 80

Inte

nsity

2θ /

Fig. 2. XRD spectrum of Mg–6Zn–1Y–0.6Ce–0.6Zr alloy.

Fig. 3. TEM micrograph of Mg–6Zn–1Y–0.6Ce–0.6Zr alloy.

Fig. 4. Stress vs. strain curves at different testing temperatures.

Fig. 5. Maximum tensile stress and elongation vs. temperature.

Table 1Maximum tensile stresses and elongations at different testing temperatures.

Temperature (�C) Maximum tensile stress, rM (MPa) Elongation

20 454.6 0.40100 326.1 0.61150 263.3 0.76200 100.4 1.54

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20 �C, 100 �C and 150 �C, a concave part is present for each curve.The concave part seems to diminish as temperature increase. Suchan effect can be attributed to the twinning in crystal lattice [16,45].As discussed in the Introduction section, Mg alloys are known to

exhibit asymmetry of flow curves in tension and compressive tests.As a consequence of twinning, the compressive tensile strength ismuch lower than in tension. As observed in [16], material formingprocesses that result in strongly directionally developed texture,like those applied in manufacturing of these specimens, favordevelopment of the plastic deformation by twinning during thecompressive loading. However, this is not the case during the ten-sile loading since the grain orientation does not favor twinning, sothe slip-type of deformation dominates in such plastic phenome-non. On account of the fact that the slipping process requires moreenergy than twinning, the tensile strength is higher than the com-pressive one. It should be emphasized that twinning is not limitedto the directional texture of wrought Mg alloys; it occurs in die castMg alloys as well [46]. If the specimen is compressive loaded thensubsequent unloading and tensile loading leads to increase instrain due to detwinning and generates a characteristic concavepart of the flow curve manifested in the tests in Fig. 4, observedas well in [4,16,17,47]. The effect is present especially in the direc-tion of extrusion while transverse to this direction such deforma-tion usually cannot be noticed [17]. Occurrence of detwinningthat follows compressive deformation leads to the pseudoelasticbehavior – nonlinearity of the unloading curve; see [48] for a sim-ilar discussion not dealing with Mg alloys. Effects of this kind canbe employed to at least partially explain dependence of Young’smodulus on the strain amplitude as documented in [4]. In this case,specimens were compressively loaded during extrusion, so ob-served effects during subsequent tension at 20 �C, 100 �C and150 �C perfectly fit within description of the plastic deformationdue to twinning. However, the tension test curve at 200 �C lacksthe concave part of the curve indicating that other deformationmechanism are dominating at elevated temperature; this is consis-tent with the notion that twinning usually occurs at room andaverage temperatures [14,49]. A similar observation was reportedrecently in research with temperatures till 125 �C [50] where twin-ning effects are becoming less pronounced as temperature in-crease. When single crystal magnesium is concerned [51], itseems that twinning is present at higher temperatures as well,although here the same trend of decreasing twinning with temper-ature rise is also present, especially after 150 �C. The effect wasquantified by Jain and Agnew [52], observing that the twin volumefraction decrease only slightly and linearly till 150 �C, with a sharpdrop by a factor of 2 after 150 �C till 200 �C. To summarize, resultspresented in Fig. 4 indicate that LCF behavior at 200 �C can be ex-pected to be different than LCF at lower temperatures. Since tensilestrength of a typical Mg alloy is in range 135–285 MPa, obtainedvalues, Table 1, show particularly high strength at room tempera-ture. Even at higher temperature (263.3 MPa at 150 �C), Mg–6Zn–1Y–0.6Ce–0.6Zr alloy exhibits tensile strength of equal magnitudelike average Mg alloy at room temperature, effectively broadeningthe field of application of the alloy. Comparison to the hot-rolledAZ31 [39] and AZ31B [38] alloys behavior at elevated tempera-tures, upon approximation reveals that tensile strength of Mg–6Zn–1Y–0.6Ce–0.6Zr at 200 �C is somewhat lower while shape ofcurves at the same temperature are similar.

Regarding the maximum elongation, the higher temperatureleads to the higher elongation. Therefore, anticipated behavior isobtained. It should be carefully noted that in the temperaturerange 20–150 �C linear variation of elongation was obtained, whileelongation at 200 �C deviates from the linear variation, Fig. 5. Thisresult is interesting since other observed behavior in this research,like the maximum tensile stress vs. temperature curve and duringlow cycle fatigue also seem to indicate different underlyingdeformation mechanics at this particular temperature. Similar con-clusions were drawn in [30]. In line with the exceptional tensilestrength, compared to most Mg alloys, elongation is also signifi-cantly above average values 2–10%, in this case 40% at room

Fig. 6. Variation of Young’s modulus with respect to temperature.

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temperature and reaching 154% at 200 �C. With respect to AZ31[39] and AZ31B [38] alloys, the Mg alloy at hand can sustain signif-icantly more strain at strain rate employed in this research.

Variation of the maximum tensile stress with respect to tem-perature can be approximated by a linear equation in the temper-ature range 20–150 �C:

rMðTÞ¼481:62�1:484T; Tð�CÞ; rMðMPaÞ; 20 �C<T<150 �C: ð1Þ

Like in the case of elongation, Fig. 5, results indicate that outsidethis range interpolation of higher order should be more appropri-ate. However, to obtain reliable approximation, more detailed re-search above 150 �C should be undertaken.

3.3. Young’s modulus

To gain an insight into the elastic behavior of Mg–6Zn–1Y–0.6Ce–0.6Zr alloy at different temperatures, Young’s moduluswas determined. Specimens were heated to the test temperatureand again held at the test temperature for 45 min to ensure ther-mal equilibrium. The tensile stress rate was kept constant at1 MPa/s during the test. The test was carried out on the single spec-imen, loaded ten times at each temperature. For all temperatures,the stress range 30–60 MPa was used to determine Young’s modu-lus. The main results are summarized in Table 2 and correspondinggraphical representation in Fig. 6. General trend of Young’s modu-lus decrease with increase in temperature is clearly noticeable. Itshould be emphasized that Young’s modulus of most Mg alloys isabout 45 GPa at room temperature (see [4,53–55] among others),while this research gives 20.7 GPa at the same temperature, differ-ent to other Mg alloys. The discrepancy was also noticed in [9]reporting 35.6 GPa in the rolling direction and 10.6 GPa in thetransverse direction; in Masaki et al. [56] 40 GPa. However, oneshould have in mind former discussion – cyclic deformation withasymmetric loops can lead to the change of Young’s modulusdepending on the strain level [4]. Furthermore, in the temperaturerange of interest the Mg alloy at hand follows almost linear varia-tion of Young’s modulus with respect to temperature; the sametrend was noticed by other researchers [54,57] in Mg alloys andfrequently in some more conventional materials like steels, see[58–60] for few examples. The physical explanation of the Young’smodulus decrease with the temperature increase must be soughtat the atomic level. With the temperature increase, atoms vibratemore intensively thus increasing the distance between neighboringatoms. This leads to the well-known expansion of materials due tothe temperature increase. However, the increase of interatomicdistance leads toward decrease of interatomic forces betweentwo atoms; see for example data for pure Mg in [61]. Such decreaseof interatomic forces leads toward less resistance to elastic defor-mation at macro level effectively reducing Young’s modulus asthe temperature increases. For most polycrystalline materials, thefunctional dependency of Young’s modulus on temperature islinear until certain limit point at higher temperature is reached.Beyond this point, more rapid and nonlinear decrease can beexpected what is usually attributed to the grain-boundary sliding[62].

Upon application of linear regression, Young’s modulus depen-dence on the temperature can be described by the equation:

EðTÞ ¼ 22:256� 0:0658T; Tð�CÞ; EðGPaÞ: ð2Þ

Table 2Young’s modulus at different testing temperatures.

Temperature (�C) 20 40 60 80Young’s modulus (GPa) 20.7 19.1 18.7 17

3.4. Low cycle fatigue tests

Low cycle fatigue tests at 20 �C, 150 �C and 200 �C were per-formed in the stress-controlled tests. Before the test, specimenswere heated at the test temperature for 45 min to ensure thermalequilibrium. During all tests, constant stress rate 20 MPa/s wasemployed. The stresses were tensile during whole test, i.e. no com-pressive stresses were applied. Therefore, the stress ratio parame-ter is always positive, R > 0. Since the specimens were held intension during the whole process, the influence of twinning-detwinning process observed in the tension–compression loading(see [17] for example) is not the driving force in this case. Instead,classical slip deformation dominated plasticity is the main sourceof damage accumulation during load cycling. The upper and lowerstresses used in each particular case are presented in Table 3. Sinceabsolute stress level used at 200 �C will result in low cycle fatigueand certainly lead to high cycle fatigue at room temperature, an-other stress measure has to be selected if one wishes to compareresults. Therefore, the relative stress level is selected as a fractionof the ultimate tensile strength at given temperature. In these par-ticular cases, relative upper stress levels were defined as 80% and90% of the maximum tensile strength and lower as 20% and 45%.

Figs. 7 and 8 presents results for two different stress levels. Nat-urally, with the increase in temperature, Mg alloy can sustain low-er absolute stress level. Apart from the curves obtained at roomtemperatures, obtained curves have the shape of the usual low cy-cle fatigue curves and resemble creep curves as well. All specimensat room temperature established stabilized cycle (cases 1–3), withratcheting strains at very low levels (Fig. 12). As visible from thefailed specimens, Fig. 9, all room temperature specimens failed atthe fillet point. Such behavior pinpoints stress concentrations asa source of failure. Therefore, the alloy at hand is highly susceptibleto stress concentrations what was not previously reported in liter-ature. Although also very close to the fillet in the specimen thatfailed at 150 �C, failure points were slightly away from the filletpoint. Interestingly, specimens 6–7 tested at 200 �C failed far fromthe fillet, almost exactly in the middle of the specimen. Such re-sults indicate the need of further research of the influence ofnotches on the Mg–6Zn–1Y–0.6Ce–0.6Zr alloy at various tempera-tures. Since the stress measure relative to the ultimate tensilestrength was used, the trend of increase of number of cycles tillfailure with the temperature increase is clearly noticeable,Fig. 10. Especial resistance toward fatigue was manifested by thespecimen tested at 200 �C, test case 6. Interesting findings were

100 125 150 175 200.6 16.0 14.5 11.4 10.5 9.4

Table 3Low cycle fatigue tests – loading parameters and results.

Test case no. Temperature (�C) Upper stress (MPa) Lower stress (MPa) Number of cyc.till failure, NF Total time (s) R

1 20 360 (�80% rM) 90 (�40% rM) 584 15772 0.252 20 360 (�80% rM) 180 (�20% rM) 1238 22287 0.503 20 410 (�90% rM) 205 (�45% rM) 755 15471 0.504 150 210 (�80% rM) 50 (�20% rM) 1802 28832 0.245 150 230 (�90% rM) 115 (�45% rM) 376 4329 0.506 200 80 (�80% rM) 20 (�20% rM) 21354 128122 0.257 200 90 (�90% rM) 45 (�45% rM) 2675 12040 0.50

Fig. 7. Evolution of strain during low cycle fatigue test, rUPPER = 80% rM. Test cases1, 4, 6.

Fig. 8. Evolution of strain during low cycle fatigue test, rUPPER = 90% rM. Test cases3, 5, 7.

Fig. 9. Fractured specimens; numbers denote test case, Table 3.

Fig. 10. Number of cycles till failure NF vs. temperature for two different upperstress values.

Fig. 11. Typical stress–strain response during the test. Test case 5.

Fig. 12. Average ratcheting strain vs. stress ratio R at different environmenttemperatures.

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also reported by Sonsino and Dieterich [27], one of the very few pa-pers dealing with fatigue at elevated temperatures. They testedhigh cycle fatigue of the creep resistant AZ 91 HP cast alloy at roomtemperature and at 130 �C. Position of the knee point at the Wöhlercurves at 130 �C was determined by the ten times more cycles thanat room temperature; corresponding fatigue strength at 130 �C was

M. Cana -dija et al. / Materials and Design 59 (2014) 287–295 293

decreased about 36% compared to room temperature. Althoughthis results in somewhat lower fatigue strength at 130 �C, it shouldbe kept in mind that the yield and ultimate stresses are also lowerat the temperature of interest.

Above observation fits well within the known sensitivity ofother Mg alloys to notches at the room temperature. The sourceof such brittle behavior is the HCP crystal structure of Mg alloysresulting in lower number of slip systems: basal slip system of Bur-gers vector hai, prismatic hai and first hai and second order hai + hcipyramidal slip systems. As noted in [63–65], the easiest glide sys-tem is the basal system, while the prism and the pyramidal slipsrequire either higher loading or, what is of special interest for thisresearch, elevated temperature. Recent research [66] confirms thatprismatic and pyramidal slips are indeed activated by the elevatedtemperature through reduction of the critical resolved shear stressof slip, thus enabling additional slip systems. Therefore, increase intemperature should reduce sensitivity to notches, what is alsonoted in current study. To conclude, machine components madeof present alloy operating at room temperature should be carefullydesigned to avoid stress concentrations, while these constraintsdecline as the operating temperature increases.

A typical stress–strain response during low cyclic fatigue is gi-ven in Fig. 11. Continuous ratcheting takes place and stabilized cy-cle was not obtained in this specimen or in any of specimensanalyzed at elevated temperatures (cases 4–7). Initial hardeningfollowing previous compressive loading due to extrusion showscharacteristic concave curve explained earlier. Subsequently, con-stant plastic strain per cycle stabilized rather fast. At the end, thisstrain starts to increase leading to the eventual failure of thespecimen. It should be noted that due to the relatively small num-ber of stored points some cycles appear as they did not reach thepeak – upper stress; this is only a post processing deficiency andthe actual peak values were reached during the test.

Fig. 13. Microstructures on the surface of the specim

Fig. 14. Microstructures on the surface of

Although not the prime goal of this research, an insight into theinfluence of the mean stress on the number of cycles till failure isvisible upon comparison of test cases 1 and 2. They were per-formed at the same temperature and with the same upper stress;however the lower stress differs, so mean stresses are 225 MPa(test case 1) and 270 MPa (test case 2). Increase in the mean stressfor 20% decreases the number of cycles till failure by almost half.Tests performed on AZ91D Mg alloy [8], although at room temper-ature, resulted in similar ratios and conclusions. For the alloy athand, research on the mean stress influence yet remains to be per-formed in more detail.

Since these tests were performed as strain controlled, ratchet-ing strain can be conveniently evaluated, Fig. 12. Average ratchet-ing strain �er was calculated as:

�er ¼e2 � e1

N2 � N1; N1 ¼ NM � 0:05NF ;

N2 ¼ NM þ 0:05NF ; NM ¼ NF=2; ð3Þ

where NF is number of cycle at failure, NM is mid-life cycle number,e1, e2 are strains at N1, N2 cycle, respectively. In that way, 10% of rat-cheting strains in the mid-life part of the tests were averaged todetermine ratcheting strain for each test case. Results indicate thatlower stress ratio R also lead to lower ratcheting strain. As alreadypresented in Figs. 7 and 8, ratcheting strain at room temperature isalmost zero, signifying stabilized cycles. Noteworthy, comparisonbetween ratcheting at 150 �C and 200 �C reveals that highertemperature results in lower ratcheting and more favorable LCFbehavior, confirming once more hypothesis that deformationprocesses at 200 �C are governed by the other mechanisms thanat lower temperatures.

Fig. 13 shows the microstructures of broken fracture and grainglide or flow on the surface of the specimen after LCF test at 150 �C.It is clear that there was no apparent grain boundary sliding

en and broken fracture after LCF test at 150 �C.

the specimen after LCF test at 200 �C.

294 M. Cana -dija et al. / Materials and Design 59 (2014) 287–295

observed on the surface test at and below this temperature(Fig. 13a). The fracture (Fig. 13b) was composed of small equiaxeddimples indicating the intragranular sliding dominate the plasticdeformation during LCF test. At 200 �C, apparent grain boundarysliding (GBS) and a few miniature cavities were observed on thesurface of tested specimen, as shown in Fig. 14a. Moreover, somelarge cavities (black region) were observed near the fracture region(Fig. 14b). It is reasonable to conclude that the miniature cavitiesform at grain boundaries and triple junctions of grain boundaries,experience nucleation, growth and sliding under the control ofgrain boundary diffusion. With the accumulation of the miniaturecavities due to diffusion resulted sliding, finally, intergranular cav-ities result in the specimen failure. The fracture of the specimentested at 200 �C composed of fine equiaxed dimples and a few deepholes. This suggested that some grains were pulled out completelyfrom the deep holes due to the GBS. Grain boundary sliding wasalso noticed in [30]. For detailed discussions about the deformationmechanisms at various temperatures, see [42].

4. Conclusions

A detailed research of the mechanical properties and LCF of Mg–6Zn–1Y–0.6Ce–0.6Zr alloy at room and elevated temperatures waspresented. Several noteworthy findings were reported. Rather hightensile strength and elongation for the alloy at room and elevatedtemperatures was observed. As expected, the tensile strength de-creases as the temperature rises. This decrease is governed by asteep linear curve, resulting in a reduction by a factor of 4.5 forthe temperature range 20–200 �C. Obtained stress–strain curvesat 20 �C, 100 �C and 150 �C exhibit expected concave part due tothe previous compressive deformation, while the curve at 200 �Clacks such shape. Judging on the maximum tensile stress and elon-gation vs. temperature curves, departure from linearity is clearlyvisible at 200 �C. Regarding low cycle fatigue with positive stressratio parameter R, obtained results indicate that Mg–6Zn–1Y–0.6Ce–0.6Zr alloy is susceptible to stress concentration at roomtemperature in the low cycle fatigue regime, so such applicationsshould be carefully set up. This is not the case at elevated temper-atures – the alloy could be easily used at elevated temperatures inthe LCF regime, especially if the applied upper stress is not exces-sive. At room temperature, stabilized cycles tend to appear, whilecontinuous ratcheting takes place at elevated temperature.Increasing temperature also has beneficial influence on ratchetingstrain. Suitability of the alloy for elevated temperature applicationis further favored by the high tensile strength and elongation.Microstructural analysis of broken fracture due to LCF alsosupports different deformation mechanisms at 200 �C. To conclude,all deformation processes at 200 �C in this alloy seem to begoverned by the other mechanisms than the processes at lowertemperatures. This indicates necessity of further research on lowcycle fatigue at temperatures above 200 �C.

Acknowledgments

We gratefully acknowledge the Science and TechnologiesFoundation of Henan of China (No. 102102210031) and the NaturalScience Foundation of Henan Educational Committee of China(2010A430008) for their financial supports.

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