Evaluation of fatigue life of AZ31 magnesium alloy fabricated by
squeeze castingMiroslava Horynov, Josef Zapletal, Pavel Dolezal,
Pavel GejdoInstitute of Material Science and Engineering, Faculty
of Mechanical Engineering, Brno University of Technology, Technick
2896/2, 616 69 Brno, Czech Republicarti cle i nfoArticle
history:Received 16 February 2012Accepted 27 August 2012Available
online 12 September 2012Keywords:Magnesium alloySqueeze
castingCyclic propertiesLow cycle fatigueHigh cycle
fatigueabstractCyclic deformation behavior and fatigue life of
squeeze-cast AZ31 magnesium alloy was studied understress
amplitude-control at room temperature. Low and high cycle fatigue
tests with engineering stressamplitudesintherangefrom40to110
MPawereconducted. Analysisofhysteresiscurveswasper-formed.
Tensioncompression asymmetry of hysteresis loops was not observed;
the alloy exhibited cyclichardening in tension and compression. The
fatigue life in the low cycle fatigue region was expressed byWhler
andderivedMansonCofncurves. Experimentaldatain both, thelow
andhighcyclefatigueregionswerettedbymeansofregressionfunctions.
SNcurvesexhibitedasmoothtransitionfromthe low to the high cycle
fatigue regions and signicant scattering of experimental points was
observed.Furthermore, metallographic and fractographic analyses
were performed. Crack initiation occurred fromthe specimen surface
or on clusters of secondary particles; the region of nal fracture
was characterizedby a transgranular ductile fracture.It can be
concluded that the fatigue properties of squeeze cast magnesium
alloy AZ31 are signicantlyimproved comparing to materials prepared
by common methods of casting. Squeeze casting also enablesthe
cost-effective fabrication of complicatedly shaped parts. 2012
Elsevier Ltd. All rights reserved.1. IntroductionMagnesium alloys
are increasingly used in the automotive andaircraft industries.
Reducing the weight of components whilemaintaining good mechanical
properties leads to reduced fuel con-sumption and more economical
and environmentally-friendlyoperation. Topredict thebehavior of
thematerial under cyclicloading, it is essential to describe the
fatigue behavior of magne-sium alloys in the low and high cycle
regions.The fatigue behavior of AZ31 magnesium alloy in the low
andhigh cycle regions has already been investigated in the
literature.However, there haveonlybeenlimitedstudies onthe
fatiguebehavior of cast magnesium alloys. The low cycle fatigue
behaviorof
extrudedAZ31wasstudiedusingstraincontrolledpushpulltests[1]androtatingbendingfatiguetests[2].
Hasegawaetal.[3]investigatedthelowcyclefatiguebehaviorofextrudedAZ31magnesiumalloyusingboth,
stress andstraincontrolledtests.The authors examined the effect of
mean stress and analyzed cyclicstressstrain behavior. Stress and
strain controlled fatigue tests ofdifferent die cast magnesium
alloys were alos performed by Son-sino and Dieterich [4]. The
inuence of notches, mean stress, andelevated temperatures was
evaluated and compared to other struc-tural materials. Zberov et
al. [5] studied the tensile, low and highcycle fatiguebehavior of
AZ31magnesiumalloyfabricatedbysqueeze casting (SC), hot rolling
(HR), and equal channel angularpressing(ECAP).
TheaveragegrainsizeforSCwas450 lm;thegrain sizeforHRrangedfrom3to20
lm, andthegrainsizeforECAPranged12 lm;yieldstrengthwas50 MPaforSC,
175forHR, and115forECAP. Thestronginuenceofproductionproce-dure on
fatigue properties was found. The fatigue limit of SC mate-rial was
40 MPa; HR and ECAP materials exhibited about 95 MPa.Chamos et al.
[6] investigated the tensile and fatigue behavior ofhot
rolledAZ31andAZ61magnesiumalloys. Longitudinal andtransverse
directions were evaluated and the fracture surfaces ofspecimens
were examined. Matsuzuki and Horibe [7] studied
theinuenceofheattreatmentonthefatiguebehaviorof
extrudedAZ31magnesiumalloy.
Plasticstraincontrolledtestswerecon-ductedonas-extrudedandannealedspecimens.
Thefatiguelifeof as-extruded material, was slightly longer than
that of heat trea-tedmaterial. Noother signicant differencewas
foundinthefatigue behavior of the experimental materials.Casting is
the most economical way to transform material intoshaped
components. The major drawback of casting techniques
istheformationof
defectssuchasporosityorsegregationdefectsthatcanbepotential
crackinitiationsitesduringthelifeofthecomponent. New casting
techniques such as squeeze casting havebeen developed to minimize
casting defects and related problems[8,9].Squeeze casting is a
technique in which the material solidiesunder
highpressureandhassignicant advantagessuchasanimprovement in
mechanical properties and the possibility to0261-3069/$ - see front
matter 2012 Elsevier Ltd. All rights
reserved.http://dx.doi.org/10.1016/j.matdes.2012.08.079Corresponding
author. Tel.: +420 54114 3147.E-mail address: [email protected]
(M. Horynov).Materials and Design 45 (2013)
253264ContentslistsavailableatSciVerseScienceDirectMaterials and
Designj our nal homepage: www. el sevi er . com/ l ocat e/ mat
desproduce parts of complicated shape. This process is also capable
ofproducingcomponentsfromdifcult-to-processmaterials. Com-ponents
fabricated by this method are ne grained with excellentsurfacenish
andhavealmostnoporosity. Theappliedpressurealsoleads
toaremarkabledecreaseinthesecondarydendritearm spacing [811].Two
methods of squeeze casting, direct and indirect, are
known.Thedirect methodrequiresapreciseamount of moltenmetal,which
is poured into the bottom part of a pre-heated die. Then thedie is
closed by the upper part and pressure is applied so that
themoltenmetalsolidiesunderpressure. Highpressureeliminatesthe
formation of a gap between the die and the casting, leading tomore
effective heat transfer and a higher rate of cooling. The
fullysolidied casting is removed from the die by ejectors. A
machinefor indirect squeeze casting has a dosing chamber which is
placedunder the die and can be tilted during lling process. When
the dos-ing chamber is lled, it is moved back to the working
position andattached to the die and molten metal is forced into the
die. Due tothe low rate of lling, no turbulent ow or oxidation
occurs. Oncethe die is lled, the maximum value of applied pressure
is achievesand the casting solidies under pressure. And again,
fully solidiedcasting isremoved from dieby ejectors.
Thedisadvantageof theindirect squeeze casting method is the
presence of a sprue and theneedfor amore sophisticateddie, which
ismore complicated toproduce than dies for the direct method. On
the other hand, directmethod requires a precise amount of molten
melt.ChadwickandYue[12]studiedthemechanical propertiesofsand-cast,
gravitydie-castasqueeze-castAZ91magnesiumalloyinas cast
stateandafter heat treatment. Inbothas-cast andheat-treated state,
the squeeze-cast material exhibited higher val-ues of ultimate
tensile strength, yield strength and elongation thanmaterials cast
by other technologies. The effect of applied pressureon the
density, macrostructure and hardness of squeeze cast AlSialloy LM13
were investigated by Maleki et al. [13]. An applied pres-sure
higher than 50 MPa resulted in an increase in density and
apressurehigherthan100 MPaledtotheeliminationof
gasandshrinkageporosities. Anincreaseinpressureupto100
MPaalsoresulted in renement of the macrostructure due to better
contactbetweenthemetalandthediesurfaceduringsolidication.
Thehardnessof thealloyincreasedwithincreasingpressureduetorenement
of the microstructure and the modication of eutecticparticles.In
the present study, low and high cycle engineering stress
con-trolledfatiguetestswereconductedonAZ31magnesiumalloyfabricated
by squeeze casting. The microstructure, cyclic deforma-tion
behavior, and low and high cycle fatigue life were
investigatedandcomparedtoavailabledataonAZ31magnesiumalloyspre-pared
by different technologies.2. Experimental details2.1. Material and
specimensTheexperimental material
usedwasAZ31magnesiumalloyfabricated by squeeze casting at ZFW GmbH
in Clausthal. The di-rect squeeze casting method was used: molten
metal wassqueezedandsolidiedat apressureof 150 MPa. Thematerialwas
supplied in the formof billet 200 mmin diameter and40 mm in height.
As squeeze casting is a method producing a nergrain and more
homogeneous structure than other casting meth-ods, noadditional
heattreatmentwasperformed. Thechemicalcomposition measured by a
Spectrumat GDS 750 optical emissionspectrometer with glow discharge
is shown in Table 1. The basicmechanical properties (Young0s
modulus, ultimate tensile strength,proof stress and elongation),
strain hardening coefcient and strainhardening exponent established
by means of tensile tests are givenin Table 2. Tensile tests were
performed according to EN ISO 6892-1[14]
onaPCcontrolledtestingdevicewithastrainrateof0.00025 s1using
cylindrical specimens with a diameter of 6 mmandgaugelengthof 30
mm. Strainwas measuredbyanaxialextensometer withagaugelengthof 30
mm. Arepresentativeengineering stressstrain curve is given in Fig.
1.As shown in Fig. 2, cylindrical specimens of two different
geom-etries were machined from experimental material and used for
fa-tigue tests. The test specimens were machined so that the axes
ofthe samples were perpendicular to the axis of the billet. The
sur-face of the gauge section was 0.4 Ra ground to remove the
machin-ing marks and to achieve a smooth surface.2.2. Experimental
procedureSpecimens for metallographic assessment were prepared in
theusual way and etched with a mixture of picric acid (5 ml acetic
acid,6 g picric acid, 10 ml water, and 100 ml ethanol).
MicrostructuralNomenclaturea () extrapolated value of the function
in the point of inec-tion for N = 1A (%) elongationb () fatigue
strength exponentc () fatigue ductility coefcientC () parameter of
KV functionE (GPa) Youngs modulusm () exponent of the RambergOsgood
functionK (MPa) strain hardening coefcientK0(MPa) cyclic hardening
coefcientn () strain hardening exponentn0() cyclic hardening
exponentN () number of cyclesNf () number of cycles to failureNt ()
transition number of cyclesR () stress ratioRm (MPa) ultimate
tensile strengthRp0.2 (MPa) proof stressR0p0:2(MPa) cyclic yield
strengthR00p0:2(MPa)
cyclicyieldstrengthestablishedbytheRambergOsgood functionVH () loop
shape parameterW (MJ/m3) area of hysteresis loopS () sum of
squareseap () plastic strain amplitudeeae () elastic strain
amplitudeeat () total strain amplitudeem () mean strainef0()
fatigue ductility coefcientra (MPa) stress amplituderf0(MPa)
fatigue strength coefcientrc (MPa)fatigue limitr1 (MPa) permanent
fatigue limitTable 1Chemical composition of AZ31 alloy (wt.%).Al Zn
Mn Si Fe Zr Mg2.86 1.07 0.35 0.005 0.004 0.01 Bal.254 M. Horynov et
al. / Materials and Design 45 (2013) 253264evaluation, local
analysis of chemical composition, and the analysisofelemental
distributionwereperformedonPHILIPSXL30scan-ning electron microscope
with EDX analyzer. Grain size was esti-matedbythelinear intercept
methodusinganOlympusGX71light microscope with Olympus Stream motion
software.Low-cycle fatigue behavior in the stress amplitude control
modewas determined on an Instron 8801 servo-hydraulic testing
system.Experiments were conducted with engineering stress
amplitudes inthe range of from45 to 110 MPa at constant frequencies
of 3 Hz and20 Hz. To assess fatigue behavior in the high-cycle
regime, fatiguetests with engineering stress amplitudes in the
range from 40 to60 MPaandatafrequencyof 130 5
HzwereconductedonanAmslerHPF1478high-resonancepulsator.
Thesymmetricalsinecycle (R = 1) was used and all tests were carried
out under labora-toryconditionsat
roomtemperatureaccordingtoASTME466-96(2002) [15] ASTM E606-92(2004)
[16]. Strain was measured byan axial extensometer with a gauge
length of 12.5 mm. Hysteresisloops were obtained using LCF
INSTRONsoftware; in addition max-imum and minimum values of stress
and strain and Young0s modu-lus were obtained for each cycle.The
fracture surfaces of fatigue specimens were examined usinga PHILIPS
XL30 scanning electron microscope with EDX analyzer toidentify
fatigue crack initiationsites andmechanisms of nalfracture.3.
Experimental results3.1. Microstructure of AZ31 magnesium alloyFig.
3 shows the secondary electron micrograph of AZ31 magne-sium alloy
and elemental maps measured by energy-dispersive X-ray
spectroscopy. According tothe results of local analysis ofchemical
composition(Table2), thestructureof
AZ31alloyisformedbythesolidsolutiondphase,
theintermetalliccphase(Mg17Al12) and the intermetallic u phase
(Mg21(Al, Zn)17), eutectic(d + c), and AlMn-based particles.Several
differently shaped AlMn particles were analyzed.According to the
AlMn binary phase diagram [17], a Mn contentof between34and 39.5
at.%correspondstothec2phase(Al8-Mn5). ThecalculatedAl/Mnratioof 1.7
0.2alsoindicatesthatthese particles are most likely Al8Mn5.The
material is heterogeneous; it is evident from the analysis
ofelement distribution (Fig. 3) that there is an Al- and Zn-rich d
phasealongthegrainboundaries. Thedifferenceinaluminumcontentbetween
thed phase and enrichedd phase was about 3 at.%. Theaverage grain
size of the material was about 50 lm.3.2. Cyclic plasticity and
low-cyclic fatigueAs the shape of hysteresis curves indicates
whether a materialundergoes cyclic hardening or softening or
remains cyclically sta-ble, hysteresis curves were
analyzed.Thecyclicresponseofthematerialundercyclicloadingwithengineeringstress
amplitudes of 65and110 MPais showninFig. 4. Tomakeevident
thechangesinshapetheof hysteresisloops,
onlycurvesforsomecyclesarepresented. Namelycyclesnumber 4, 40, 400,
4000, and20000andcyclesnumber 2, 16,160, and 800 are shown for the
two stress amplitudes respectively.Cycles No. 4 and 2 roughly
correspond to Nf/1000; cycles No. 20000and 800 roughly correspond
to Nf/2.The elastic parts of the hysteresis loops are rather short,
com-pared to the plastic parts. In both tension and compression
regions,symmetric deformation response occurs except for the rst
cyclesat higher stress amplitudes. For higher stress amplitudes,
concaveregions can be observed in tension and compression (e.g. the
sec-ond and sixteenths cycle in Fig. 4b).At each stress amplitude,
the width of the loops decreases withincreasing number of cycles
suggesting that the material cyclicallyhardens.Loop shape
parameters VH were obtained using Eq. (1). The areaof hysteresis
loop W, representing the specic energy dissipated ina material
within one cycle was determined by numerical integra-tion using the
trapezoidal rule. Fig. 5 shows the dependence of
hys-teresisloopareaW,
plasticstrainamplitudeeapandloopshapeparameters VH on the number of
cycles. As the plastic strain ampli-tude and area of the hysteresis
loop decrease, the material hardenswithoutsaturation.
TheparameterVHincreasegraduallyafterasteeper increaseduringtherst
cycles; nolocal minimawereobserved.VH W4eap ra1Fig. 6 shows the
evolution of plastic strain amplitude with num-ber of cycles at
different stress amplitudes; the evolutions of
totalandmeanstrainareillustratedinFigs. 7and8, respectively.
Achange of 5% in the effective modulus was used as a criterion
fordetermining Nf.Table 2Tensile properties of AZ31 alloy.E (GPa)
Rp0.2 (MPa) Rm (MPa) A (%) K (MPa) n40.66 55 174 8.8 404.8 0.32Fig.
1. Tensile stressstrain curve of AZ31 magnesium alloy.Fig. 2. Shape
and dimension of specimens for (a) high-resonance pulsator and
(b)servo-hydraulic testing system.M. Horynov et al. / Materials and
Design 45 (2013) 253264 255With an increasing number of cycles, the
plastic and total strainamplitudes gradually decrease until the
rapid increase due to crackopening. Though the experimental
material was cast, curves showthe same trend for all stress
amplitudes and no inconsistency wasobserved. As seen from
comparison of Figs. 6 and 7, plastic defor-mation is greater than
elastic deformation during the rst cycles.The mean strain is
constant or very gradually decreases until therapid increase due to
crack opening.At thehighest stressamplitudes, signicant
plasticresponseduring the rst half cycle occurs. The material
exhibits symmetri-cal behavior in both compression and tension with
a subsequentdecrease in mean strain. As the material is cast, the
values of meanstrain are not proportional to stress amplitude.The
cyclic deformation curve (CDC), expressed by the regressionfunction
in loglog coordinates, is plotted in Fig. 9a. Parameters Kand n
were obtained using Eq. (2) while Eq. (3) was used to estab-lish
the cyclic yield point R0P0:2 (Table 4). For comparison, the
statictensile curve and the cyclic stressstrain curve, together
with thecyclic yield strength R00P0:2, are shown in Fig. 9b. The
experimentalpoints of the CDC were tted to a modied RambergOsgood
func-tion(4); valueof
Young0smodulususedintheregressionwasdeterminedfromthetensiletest.
Theobtainedparameters aregiven in Table 3. The comparison shows
that the cyclic
deformationcurvehasagreaterslopeintheelasticplasticregionthanthestatic
tensile curve. Thus, the experimental material
cyclicallyhardens.Fig. 3. (a) Secondary electron micrograph of AZ31
magnesium alloy and elemental distribution maps measured by
energy-dispersive X-ray spectroscopy: (b) Al, (c) Mn, (d)Zn, (e)
EDX spot analysis of AlMn based particle.256 M. Horynov et al. /
Materials and Design 45 (2013) 253264ra K0 en0ap2R0p0:2 K0
0:002n03er rE rr0 m4TheWhlercurve(Fig.
10)andderivedMansonCofncurve(Fig. 11) were tted by means of
regression analysis according topower laws (5) and (6),
respectively [4,17]. Only specimens testedat low frequencies were
considered in this regression analysis. Eq.(7) [4] was used to plot
the dependence of total strain amplitudeeat,
plasticstrainamplitudeeap, andelasticstrainamplitudeeaeon the
number of cycles to failure (Fig. 11). The material parame-ters
found by the tting procedure are summarized in Table 5.
Fur-thermore, thetransitionnumber of cycleswasalsoestablished(Nt =
1025). The intersection of plastic and elastic strain
amplitudedependence on number of cycles is located in the region
that is notcovered by experimental data. The amplitudes of elastic
and plasticstrainwerethereforeextrapolatedtoNf = 200. Thevalueof
theplastic strain amplitude eap = 6 106for 107cycles was
obtainedfrom Eq. (6).ra r0f 2Nfb5Fig. 4. Hysteresis loops for (a)
65 MPa and (b) 110 MPa. Cycle No. 20000 (65 MPa)and 800 (110 MPa)
roughly correspond to Nf/2.Fig. 5. Dependence of hysteresis loop
area W, plastic strain amplitude eap and loopshape parameters VH on
number of cycles. Each set of symbols represents differentstress
amplitude.Fig. 6.
Evolutionofplasticstrainamplitudeinengineeringstresscontrolledlowcycle
fatigue tests.Fig. 7. Evolution
oftotalstraininengineeringstresscontrolledlowcyclefatiguetests.Fig.
8. Evolution of mean strain in engineering stress controlled low
cycle fatiguetests.M. Horynov et al. / Materials and Design 45
(2013) 253264 257eap e0f 2Nfc6eat r0fE 2Nfbe0f 2Nfc73.3. Regression
analysis of SN curvesExperimental data were tted to a smooth
continuous functionusing the LevenbergMarquardt nonlinear least
squares algorithm.Commonlyusedregressionfunctions wereapplied,
using1000iterations and 106tolerance.The dependence of stress
amplitude ra on the number of cyclesto failure Nf is given in Fig.
12. Experimental data in the low
andhigh-cyclefatigueregionswerettedbytheStromayer(8)andthe
KohoutVechet (KV) functions (9) [18]. The regression param-eters
are summarized in Table 6. As can be seen from the table, thesum of
squares of the Stromayer function is lower than the sum ofthe
squares of the KV function. This indicates that the
Stromayerfunction better describes the fatigue behavior of the
experimentalmaterial. On the other hand, comparing parameter b of
the Basqu-inefunction(Table5)withthecorrespondingparametershowsthat
the KV function is the most suitable for describing
low-cyclefatigue behavior.ra a Nbf r18ra r1 NfNf C b93.4.
Fractographic analysisThe fracture surfaces were examined using SEM
and energy-dis-persive X-ray spectroscopy analysis.The inuence of
stress amplitude on the characteristics of thefracture surface is
given in Fig. 13. The number of initiation sites,markedwitharrows,
increasedwithincreasingstressamplitudeas well as the size of region
of the nal fracture. Fatigue cracks ini-tiated from the specimen
surface except for specimens where crackinitiation took place at
clusters of secondary particles.Fig. 14 shows fracture surface with
initiation on cluster of sec-ondary particles. Maximumsize of
cluster is approximately550 lm. The area containing increased
number of secondary parti-cles is clearly visible on back-scattered
electron micrographs(Fig. 14b and c). The EDX spot analysis (Fig.
14d) shows that thoseparticles, of approximately 3 lm in size, are
AlMn based. Clustersof AlMn particles were also observed in
longitudinal section of
fa-tiguedspecimensatvariousdistancesfromthefracturesurface.Therewasnoevidenceoffatiguecracksneartheclusterswhichwere
further from the specimen surface.The fracture surface of a sample
loaded with a stress amplitudeof 65 MPa is shown in Fig. 15. As can
be seen in Fig. 13,
multiplecrackinitiationoccurredatthisstressamplitude.
Fatiguecracksinitiated from the specimen surface, initiation sites
are character-ized by cleavage-like facets. Initiation sites are
very similar, differ-ingonlyinthesizeof thefacets. Detail
fromtheinitiationsitepresentedinFig. 15ashowscleavageplanewithriver
pattern.Fig. 15b presents higher magnication image of the beginning
ofthe crack propagation area where ridges parallel to the crack
prop-agation direction can be seen. Striations perpendicular to the
crackpropagation direction were observed in the crack propagation
area(Fig. 15c). The region of nal fracture exhibits a transgranular
duc-tile fracture with a pronounced dimple pattern (Fig. 15d).Fig.
16showsthefracturesurfaceof thespecimentestedat110 MPa. As can be
seen from Fig. 16a, the fracture surface is moreruggedthanthat of
specimentestedat 65 MPa. MultiplecrackFig. 9. (a) Cyclic
deformation curve in loglog coordinates (b), comparison of
statictensile curve and cyclic stressstrain curve tted to
RambergOsgood function.Table 3Composition of constituent phases in
squeeze cast AZ31 magnesium alloy (at.%).Mg Al Mn Znd phase 98.72
1.14 0.04 0.10c phase Mg17Al1277.11 21.42 1.47u phase Mg21(Al,
Zn)1775.22 19.24 5.54AlMn particle 63.46 24.02 12.52 Table 4Summary
of material parameters.Power law RambergOsgood functionK0(MPa)
n0R0p0:2(MPa) r0 (MPa) m R00p0:2(MPa)445.1 0.23 106 469 4.03
101Fig. 10. Wohler curve in loglog coordinates tted by power
low.258 M. Horynov et al. / Materials and Design 45 (2013)
253264initiationwithpronouncedratchetmarksbetweentheinitiationsites
was observed. Initiation sites were mostly similar to that
ob-servedforthespecimentestedat65 MPa. Thebeginningofthecrack
propagation area with a secondary crack is shown inFig. 16b.
Asintheprevioussample, ridgesparallel tothecrackpropagation
direction were observed. Propagation area is charac-terized by
striations perpendicular to the crack propagation direc-tion (Fig.
16c). The region of nal fracture exhibits a transgranularductile
fracture (Fig 16d) similar to the fracture observed for thespecimen
tested at 65 MPa.4. DiscussionThemicrostructureof
AZ31magnesiumalloyconsists of thesolidsolutiondphase,
theintermetalliccphase(Mg17Al12) andtheintermetallicuphase(Mg21(Al,
Zn)17), eutectic(d + c), andAlMn-based secondary particles.
According to EDX analysis, theseparticles are supposed to be
Al8Mn5,which is in agreement withthe kinetic model of the
solidication of AZ31 in a previous study[19]. However, another
solidicationmodel [20] suggests,
thatAl8Mn5transformstoAl11Mn4andthen, below 250 C, trans-forms to
AlMn4.BasedonbinaryMgAl (Fig. 17) [21] andternaryMgAlZn(Fig. 18)
diagrams [22], the presence of eutectic and such amountsof
cphaseare notsupposed. Themicrostructureofwrought andcast AZ31
magnesium alloy has been investigated in the literature.According
to the authors of articles [2325] the microstructure ofwrought
alloy consists of solid solution and AlMn(Fe) based par-ticles,
Song and Xu [26], Cheng et al. [27] and Shahri et al. [28]
alsofound a small amount of intermetallic c phase. Uniformly
distrib-uted ne particles of intermetallic c phase with maximum a
vol-umefractionof 5%wereobservedincast AZ31alloy[29]. Thepresence
of eutectic and a higher amount of c phase in the exper-imental
material
canbeexplainedbyahighcoolingratewhenusingthesqueezecastingmethod.
Duetothehighcoolingrate,segregationandstructural
heterogeneityoccurs andlocallyin-creased concentrations of aluminum
result in the precipitation ofeutectic and a higher amount of c
phase.A minimumof casting defects was observed in the
experimentalmaterial. Clustersof
AlMnbasedparticleswerelocallyfound,which may play the role of
initiation sites during the fatigue pro-cess. Even so, the presence
of manganese in AZ magnesium alloysis desirable. It improves the
corrosion resistance of AZ type magne-sium alloys by lowering the
corrosion potential of secondary phasewith iron [30,31].The basic
mechanical properties of the experimental
material,establishedviatensiletests, areworsethanthoseof hot
rolled[3,25] and extruded [1] AZ31 alloys. Higher strength of
wrought al-loysiscausedbynergrain,
ahigherdislocationdensityandamore homogeneous structure compared to
our experimental mate-rial. Because of the experimental materials
structural heterogene-ity, fracture occurred without the formation
of neck which resultsin lower ductility of the studied
material.From an analysis of the hysteresis curves it was
established thatthematerial undergoesmarkedcyclichardeningat
eachstressamplitudewhilecyclicsofteningwasnotobserved. Thisndingis
in agreement with literature [1,3,5,7,32]. It is well known,
thatmaterial undergoes cyclic softening when containing high
disloca-tion density (e.g. wrought materials) or being age
hardened. Cyclicloadingleadstotheformationof
complicateddislocationstruc-tures such as persistent slip bands
with different dislocationarrangement comparedtothematrix.
Persistent slipbandsaresofterthanthematrix,
whichleadstothelocalizationofplasticstrainandcyclicsoftening.
Agehardenedmaterialssoftenwhencoherent precipitates are cut by
dislocations and subsequently dis-solve because the precipitate
fragments are smaller than the criti-cal size, thus
thermodynamically unstable. However, extruded AZtype magnesium
alloys do not give an impression of cyclic soften-ing [1,3,7].
Furthermore, as the c phase precipitates rstly in theform of
incoherent precipitates [33,34], AZ type magnesium alloysdo
notcyclically soften by the dissolution of precipitates and
forAZ91-T6 [35] only cyclic hardening has been observed.Also, there
was a clear increase in cyclic yield strength(101 MPa) versus
static yield strength (55 MPa) for our experimen-tal material.The
shapes of hysteresis curves were also studied for the pur-pose of
explaining deformation mechanisms. The deformationmodes of
magnesium and its alloys are dislocation slip in the
basal,prismatic, pyramidal I andpyramidal II slipsystems
andalsomechanical tensile twinning f11
20gh10
1
1i and compressivetwinning 10
1
1h10
12i. Mechanical twinning occurs, when theFig. 11. Dependence of
total strain amplitude eat, plastic strain amplitude eap,
andelastic strain amplitude eae on the number of cycles in loglog
coordinates.Table 5Low cycle fatigue parameters.r0f(MPa) b
e0fc410.52 0.169 1.45 0.729Fig. 12. SNcurvesofAZ31magnesiumalloyatR
= 1ttedtoStromayerandKohoutVechet functions.Table 6Summary of
regression parameters.Stromayer Kohout-Vecheta 1075.3 b 0.38 0.17C
2.03 105r1 (MPa) 43.3 45.8S 1473.5 1531.5M. Horynov et al. /
Materials and Design 45 (2013) 253264
259c-axisisparalleltothetensileaxis
orperpendiculartothecom-pression axis [3640].As only pyramidal
slip, which is in magnesium alloys activatedat about 200 C
[38,41,42], can fulll the Von Misses criterion
with-outtheactivationof otherdeformationmodes, variousprimaryand
secondary slip and twinning mechanisms can and have to beactivated
at the same time at room temperature [4345]. Evidenceof pyramidal
slip at room temperature was found for MgLi
singlecrystalbyAgnewetal. [36]andAndoandTonda[46]. Assinglecrystal
was strained along the direction, the Schmid factors for ba-sal and
pyramidal II slip were nearly identical and pyramidal slipoccurred
at roomtemperature. For polycrystalline magnesiumand its alloys,
combinations of other deformation modes were ob-served at room
temperature. Gharghouri et al. [37] observed basalslip and tensile
and compressive twining for Mg7.7Al. Basal slipwith tensile
twinning was reported for AM60 [42], AZ31 [47] andalso for pure Mg
[40]. Basal slip and tensile twining were consid-ered as major
deformation modes for extruded AZ31 magnesiumalloy [39]. Only in
the case that the c-axis was nearly perpendicularto the loading
direction, the Schmid factors of the three basal
slipsystemsissosmall, thatprismaticandpyramidal I slipsystemcould
be additionally activated. Basal slip and twining can
there-forebeconsideredasthemostimportantdeformationmodesatroom
temperature for polycrystalline magnesium alloys with ran-domly
oriented grains.Symmetric hysteresis curves with respect to tension
and com-pression are typical for deformation predominantly
accomplishedby means of dislocation slip [48]. On the other hand,
asymmetricand sigmoidal-shaped curves are the result of mechanical
twining[49].At all stressamplitudes, except fortherst cyclesat
higherstressamplitudes, hysteresisloopsweresymmetric,
whichsug-gestsslip-dominatedplastic deformation. Hasegawaetal.
[3]re-ported asymmetric hysteresis curves until half-life using
thestresscontrolmodeandasymmetryuntiltheendofthetestforthe strain
control mode. Since stress control mode tests are not fre-quently
used, the shape of hysteresis curves were further discussedwith
strain control mode fatigue test results.Symmetric hysteresis
curves were reported for die cast [50,51]and thixomolded [32]
magnesium alloys (AZ91, AE42) subjected tothestraincontrol
modefatiguetest whileextruded[1,7,51,52]magnesium alloys (AZ31,
AZ80, AM30) exhibited asymmetric hys-teresiscurves. Asthemechanical
twiningdependsoncrystallo-graphic orientation, strong texture of
extrudedalloys leads totensile-compression asymmetry. Cast alloys
have no preferentiallyoriented grains and hysteresis curves are
therefore symmetric.Fortherstcyclesathigherstressamplitudes,
thehysteresiscurves were observed to have a sigmoidal shape, which
is in accor-dance the with ndings of Matsuzuki and Horibe [7] who
reportedconcaveloopregionsresultingfromtwiningasthepredominanteffect
at higher strain
amplitudes.Gradualcyclichardeningisalsoevidentfromthedecreaseinplastic
strain with increasing number of cycles which correspondsto a
decrease in the area of hysteresis loop W. On the other
hand,theloopshapeparameterVHispracticallyconstantfordifferentstress
amplitudes. This can be explained by the uniform
deforma-tionoftheexperimental material
withoutsignicantlylocalizedplastic deformation.Thevalueof
parametern0of theexperimental material
washigherthanvaluesreportedforextrudedAZ31alloy[1]. Thisiscaused by
the greater plastic deformation capacity of cast material.For
wrought materials, a portion of the plastic deformation capac-ity
is used during fabrication.The low-cycle fatigue parameters e0f, c,
r0fand b were obtainedby means of the regression function.
Parametersb andcwere
inagreementwithvaluesreportedforextrudedAZ31alloy[3]andslightlyhigherthanthatofdie-castA91HP[35,53].
Parameter e0fof our experimental material is higher than that of
extrudedAZ31 alloy [3] and die-cast A91HP [35,53] due to the higher
plasticresponse.According to the regression analysis of the whole
fatigue life itwas established that the Stromayer function isthe
most
suitablefordescribingthefatiguebehavioroftheexperimentalmaterial.A
comparison of parameter b with the Basquin function shows thatthe
KV function better describes the fatigue behavior in the low-cycle
region.Fig. 13.
InuenceofstressamplitudeoncharacteristicsoffracturesurfaceofAZ31magnesiumalloy.
Initiationsitesaremarkedbyarrows, dashedlineindicatesthetransition
between propagation area and area of nal fracture.260 M. Horynov et
al. / Materials and Design 45 (2013) 253264Fig. 14. (a) Secondary
electron micrograph of fatigue fracture surface with initiation
site on cluster of secondary particles, (b) back-scattered electron
micrograph of initiationsite, (c) more detailed back-scattered
electron micrograph of area with AlMn based particles, and (d) EDX
spot analysis of area with AlMn based particles.Fig. 15. Secondary
electron micrographs of fatigue fracture surfaces of sample loaded
with stress amplitude of 65 MPa. (a) Initiation site, (b) beginning
of propagation area,(c) striations, and (d) region of nal
fracture.M. Horynov et al. / Materials and Design 45 (2013) 253264
261Thevaluesof fatiguelimitsfor108cycleswerebasicallythesame when
tted to each function. The value of the fatigue limit(rc = 45 MPa)
andthefatigueratio(rc/Rm = 0.26) were slightlyhigher thanthose
reportedfor squeeze-cast AZ31magnesiumalloybyZberovet al. [5].
ComparingtoAZtypemagnesiumalloys preparedby commonmethods of
casting, the value offatiguelimitofourexperimental material
issignicantlyhigherthanthatofas-castAZ61magnesiumalloy[54]andcomparablewithgravity-cast
[55] andhigh-pressuredie-cast [56,57] AZ91magnesiumalloy.
Itshouldbenoted, thatincreasingaluminumcontent
inAZtypemagnesiumalloys results inhigher fatiguestrength; Chamos et
al. [6] reported, that fatigue limit
forAZ61magnesiumalloyis23%highercomparedtoAZ31magne-siumalloys.Fig.
16. Secondary electron micrographs of fatigue fracture surfaces of
sample loaded with stress amplitude of 110 MPa (a) initiation site,
(b) beginning of propagation areawith secondary crack, (c)
striations, and (d) region of nal fracture.Fig. 17. MgAl phase
diagram [20].262 M. Horynov et al. / Materials and Design 45 (2013)
253264The fracture surfaces of selected specimens were examined.
Fa-tigue cracks initiated from the specimen surface or from the
clus-ters of AlMn based particles. Those clusters were observed
alongthelongitudinal sectionof thefatiguedspecimenandbecausethere
was no evidence of fatigue cracks near the non-surface clus-ters,
we can conclude, that those clusters act as initiation sites
onlywhen placed at specimen surface. This is in agreement with
ndingof Patel etal. [32]whoreportednearsurfaceAlMnparticleinAZ91
magnesium alloy which did not act as an initiation site.
Doc-umented fatigue crack initiated at afree surface and changed
itsdirection when reached AlMn particle.Initiation sites of
examined samples were mostly similar,
char-acterizedbycleavage-likefacets andinsomecases byratchetmarks.
As ratchet marks indicate relatively high stresses, its pres-ence
was observed only for specimen tested at 110
MPa.Ridgesparalleltothecrackpropagationdirectionwhichhavebeenobservedat
thebeginningof thecrackpropagationareasare most probably formed by
local shears that merge plateaus cor-responding to different
fracture plains in individual grains [58].5. ConclusionsLow andhigh
cycle engineering stresscontrolledfatigue testswere conducted on
AZ31 magnesium alloy fabricated by squeezecasting. The
microstructure, cyclic deformation behavior, and
lowandhighcyclefatiguelifewereinvestigatedandcomparedtoavailabledataonAZ31magnesiumalloyspreparedbydifferenttechnologies.
The conclusions obtainedcanbe
summarizedasfollows:(1)Themicrostructureof squeeze-cast
AZ31alloyexhibitsahigh level of microstructural and chemical
heterogeneity. Itis formed by solid solution, c and u phase,
eutectic and Al8-Mn5 particles. The average grain size of the
material is about50 lm.(2)Thematerial cyclicallyhardensat all test
conditionsandexhibits symmetrical behavior in compression and
tension.Sigmoidal shape of hysteresis curves, which suggests
twin-ingas apredominant mechanismof plasticdeformation,appears at
rst cycles at higher stress amplitudes.(3)The Stromayer function is
optimal for describing the wholefatigue life of squeeze-cast AZ31
alloy, while the KV
func-tionbetterdescribesthefatiguebehaviorinthelow-cycleregion.(4)Fatiguecracksinitiatesfromspecimensurfaceorfromthesurface
clusters of AlMn based particles.(5)Even though the squeeze cast
material exhibits a high levelof heterogeneity,
nosignicantscatteringof experimentalpoints was observed.(6)The
squeeze casting method enables the fabrication of
com-plicatedlyshapedpartswithmechanical propertiesworsethanthose of
wrought materials but signicantlybetterthanthoseof material
producedbycommonmethodsofcasting.Fig. 18. MgAlZn system computed
isothermal section at 335 C [21].M. Horynov et al. / Materials and
Design 45 (2013) 253264 263AcknowledgmentsThe work was nanced by
the Czech Science Foundation, Pro-ject No. 101/09/P576 and was also
carried out in the frame of theNETME centre supported by the
European Regional DevelopmentFund and CEITEC Central European
Institute of Technology
withresearchinfrastructuresupportedbytheProject
CZ.1.05/1.1.00/02.0068 nanced from the European Regional
Development Fund.TheauthorswouldalsoliketothankProf. TomKruml
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