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Materials Science and Engineering A 528 (2011) 7124– 7132

Contents lists available at ScienceDirect

Materials Science and Engineering A

journa l h o me pa ge: www.elsev ier .com/ locate /msea

atigue behaviour and crack growth rate of cryorolled Al 7075 alloy

rosenjit Dasa,b, R. Jayaganthanb,∗, T. Chowdhuryc, I.V. Singhd

Central Mechanical Engineering Research Institute (CSIR), Durgapur 713209, IndiaDepartment of Metallurgical and Materials Engineering, IIT Roorkee, Roorkee 247667, IndiaSchool of Material Science and Engineering, Bengal Engineering and Science University, Shibpur, IndiaDepartment of Mechanical & Industrial Engineering, IIT Roorkee, Roorkee 247667, India

r t i c l e i n f o

rticle history:eceived 24 February 2011eceived in revised form 26 March 2011ccepted 12 May 2011vailable online 12 June 2011

eywords:echanical characterizationltrafine-grained Al 7075 alloyryorollingatigue life

a b s t r a c t

The effects of cryorolling (CR) on high cycle fatigue (HCF) and fatigue crack growth rate behaviour of Al7075 alloy have been investigated in the present work. The Al 7075 alloy was rolled for different thicknessreductions (40% and 70%) at cryogenic (liquid nitrogen) temperature and its tensile strength, fatigue life,and fatigue crack growth mechanism were studied by using tensile testing, constant amplitude stresscontrolled fatigue testing, and fatigue crack growth rate testing using load shedding (decreasing �K)technique. The microstructural characterization of the alloy was carried out by using Field emissionscanning electron microscopy (FESEM). The cryorolled Al alloy after 70% thickness reduction exhibitsultrafine grain (ufg) structure as observed from its FESEM micrographs. The cryorolled Al 7075 alloysshowed improved mechanical properties (Y.S, U.T.S, Impact energy and Fracture toughness are 430 Mpa,530 Mpa, 21 J, 24 Mpa m1/2 for 40CR alloy) as compared to the bulk 7075 Al alloy. It is due to suppressionof dynamic recovery and accumulation of higher dislocations density in the cryorolled Al alloys. Thecryorolled Al alloy investigated under HCF regime of intermediate to low plastic strain amplitudes has

shown the significant enhancement in fatigue strength as compared to the coarse grained (CG) bulk alloydue to effective grain refinement. Fatigue crack growth (FCGR) resistance of the ufg Al alloy has been foundbe higher, especially at higher values of applied stress intensity factor �K The reasons behind such crackgrowth retardation is due to diffused crack branching mechanism, interaction between a propagatingcrack and the increased amount of grain boundaries (GB), and steps developed on the crack plane duringcrack-precipitate interaction at the GB due to ultrafine grain formation.

. Introduction

Polycrystalline materials with ultrafine-grained (ufg)icrostructure exhibit enhanced mechanical properties as

ompared to the conventional bulk materials [1]. Due to thever-growing demand for superior mechanical properties of Allloys, a new processing route, namely severe plastic deformationechnique (SPD) has been used extensively to produce this alloyith the ultrafine-grained microstructures due to the limitation of

onventional processing [2,3]. The processing of bulk aluminumlloys to ultrafine grain sizes through the conventional route isery difficult due to its high stacking fault energy. SPD processesuch as equal channel angular pressing (ECAP), multiple com-ression, accumulative roll bonding, and torsional straining are

sed to produce bulk nanostructured/ultrafine-grained metals fortructural and functional applications. However, majority of theseethods require large plastic deformations with strains much

∗ Corresponding author. Tel.: +91 1332 285869; fax: +91 1332 285243.E-mail address: rjayafmt@iitr.ernet.in (R. Jayaganthan).

921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2011.05.021

© 2011 Elsevier B.V. All rights reserved.

larger than unity. Nanostructured/ultrafine grained pure metalssuch as Cu, Al, Ni [4–6] and Al [7–10] alloys are produced from itsbulk metals/alloy by deforming them at cryogenic temperatureusing cryorolling technique. Rolling of pure metals and alloysin cryogenic temperature suppresses dynamic recovery and thedensity of accumulated dislocations reaches a higher steady statelevel and with the increasing number of cryorolling (CR) passes,these dislocation cells rearrange themselves into ultrafine-grainedstructures with high angle grain boundaries as reported in theliterature [2,11].

The development of high strength Al alloys for aerospace andautomobile applications is ever growing for extending life periodof the structural components fabricated from these alloys. The alu-minum alloys (7XXX) have been widely used as structural materialsdue to their excellent properties such as low density, high strengthto weight ratio, ductility, toughness, and resistance to fatigue[12–14]. The cryorolled (CR) Al 7075 alloy exhibited improved ten-

sile, hardness, impact properties compared to room temperaturerolled Al alloy as reported in the earlier work done by our researchgroup [10,15]. Increased fracture toughness of the cryorolled Alalloy, observed in our earlier work [16] was due to high density

P. Das et al. / Materials Science and Engineering A 528 (2011) 7124– 7132 7125

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Fig. 1. Schematic diag

f dislocations, ultrafine grain formation, grain boundary sliding,nd increased fracture stress (�f). The effect of grain size on cycliclasticity, fatigue life and crack growth rate of materials such asteel, copper, nickel, titanium and magnesium based alloys haseen reported [17–19]. Two major conclusions based on these stud-

es are drawn such as (i) the fatigue limit of pure fcc metals withelatively high stacking fault energy and wavy slip behaviour areot affected by the grain size; (ii) the fatigue strength of materi-ls exhibiting planar slip, increases with decreasing grain size andollows the Hall–Petch relationship in the same way as the yieldtress in conventional polycrystalline metals [20]. However, theres no reported literature on high cycle fatigue and fatigue crackrowth rate (FCGR) of cryorolled Al 7075 alloy. Therefore, aim of theresent work was to study the effect of cryorolling on fatigue limitnd fatigue crack growth resistance of Al 7075 alloy. The fatiguetrength of cryorolled Al alloys is strongly dependent on grain sizeimilar to the yield strength as observed in the present work. Themproved fatigue life and endurance limit has been observed in casef ufg Al 7075 alloy. FCGR tests were carried for tension–tensionatigue loading using compact tension (CT) specimens of ufg Allloy. A significant improvement in fatigue crack growth resis-ance was observed for ufg Al alloy in the stage II region of Parisurve as compared to bulk Al alloy, due to effective grain refine-ent. SEM characterization of the samples fractured under fatigue

oading has been carried out to reveal the transition in fractureorphology from the high to low stress region. Measurement of

triation spacing has been carried out by using SEM images for FCGRested samples, to investigate the crack growth resistance obtainedxperimentally.

. Experimental procedure

The Al 7075 alloy with the chemical composition of 6.04 Zn,.64 Mg, 1.76 Cu, 0.50 Cr, 0.2 Si, 0.15 Mn, 0.57 Fe, and Al balance

n the form of extruded ingot with the diameter of 50 mm, usedn the present work, has been procured from Hindustan Aeronau-ics Ltd., Bangalore, India. The as received Al extruded ingot was

achined into small plates and then solution treated (ST) at 490 ◦Cor 6 h followed by quenching treatment in water at room temper-ture. The solution treated Al 7075 alloy plates were subjected toolling at cryogenic temperature to achieve 40% and 70% thicknesseduction. The samples were soaked in liquid nitrogen taken in theryocan for 30 min prior to each roll pass during the rolling pro-ess. The diameter of the rolls and the rolling speed were 110 mmnd 8 rpm, respectively. The temperature before and after rollingf the samples was −190 ◦C and −150 ◦C, respectively, in each pass.

t may be mentioned that the time taken for rolling and puttingack the samples into cryocan was less than a 40–50 s during eachass in order to preclude the temperature rise of the samples. Theolid lubricant, MoSi2, has been used during the rolling process to

f cryorolling process.

minimize the frictional heat. The thickness reduction per pass was5% but many passes were given to achieve the required reductionof the samples. A schematic diagram of the cryorolling process isshown in Fig. 1.

Vicker’s macro hardness and tensile tests were performed todetermine the mechanical properties of the CR Al 7075 alloy sub-jected to various % reductions achieved by cryorolling treatment.Vickers macro hardness (HV) was measured on the plane parallelto longitudinal axis (rolling direction) by applying a load of 15 kgfor 15 s. The tensile test was performed after polishing the samplesprepared in accordance with ASTM Standard E-8/E8M-09 [17] sub-size specifications parallel to the rolling direction with a 25 mmgauge length in air at room temperature using a S series, H25K-Smaterials testing machine operated at a constant cross-head speedwith an initial strain rate of 5 × 10−4 s−1 [15]. XRD analysis was car-ried by Bruker AXS D8 advance instrument using Cu K� radiationfor identifying the presence of different phases in the staring bulkalloy and cryorolled samples.

Fatigue life characterization in terms of nominal stress (S–Ncurve characterization) was performed using a 100 kN servohydraulic material testing machine at a cross-head speed of3 mm/min under load control condition. In the tensile range, sinu-soidal load cycles at a stress ratio of R = 0.2 and at a frequency of20 Hz were used for all the tests. The samples for fatigue testingwere machined along longitudinal axis of the supplied extrudedingot for the bulk 7075 alloy and along rolling direction in case ofufg alloy, according to ASTM E 468-04 and E 466-07 [21,22] andtested in air. The sample dimensions are shown in Fig. 2(a). Prior totesting, in order to minimize the introduction of residual stressesthroughout the machining operation of the specimens, the sampleswere polished in air at room temperature. It enabled the elimi-nation of the remaining circumferential notches that could act asstress concentrators during the fatigue tests.

Fatigue crack growth rate (FCGR) tests have been carried out onas-received and cryorolled Al 7075 alloy, as per ASTM E647-08 [23]standard and the sample dimensions are shown in Fig. 2(b). Fortension–tension fatigue loading of compact tension (CT) specimen,the clevis loading fixtures were used. For this type of loading, boththe maximum and minimum loads are tensile, and the load ratio,R = Pmin/Pmax, is in the range of 0 < R < 1. A ratio of R = 0.1 is com-monly used for developing data for comparative purposes. Cyclicloading may involve various waveforms for constant-amplitudeloading. A measure of the resistance of a material to crack exten-sion is expressed in terms of the stress intensity factor. Initially, thespecimens were fatigue pre-cracked to either 0.1 B or h or 1.0 mm,whichever is greater and then growing the pre-crack under given

loading conditions (which includes frequency 20 Hz, load ratio0.1, load amplitude 3 kN and initial and final crack sizes, 17 mm(a/w = 0.34) and 24 mm, respectively. Crack length was measuredby means of travelling microscope. To facilitate easy crack length

7126 P. Das et al. / Materials Science and Engineering A 528 (2011) 7124– 7132

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ig. 2. (a) Dimensional details of high cycle fatigue specimen, (b) dimensionaletails of CT specimen (All dimensions are in mm).

easurement, lines were marked on the specimen surfaces at equalntervals of 0.5 mm across the direction of crack propagation [24].oad shedding technique (�K − decreasing) was used to determinehe crack growth rates at different �K values and Paris constantsc’ and ‘m’ in Paris equation (da/dN = C�Km). Tests were terminated

hen physical value of crack growth becomes negligible.

. Results and discussion

.1. Microstructure characterization

The optical micrograph of the bulk Al alloy (starting material)nd SEM micrographs of the cryorolled Al 7075 alloy after 40% and0% thickness reduction are shown in Fig. 3(a–c). The microstruc-ure of the bulk Al alloy exhibits lamellar grains lying parallel to thengot axis. The average grain size is around 40 �m. The grain sizes reduced to around 950 nm and 600 nm for the CR samples sub-ected to 40% and 70% thickness reduction, respectively as observedrom the Fig. 3(b and c). Since the dynamic recovery is effectivelyuppressed by rolling at liquid nitrogen temperature (−190 ◦C), theR Al 7075 alloy shows ultrafine grains or fragmented grains withigh fraction of high angle grain boundaries.

.2. Mechanical properties

A significant improvement in mechanical properties of Al 7075lloy obtained after cryorolling compared to starting bulk 7075l alloy is due to higher dislocation density and ultrafine grains.able 1 shows the effect of cryorolling over tensile and hardnessroperties of the alloy, which are reported in our earlier work15,16].

.3. XRD patterns

The XRD peaks of starting bulk Al 7075 alloy and cryorolled Al

lloy at different thickness reductions are shown in Fig. 4. Pres-nce of Fe rich (AlFeSi) and Mg rich (MgZn2) phases can be seenn case of bulk alloy. Amount of AlFeSi is much lower in case ofryorolled alloy compared to its bulk form due to solution treat-

Fig. 3. (a) Optical micrograph of starting material and FESEM image of (b) 40 CR and(c) 70 CR.

ment given prior to cryorolling. In case of 40% reduced samples,Mg rich (MgZn2) phase was not observed, as it got completelydissolved in the Al matrix due to induced cryorolling strain. Thepeak for AlCuMgSi and reappearance of MgZn2 precipitate peakare observed in the 70% cryorolled material. It is because of thesuppression of dynamic recovery in the alloy, due to cryorolling(rolling at liquid nitrogen atmosphere), which leads to accumu-lation of high amount of dislocation density in the 70% cryorolledsamples. When the cryrolled alloy containing high dislocation den-sity (at high strain, ε ≥ 1.8) were placed at room temperature, theaccumulated dislocations may act as a short-circuit path for solutes

and atomic migration [25] and facilitates easy nucleation of theprecipitates due to the higher driving force available at room tem-perature.

P. Das et al. / Materials Science and Engineering A 528 (2011) 7124– 7132 7127

Table 1Mechanical properties of the 7075 Al alloy for different processing conditions.

Al 7075 alloy E (Gpa) Poission’s ratio (�) Hardness (VHN) �ys (Mpa) �UTS (Mpa) % Elongation

Starting bulk alloy 72 0.33 81 260 510 1540% room temperature rolled 72 0.33 137 395 517 1240% cryorolled 72 0.33 155 430 530 1070% room temperature rolled 72 0.33 170% cryorolled 72 0.33 1

Fc

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3.5. Fatigue fracture surface morphology

ig. 4. XRD pattern of (a) Starting Bulk alloy,(b) 40% cryolled alloy and (c) 70%ryorolled alloy.

.4. High cycle fatigue properties

Fig. 5(a) shows the stress versus number of cycles to failure curveor the bulk Al 7075 alloy (at R = 0.2). It is observed that at an alter-ating stress level of 240 MPa, the number of cycles to failure is.68 × 105; whereas, at alternating stress of 140 MPa, the numberf cycles to failure is increased to 2.8 × 107 for bulk Al alloy. Onhe other hand, for 40% cryorolled alloy at stress level of 380 MPa,he number of cycles to failure is 1.1 × 106; whereas at 230 MPa it

ndergoes 3.1 × 108 cycles to failure. The values obtained in casef cryorolled Al alloy (70% reduction) are even higher, which are.3 × 107 cycles at 480 MPa, and undergoes 2.3 × 109 cycles to fail-

Fig. 5. (a) Comparative S–N curve for all the materials, (b) schema

60 510 525 873 540 550 5

ure at 340 MPa. Hence, a gradual improvement in fatigue life isobserved due to formation of ultrafine grains in the cryorolled Alalloy.

The higher HCF resistance of ufg alloy can be explained by thetotal strain fatigue life diagram shown schematically in Fig. 5(b).The fatigue life relation shown in the figure is based on the split-ting of the total strain amplitude �εt/2 into its elastic and plasticcomponents, �εel/2 and �εpl/2, respectively. The first term refersto HCF (Basquin relation) and is usually expressed in terms of thefatigue strength coefficient �f, the elastic modulus E, the fatiguestrength exponent b and Nf, whereas the second term, referringto LCF (Coffin–Manson relation), is expressed via the fatigue duc-tility coefficient � ′

f, the fatigue ductility exponent c and Nf. After

cryorolling, ufg alloy exhibits high � ′f/E value, which contributes

for their increased fatigue strength [26]. Fig. 6 shows the improvedhigh cycle fatigue properties of the ufg Al 7075 alloy compared toother Al alloys used for structural applications. The TEM observa-tions of ultrafine-grained Ni, Ti and Cu alloys microstructure afterstress controlled fatigue loading reported by earlier researchersrevealed a more clear dislocation array in the vicinity of grainboundaries, whereas no dislocation substructure has formed insidethe grains. Also, twins formation was observed but it is very lowdensity led to the conclusion that such defects do not have influenceon the fatigue behaviour [27,28]. Hence, a lack of shear band-ing, grain coarsening and dislocation substructure formation wasobserved in case of ufg metals and alloys as compared to their bulkcounterpart. Severe straining associated with ufg microstructureformation is responsible for inability of the material to undergofurther cyclic hardening in case of stress controlled fatigue (HCF)loading [17]. The similar phenomenon may occur in case of ufg 7075Al alloy.

The fracture surface of the specimens, tested for generation ofS–N curve, has been examined under SEM and the fractographs

tic total strain fatigue life diagram for ufg and CG material.

7128 P. Das et al. / Materials Science and Engi

Fa

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ig. 6. Comparative S–N plot of 70% cryorolled (ufg alloy) with other series of Allloys.

btained is shown in Figs. 7 and 8. A significant change in theatigue fracture behaviour is observed as evident from fractographsf all the three materials tested under two different stress ranges.he fracture surfaces of bulk Al alloy shows dimpled ductile frac-

ig. 7. Fracture surface morphology of Al 7075 alloys, cryorolled at different percentage ofnd (c) 70% cryorolled alloy.

neering A 528 (2011) 7124– 7132

ture with little amount of facets, dimple size gradually decreaseswith increasing percentage of thickness reduction attained due tocryorolling as shown in Figs. 7 and 8. Fatigue striations were notobserved in the high stress range for any of the materials due tohigher amplitude of the applied load facilitates dimpled rupture,whereas in the low stress range, lower stress amplitude facilitatesimproved fatigue ductility, which governs generation of ductilestriations observed in the fracture surfaces of both bulk and ufgAl alloy [29,30].

3.6. Fatigue crack growth rate

The compact tension (CT) specimens used for FCGR (da/dN)determination are prepared in longitudinal direction with notchand intended direction perpendicular to the rolling direction asper ASTM E-1820 and ASTM E-647 [24,31,32] standards as shownin Fig. 2(b). The ratio of W/B is 4 for FCGR specimens. The polishedand degreased CT specimens were then pre-cracked to a total cracklength of nearly 17 mm (a/w = 0.34) prior to the actual test. Thepresent work presents the experimental results of fatigue crackgrowth resistance of ultrafine-grained (ufg) Al 7075 alloy. StageII regime of growth rate is explored for all the materials and the

results are shown in the form of traditional log–log da/dN − �Kdiagrams as shown in Fig. 9. Fig. 9(a) describes the effect of stressratio over FCGR in case of bulk alloy, which shows enhanced crackgrowth rate with the increase in stress ratio (R). Values of Paris

reduction, for high stress region of fatigue loading: (a) Bulk alloy, (b) 40% cryorolled

P. Das et al. / Materials Science and Engineering A 528 (2011) 7124– 7132 7129

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ig. 8. Fracture surface morphology of Al 7075 alloys, cryorolled at different percennd (c) 70% cryorolled alloy.

onstants for bulk Al alloy are plotted in Fig. 9(b). Comparative plotf FCGR and Paris constants (c and m) for all three materials arehown in Fig. 9(c and d). The results are varying from the few exper-mental data available for other ufg materials like high purity Ni, Ti17,33]; whereas, it is in good agreement with the results obtainedor commercially pure Cu [18,19]. In fact, the cryorolled Al shows aelatively high fatigue crack growth resistance with respect to thenprocessed coarse grained Al alloy, especially at high values ofpplied stress intensity factor, �K. A numerically larger �K repre-ents higher mechanical driving force to propagate the crack, whichignifies that for same da/dN, a much higher �K is needed in casef ufg Al alloy compared to bulk Al alloy.

From the point of view of fatigue mechanism, it has beenbserved that the failure is dominated by fatigue resistance ofhe grains with no influence of the grain boundaries in the casef bulk polycrystalline Al alloy with grain size ranging from 30o 50 �m. On the other hand, the fracture mechanism of the ufgl alloy is dominated by the interaction between a propagatingrack and the grain boundaries (GBs) structure, the voids created

y grain boundary sliding, which leaves wedges at the points ofriple junction and also enhanced plastic zone size ahead of therack tip. All of the conditions stated above can produce retarda-ion in the crack growth rate in case of ufg Al 7075 alloy. Moreover,

f reduction, for low stress region of fatigue loading: (a) Bulk alloy, (b) 40% cryorolled

in comparison with the coarse grain structure, a small grain sizecan potentially result in more homogeneous deformation, whichcan retard crack nucleation by reducing stress concentrations andultimately could raise the fatigue limit of the ufg structure. It canbe considered that, in most planar slip materials, GB provides a“topological obstacles to the slip” [34]. In case of ufg Al alloy, theprecipitates are smaller in size and well dispersed, crack devel-ops steps on the crack plane while bypassing the precipitates dueto crack-precipitate interaction at the GBs. It results in a fatiguecrack retardation and deflection at GB, which leads to an increaseof the free crack surface, contribute to significant suppression ofcrack propagation rate. Fig. 10(a) explains the transition in crackgrowth mechanism from transgranular in case of coarse grainedbulk Al alloy to intergranular in case ufg Al alloy. Fig. 10(b) showsthe experimental observation of crack deflection due to diffusedbranching mechanism seen in case of ufg Al alloy samples at higheraverage �K due to effective crack and grain boundary interaction.

3.7. Fracture surface morphology of crack growth tested samples

The fracture surfaces of CT specimens have been examinedunder SEM and are presented in Fig. 11. FCGR tests are carriedout using decreasing �K technique, SEM results for all the speci-

7130 P. Das et al. / Materials Science and Engineering A 528 (2011) 7124– 7132

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Fi

ig. 9. (a) Fatigue crack growth curve of bulk alloy at different stress ratio, (b) plot orack growth curve and (d) comparative plot of Paris constants for all three materia

ens are given for high initial �K and final lower �K. Fractographseveals that crack growth occurs by forming ductile striations. Theequirement for striation formation is ductility, which is adequatelyet in the ufg Al alloy investigated in this work. Fracture sur-

aces signify the improved fatigue crack growth resistance of thefg Al alloy with increasing % reduction achieved by cryorollingechnique. It may be noticed that at high initial �K, the fracture

ppears to be cyclic-cleavage with ductile striations; whereas atnal lower �K, the ductile striation is dominating feature for allhe materials. The amount of ductile striations is more in case offg Al alloy, which indicates improved crack tip plasticity due to

ig. 10. (a) Crack paths for bulk (transgranular) and UFG (intergranular) 7075 Al alloy and (n case of UFG alloy.

constants for the bulk alloy at different stress ratio, (c) Comparative plot of Fatigue

effective grain refinement. The striations bear nearly one-to-onerelation to macroscopic measurements of da/dN. The average stri-ation widths calculated from the SEM images are in good agreementwith the measured crack growth rate. For same �K range, the stri-ation spacing is relatively smaller in case of ufg Al alloy due to theinteraction between growing crack and increased amount of grainboundaries, hindrance of the crack in presence of small sized pre-

cipitations at the GB. Several small secondary cracks can be clearlyseen in perpendicular and parallel direction with respect to themain crack path direction, even at some distance from it. The simi-lar mechanism has already been reported for commercial pure UFG

b) experimental observation of crack deflection due to diffuse branching mechanism

P. Das et al. / Materials Science and Engineering A 528 (2011) 7124– 7132 7131

Fig. 11. SEM factographs of FCGR tested C.T specimens showing striations: (A) Bulk alloy(R = 0.1).

Table 2Comparison of striation width for all conditions.

Al 7075 alloy �K(Mpa√

m) Striation width (mm/cycle)

Bulk alloy (R = 0.3) 17 8 × 10−4

10.8 1.1 × 10−4

Bulk alloy (R = 0.1) 17 6 × 10−4

10.8 0.8 × 10−4

40% cryorolled (R = 0.1) 24 5 × 10−4

15 0.6 × 10−4

Tgam

ttcata

4

cunag

70% cryorolled (R = 0.1) 29 7 × 10−4

19 1 × 10−4

i [35], UFG Cu [36], and AA6063 aluminum alloy [37]. Microcracksenerated to accommodate excessive strain in the crack vicinity for

decrease in the further strain hardening capability of the ufg alloyay be due to the severe grain refinement.Table 2 shows the average value of striation width with �K (ini-

ial and final) for all the conditions. The cryorolled Al alloy with 70%hickess reduction shows a slightly higher value of striation widthompared to bulk Al alloy at same stress ratio; but its �K range islso higher, revealing a significant increase in crack growth resis-ance due to the formation of ultra fine grains in the cryorolled Allloy.

. Conclusions

In the present work, the experimental characterization of highycle fatigue properties and fatigue crack growth resistance of

ltrafine-grained Al 7075 alloy produced by cryorolling tech-ique has been investigated. A substantial increase in mechanicalnd fatigue properties and significant retardation in fatigue crackrowth rate has been observed in case of cryorolled Al alloy sam-

(R = 0.1), (B) bulk alloy (R = 0.3), (C) 40% cryorolled (R = 0.1) and (D) 70% cryorolled

ples due to high density of dislocations, grain boundary sliding,increased amount of grain boundaries and significant grain refine-ment with the increasing amount of % reduction achieved bymultiple cryorolling passes. XRD patterns confirm the presence ofsmall sized and well dispersed AlCuMgSi and MgZn2 precipitatesat the 70% cryorolled, fully formed ultrafine-grained 7075 Al alloy,which aid for enhancement in fatigue, mechanical properties andcrack growth resistance.

The dependence of fatigue strength on grain size is similar to theyield strength, which is explained by the total strain fatigue life dia-gram following Basquin relation. Smaller grain size of cryorolled Al7075 alloy leads to more homogeneous deformation, which retardcrack nucleation by reducing stress concentrations and ultimatelyraise the fatigue limit of the ufg structure. The improvement infatigue crack growth resistance of the ufg Al alloy is due to interac-tion between a propagating crack and the higher amount of grainboundaries (GBs) structure, enhanced plastic zone size ahead of thecrack tip and crack-precipitate interaction at the GB.

Fractographs of all the tested materials fractured undertension–tension fatigue loading signifies transition in fracture mor-phology from dimpled rupture at the high stress range to ductilestriations at lower stress level. SEM images used to calculate stria-tion spacing in case FCGR tested samples confirms that it is nearlyone-to-one relation with the macroscopic measurements of crackgrowth per cycle. For same �K range, striation spacing is relativelysmaller in case of ufg Al alloy, which explains the improved fatiguecrack growth resistance attained by effective grain refinement inthe alloy during cryorolling.

Acknowledgements

The author, Mr. Prosenjit Das, would like to thank National Met-allurgical Laboratory, Jamshedpur, especially Dr. S. Tarafdar and

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[35] Y. Estrin, A. Vinogradov, Int. J. Fatigue 32 (6) (2010) 898–907.

132 P. Das et al. / Materials Science an

r. S. Sivaprasad for their cordial help to carry out Fatigue crackrowth rate experiments. The author, Dr. R. Jayaganthan, wouldike to thank DST, New Delhi for their financial support to this workhrough grant no: DST-462-MMD.

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