Materials and Design 32 (2011) 1298–1305

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Materials and Design

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Tensile and impact-toughness behaviour of cryorolled Al 7075 alloy

P. Das a, R. Jayaganthan a,⇑, I.V. Singh b

a Department of Metallurgical and Materials Engineering & Centre of Nanotechnology, Indian Institute of Technology Roorkee, Roorkee 247667, Indiab Department of Mechanical and Industrial Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, India

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

Article history:Received 7 June 2010Accepted 21 September 2010Available online 25 September 2010

Keywords:A. Non-ferrous metals ad alloysC. Heat treatmentE. Mechanical

0261-3069/$ - see front matter � 2010 Elsevier Ltd. Adoi:10.1016/j.matdes.2010.09.026

⇑ Corresponding author. Tel.: +91 1332 285869; faxE-mail address: rjayafmt@iitr.ernet.in (R. Jayagant

The effects of cryorolling and optimum heat treatment (short annealing + ageing) on tensile and impact-toughness behaviour of Al 7075 alloy have been investigated in the present work. The Al 7075 alloy wasrolled for different thickness reductions (40% and 70%) at cryogenic (liquid nitrogen) temperature and itsmechanical properties were studied by using tensile testing, hardness, and Charpy impact testing. Themicrostructural characterization of the alloy was carried out by using field emission scanning electronmicroscopy (FE-SEM). The cryorolled Al alloy after 70% thickness reduction exhibits ultrafine grain struc-ture as observed from its FE-SEM micrographs. It is observed that the yield strength and impact tough-ness of the cryorolled material up to 70% thickness reduction have increased by 108% and 60%respectively compared to the starting material. The improved tensile strength and impact toughness ofthe cryorolled Al alloy is due to grain refinement, grain fragments with high angle boundaries, and ultra-fine grain formation by multiple cryorolling passes. Scanning electron microscopy (SEM) analysis of thefracture surfaces of impact testing carried out on the samples in the temperature range of �200 to 100 �Cexhibits ductile to brittle transition. cryorolled samples were subjected to short annealing for 5 min at,170 �C, and 150 �C followed by ageing at 140 �C and 120 �C for both 40% and 70% reduced samples.The combined effect of short annealing and ageing, improved the strength and ductility of cryorolledsamples, which is due to precipitation hardening and subgrain coarsening mechanism respectively. Onthe otherhand, impact strength of the cryorolled Al alloy has decreased due to high strain rate involvedduring impact loading.

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1. Introduction

Severe plastic deformation (SPD) techniques are now widelyused for the production of ultrafine grained (UFG) microstructuresin bulk metals. A unique advantage of these techniques is due tothe possibility of developing fully dense nanostructured and ultra-fine grained materials without the introduction of any contami-nants. For example, severe plastic deformation (SPD) processessuch as equal channel angular pressing (ECAP), multiple compres-sion, accumulative roll bonding, and severe torsional straining aregiven lot of focus for the development bulk nanostructured metalsfor structural and functional applications. The SPD methods impartvery large deformations to the samples at relatively low tempera-tures under high pressures [1,2]. However, majority of these meth-ods require large plastic deformations with strains much largerthan unity and scaling up of these processes is difficult. Almost46% of aluminum alloys are used in the form of sheet and foil asreported in the literature [3]. Conventional rolling could be a suit-able technique for the commercial production of bulk ultrafine

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grained Al alloys sheets but due to dynamic recovery and highstacking fault energy of Al and its alloys, it is difficult to produceultrafine grained microstructures in the samples. To overcomethese constraints, cryorolling has been identified as one of the po-tential routes for producing nanostructured/ultrafine grained puremetals Cu, Al, Ni [1,2,4] and Al alloys [5–7] from its bulk alloys.Rolling of pure metals and alloys in cryogenic temperature sup-presses dynamic recovery and the density of accumulated disloca-tions reaches a higher steady state level as compared to roomtemperature rolling. With the multiple cryorolling (CR) passes,these higher density of dislocations rearrange themselves intosub-structures followed by the formation of ultrafine grain struc-tures (ufg) with high angle grain boundaries [8,9].

Fracture and impact-toughness behaviours of aluminum alloysare of great technological importance for ensuring safe materialdesign in structural applications. The aluminum alloys (7XXX)have been widely used as structural materials due to their excel-lent properties such as low density, high strength, ductility, impacttoughness, fracture toughness and resistance to fatigue [10–14].The cryorolled (CR) Al 7075 alloy exhibited the improved tensile,and hardness properties compared to room temperature rolled Alalloy as reported in the literature [3]. Precipitation kinetics of the

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CR Al 7075 alloy revealed that ultrafine grained alloy exhibits high-er driving force for the precipitate formation than its bulk Al alloys[3].

There is no reported literature on the impact toughness of cryo-rolled Al 7075 alloy. Therefore, the present work has been envis-aged to study the impact toughness, tensile properties, effect ofoptimum heat treatment over impact and tensile properties of CRAl 7075 alloy by Charpy impact and tensile testing, respectively.The microstructure of the CR Al alloy and post-heat treated CR Alalloy is characterized by FE-SEM for correlating it with their ob-served mechanical properties. The fracture surface of tensile, im-pact test samples was characterized by FE-SEM to reveal themode of failure in the CR, and post-heat treated CR Al alloys.

Fig. 1. (a) Optical micrograph of starting material and SEM micrograph of (b) CRmaterials after 40% thickness reduction (c) CR material after 70% thicknessreduction.

2. Experimental procedure

The Al 7075 alloy with the chemical composition of 6.04 Zn,3.64 Mg, 1.76 Cu, 0.50 Cr, 0.2 Si, 0.15 Mn, 0.57 Fe, and Al balancein the form of extruded ingot with the diameter of 50 mm, usedin the present work, has been procured from Hindustan Aeronau-tics Ltd., Bangalore, India. The as received Al extruded ingot wasmachined into small plates and then solution treated (ST) at490 �C for 6 h followed by quenching treatment in water at roomtemperature. The solution treated Al 7075 alloy plates were sub-jected to rolling at cryogenic temperature to achieve 40% and 70%thickness reduction. The samples were soaked in liquid nitrogentaken in the cryocan for 30 min prior to each roll pass duringthe rolling process. The diameter of the rolls was 110 mm andthe rolling speed was 8 rpm. The temperature before and afterrolling of the samples was �190 �C and �150 �C, respectively, ineach pass. It may be mentioned that the time taken for rollingand putting back the samples into cryocan was less than a 40–50 s during each pass in order to preclude the temperature riseof the samples. The solid lubricant, MoSi2, has been used duringthe rolling process to minimize the frictional heat. The thicknessreduction per pass was 5% but many passes were given to achievethe required reduction of the samples. In order to improve themechanical properties, cryorolled samples were subjected toshort annealing for 5 min at, 190 �C, 170 �C and 150 �C followedby ageing at 160 �C, 140 �C and 120 �C for both 40% and 70% re-duced samples.

Micro hardness and tensile tests were performed to determinethe strength and ductility of the CR Al 7075 alloy subjected to var-ious strains, annealing and ageing treatment. Vickers macro hard-ness (HV) was measured on the plane parallel to longitudinal axis(rolling direction) by applying a load of 15 kg for 15 s. The surfaceof the specimen was polished mechanically using emery paperprior to each HV measurement to ensure its clean surface. Thehardness value is an average of ten measurements made on surfaceof each specimen. The tensile specimens were prepared in accor-dance with ASTM Standard E-8/E8M-09 [15] sub-size specifica-tions parallel to the rolling direction with a 25 mm. gauge length.The tensile test was performed after polishing the samples in airat room temperature using a S series, H25K-S materials testing ma-chine operated at a constant crosshead speed with an initial strainrate of 5 � 10�4 s�1. The samples with different percentage ofthickness reduction, after cryorolling, were machined to the samelength, without changing the thickness for tensile test.

For impact testing test, CR Al alloy and original samples werecut from longitudinal (rolling) direction. The in-plane specimendimension is 10 mm � 55 mm with a 2 mm deep, 45� V-notch hav-ing a 0.25 mm tip radius at the center of the specimen, prepared asper the ASTM Standard E-23-07ae1 [16]. Impact tests were con-ducted at temperatures ranging from �200 �C to 100 �C to investi-gate the ductile–brittle transition behaviour of the Al 7075 alloy

and CR Al alloy. Each sample was cooled or heated to temperaturesat least 50 �C above or below the desired testing temperature. Itprovides enough time for each sample to get properly positionedin the testing anvil, as well as for the equilibration of the interiorand exterior temperature. The microstructural characteristics ofthe starting bulk Al alloy and cryorolled Al 7075 alloys and alsotheir fracture surfaces were characterized by using FE-SEM.

Fig. 2. Rectangular tension test specimens (dimensions in mm).

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3. Results and discussion

3.1. Microstructure characterization

The optical micrograph of the bulk alloy (starting material) andSEM micrographs of the cryorolled Al 7075 alloy after 40% and 70%thickness reduction are shown in Fig. 1a–c. The microstructure ofthe bulk alloy exhibits lamellar grains lying parallel to the ingotaxis. The average grain size is around 40 lm. The grain size is re-duced to around 950 nm and 600 nm for the CR samples subjectedto 40% and 70% thickness reduction, respectively as observed fromthe Fig. 1b and c. Since the dynamic recovery is effectively sup-pressed by rolling at liquid nitrogen temperature (�190 �C), theCR Al 7075 alloy shows high fraction of high angle grain bound-aries and high amount of dislocation density as reported in our ear-lier work [17,18].

3.2. Tensile properties

The tensile specimens were machined as per ASTM E-8/E8M-09sub-size specifications parallel to the rolling direction with gaugelengths of 25 mm. A schematic diagram of tensile specimens isshown in Fig. 2. Fig. 3 shows the tensile properties of Al 7075 alloycryorolled at different percentage of thickness reduction (40% and70%). It is observed that the tensile strength (UTS) of CR sampleshas increased from 500 MPa to 530 MPa (nearly 6% increase),whereas yield strength (YS) has increased from 260 MPa to430 MPa (nearly 66% increase) for 40% thickness reduction. Simi-larly, for the CR samples with 70% thickness reduction (e = 1.8),UTS has increased from 500 MPa to 550 MPa (nearly 10% increase)and YS has increased from 260 MPa to 540 MPa (nearly 108% in-crease). The enhancement of tensile strength in CR Al alloys isdue to the effective suppression of dynamic recovery, which leadsto high amount of dislocation density in the samples [17]. Theinfluence of cryorolling treatment on yield strength (YS) is sub-stantial as compared to that of it on ultimate tensile strength of

Fig. 3. Tensile properties of Al 7075 alloy after different percentage of thicknessreduction.

the samples due to effective grain refinement. However, % elonga-tion of the samples decreases with cryorolling as shown in Fig. 3. Itcan be improved further by optimum heat treatment i.e., shortannealing and ageing.

3.3. Optimum heat treatment

The phenomena such as recovery, recrystallization, and precip-itation hardening ought to be controlled through the optimum HT(annealing + ageing) conditions for enhancing both strength andductility simultaneously to a considerable extent. Hence, the CRAl 7075 alloy samples, with 40% and 60% thickness reduction, were

Fig. 4. Hardness of CR 7075 alloy after short annealing + aging at 150 �C for 5 min.+120 �C up to 8 h: (a) 40% reduction and (b) 70% reduction sample.

Fig. 5. Tensile properties of CR 7075 Al alloy subjected to various conditions: (1)starting material, (2) CR at 40% reduction, (3) CR (40%) 150 �C for 5 min. +120 �C for3 h, (4) CR at 70% reduction and (5) CR (70%) 150 �C for 5 min. +120 �C for 3 h.

Fig. 7. Effect of cryorolling on the impact energy of 7075 Al alloy.

Fig. 8. Effect of cryorolling and CR + optimum HT on the impact energy of 7075 Alalloy.

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subjected to short annealing at 190 �C, 170 �C and 150 �C for 5 minfollowed by ageing at 160 �C, 140 �C and 120 �C up to 8 h. Fig. 4shows the variation of hardness with ageing time at the shortannealing temperature of 150 �C combined with different ageingtemperatures. It is evident that hardness decreases with ageingtime at the ageing temperatures of 120 �C, 140 �C and 160 �C. Itis because of the dynamic recovery of the CR samples subjectedto these short annealing and ageing treatments. The effect of thesubstructure coarsening is prevalent as compared to precipitationhardening mechanism in the earlier ageing time; afterwards hard-ness is increasing due to precipitation hardening mechanism. Iden-tified peak aged condition for all annealed state of the cryorolled7075 Al alloy specimens was 150 �C for 5 min + 120 �C for 3 h forboth 40% and 70% thickness reduction and this condition was cho-sen for the tensile testing and the results are shown in Fig. 5.

The cryorolled samples after peak ageing shows a significant in-crease in YS and UTS with a slight increase in ductility. The YS, UTSand ductility of the CR materials with the peak heat treated condi-tion have increased from 430 MPa to 450 MPa, 530 MPa to542 MPa and 10% to 12% respectively, for the 40% thickness reduc-tion. Similarly, for the cryorolled samples with 70% reduction, YS,UTS and ductility have increased from 540 MPa to 560 MPa,550 MPa to 573 MPa and 5% to 8% respectively. The simultaneousimprovement in strength and ductility of CR Al 7075 alloy sub-jected to post-heat treatment is due to the combined effect of dy-namic recovery, grain refinement, and precipitation hardening asreported in our earlier work [3,18].

3.4. Impact properties

For impact testing, samples were cut from longitudinal (rolling)direction. The in-plane specimen dimensions are 10 mm � 55 mmwith a 2 mm deep, 45� V-notch having a 0.25 mm tip radius at thecenter of the specimen, which is prepared as per the ASTMStandard E-23-07ae1 (Fig. 6). Fig. 7 shows the impact toughness

Fig. 6. Schematic diagram

properties of Al 7075 alloy cryorolled at different percentage ofthickness reductions (40% and 70%). It is observed that the impacttoughness of the CR samples increases due to breakage of the largealuminum dendrites, grain fragment with high angle boundariesand formation of ultrafine grains with the increasing number ofcryorolling passes [3,19–21]. Impact energy of starting bulk Al al-loy is 17 J, and it has increased to 21 J (24% increase) and 27 J(nearly 60% increase) after 40% and70% thickness reductions,respectively. The effect of optimum heat treatment (HT) conditions(short annealing + ageing) conditions is not well pronounced overthe impact toughness properties of the CR Al 7075 alloy with dif-ferent percentage of thickness reductions as shown in Fig. 8. It isclear from this figure that precipitation hardening does not im-prove impact strength of CR Al alloy, which may be due to highstrain rate involved in impact testing and preferential fracture pathfacilitated by precipitates.

3.4.1. Ductile to brittle transitionAlthough Al alloys do not show significant effect of ductile to

brittle transition on the impact strength, sub-zero properties ofthese alloys is limited. Hence, the influence of ductile to brittletransition on the impact strength of the cryorolled Al alloy, atsub-zero temperature (�200 �C, �150 �C, �80 �C, �50 �C) has beeninvestigated in the present work. Specimens oriented in rollingdirection were tested. It is observed that CR samples show a

of impact specimens.

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profound effect upon impact toughness that enhances with in-crease in thickness reduction due to cryorolling. Different criteriaare used to find out the transition temperature of CR Al alloysdepending upon applications. For example, reducing the grain sizer affects the DBTT, which shifts the curve towards left and gives awider range of service temperature for the material as shown inFig. 9. The improved impact toughness of CR Al alloy due to grainrefinement ensures higher absorbed energy compared to bulk alloyat the same temperature and subsequently DBTT curve shifts to-wards the left what marks the usefulness of the UFG alloy in thesub-zero temperature applications.

3.4.2. Fracture appearance transition temperature (FATT)Different criteria are used to determine the transition tempera-

ture, depending on nature of the applications such as (i) T1 is thetransition temperature at which fracture is 100% ductile (fibrous),it is called fracture transition plastic (FTP), (ii) T2 is transition tem-perature at which fracture is 50% cleavage and 50% ductile, (iii) T3

is the transition temp. at which average energy absorption of uppershelf (ductile regime) and lower shelf (brittle regime) took place,(iv) T4 is transition temperature defined at Cv = 20 J and (v) T5 isthe transition temperature at which fracture is 100% cleavage,called Neal ductility temperature (NDT) [22]. In the present work,transition temperature is considered as the temperature at which

Fig. 9. (a and b) Variation of impact energy with test temperature.

fracture manifests as 50% cleavage and 50% ductile. Fig. 10a–cshows the FATT also called ductile–brittle Transition Temperature(DBTT) for cryorolled samples and the starting bulk Al alloy. It isevident from this figure that grain size affects transition tempera-ture; the DBTT curve is shifted to the left and enables wider rangeof service temperatures for the material. So, sub-zero performanceof the Al alloy has improved with increasing percentage of thick-ness reduction during cryorolling.

Fig. 10. (a–c) Variation of FATT with increasing % reduction of Al alloys: (a) startingmaterial, (b) 40% reduction and (c) 70% reduction.

Fig. 11. Fracture surface morphology (room temp.) of 7075 Al alloys under tensileloading cryorolled at different percentage of reduction: (a) starting material, (b) 40%reduction, (c) 70% reduction and (d) 70% reduction after aging.

Fig. 12. Fracture surface morphology of Al 7075 alloys, under impact loading,cryorolled at different percentage of reduction (a) Starting material, (b) 40%reduction, (c) 70% reduction and (d) 70% reduction after aging.

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Fig. 13. Fracture surface morphology of 7075 Al alloys showing ductile to brittletransition: (a) starting material at �200 �C, (b) starting material at 100 �C, (c) 70%reduction at �200 �C and (d) 70% reduction at 100 �C.

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3.5. Fracture surface morphology

3.5.1. Tensile fractureThe SEM fractographs of tensile samples of the Al 7075 alloy

cryorolled at different percentage of thickness reduction (40%and 70%) are shown in Fig. 11. It reveals that starting bulk Al alloyspecimens is fractured in a total ductile manner, consisting of well-developed dimples over the entire surface. The average dimple sizeof the starting materials is 5 lm and then it gradually decreaseswith increasing percentage of thickness reduction attained due tocryorolling as shown in Fig. 11. The dimple size gets reduced to lessthan 1 lm after 70% thickness reduction in the cryorolled samples.A continuous decrease in dimple size observed in the cryorolledsamples may be due to the grain refinement and work hardening,which are in tandem with the similar features reported for the se-verely deformed samples reported in the literature [23,24].

3.5.2. Impact fractureFig. 12 shows the SEM fractographs of samples, tested under

impact loading, of the Al 7075 alloy cryorolled at different percent-age of thickness reduction. The fracture behaviour of the samplestested under impact load is different from that of tensile test dueto different load forms [25–27]. The basic difference between bothtypes of fracture surfaces is due to the fact that the tensile speci-mens results in tensile fracture; whereas, Charpy impact speci-mens exhibits shear induced fracture. Due to high strain rateinvolved in the impact specimens, the fracture surface shows com-plete dimple fracture as compared to some quasi-cleavage regionspresent in the case of tensile fracture surface with increasing %reduction obtained by cryorolling. The fracture morphologies aredifferent in different areas of the impact specimen, due to non-uni-form stress distribution in the specimens. The impact stress causesenergy accumulation near the rear side of the specimen, whichleads to the triaxial stresses. Consequently, deformation of the rearpart is larger than the impact part.

3.5.3. Ductile to brittle transition in case of impact fractureFig. 13 signifies ductile to brittle transition in the fracture surface

of both starting bulk alloy and ultrafine grained Al alloy. The fracturesurface at�200 �C for bulk alloy and CR Al alloy shows tearing ridgesand quasi cleavage brittle appearance, respectively, whereas at100 �C, both samples exhibit a dimple fracture upon failure. This isdue to limited active slip systems operating at low temperature,which caused low plastic deformation. However, increasing temper-ature facilitates activation of more slip systems resulting in highplastic deformation and thereby impact energy also increases.

4. Conclusion

Tensile, impact-toughness, and fracture behaviour of cryorolledAl 7075 alloy have been investigated in the present work. A substan-tial increase in yield strength and significant enhancement in impacttoughness of the cryorolled Al alloy samples was observed due tohigh density of dislocations, grain boundary sliding and significantgrain refinement with the increasing amount of % reductionachieved by multiple cryorolling passes. An increase in % elongationin CR Al alloys has been observed upon subjecting them to optimumheat treatment (short annealing + ageing) of 150 �C for 5 min +120 �C for 3 h. Ageing effect is not pronounced on the impact tough-ness due to the high strain rate involved during impact loading.

The effect of ductile to brittle transition on the impact strengthhas been investigated to evaluate the sub-zero properties of Al7075 alloy. The tests were conducted at temperatures ranging from�200 �C (liquid nitrogen temperature) to 100 �C. The DBTT shifts

P. Das et al. / Materials and Design 32 (2011) 1298–1305 1305

the curve towards left with the grain size reduction. The improvedimpact strength due to grain refinement in CR Al 7075 may beexploited for sub-zero temperature applications.

The SEM fractographs of tensile and impact samples of startingAl alloy and CR Al alloys at different percentage of thickness reduc-tion reveals that the former specimens are fractured in a total duc-tile manner, consisting of well-developed dimples over the entiresurface, but the dimple size gets reduced to less than 1 lm forthe latter. A continuous decrease in dimple size in the cryorolledsamples, in the present work, may be due to the grain refinementand work hardening. The ductile to brittle transition observed inthe fracture surfaces of the samples of both bulk alloy and CR Al al-loys reveals that the fracture occurs over a range of temperaturefrom �200 �C to 100 �C.

Acknowledgement

The author, Dr.R. Jayaganthan, would like to thank DST, NewDelhi for their financial support to this work through Grant No:DST-462-MMD.

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