Annealing Behavior of 6061 Al Alloy Subjected to Differential Speed Rolling Deformation

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Article Annealing Behavior of 6061 Al Alloy Subjected to Differential Speed Rolling Deformation

Young Gun Ko 1 and Kotiba Hamad 2,* 1 School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Korea; [email protected] 2 School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 16419, Korea * Correspondence: [email protected]; Tel.: +82-31-290-7401

Received: 27 October 2017; Accepted: 6 November 2017; Published: 10 November 2017

Abstract: This study investigated the effects of heat treatment (annealing) on the microstructure of ultrafine grained 6061 Al alloy samples fabricated by a differential speed rolling (DSR) process. The samples were fabricated using two passes DSR with 75% thickness reduction and a speed ratio of 1:4. The DSR-deformed 6061 Al alloy sample exhibited a lamellar boundary structure composed mostly of subgrains surrounded by low-angle grain boundaries. After annealing, the DSR-deformed 6061 Al alloy samples exhibited coarse grained structure and transformed from lamellar to equiaxed, where both the grain size and grain shape aspect ratio increased with increasing annealing temperature. The fraction of grain boundaries with high misorientation angles increased progressively during annealing, to ~77% at annealing temperature of 350 ◦C.

Keywords: differential speed rolling; 6061 Al alloy; annealing; microstructure

1. Introduction The interest in ultrafine grained (UFG) materials with exceptional properties has increased rapidly because of their scientific and technological importance. In recent studies, metals with an UFG structure (mean grain size < 1 µm) showed extraordinary performance, such as improved strength and low-temperature super-plasticity [1,2]. One method to achieve the UFG structure is severe plastic deformation (SPD). Several methods have been applied successfully to fabricate UFG materials using SPD. Those are equal-channel angular pressing (ECAP), accumulative roll bonding (ARB), high pressure torsion (HPT) and differential speed rolling (DSR) [3–9]. Among them, DSR is the method that shows a high capability to produce UFG sheets continuously. This is usually carried out by imposing uniform shear strain in the thickness direction of the sheets through two identical rolls with different rotation speeds [7–9]. Aluminum alloys are used for applications such as food packaging, sports utilities, automotive and aircraft because of their high specific strength (strength/density ratio) and corrosion resistance. Alloys with the base composition of ternary aluminum-magnesium-silicon, which are called 6xxx series, possess good strength, heat-treatability, excellent formability and acceptable corrosion resistance [10]. In this series, 6061 Al alloy is the most commonly used alloy in many applications and can be fabricated easily by extrusion, forging or rolling. To enhance the property of the 6061 Al alloy for specific applications, grain refinement is used. Kim et al. [11] reported that the mechanical properties of UFG 6061 alloy subjected to DSR deformation was higher than those fabricated by ECAP and HPT. The excellent strength achieved after deformation by DSR is usually conjugated with the poor ductility which has limited the material applicability in the industrial fields. For improving the strength and ductility of the UFG materials, an appropriate thermal treatment following plastic deformation is required. Several studies examining the effects of annealing have been carried out. Kim et al. [12] reported that the thermal treatment of an ECAP deformed 6061 Al alloy was effective in improving

Metals 2017, 7, 494; doi:10.3390/met7110494 www.mdpi.com/journal/metals MetalsMetals2017 2017,,7 7,, 494 494 22 ofof 1010

Kim et al. [12] reported that the thermal treatment of an ECAP deformed 6061 Al alloy was effective thein improving strength whilst the strength retaining whilst moderate retaining tensile moderate ductility. tensile Rezaei ductility. et al. Rezaei [13] reported et al. [13] the reported improved the performanceimproved performance (strength and (strength ductility) and of ductility) ARB-deformed of ARB-deformed sheets aftera sheets thermal after treatment. a thermal On treatment. the other hand,On the there other has hand, been there no conclusive has been no research conclusive on the research microstructural on the microstructural evolution of UFG evolution 6061 Al of alloyUFG deformed6061 Al alloy by DSR deformed during by annealing. DSR during Therefore, annealing. the aimTherefore, of this workthe aim is to of investigate this work microstructuralis to investigate evolutionmicrostructural of DSR-deformed evolution of 6061 DSR-deform Al alloy duringed 6061 annealing. Al alloy during annealing. 2. Experiments 2. Experiments The chemical composition of the 6061 Al alloy sheets used in the present work was (in wt %): The chemical composition of the 6061 Al alloy sheets used in the present work was (in wt %): 0.9 0.9 Mg, 0.71 Si, 0.5 Fe, 0.24 Cu, 0.19 Cr, 0.12 Mn, 0.05 Zn, 0.05 Ti, with Al comprising the balance. Mg, 0.71 Si, 0.5 Fe, 0.24 Cu, 0.19 Cr, 0.12 Mn, 0.05 Zn, 0.05 Ti, with Al comprising the balance. The as- The as-received sheets were cut into small plates, 0.4 cm (thickness), 4 cm (width) and 10 cm (length). received sheets were cut into small plates, 0.4 cm (thickness), 4 cm (width) and 10 cm (length). The The plates were then homogenized for 3 h at 550 ◦C and air cooled to obtain a fully annealed structure plates were then homogenized for 3 h at 550 °C and air cooled to obtain a fully annealed structure (Figure1). The homogenized plates were then subjected to the room-temperature DSR deformation (Figure 1). The homogenized plates were then subjected to the room-temperature DSR deformation using two identical rolls (220 mm in diameter). To impose the shear deformation by DSR, a speed using two identical rolls (220 mm in diameter). To impose the shear deformation by DSR, a speed ratio of 1:4 was used, where the lower roll speed was fixed to ~3 m/min. The plates were subjected ratio of 1:4 was used, where the lower roll speed was fixed to ~3 m/min. The plates were subjected to to two passes with 50% thickness reduction per pass, so that the total thickness reduction was 75% two passes with 50% thickness reduction per pass, so that the total thickness reduction was 75% after after the DSR deformation (two passes). During the DSR deformation, the plates were rotated by 180◦ the DSR deformation (two passes). During the DSR deformation, the plates were rotated by 180° around their rolling direction between the adjacent passes. This route was found to be beneficial for around their rolling direction between the adjacent passes. This route was found to be beneficial for controlling the material with a fine grain structure [14,15]. The heat treatment was carried out on the controlling the material with a fine grain structure [14,15]. The heat treatment was carried out on the DSR-deformed plates for 1 h at various temperatures of 150–400 ◦C with an interval of 25 ◦C. DSR-deformed plates for 1 h at various temperatures of 150–400 °C with an interval of 25 °C.

(a) (b)

FigureFigure 1.1. ((aa)) HomogenizationHomogenization conditionsconditions ofof thethe as-receivedas-received 60616061 AlAl alloyalloy sheetssheets andand ((bb)) thethe obtainedobtained microstructuremicrostructure (DSR:(DSR: DifferentialDifferential Speed Speed Rolling; Rolling; A.C.: A.C.: Air Air Cooled). Cooled).

The structural features of the deformed and annealed plates were observed using electron The structural features of the deformed and annealed plates were observed using electron backscatter diffraction (EBSD, Hitachi S-4300 FESEM; Hitachi, Tokyo, Japan). For EBSD observations, backscatter diffraction (EBSD, Hitachi S-4300 FESEM; Hitachi, Tokyo, Japan). For EBSD observations, sample cut from the normal direction-rolling direction (ND-RD) plane, were electro-polished using sample cut from the normal direction-rolling direction (ND-RD) plane, were electro-polished using a solution of 20% perchloric acid in methanol at 25 V for 10 s. The EBSD observations were conducted a solution of 20% perchloric acid in methanol at 25 V for 10 s. The EBSD observations were conducted at a step size of 0.02 µm. The EBSD data was analyzed using a TSL OIM 6.1.3 (Hitachi, Tokyo, Japan) at a step size of 0.02 µm. The EBSD data was analyzed using a TSL OIM 6.1.3 (Hitachi, Tokyo, Japan) and the evaluation of misorientation distribution was made based on the assumption that the and the evaluation of misorientation distribution was made based on the assumption that the misorientation angles from 2° to 15° indicated the low angle-grain boundaries (LAGBs) whereas the misorientation angles from 2◦ to 15◦ indicated the low angle-grain boundaries (LAGBs) whereas the misorientation angles higher than◦ 15° implied the high-angle grain boundaries (HAGBs). misorientationTransmission angleselectron higher microscopy than 15 impliedobservations the high-angle (TEM, Philips grain boundaries TECNAI (HAGBs).G2F20; Drachten, Transmission The electronNetherland) microscopy of the annealed observations plates (TEM, were Philips performed TECNAI at an G2F20; acceleration Drachten, voltage The Netherland)of 200 kV. For of thethis annealedmeasurement, plates foils were cut performedform the annealed at an acceleration plates were voltagethinned ofby 200a focused kV. For ion thisbeam. measurement, To measure foilsthe Vickers cut form microhardness, the annealed the plates annealed were thinned6061 Al byallo ay focusedplates with ion mirror-like beam. To measuresurfaces thewere Vickers tested microhardness,under a load and the dwell annealed time of 6061 200 Alg and alloy 10 plates s, respectively. with mirror-like In order surfaces to avoid were the occurrence tested under of natural a load andaging, dwell the timemicrostructure of 200 g and observations 10 s, respectively. and hard Inness order measurements to avoid the occurrencewere carried of out natural right aging, after thedeformation microstructure and annealing observations experiments. and hardness measurements were carried out right after deformation and annealing experiments.

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3. ResultsMetals 2017 and, 7 Discussion, 494 3 of 10 3. Results and Discussion 3.1. Microstructural Features of the Differential Speed Rolling-Deformed 6061 Al Alloy Figure3.1. Microstructural2 shows the Features EBSD mapsof the Differential obtained Speed from Ro thelling-Deformed near-surface 60 region61 Al Alloy of the 6061 Al alloy plate deformedFigure by the 2 DSRshows under the EBSD the investigatedmaps obtained conditions from the ne (twoar-surface passes, region 75% of andthe 6061 1:4). Al The alloy EBSD plate maps (inversedeformed pole figure by the (IPF) DSR andunder grain the investigated boundaries condit (GBs))ions in Figures(two passes,2a and 75%1b and show 1:4). a lamellarThe EBSD boundary maps structure(inverse composed pole figure mostly (IPF) and of subgrains grain boundaries with high (GBs)) fraction in Figures of low-angle2a and 1b show grain a lamellar boundaries boundary (LAGBs), as shownstructure by thecomposed dashed mostly rectangle of subgrains A in Figure with high2a. Infraction addition, of low-angle a lower grain fraction boundaries of ultrafine (LAGBs), grains surroundedas shown by by boundaries the dashed with rectangle an angle A in of Figure misorientation 2a. In addition, (>15 ◦a) lower are also fraction observed of ultrafine near the grains lamellar surrounded by boundaries with an angle of misorientation (>15°) are also observed near the lamellar boundaries (dashed rectangle B in Figure2a). This was also confirmed by the grain boundaries map boundaries (dashed rectangle B in Figure 2a). This was also confirmed by the grain boundaries map presented in Figure2b. presented in Figure 2b.

FigureFigure 2. ( a2.) ( IPFa) IPF (Inverse (Inverse Pole Pole Figure), (b ()b grain) grain boundaries boundaries and ( andc) partitioned (c) partitioned IPF maps IPF of the maps DSR- of the deformed 6061 Al alloy. DSR-deformed 6061 Al alloy. The nucleation of such ultrafine grains around the elongated grains leads to a necklace-like Thestructure, nucleation which was of such also reported ultrafine in grains bcc material arounds after the deformation elongated by grains DSR leadsat different to a necklace-likethickness structure,reductions which [16,17]. was also The reportedevolution inof bccthe materialsnecklace-like after structure deformation is typical by of DSR a continuous at different dynamic thickness reductionsrecrystallization [16,17]. The (CDRX) evolution process of at thean intermediate necklace-like stage structure of deformation is typical by of SPD a continuous [18]. This can dynamic be recrystallizationattributed to (CDRX)the rapid processevolution at of an strain intermediate gradients near stage grain of deformationboundaries, leading by SPD to the [18 ].formation This can be attributedof large to misorientations the rapid evolution in the vicinities of strain of gradients the boundaries. near grain Therefore, boundaries, it is deduced leading that to different the formation grain of refinement mechanisms can occur, depending on the materials and processing conditions. large misorientations in the vicinities of the boundaries. Therefore, it is deduced that different grain The fraction of ultrafine grains (~23%) in the deformed microstructure was determined by refinementpartitioning mechanisms the ultrafine can grains occur, using depending the criteria on theof the materials grain orientation and processing spread (GOS) conditions. [19], as shown Thein Figure fraction 2c. ofIn ultrafineaddition, the grains individual (~23%) layers in the formed deformed between microstructure the lamellar boundaries was determined were by partitioningcharacterized the ultrafine by low-angle grains grain using boundaries the criteria (LAGBs) of the and grain intra-layer orientation orientation spread gradients (GOS) [19 resulting], as shown in Figurein the2 c.(sub) In grain addition, structure. the After individual the DSR layersdeformat formedion, the betweenpresence of the LAGBs lamellar inside boundaries the elongated were characterizedgrains is normally by low-angle observed grain after boundaries SPD deformation. (LAGBs) To andexplain intra-layer the development orientation of the gradients (sub) grained resulting in thestructure (sub) grain inside structure. the individual After layers, the DSR the deformation,misorientation theprofiles presence were ofrecorded LAGBs along inside the therectangle elongated grainsA isin normally Figure 2a. observedThe misorientation after SPD profiles deformation. (point-to-origin To explain and thepoint-to-point) development measured of the (sub)along grainedthe structuredashed inside line the(L) are individual presented layers, in Figure the 3. misorientation The results showed profiles large were variat recordedion across along the individual the rectangle layer, where average values of 4.06° and 6.77° were recorded for point-to-origin and point-to-point A in Figure2a. The misorientation profiles (point-to-origin and point-to-point) measured along the misorientation, respectively. The increase in the intra-layer misorientation is associated with the dashedformation line (L) of are a (sub) presented grain structure. in Figure On3 .the The other results hand, showed low misorientation large variation regions across in this the profile individual are ◦ ◦ layer,associated where average with the values formation of 4.06 of dislocation-celland 6.77 were structure. recorded Based for on point-to-origin the EBSD measurements and point-to-point of the misorientation,DSR-deformed respectively. sample, it is Theworth increase pointing in out the that intra-layer the traditional misorientation definition of the is associated grain size is with not the formationenough of here, a (sub) where grain sub structure. structures On with the clear other orientation hand, low gradients misorientation are formed regions after inthe this DSR profile are associated with the formation of dislocation-cell structure. Based on the EBSD measurements of the DSR-deformed sample, it is worth pointing out that the traditional definition of the grain size Metals 2017, 7, 494 4 of 10 is not enough here, where sub structures with clear orientation gradients are formed after the DSR Metals 2017, 7, 494 4 of 10 deformation.Metals 2017, Due7, 494 to these gradients, two (non-neighboring) points belonging to the same4 grain of 10 can be highlydeformation. misoriented. Due to Consequently, these gradients, the two calculation (non-neighboring) of grain points size maybelonging characterize to the same both grain grains can and subgrains.deformation.be highly The misoriented. distribution Due to these Consequently, of gradients, the (sub) two the grain (non-neighboring)calculatio size ofn the of grain 6061 points size Al alloymay belonging characterize deformed to the by bothsame the grains grain DSR andcan process (Figurebesubgrains.4 highlya) shows misoriented. The that distribution a significant Consequently, of the fraction (sub) the grain (~19%)calculatio size of ofngrains theof grain 6061 have Alsize alloy diameters may deformed characterize less by than theboth 0.1DSR grainsµ processm, and whereas subgrains.(Figure 4a) The shows distribution that a significant of the (sub) fraction grain (~19%) size ofof thegrains 6061 have Al alloydiameters deformed less than by the 0.1 DSRµm, whereasprocess the average diameters (dave) lies within 0.4–0.5 µm. In addition, ~40% of the grain boundaries have misorientation(Figurethe average 4a) anglesshows diameters that higher a ( significantdave than) lies 15 within◦ fraction(high-angle 0.4–0.5 (~19%) µm. grain of Ingrains addition, boundaries have diameters~40% (HAGBs)), of the less grain than as boundariesshown0.1 µm, bywhereas Figurehave 4b. themisorientation average diameters angles higher(dave) lies than within 15° (high-angle 0.4–0.5 µm. grain In addition, boundaries ~40% (HAGBs)), of the grain as shownboundaries by Figure have The precipitation behavior of 6061 Al alloy subjected to DSR deformation was investigated misorientation4b. angles higher than 15° (high-angle grain boundaries (HAGBs)), as shown by Figure by Kim et al.[11]. They found that the features of precipitates after the DSR deformation, 4b. The precipitation behavior of 6061 Al alloy subjected to DSR deformation was investigated by includingKim Theet number, al. precipitation [11]. They size, foundbehavior shape that and of the 6061 distribution, features Al alloy of subjectedprecipitates are mainly to DSRafter related deformationthe DSR to thedeformation, was initial investigated microstructure including by (beforeKimnumber, deformation). et al. size, [11]. shapeThey Their foundand results distribution, that revealedthe features are that mainlyof the precipitates sample related with afterto athe the fully-annealed initial DSR deformation,microstructure structure including (before possessed a smallnumber,deformation). number size, of Theirshape precipitates results and revealeddistribution, after that the theare DSR sample mainly deformation. with related a fully-annealed to Inthe the initial presented structure microstructure possessed work, accordingly,(before a small the sampledeformation).number was of precipitates subjected Their results after to revealed a the heat DSR treatmentthat deformation. the sample at 550 withIn ◦theC a fully-annealed forpresented 3 h followed work, structure accordingly, by airpossessed cooling the asample small to obtain a fully-annealednumberwas subjected of precipitates structure to a heat (Figure aftertreatment the1 ).DSR at In 550 addition,deformation. °C for 3 the h followed In rolling the presented temperatureby air cooling work, (RT)to accordingly, obtain employed a fully-annealed the here sample is a less than idealwasstructure subjected aging (Figure temperature to a 1). heat In addition,treatment of 6061 the at Al rolling550 alloy °C temperaturefor (~170 3 h followed◦C). (RT) by employed air cooling here to isobtain a less a than fully-annealed ideal aging structure (Figure 1). In addition, the rolling temperature (RT) employed here is a less than ideal aging Basedtemperature on this of discussion, 6061 Al alloy it can(~170 be °C). noted that the effect of the DSR deformation and pre/post heat temperatureBased on of this 6061 discussion, Al alloy (~170 it can °C). be noted that the effect of the DSR deformation and pre/post heat treatment on the precipitation behavior of Al alloys should be investigated in more detail using high treatmentBased on on the this precipitation discussion, it behavior can be noted of Al that alloys the should effect of be the investigated DSR deform ination more and detail pre/post using heathigh resolutiontreatmentresolution TEM. on TEM. the precipitation behavior of Al alloys should be investigated in more detail using high resolution TEM.

Figure 3. Misorientation profile along the dashed line taken from the rectangle A in the DSR-deformed Figure 3. Misorientation proﬁle along the dashed line taken from the rectangle A in the DSR-deformed Figure6061 Al 3. alloy Misorientation (Figure 2a). profile along the dashed line taken from the rectangle A in the DSR-deformed 6061 Al alloy (Figure2a). 6061 Al alloy (Figure 2a).

Figure 4. (a) Grain size distribution and (b) grain boundaries misorientations of the DSR-deformed Figure6061 Al 4. alloy. (a) Grain The greensize distribution line in Figure and 4a ( showsb) grain additionally boundaries the misorientations grain size distribution. of the DSR-deformed Figure 4. (a) Grain size distribution and (b) grain boundaries misorientations of the DSR-deformed 6061 Al alloy. The green line in Figure 4a shows additionally the grain size distribution. 6061 Al alloy. The green line in Figure4a shows additionally the grain size distribution.

MetalsMetals2017 2017, 7, ,7 494, 494 55 of of 10 10

3.2. As-Annealed Samples 3.2. As-Annealed Samples 3.2.1. Microhardness 3.2.1. Microhardness Figure 5a shows microhardness of the DSR-deformed 6061 Al alloy samples as a function of the Figure5a shows microhardness of the DSR-deformed 6061 Al alloy samples as a function of annealing temperature. The curve obtained in Figure 5a exhibited the following three different the annealing temperature. The curve obtained in Figure5a exhibited the following three different regions (i) the microhardness decreases slightly until ~225 °C. This range corresponds to the recovery regions (i) the microhardness decreases slightly until ~225 ◦C. This range corresponds to the recovery in the materials. (ii) The microhardness decreases sharply, i.e., at a steeper slope on the curve, from in the materials. (ii) The microhardness decreases sharply, i.e., at a steeper slope on the curve, 225 to 350 °C and (iii) the curve levels off at temperatures higher than 350 °C. The sharp decrease of from 225 to 350 ◦C and (iii) the curve levels off at temperatures higher than 350 ◦C. The sharp decrease the microhardness in the second region of the curve suggests the occurrence of material softening of the microhardness in the second region of the curve suggests the occurrence of material softening induced by recrystallization [20]. To show the effect of the annealing temperature on the softening induced by recrystallization [20]. To show the effect of the annealing temperature on the softening behavior of the DSR-deformed 6061 Al alloy, the softened fraction (X) was calculated as follows behavior of the DSR-deformed 6061 Al alloy, the softened fraction (X) was calculated as follows [21,22]: [21,22]: H − H = HHd − t X X = dt (1) Hd −−H0 (1) HHd0 where Hd, Ht and H0 are the microhardness after DSR deformation, the instantaneous microhardness where Hd, Ht and H0 are the microhardness after DSR deformation, the instantaneous microhardness at any temperature, and the microhardness of the initial sample prior to DSR, respectively. Figure5b at any temperature, and the microhardness of the initial sample prior to DSR, respectively. Figure 5b shows the softened fraction at the various annealing temperatures used in this work. After annealing shows the softened fraction at the various annealing temperatures used in this work. After annealing at 225 ◦C for 1 h, the estimated softened fraction of the annealed sample showed a ~2.99% reduction at 225 °C for 1 h, the estimated softened fraction of the annealed sample showed a ~2.99% reduction compared to the DSR-deformed sample. The softened fraction of the sample increased signiﬁcantly compared to the DSR-deformed sample. The softened fraction of the sample increased significantly after annealing at temperatures between 225 and 350 ◦C, where the samples annealed at 275, 325 and after annealing at temperatures between 225 and 350 °C, where the samples annealed at 275, 325 and 350 ◦C showed ~45%, ~79.8% and ~99.8% softened fraction compared to the as-deformed sample. 350 °C showed ~45%, ~79.8% and ~99.8% softened fraction compared to the as-deformed sample.

FigureFigure 5. 5.Effect Effect of of the the annealing annealing temperature temperature on on ( a()a) microhardness microhardness and and ( b(b)) softened softened fraction fraction of of the the DSR-deformedDSR-deformed 6061 6061 Al Al alloy. alloy. The The different different color color remarks remarks along along the the dashed dashed line line in in this this ﬁgure figure indicate indicate thethe annealed annealed samples samples used used to to investigate investigate the the microstructural microstructural evolution evolution during during annealing. annealing.

3.2.2. Microstructure 3.2.2. Microstructure The microstructural evolution of the samples annealed at 225 °C, 275 °C, 325 °C and 350 °C, The microstructural evolution of the samples annealed at 225 ◦C, 275 ◦C, 325 ◦C and 350 ◦C, which which belong to the second region (ii) as indicated by the colored symbols in Figure 5a, were belong to the second region (ii) as indicated by the colored symbols in Figure5a, were investigated to investigated to understand the softening behavior of the DSR-deformed 6061 Al alloy plates during understand the softening behavior of the DSR-deformed 6061 Al alloy plates during the heat treatment. the heat treatment. The EBSD maps (grain boundaries maps) in Figure 6 showed that the lamellar The EBSD maps (grain boundaries maps) in Figure6 showed that the lamellar structure was gradually structure was gradually broken-down to form the equiaxed grains. On the other hand, at 275 °C, broken-down to form the equiaxed grains. On the other hand, at 275 ◦C, some areas showed the some areas showed the lamellar structure obtained by the initial DSR deformation, as indicated by lamellar structure obtained by the initial DSR deformation, as indicated by the dashed rectangles in the dashed rectangles in Figure 6b. This was shown further by partitioning of the recrystallized grains Figure6b. This was shown further by partitioning of the recrystallized grains of the samples treated at of the samples treated at 225, 275, 325 and 350 °C using the criteria of GOS [19] (Figure 6e–h). The 225, 275, 325 and 350 ◦C using the criteria of GOS [19] (Figure6e–h). The most obvious and expected most obvious and expected observation from Figure 6e–h is that the fraction of the recrystallized observation from Figure6e–h is that the fraction of the recrystallized grains increased with increasing grains increased with increasing the temperature, where recrystallizations corresponding to ~18%,

Metals 2017, 7, 494 6 of 10 theMetals temperature, 2017, 7, 494 where recrystallizations corresponding to ~18%, ~31%, 52% and ~84% were measured6 of 10 after the heat treatment at 225, 275, 325 and 350 ◦C, respectively. In addition, the larger fraction ~31%, 52% and ~84% were measured after the heat treatment at 225, 275, 325 and 350 °C, respectively. of recrystallized microstructures of the sample annealed at 350 ◦C (Figure6h) resulted in the low In addition, the larger fraction of recrystallized microstructures of the sample annealed at 350 °C microhardness values, as shown in Figure5a. (Figure 6h) resulted in the low microhardness values, as shown in Figure 5a.

Figure 6. Grain boundaries maps and recrystallized microstructures of the DSR-deformed 6061 Al Figure 6. Grain boundaries maps and recrystallized microstructures of the DSR-deformed 6061 Al alloy alloy annealed for 1 h at (a,e) 225 °C (b,f) 275 °C (c,g) 325 °C (d,h) 350 °C. annealed for 1 h at (a,e) 225 ◦C(b,f) 275 ◦C(c,g) 325 ◦C(d,h) 350 ◦C. Figure 7 presents the microstructural parameters of the annealed plates, including the grain size, grainFigure shape7 presentsaspect ratio the and microstructural grain boundary parameters misorientations. of the annealed The grain plates, size includingdistributions the grainof the size,annealed grain plates shape shown aspect in ratio Figure and 7b grain clearly boundary show that misorientations. after annealing Theat 225 grain °C and size 275 distributions °C for 1 h, the of ◦ ◦ thegrain annealed size increased plates shownfrom ~0.9 in Figureµm, which7b clearly are still show submicron that after grains, annealing to ~2.2 at µm 225 (FigureC and 7b,d). 275 CAfter for 1annealing h, the grain at size325 increased°C and 350 from °C, ~0.9 a significantµm, which change are still in submicron grain morphology grains, to ~2.2was µobserved,m (Figure where7b,d). ◦ ◦ Afterequaixed annealing grains at with 325 shapeC and aspect 350 C,ratios a signiﬁcant of 0.6 and change 0.62 were in grain obtained morphology after annealing was observed, at 325 °C where and ◦ equaixed350 °C, respectively grains with (Figure shape 7a,d). aspect The ratios distribution of 0.6 ands of 0.62 the weregrain obtainedboundary after misorientations annealingat in 325FigureC ◦ and7c revealed 350 C, respectivelythat the fraction (Figure of HAGBs7a,d). The evolved distributions during the of the heat grain treatment boundary of the misorientations deformed plates in Figureincreased7c revealed with increasing that the fractionthe temperature, of HAGBs evolvedwhere ~44%, during ~60%, the heat ~68% treatment and ~77% of the of deformedthe grain platesboundaries increased in the with samples increasing annealed the temperature,at 225 °C, 275 where°C, 325 ~44%, °C, and ~60%, 350 °C ~68% for 1 and h, respectively, ~77% of the grainhad a ◦ ◦ ◦ ◦ boundarieshigh angle of in misorientation the samples annealed (>15°). After at 225 annealingC, 275 atC, 225 325 °C,C, the and dislocations 350 C for generated 1 h, respectively, by the plastic had ◦ ◦ adeformation high angle of(DSR) misorientation were absorbed (>15 at). the After boundaries. annealing At at 225the sameC, the time, dislocations this absorption generated led by to the an plasticincrease deformation in the misorientation. (DSR) were After absorbed annealing at the at boundaries. 275 °C, however, At the samesome time,LAGBs this are absorption still observable led to ◦ anwithin increase the deformed in the misorientation. grains, which After might annealing be attributed at 275 toC, the however, non-absorbed some LAGBs dislocations. are still In observable addition, the fraction of LAGBs observed in the samples after annealing at 325 °C and 350 °C led to the evolution of coarse grains surrounded by HAGBs, as shown by the arrows in Figure 6c,d.

Metals 2017, 7, 494 7 of 10 within the deformed grains, which might be attributed to the non-absorbed dislocations. In addition, the fraction of LAGBs observed in the samples after annealing at 325 ◦C and 350 ◦C led to the evolution of coarse grains surrounded by HAGBs, as shown by the arrows in Figure6c,d. Metals 2017, 7, 494 7 of 10

Figure 7. (a) Grain shape aspect ratio, (b) grain size, (c) grain boundaries misorientation and (d) Figure 7. (a) Grain shape aspect ratio, (b) grain size, (c) grain boundaries misorientation and (d) average average aspect ratio and grain size values of the DSR-deformed 6061 Al alloy annealed at different aspect ratio and grain size values of the DSR-deformed 6061 Al alloy annealed at different temperatures for temperatures for 1 h. The columns used in Figure 7d are corresponding to the investigated samples 1 h. The columns used in Figure7d are corresponding to the investigated samples remarked in Figure5a. remarked in Figure 5a.

◦ After annealingannealing at at 225 225C, °C, local local imbalances imbalances in surface in surface tension tension resulting resulting from thefrom transverse the transverse LAGBs canLAGBs lead can to pinch-offlead to pinch-off of the lamellar of the lamellar HAGBs HAGBs formed formed after DSR after deformation DSR deformation and subdivision and subdivision of the elongatedof the elongated grains intograins shorter into shorter segments segments (Figure 8(Figa). Examplesure 8a). Examples of this behavior of this canbehavior be seen can clearly be seen in theclearly high in magnificationthe high magnification TEM images TEM presented images presented in Figure in8b. Figure In addition, 8b. In addition, the very the fine very new fine grains new (<100grains nm,(<100 indicated nm, indicated by the by arrows the arrows in Figure in Figure8c) observed 8c) observed near the near grain the gr boundariesain boundaries confirmed confirmed that ◦ thethat recrystallization the recrystallization was activatedwas activated after 225after C.225 The °C. recrystallized The recrystallized grains grains typically typically grow fromgrow nuclei from formednuclei formed by recovery by withinrecovery the within rolling componentsthe rolling [components23]. Generally, [23]. the microstructureGenerally, the evolution microstructure during annealingevolution during of Al alloys annealing subjected of Al to alloys ARB subjected deformation to ARB is characterized deformation by is characterized nucleation and by growth nucleation [24]. However,and growth the [24]. present However, results the showed present a differentresults showed trend, ina different which the trend, lamellar in which HAGBs the pinching-offlamellar HAGBs and nucleationpinching-off followed and nucleation by growth followed co-exist. by This, growth in turn, co-exist. is attributed This, toin processingturn, is attributed characteristics to processing of these methods.characteristics In ARB, of these the sample methods. is subjected In ARB, to the a highsample amount is subjected of strain, to which a high enhances amount theof strain, recovery which rate andenhances increases the therecovery fraction rate of HAGBsand increases as a result. the frac However,tion of inHAGBs the DSR as process,a result. the However, quick dissipation in the DSR of deformationprocess, the andquick friction dissipation heats fromof deformation a thin sheet and with friction a high surface/volumeheats from a thin ratio sheet to thewith cold a high rolls leadsurface/volume to reduction ratio of recovery to the cold rate. rolls Additionally, lead to reduction the fraction of recovery of HAGBs rate. Additionally, achieved by the the DSR fraction is still of largerHAGBs than achieved those achievedby the DSR by is conventional still larger than methods those (symmetric achieved by cold conventional rolling) [9 ].methods In accordance (symmetric with thecold previous rolling) ideas,[9]. In theaccordance mechanism with that the is previous responsible ideas, for the the mechanism structure evolution that is responsible during annealing for the ofstructure the DSRed evolution 6061 Alduring alloy annealing samples isof related the DSRed to the 6061 medium Al alloy fraction samples of HAGBs.is related At to thisthe medium fraction, thefraction HAGBs of HAGBs. pinching-off At this to divide fraction, the the lamellar HAGBs structure pinching-off and nucleation to divide at the the lamellar highly-deformed structure areas and simultaneouslynucleation at the occur. highly-deformed areas simultaneously occur.

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Figure 8. TEM (Transmission Electron Microscopy) images of the DSR-deformed 6061 Al alloy plates Figure 8. TEM (Transmission Electron Microscopy) images of the DSR-deformed 6061 Al alloy plates annealed at 225 °C for 1 h: (a) general microstructure; (b) pinching-off of the lamellar HAGBs (high- annealed at 225 ◦C for 1 h: (a) general microstructure; (b) pinching-off of the lamellar HAGBs angle grain boundaries) formed after DSR deformation and subdivision of the elongated grains into (high-angle grain boundaries) formed after DSR deformation and subdivision of the elongated grains shorter segments; (c) evolution of fine recrystallized grains. into shorter segments; (c) evolution of fine recrystallized grains. In general, the EBSD maps and microstructural parameters of the 6061 Al alloy samples during annealingIn general, at different the EBSD temperatures maps and microstructuralfor 1 h showed parametersthat the microstructure of the 6061 Alevolves alloy samplesin a continuous during annealingmanner, in at that different no local temperatures transformation for 1 front h showed can be that observed. the microstructure This behavior evolves was also in a reported continuous by manner,the present in that authors, no local intransformation which the texture front evolut can beion observed. during Thisthe behaviorannealing was treatment also reported of the by DSR- the presentdeformed authors, 6061 Al in alloy which samples the texture was evolutioninvestigated during [25]. theThe annealing results showed treatment that of the the manner DSR-deformed in which 6061the various Al alloy components samples was of investigated rolling, shear [25 ].and The recrys resultstallization showed thattextures the mannerchanged, in was which related the various to the componentsoperation of ofcontinuous rolling, shear recrystallization. and recrystallization textures changed, was related to the operation of continuousThe results recrystallization. presented in this work mostly involve the grain structure evolution, but the precipitationThe results behavior presented upon inannealing this work of the mostly DSR-deformed involve the samples, grain structure which is evolution,also one of butthe key the precipitationaspects of the behavior microstructure upon annealingfor 6xxx series of the Al DSR-deformedalloys, has not been samples, discusse whichd. Accordingly, is also one offuture the keyexperiments aspects ofwill the be microstructure designed to address for 6xxx this series point. Al alloys, has not been discussed. Accordingly, future experiments will be designed to address this point. 4. Conclusions 4. Conclusions Ultrafine grained 6061 Al alloy plates (0.9 wt % Mg) manufactured by room-temperature DSR Ultrafine grained 6061 Al alloy plates (0.9 wt % Mg) manufactured by room-temperature DSR deformation were annealed isochronally at temperatures between 150 and 400 °C for 1 h. The basic deformation were annealed isochronally at temperatures between 150 and 400 ◦C for 1 h. The basic results and conclusions from the present investigation are as follows: results and conclusions from the present investigation are as follows: 1. The DSR-deformed 6061 Al alloy showed a lamellar boundary structure composed mostly of 1. subgrainsThe DSR-deformed with high 6061fraction Al alloyof LAGBs. showed a lamellar boundary structure composed mostly of 2. Thesubgrains hardness with of high the fractionDSRed plates of LAGBs. slightly decreased with increasing temperature until 225 °C 2. andThe decreased hardness of sharply the DSRed after platesannealing slightly at temperatures decreased with higher increasing than 225 temperature °C. until 225 ◦C 3. Theand decreasedmicrostructure sharply of afterthe annealingDSR-fabricated at temperatures plates transformed higher than from 225 ◦theC. lamellar boundary 3. structureThe microstructure into a coarse-grained of the DSR-fabricated structure during plates transformed annealing and from the the extremely lamellar fine boundary grains structuredetected ininto the a coarse-grainedsample annealed structure at 225 during °C suggested annealing that and the the recrystallization extremely fine grains became detected active in after the annealingsample annealed at this temperature at 225 ◦C suggested (225 °C). that the recrystallization became active after annealing at this temperature (225 ◦C).

Metals 2017, 7, 494 9 of 10

Acknowledgments: This research was supported by National Research Foundation (NRF) of Korea (2017R1C1B5017204). Author Contributions: Young Gun Ko conceived and designed the experiments; Kotiba Hamad performed the experiments; analyzed the data; and wrote the paper. Both authors discussed the results. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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