Equal Channel Angular Pressing Of A Newly Developed .

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Equal Channel Angular Pressing of a NewlyDeveloped Precipitation HardenableScandium Containing Aluminum AlloyJahanzaib Malik, Wahaz Nasim, Bilal Mansoor, Ibrahim Karaman,Dinc Erdeniz, David C. Dunand, and David N. SeidmanAbstractPrecipitation hardenable aluminum alloys are well-knownfor their high strength-to-weight ratio, good thermalstability, electrical conductivity, and low cost. Equalchannel angular pressing (ECAP) is proven to furtherimprove the mechanical properties of metallic alloysthrough microstructure modification. In this work, ECAPof a recently developed, precipitation hardenable, castAl–Er–Sc–Zr–V–Si alloy in peak-aged condition by route4Bc was carried out to create an alloy with ultra-fine grainstructure. The combined effect of grain refinement andprecipitation on the tensile behavior and thermal stabilityof the ECAPed alloy is reported here. Improvement inyield strength and lack of strain hardening in ECAPedalloy were as expected. Microhardness contour plots witha narrower spread indicated enhancement in microstructural homogeneity after four ECAP passes as compared tothe peak-aged condition. The variations in microhardnessafter annealing heat treatments at different temperatureshighlighted the important role precipitates play in maintaining microstructure stability up to 250 C in theECAPed material.Keywords Aluminum alloy Precipitation strengtheningECAP MicrostructureJ. Malik W. Nasim B. Mansoor (&) I. KaramanDepartment of Materials Science and Engineering, Texas A&MUniversity, 575 Ross Street, College Station, TX 77843, USAJ. Malike-mail: jmalik@tamu.eduB. MansoorMechanical Engineering Program, Texas A&M University atQatar, Education City, Doha, QatarD. Erdeniz D. C. Dunand D. N. SeidmanDepartment of Materials Science and Engineering, NorthwesternUniversity, 2220 Campus Drive, Evanston, IL 60208, USAIntroductionStrengthening of materials at room temperature and elevatedtemperature is a comprehensive subject in materials sciencewith a strong practical importance. The ultimate aim generally is to have materials with high strength-to-weight ratio,in order to achieve better efficiencies in high end applications. Precipitation hardening is one of the most effectivestrengthening processes which relies on alloying additionsand heat-treatments to improve the mechanical response ofmetallic alloys [1]. Precipitation relies on solutes that havehigher solubility at elevated temperatures and decreasingsolubility at low temperatures [2].Scandium containing aluminum alloys are readily usedfor high temperature applications due to their improvedstrength, toughness, creep resistance and good electricalproperties [3]. The strengthening in these alloys is due tonano-sized and coherent tri-aluminide phases such as Al3Sc.The mechanism of strengthening at ambient temperature iscontrolled by dislocation-precipitate interaction. Dislocations bypass the precipitates by shearing or Orowan loopingmechanisms [4]. Continuous research in the optimization ofthe composition has allowed to develop aluminum alloyswhich can sustain temperatures as high as 400 C. This hasbeen made possible by the alloying additions of severalrare-earth and transition metal elements that form L12ordered trialuminide precipitates [5]. Additions of Zr, Er,and V has been reported to have a profound effect on thecoarsening resistance of the precipitates due to theircore/shell morphology [6–8]. Erdeniz et al. [9] has recentlydeveloped an aluminum alloy with micro-additions of Er,Sc, Zr, V, and Si. The authors reported a maximum hardnessof *600 MPa at ambient temperature after optimizing thepeak-aging parameters. This is attributed to the precipitationstrengthening due to the formation of L12-ordered Al3(Sc,Er,Zr,V) nano-precipitates.In addition to precipitation strengthening, severe plasticdeformation (SPD) can be employed to further enhance The Minerals, Metals & Materials Society 2018O. Martin (ed.), Light Metals 2018, The Minerals, Metals & Materials Series,https://doi.org/10.1007/978-3-319-72284-9 57423

424J. Malik et al.strength through grain refinement in accordance with theHall-Petch relationship [10–14]. Equal channel angularpressing (ECAP) is a popular SPD technique largely due toits strengthening effectiveness and processing efficiency [15–18]. Pre-ECAP aging and post-ECAP aging are the two mainprocessing sequences that are used to optimize thestrengthening of precipitation hardenable alloys. In thepost-ECAP aging category, many authors have investigatedthe effect of ECAP on precipitation kinetics and the linkbetween high density of lattice defects and diffusion controlled processes [19–21]. In the pre-ECAP aging, majorareas of concern are the influences of deformation duringECAP on the precipitates distribution [22], fracturing [23],coarsening/dissolution [24], and subsequent microstructurestability at elevated temperatures.In this study, a newly developed precipitation hardenedAl–Sc–V–Er–Zr–Si alloy by Erdeniz et al. [9], in peak agedcondition was subjected to equal channel angular pressingusing route 4Bc. It is to be noted that only the effects ofECAP on aged alloy has been reported here. ECAP of thehomogenized alloy is another extensive study and will bereported elsewhere. The effect of ECAP on the grainrefinement and subsequent improvement in mechanicalproperties were studied using tensile tests and microhardnesscontour maps. In addition, thermal stability of ECAPedmicrostructure was evaluated by taking microhardnessmeasurements after conducting annealing heat treatments atvarious temperatures. Lastly, scanning electron microscopy(SEM) was used to evaluate the influence of ECAP onprecipitate distribution, coarsening/dissolution, and fracturing, if any.Experimental ProcedureAlloy Production and Aging Heat TreatmentsThe alloy used in this study is cast aluminum withmicro-alloying additions of Sc, Er, Zr, V, and Si. Nominalcomposition of Al–Sc–Er–Zr–V–Si is given in Table 1. Thedetailed casting procedure for this alloy has been reported inthe previous study [9]. Three as-cast billets weresolution-treated at 640 C for 4 h. One of the billets waskept in the homogenized (H) state for reference. The othertwo billets were aged using a two-step aging treatment of350 C/16 h followed by 400 C/12 h. The effectiveness ofthis double aging treatment has been previously reported byTable 1 Composition in at. %,of alloying additions in aluminumErdeniz et al. [9]. All heat-treated billets were quenched inwater to terminate the treatment.ECAP ProcessingOne of the two aged billets was subjected to ECAP at roomtemperature, with a pressing speed of 1 mm/min, using a 90 die channel. This billet will be referred as PA-ECAP. Aluminum foil wrapping was used to reduce the frictionbetween the billet and the die. The schematic of ECAPprocess is shown in Fig. 1. Three directions (extrusion,normal and flow direction) and their corresponding orthogonal planes are also given in the figure. Only flow plane wasused for microstructure characterization and microhardnessmeasurements. Route Bc with four passes was utilized in thisstudy, which corresponds to the rotation of billet by 90 aftereach pass. This particular route was selected based on itsestablished efficacy in obtaining uniform ultra-fine grainstructure with high angle of misorientation as reported byseveral researchers for a range of different metallic alloysincluding aluminum [14, 25, 26]. Furthermore, four andmore passes can accumulate sufficient strains to inducehigh-angle grain boundaries [27].Mechanical CharacterizationRoom temperature tensile tests and Vickers microhardnessmeasurements were carried out on three specified conditions:(1) Homogenized only (H), (2) Peak-aged (PA), and(3) Peak-aged followed by ECAP (PA-ECAP). Tensile testswere conducted at a constant strain rate of 1 10 3 s 1using an electromechanical MTS Insight 30 kN machine.Schematic of the miniature tensile specimen used in thisstudy is given in Fig. 2. Two specimens were tested at thiscondition to ensure repeatability.To study the hardness distribution and homogeneity,microhardness measurements on above-mentioned threeconditions were carried out using 50 gf load and an indentation time of 10 s. Specimens of size 1 1 cm size werecut and polished down to 1 µm diamond polish to obtain amirror-like finish. A grid profile was made with 1 mm stepsize in both x and y directions and then represented usingcontour plots.Microhardness measurements after annealing heat treatments on PA and PA-ECAP specimens were taken toScErZrVSiAl0.0130.0070.0710.0740.054Bal.

Equal Channel Angular Pressing of a Newly Developed Fig. 1 Schematic of an ECAPprocess illustrating orthogonaldirections and planes. For routeBc, the billet was rotated 90 along y-axis, after each such passas shown in the figure425PressYX90 ZX: Extrusion direc onY: Normal direc onZ: Flow direc onYZ: Extrusion planeZX: Normal planeYX: Flow planeFig. 2 Sketch with dimensions of a miniature specimen used fortensile testing. All dimensions are in millimetersevaluate the thermal stability and grain coarsening resistanceat elevated temperatures. Annealing heat treatments werecarried out using the temperature range from 50 to 550 Cwith 50 C increments and holding time of 3 h at eachtemperature. In some limited cases, annealing data from anECAPed (route 4Bc), commercially pure aluminum alloy(Al-1100) was included as a reference.Microstructure CharacterizationAll specimens for microstructure characterization were cutfrom the center of the cast billet and the flow plane of theECAPed billet, using electro discharge machining. Specimens were ground down to 1200 grit size, followed bypolishing using 3 and 1 µm diamond paste and 0.04 µmcolloidal silica. Optical microscopy was employed to measure the grain sizes of H and PA specimens. Specimens wereetched using Keller’s reagent (20 mL HNO3, 20 mL HCl,45 90 2nd pass5 mL and 20 mL H2O) and observed under the lightmicroscope to reveal the grain boundaries.EBSD analysis was performed to observe themicrostructural changes and obtain a map of grain size andorientation after ECAP. A FERA SEM with an EBSDattachment was used for mapping. The PA-ECAP specimenof area 25 25 µm was scanned using at a step size of0.1 µm. Map noise reductions was conducted to the fifthdegree before grain size and misorientation measurements.In addition, a FEI Quanta 400 SEM was employed to studythe distribution of precipitates in the annealed PA-ECAPspecimens and to explain their role in maintaining thermalstability. Imaging was done using a backscattered electrondetector and an accelerating voltage of 20.0 kV.Results and DiscussionThe comparison of grain sizes in the optical images of H andPA, and EBSD map of PA-ECAP specimens are shown inFig. 3. Average grain size of H is 411 232 µm and of PAis slightly coarser, 656 385 µm. Larger grain size in PAwas caused by the two-step aging treatment after homogenized condition. Another observation on H and PA micrographs is the subgrain structure seen (approximately 40 µm)in some of the grains, which may have been caused by theremnant primary precipitates that formed during solidification. These precipitates may inhibit recrystallization duringthe homogenization and aging treatments, as stated in previous studies [28, 29]. EBSD maps of the flow plane ofPA-ECAP is shown in Fig. 3c. Color mapping orientationwas with respect to the Z direction for each sample (out of

426J. Malik et al.(a)(b)(c)SubgrainsSubgrains500μm500 μm500500 μmμm5 μmFig. 3 Optical micrographs of a Homogenized (H) and b Peak-aged (PA) specimens, at a magnification of 5X. EBSD map of c PA-ECAP (flowplane) specimen, showing grains oriented along the shear plane. Theoretical shear plane at 45 is shown by white line in (c)the page). Ultrafine grain size of 0.78 0.44 µm wasobtained after ECAP. Grains and sub grains can be clearlyseen oriented and elongated along the shear plane.Room temperature stress vs. strain plots obtained at aconstant strain rate of 1 10 3 s 1, are shown in Fig. 4.The relevant values of tensile data are given in Table 2.Three cases of strengthening can be seen here: (1) Solutestrengthening (H), where most of the alloying elements arepresent in the form of a solid solution. The strength is lowand mainly due to the interaction of dislocation with soluteatoms, but large ductility of 31.6% is obtained. (2) Precipitation strengthening (PA), where most of the solute atomshave precipitated out after aging treatments, in the form oftri-aluminide precipitates. These precipitates are elasticallyhard, coarsening resistant and coherent in nature, and exhibit200HPAPA-ECAPStress (MPa)15010050005101520253035Strain (%)Fig. 4 Tensile tests at a constant strain rate of 10 3 s 1 for threedifferent processing conditions. A general trend of high yield stress andlow ductility is observed after successive strengthening processeshigh strength due to elastic interactions exerted by themismatch in lattice parameter and precipitate-matrix modulus [30]. As a result, we see a 2.3 times increase in yieldstrength as compared to the homogenized (H) condition,along with noticeable work hardening, and 6% decrease inductility. (3) Combination strengthening (PA-ECAP), wheremajor source of strength is due to grain refinement, inaccordance with Hall-Petch relationship, and some contribution is possibly due to the precipitate hardening. Consequently, maximum yield strength of 178.3 MPa wasobtained at the expense of ductility. The decrease in ductilitywith increasing strength is due to insufficient strain hardening, as explained by Wang and Ma [31].Figure 5 shows the contour plots of microhardness of thethree conditions. This was done to illustrate the microhardness homogeneity after aging and ECAP. A very smallmicrohardness spread of 27–30 HV in H shows that theprimary precipitates which may have formed during solidification and remained undissolved during the homogenization treatment, have negligible effect on localized or overallstrengthening. For PA specimen, the microhardness distribution across the cross-section ranging from 48 to 55 HV,indicates slight inhomogeneity with high values at somelocal points, highlighted in orange color on the contourmap. After ECAP there is homogeneity in the microstructurewith microhardness distribution in the range of 59–63 HV.This is mainly attributed to the uniform grain refinementafter ECAP. Xu and Langdon [32] studied the hardnesshomogeneity after ECAP in aluminum and its alloy andconcluded that it gradually increases with number of passes.Figure 6 shows the annealing heat treatment curves forPA, PA-ECAP and Al-1100-ECAP. Pure aluminum alloyshows a sudden drop in microhardness after annealing at150 C for 3 h, followed by a sharp drop to 30 HV after250 C for 3 h, and ultimately reaching the nominalmicrohardness of 25 HV after 400 C/3 h. This trend is

Equal Channel Angular Pressing of a Newly Developed 427Table 2 Summary of the results from tensile tests, microhardness measurements and microstructure analysisDesignationYield strength at 10 3 s 1 strain rate(MPa)Elongation at 10 3 s 1 strainrate (%)Mean microhardness(HV)Average grain size(µm)H50.731.629.4411 232PA115.624.252.4656 385PA-ECAP178.310.462.30.78 0.44Y (mm)(a) Homogenized only1.00.90.80.70.60.50.40.30.20.1(c) Peak-aged ECAP(b) Peak-aged onlyHVHVHVHV0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9X (mm)X (mm)X (mm)70656055504540353025Fig. 5 Contour plots showing Vickers microhardness distribution in 1 1 cm cross-section of a H, b PA, and c PA-ECAP70PAPA-ECAPAl-1100-ECAPMicrohardness (Hv)60504030200100200300400500600Temperature (ºC)Fig. 6 Microhardness measurements after annealing heat treatmentson PA-ECAP and Al-1100-ECAP. Commercially pure Al-1100 wasalso ECAPed using route 4Bc and used here as a referencequite expected and is mainly due to rapid recrystallizationand grain growth during annealing. Contrasting trend ofmicrohardness can be seen in the newly developed alloy inPA and PA-ECAP conditions. PA shows slight increase upto 100 C, thereafter displays a fairly small decrease inmicrohardness up until 400 C. Sharp drops in microhardness beyond 400 C corresponds to coarsening and dissolution of precipitates [9]. Nevertheless, annealing trend ofPA indicates the potential of precipitates in maintaining thestrength of the alloy when exposed to elevated temperatures.PA-ECAP alloy has fine grain structure and homogenousdistribution of precipitates after ECAP, as illustrated byEBSD map and microhardness contour plots. Annealing heattreatments on PA-ECAP shows a slight increase in microhardness at 150 C and thermal stability up until 250 C.The main difference between PA-ECAP and Al-1100 is ofcourse the presence of finely distributed, nanosized, coarsening resistant tri-aluminide precipitates. Thus, it is evidentthat when exposed to elevated temperatures these precipitates play a role of inhibiting, restraining and delaying thegrain growth, therefore maintaining the combined strengthening [29]. Microhardness eventually drops at temperaturesgreater than 300 C, this is due to concurrent recrystallization, grain growth, and to a lesser extent, precipitate coarsening at elevated temperatures.Size and distribution of precipitates after annealing atgiven temperatures is shown in Fig. 7. All the SEM imagesare captured from the flow plane of the ECAPed billet. Thearrangement of precipitates follows a definite trend and aredistributed along the shear plane, which is at *45 anglefrom the longitudinal plane as illustrated in Fig. 7a. Elongated grains in the same direction are also visible (Fig. 7d).The precipitates can also be seen decorating along the grainboundaries, more prominently in Fig. 7e, f. These precipitates play a significant role in restraining the grain growthduring annealing, thus maintaining the thermal stability ofthe microstructure. Precipitates sizes in these alloys are in

428J. Malik et al.(a)50 C(b)150 C(c)250 C350 C(e)450 C(f)550 C45 (d)Fig. 7 Scanning electron microscopy images after various step of annealing heat treatment. At each temperature, the specimen was held for 3 hand then quenched in waterConclusions1. Grain size refinement from 656 385 µm to0.78 0.44 µm resulted after ECAP. Extensive subgrain formation was observed. Grains were orientedalong the shear plane.2. Enhancements in mechanical properties after ECAP wasas expected, with a *55% increase in yield strengthfrom peak-aged condition. This was attributed to thecombined strengthening due to grain refinement andprecipitate hardening. However, insufficient strain hardening capability resulted in the reduction of ductility.3. Homogenous microstructure and distribution of precipitates after ECAP of peak-aged specimen resulted inhardness homogeneity across the specimen.4. Annealing heat treatments show that precipitates play avital role in maintaining microhardness by inhibiting andrestraining grain growth at elevated temperatures.Therefore, the ECAPed alloy maintains thermal stabilityup to 250 C/3 h.The effect of ECAP on the grain refinement and subsequentimprovement in mechanical properties of Al–Sc–V–Er–Zr–Si alloy was studied using tensile tests, microhardness contours, heat treatments and microscopic imaging. Followingconclusion can be drawn from this study:Acknowledgements This work was made possible by a NationalPriorities Research Program grant from the Qatar National Research Fund(a member of The Qatar Foundation), under grant number NPRP7-756-2-284. The statements made herein are solely the responsibility ofthe authors.the range of 50–500 nm, although sizes observed inprevious studies are in the range of 10–100 nm [3, 8, 33].Due to the fine scale of precipitates, the true numberdensity of precipitates cannot be resolved in Fig. 7a–d.These precipitates become visible at higher temperatures(Fig. 7e–f) as a result of

stability, electrical conductivity, and low cost. Equal channel angular pressing (ECAP) is proven to further improve the mechanical properties of metallic alloys through microstructure modification. In this work, ECAP of a recently developed, precipitation hardenable, cast Al–Er–Sc–Zr–V–Si alloy in peak-aged condition by route

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