The Effect of Cold-Worked Ratio on Corrosion Behavior of T8 Tempered Al Si Mg/SiC p Metal Matrix Composites

Transcription

1 The Effect of Cold-Worked Rtio on Corrosion Behvior of T8 Tempered Al Si Mg/SiC p Metl Mtrix Composites Burk Dikici 1, Cgri Tekmen 2, Mehmet Gvgli 3, Yoshiki Tsunekw 2 1 Yuzuncu Yil University, Ercis Technicl Voctionl School of Higher Eduction, Ercis-Vn, TURKEY 2 Toyot Technologicl Institute, Mterils Processing L, , Ngoy, JAPAN 3 Atturk University, Deprtment of Mechnicl Engineering, Erzurum, TURKEY Keywords: T8 temper, corrosion, metl mtrix composite, SiC prticle, cold working Astrct The corrosion ehviors of T8 tempered Al Si Mg/SiC p metl mtrix composites (MMCs) were investigted in erted nd deerted 3.5wt% NCl solutions y potentiodynmic polriztion technique in this study. In order to determine the effect of cold work on the electrochemicl ehvior, the composites were cold-worked to plstic strin of 4, 6, 10, 25 nd 50%. A mjor chnge in the corrosion morphologies were oserved cold working rtio incresed from 4 to 50%. For exmple, electrochemicl nlysis indicted tht the corrosion current densities (i cor ) of 20% vol. SiC composite incresed from 0.8 to 5.9 with incresing cold-worked rtio fter T8 temper. In ddition, we find tht there is significnt colortion etween corrosion susceptiility nd level of cold work. 1. Introduction Nowdys, luminum sed metl mtrix composites (AMMCs) cn e strengthen with precipittion hrdening (ging), work hrdening, reinforcement with intermetllic compounds, solid solution strengthening. One of the min dvntges of prticle reinforced AMMCs is tht illets of the composite cn e mechniclly processed using technologies developed for monolithic lloys; for exmple, extrusion, forging or hot nd cold rolling. Therefore, the composites re very commonly used in erospce nd utomotive industry due to these multi-dvntges. However, mechnicl properties nd the corrosion resistnces of these type composites re depending on complex fctors such s rtio of deformtion nd time of geing esides the microstructure of mteril. In ddition, the fctors re lso considerle scientific nd technicl importnce. It is ccepted tht the presence of the reinforcing phse ffects oth the deformtion nd geing ehvior of the metl mtrix due to the ltered re-crystlliztion kinetics round the reinforcement prticles [1 9]. Trowsdle et.l. [3] found tht the polriztion curves of 50% cold-deformed composite fter het tretment were very similr to those in the sence of cold work; in prticulr, there ws no chnge in the pitting potentil. Briefly, ccording to the uthors, the residul stresses nd disloction density re ineffective on the corrosion resistnce of Al/SiC composites. Quinoo et. l.[6] reported tht the precipittion kinetics in lloy during the ged stges, re fster with incresing level of cold work thus less energy will required for the formtion of precipittion phses. So, the required ctivtion energy to formtion precipittion phses will decrese incresing level of cold work. However, the effects of reinforcements on the precipittion kinetics in MMCs re more complex. Compred to the unreinforced lloys, disloctions creted from the therml mismtch etween the mtrix nd the reinforcements provide dditionl sites for precipittion nd enhnce pipe diffusion of solute toms [8]. For exmple, the ging mechnism for Al Si Mg lloys is developed y controlled decomposition of unstle supersturted solid solution. The precipittion sequences in the ternry system re GP1 β β β (Mg 2 Si) (Eq. 1) [6, 8]. This phse my lter the corrosion chrcteristics of the mtrix lloy nd composite ecuse the Mg concentrtion in solid solution fter ged process is reduced nd Mg 2 Si precipittions lso exhiit cthodic ehvior ccording to Al sed mtrix lloy [10]. Si + 2Mg Mg 2 Si (1) Additionlly, ccording to some reserch groups, the formtion of highly nodic precipittion phses cn led to n incresed susceptiility to stress corrosion crcking (SCC) [3, 10 12]. An nother

2 possiility, the corrosion resistnce of the composites fter T8 process my influence due to the incresing of formed vcncies t disloctions nd lso the rection products occurred t the Al/SiC interfces [2]. Pulished dt indicte tht the formtion of rection products my hve n importnt influence on the corrosion ehvior of the mtrix lloy nd composites. For exmple, SiC cn rect with molten luminum nd producing luminum-cride (Al 4 C 3 ), ccording to the following rection (Eq. 2) [3, 13]. Composites contining Al 4 C 3 re more susceptile to corrosion ecuse it is n extremely rittle phse nd wter rective, which my lter the corrosive sensitivity of the composite [14]. 4Al + 3SiC Al 4 C 3 + 3Si (2) The friction method nd mechnicl properties of AMMCs re now well documented. But, the corrosion susceptiilities of T8 tempered AMMCs hve not een fully explined up to now. So it is n urge to understnd the effects on corrosion ehvior of these composites of reltion etween cold working nd ging. In our previous studies [8, 9], the role of reinforcement volume frction nd ged het tretment on the corrosion ehvior of the composites ws determined, respectively. Therefore, the present investigtion is imed to determine pitting resistnce nd to study the corrosion chrcteristics of Al Si Mg/SiC P MMCs in T8 het-treted conditions. 2. Experimentl Procedure Silicon cride prticle (SiC p ) reinforced composites consisting of 10 nd 20 vol% SiC p s re produced y the compocsting technique. In order to determine the effect of cold work on the electrochemicl ehvior, the composites were cold-worked to plstic strin of 4, 6, 10, 25 nd 50%. In ll these type composites, the mtrix/reinforcement interfces is hve significnt effect on the composite corrosion performnce, so the surfce of SiC prticles efore csting ws coted with thin silicon-dioxide (SiO 2 ) lyer for oth improve of dhesion nd prevent the formtion of Al 4 C 3. The detils of the production method nd T8 process re given in previous studies [8, 15]. The electrochemicl ehviours of the composites hs een nlyzed in erted nd deerted 3.5 wt.% NCl queous solutions t room temperture in Pyrex glss cell. In this study, ASTM stndrds (G 1-03, G5-94) re used oth checked of experimentl technique nd instrumenttion nd prepred of corrosion test specimens [16, 17]. The ll electrochemicl mesurements were crried out using PGS95 potentiostt/glvnostt (BANK Inc., Germny). An Ag/AgCl electrode nd pltinum (Pt.) electrode were used s reference nd uxiliry electrodes respectively. The solution ws deerted to remove oxygen with N 2 gs. The deerted process strted t 60 minutes prior to mesurement nd continued until the end of the experiment. The corroded surfces of the composites fter the corrosion tests re oserved y using scnning electron microscopy (SEM). Also, X-ry diffrction (XRD) technique hs een used in order to determine phses occurred t the mtrix/sic interfce. 3. Results In erted conditions, potentiodynmic polriztion curves of T8 tempered mtrix lloy nd 20% SiC composites re given in Fig. 1 which shows the effect of cold-worked rtios on the corrosion resistnce. 50% Reduction 0,000 0,001 0,010 0,100 1,000 10,000 50% Reduction 0,000 0,001 0,010 0,100 1,000 10,000 4% Reduction 6% Reduction 10% Reduction 25% Reduction 4% Reduction 6% Reduction 10% Reduction 25% Reduction Fig.1. Potentiodynmic polriztion curves of ) mtrix lloy nd ) 20% SiC composite ccording to cold-worked rtios in erted 3.5% NCl solution

3 The nodic nd cthodic polriztion curves recorded on mtrix lloy nd the composites re compred in Fig. 2 in the cse of 4 nd 25% coldworked rtio, respectively. The E corr, E pit, i corr vlues clculted from the polriztion curves, crried out in 3.5% NCl/lortory tmosphere, re collected in Tle 1. 20% SiC MMC 0,000 0,001 0,010 0,100 1,000 10,000 Mtrix lloy %10 SiC MMC Mtrix lloy %10 SiC MMC 20% SiC MMC 0,000 0,001 0,010 0,100 1,000 10,000 Fig. 2. Potentiodynmic polriztion curves of coldworked mterils erted in 3.5% NCl solution ) 4% nd ) 25% Tle 1. E corr, E pit nd i corr vlues of the mtrix lloy nd composites in erted 3.5% NCl solution Mteril Reduction E corr E pit i corr (%) (mv) (mv) ( A/cm 2 ) Mtrix Alloy % SiC p % SiC p Tle 1 indictes tht the corrosion resistnces (i cor ) of unreinforced mtrix lloy nd the composites increse with the increse of cold-work level. For exmple, the verge vlue of the 20% SiC p composite ws clculted to e 3.2µA/cm 2 for the 25% cold-worked specimens (Tle 1), which is igger thn for the 4% cold-worked composites (1.7µA/cm 2 ). Menwhile, mjor chnge in the E corr nd E pit vlues for ech composite type were not oserved cold working rtio incresed from 4 to 50%. Proly, the corrosion potentil pproches closely the pitting potentil ecuse of sutle polriztion effect my e msked y employing erted solution (Fig. 2). Therefore, in the second stge of the study, ll electrochemicl tests were repeted under nitrogen tmosphere. Polriztion curves of T8 tempered mtrix lloy nd 20% SiC composite fter deerted process compre in Fig. 3. In those curves, the pitting potentils (E pit ) were demrcted y mrked rise in the nodic current density for the ech plstic strin rtio ,0000 0,0001 0,0010 0,0100 0,1000 1,0000 Fig. 3. Potentiodynmic polriztion curves of 20% SiC composite ccording to cold-worked rtios in deerted 3.5% NCl solution Fig. 4 shows the vrition of the potentils versus current densities of the mterils s function of SiC prticle content for 4 nd 25% reduction of cold work. The sme s erted conditions, the E corr, E pit, i corr vlues clculted from the polriztion 4% 6% 10% 25% 50% 4% 6% 10% % 50% ,0000 0,0001 0,0010 0,0100 0,1000 1,0000

4 curves re collected lso in Tle 2 for nitrogen tmosphere tests ,0000 0,0001 0,0010 0,0100 0,1000 1, Fig. 4. Potentiodynmic polriztion curves of cold-worked mterils deerted in 3.5% NCl solution ) 4% nd ) 25% Tle 2. Corrosion (E corr ), pitting (E pit ) potentils nd current density (i corr ) vlues of the mtrix lloy nd composites in deerted 3.5% NCl solution Mteril Mtrix Alloy 10% SiC p 20% SiC p Mtrix Alloy 10% SiC MMC 20% SiC MMC Reduction (%) E corr E pit Mtrix Alloy 10% SiC MMC 20% SiC MMC ,0000 0,0001 0,0010 0,0100 0,1000 1,0000 i corr ( A/cm 2 ) (mv) (mv) , Tle 2 indictes tht the i corr vlues of the composites increse with the increse of cold-work level, whose mximum vlues rech 4.0, 5.0 nd 5.9 µa/cm 2 for mtrix lloy, 10%SiC nd 20% SiC MMCs, respectively. Briefly, it cn e sid tht the current densities (i cor ) of the unreinforced mtrix lloy nd the composites incresed with oth incresing SiC prticle frction nd cold-worked rtios. In other words, there is significnt colortion etween corrosion susceptiility nd level of cold work. Fig. 5 demonstrtes the Mg nd Si distriutions tht hs een otined for 25% coldworked 20% vol. SiC p composite. The results of X- ry mps show tht the concentrtion of Mg is nerly homogeneous in the mtrix lloy (Fig.5-). But, Si hs een oserved tht preferentilly incresing t mtrix/sic prticle interfces (Fig. 5-c). ) 1 µm 1 µm Fig. 5. ) SEM microgrph nd X-ry mps of ) Mg, c) Si fter T8 temper of the 20% vol. composite Fig.6 shows the SEM imges of the unreinforced mtrix lloy nd the composites fter corrosion tests, which hve developed principlly loclized corrosion t the mtrix lloy (Fig. 6-) nd the pitting corrosion t the composites (Fig. 6--c). In composites, the pitting corrosion hs propgted principlly t the mtrix/sic p interfce (Fig. 7). At the dvnced stges of the corrosion, the ttck ws rther dense round the SiC prticles interfce (Fig. 8-). As result, this kind of ttck my ffect the norml pitting corrosion ehvior of the mteril. The formtion of pits or holes nd free fcetted silicon cride due to intensive mtrix corrosion is shown in Fig. 8. There is not ny indiction of corrosion on the SiC p surfce (Fig. 8-) c) 1 µm

5 c Fig. 8. ) Holes round of SiC prticle due to intensive mtrix corrosion, ) nd free fcetted of SiC prticle in T8 tempered 20% vol. SiCp composite fter corrosion Fig. 6. SEM imges of the 25% cold-worked ) mtrix lloy nd ) 10%, c) 20% vol. SiC composites fter corrosion tests Fig. 7. Preferentilly corrosion formed round SiC prticles in 4% cold-worked 20% vol. SiCp composite In our previous studies, XRD results indicted tht the Al 4 C 3 rection product ws not formed which mens continuous SiO 2 lyer is exist on the reinforcement prticle surfce [9, 18]. 4. Discussion Figs 1 4 shown tht there is distinct colortion etween the corrosion susceptiilities nd cold work levels of the T8 tempered mterils. Also, the corrosion resistnce of the mterils decreses with incresing degree of cold work. In rel interfce systems, disloctions form t or ner the interfce to relief the strin energy due to lttice mismtch. According to different uthors [3, 19], the resons of rpid dissolution of toms in the composite could fvour due to the ccumultion of very lrge disloction density nd high stress concentrtions in the Al/SiC interfces compred to mtrix lloy. Therefore, corrosion resistnce of AMMCs my e ltered SiC prticle volume frction nd high disloction densities formed in the vicinity of SiC prticles. The current density vlue is directly relted to the electrode potentil, nd it cn provide more relistic results relted to electrochemicl ehvior of the composites [20]. In Tle 1 nd 2, the i cor vlues of the composites increse mrkedly with incresing oth degree of cold work nd SiC volume friction for ech mteril type. There re two importnt questions

6 in this stte, either SiC p volume frction or coldworked rtio my hve een ltered the corrosion resistnce of the composites. Our previous studies [9, 18] indicted tht i cor vlues of the composites re ~0.5µA nd 0.8 µa/cm 2 fter SiC prticle ddition nd ged het tretment, respectively. Wheres, incresing the cold-worked rtio increse the i corr vlues from 0.8 to 5.9 µa/cm 2 in the present study. In other words, the corrosion effect formed owing to cold working is much more thn tht due to oth SiC prticle ddition nd ged het tretment. The incresing of the i corr vlues fter T8 process my e due to the point defects introduced into the mterils y cold working which provide nucletion sites for pits nd thus fcilitte corrosion. In ddition, this would support the mechnism of stress concentrtion t the interfce in the stimultion of the corrosion process [19]. The corrosion ttcks my strt t Al/SiC interfces owing to the semi-conducting properties of SiC (Fig. 7 nd 8-). Becuse SiC p reinforcement hs much higher n electricl resistnce (160 m) compred to the A356 lloy (4x10-8 m) [12, 21]. Thus, glvnic coupling effect would e possile etween SiC nd the ctive the mtrix [22]. However, the T8 temper nd incresing SiC volume frction decresed lso the size of pits ut not their numer, s shown in Fig. 6--c. These results re in ccordnce with composite theory ecuse it is well known tht SiC prticles cting s physicl rriers to the pit growing [3, 5, 12, 22]. Consequently, reinforcement nd mtrix lloy could e the detchment due to pplied reduction nd the development of possile crevice effect in T8 conditions (Fig. 8-) [19]. This illustrtes tht the decrese in corrosion resistnce of the cold worked smple is due to the incresed mount of disloction nd point defects introduced into the mterils, which serve s high energy sites for the nucletion of pits. In ddition, such phenomen cn e explined y microcrcks nd stresses tht exist in the composite. Recent studies conducted on the production of MMC indicte tht due to the difference in therml expnsion coefficients of the reinforcement (4.5x10-6 /K for SiC) nd mtrix lloy (21.5x10-6 /K for A356-T6), some micro-crcks my occur where the discontinuity is present during solidifiction. The increse in disloction density round the precipittes outcome from ge hrdening nd residul stresses my increse these micro-crcks. As result, res which hve different potentils my increse nd when the potentil exceed E corr vlue, these res my ct s corrosion nucletion sites [12]. However, it should e expressed tht this result is in contrdiction with the generl point of view tht residul stresses nd disloction density re ineffective on the corrosion resistnce of Al/SiC composites [3, 12]. Another possile explntion of incresed corrosion susceptiility is the distriution of the crcked prticles. It is well known tht crcked powders possess generlly higher surfce energy thn lrge prticles [23]. The presence of micro-crcked prticles cn mke preferentilly corrosion mechnism the corrosion will continue round these prticles nd lso provide the development of min pits. Menwhile, the structure tends to induce mny micro-sumicro locl tteries, nd mke the mtrix grins potentilly differentite, thus increse the pitting potentil. Becuse, SiC is nole mteril nd will not undergo corrosion (Fig. 8-). But this my cuse to form the smll node-ig cthode corrosion type nd the corrosion increse significntly. Thus, loclized corrosion in the Al mtrix lloy will progress quickly nd in depth. This type of corrosion known s pitting nd even it cuse less mteril loss, it my dmge the construction. As result, this kind of constitutions my ffect the norml pitting corrosion ehvior of the mteril (Fig. 4-). Trowsdle et. l. [3] reported tht the polriztion curves of 50% cold deformed composite were similr to un-deformed composite. This pproch ws focused only to one deformtion rtio. Therefore, it is not sid tht deformtion is not effected corrosion ehvior of the AMMCs. Becuse, it could e seen tht the corrosion susceptiilities oth of the composites nd of unreinforced mtrix were incresed y incresing the level of cold work even though composites were cold-worked to plstic strin of 4% in this study. 5. Conclusions We hve performed n experimentl study in order to understnd the effect of cold working on the corrosion of Al Si Mg/SiC MMCs. The following conclusions cn e drwn from the ove experimentl study. 1. The results show tht the cold deformtion nd its rtio hve gret effect on the electrochemicl ehviors of AMMCs. 2. Deerted process must e used to determine the tendency of the ctive metl dissolution in the NCl solutions.

7 3. The corrosion prmeters clculted from Tfel plot indictes tht the corrosion resistnces (i cor ) of the composites increse with the increse of cold-work level, whose mximum vlue rech 4.0, 5.0 nd 5.9 µa/cm 2 for mtrix lloy, 10%SiC nd 20% SiC MMCs, respectively in deerted 3.5% NCl solution. 4. The corrosion effect formed owing to cold working is much more thn tht due to oth SiC prticle ddition nd ged het tretment. 5. The SEM imges show preferentilly corrosion formed round the SiC prticles in the composites (Fig. 8). 6. Both the cthodic ehvior of Mg 2 Si phse nd ccelerted precipittion kinetics t Al/TiC interfce my reduce i corr current densities of the composites in the T8 conditions. 7. In ddition, high disloction density, residul stresses nd micro-crcks my cuse to form smll nodes thus ffect unfvorle the corrosion chrcteristics. Acknowledgements This reserch project ws finnced y Atturk University under BAP 2003/66 in Turkey. The uthors would like to thnk Prof. Dr. Hnifi Src nd Prof. Dr. Sinn Ypici for lortory contriution. References [1]. M. Ferry nd P. R. Munroe, Recrystlliztion kinetics nd finl grin size in cold rolled prticulte reinforced Al-sed MMC, Composites Prt A: Applied Science nd Mnufcturing, Volume 35, Issue 9, Septemer 2004, Pges [2]. Dvid Tlot, Jmes Tlot, Corrosion Science nd Technology, ch.5-9, CRC Press, Boc Rton, Florid, [3]. Trowsdle, A.J., Nole, B., Hris, S.J., Giins, I.S.R., Thompson, G.E., nd Wood G.C., The influence of silicon cride reinforcement on the pitting ehviour of luminium., Corrosion Science, 38 (2), 1996, [4]. Trzskom, P.P., Pit morphology of luminum lloy nd silicon cride/luminum lloy metl mtrix composites, Corrosion, 46(5), 1990, [5]. Trzskom, P.P., McCfferty, E., Crowe, C.R., Corrosion Behvior of SiC/Al Metl Mtrix Composites., Elektrochemicl Society of Journl, 130(9), 1983, [6]. G. K. Quinoo, S. Ynncopoulos, The effect of cold work on the precipittion kinetics of Al6111 luminum, Journl of mterils Science, 39, 2004, [7]. Liu, Y.L., Hnsen, N., Jensen, D.Juul, Recrystlliztion microstructure in cold-rolled luminum composites reinforced y silicon cride whiskers, Metllurgicl trnsctions. A, Physicl metllurgy nd mterils science, Volume 20 A, Issue 9, Septemer 1989, Pges [8]. C. Tekmen, U. Cocen, Role of cold work nd SiC volume frction on ccelerted ge hrdening ehvior of Al Si Mg/SiC composites, Journl of Mterils Science Letters 22, 2003, [9]. B. Dikici, M. Gvgli, nd C. Tekmen, Corrosion Behvior of n Artificilly Aged (T6) Al Si Mg-sed Metl Mtrix Composite, Journl of Composite Mterils : [10]. De Slzr, J.M.G., Uren, A., Mnznedo, S., Brren, M.I, Corrosion ehviour of AA6061 nd AA7005 reinforced with Al 2 O 3 prticles in erted 3.5% chloride solutions: potentiodynmic mesurements nd microstructure evlution, Corrosion Science, 41, 1999, [11]. Shreih, L.L., Corrosion. Tien Wh Pres (Pte) Ltd., 1, 4:3-4:30, Singpore.,1979 [12]. Lucs, K.A., nd Clrke, H., Corrosion of Aluminium-sed Metl Mtrix Compoites. Reserch Studies Pres Ltd, p.1-137, Englnd, [13]. Yghmee, M.S., nd Kpty, G., On the stility rnge of SiC in ternry liquid Al-Si- Mg lloy. Mterils Worlds, Proceeding of Hungrin Mterils Science Society, Proceeding of Hungrin Mterils Science Society, e-journl: 2(3), [14]. Lloyd, D.J. The Solidifiction Microstructure of Prticulte Reinforced Aluminium/SiC Composites, Composites Science nd Technology 35,159-16, [15]. C. Tekmen nd U. Cocen, The Effect of Si nd Mg on Age Hrdening Behvior of Al-SiCp

8 Composites, Journl of Composite Mterils : [16]. ASTM G1-90, Stndrd Prctice for Prepring, Clening, nd Evluting Corrosion Test Specimens, ASTM Interntionl. USA, 15-20, 03.02, [17]. ASTM G5-94, Stndrd Reference Test Method for Mking Potentiosttic nd Potentiodynmic Anodic Polriztion Mesurements, ASTM Interntionl. USA, 58-69, 03.02, [18]. Gvgli, M., Dikici, B., Tekmen, C., The effect of SiCp reinforcement on the corrosion ehviour of Al sed metl mtrix composites, Corrosion Reviews, vol. 24 (1-2), 27-37, [19]. C. Monticelli, F. Zucchi, G. Brunoro nd G. Trnelli, Stress corrosion crcking ehviour of some luminium-sed metl mtrix composites, Corrosion Science, Volume 39, Issues 10-11, Octoer-Novemer 1997, Pges [20]. J.R. Glvele, Tfel s lw in pitting corrosion nd crevice corrosion susceptiility Corrosion Science, Volume 47, Issue 12, Decemer 2005, Pges [21]. ASM Hndook, ASM Interntionl, 2, 1-215, Ohio/USA, 1990 [22]. Turnull, A., Review of corrosion studies on luminium metl mtrix composites. British Corrosion Journl, 27(1), 27-35, [23]. Aiju Li, Yuhu Zhen, Qing Yin, Lipeng M nd Ynsheng Yin, Microstructure nd properties of (SiC, TiB 2 )/B 4 C composites y rection hot pressing, Cermics Interntionl, Volume 32, Issue 8, Decemer 2006, Pges