THE EFFECTS OF WITHDRAWAL AND MELT OVERHEATING HISTORIES ON THE MICROSTRUCTURE OF A NICKED-BASED SINGLE CRYSTAL SUPERALLOY

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1 THE EFFECTS OF WITHDRAWAL AND MELT OVERHEATING HISTORIES ON THE MICROSTRUCTURE OF A NICKED-BASED SINGLE CRYSTAL SUPERALLOY Lin Liu, Tiwen Hng, Minming Zou, Weiguo Zhng, Jun Zhng, Hengzhi Fu Stte Key Lortory of Soliifition Proessing, Northwestern Polytehnil University, Xi n , PR of Chin Keywors: Diretionl soliifition, Melt overheting, Withrwl veloity, Denriti sping, Superlloy. Astrt The soliifition interfe morphologies of single rystl superlloy DD3 hve een stuie over rnge of ooling rte with lrge vritions in withrwl spees in high-therml grient iretionl soliifition. The effet of withrwl n therml histories on the enrite morphologies n sping is lso stuie. The results show tht inresing either therml grient or withrwl rte will refine the enrite. Espeilly, super-fine enrite struture with suppresse sie rnhes ws foun uner the ooling rte s high s 10K/s uring high therml grient iretionl soliifition. The initil withrwl histories n melt superhet tretment on enrite morphologies re signifint: with the inrese in the initil elertion withrwl spee, the primry enrite rm sping in stle growth zone ereses. By growth veloity trnsition from higher one to lower one ruptly, the primry enrite sping, n onstitutionl segregtion in the stle growth zone n e reue n vie vers. Melt superheting tretment prior to soliifition n stilize liqui/soli interfe n refine primry enrite n γ phse oviously. Wheres, the seonry enrite rms seem not sensitive neither to melt superheting tretment, nor initil elertion withrwing. Introution It eomes ommon proess of turine les y investment sting of the superlloys, very often into single-rystl form. Brigmn high rte soliifition tehnique (HRS), onsequently, is preominnt mnufture proess of single rystl les. During iretionl soliifition, therml grient (G) n growth veloity (V) influene the sle of enrite struture whih n e hrterize y their mirostruture prmeters, suh s primry enrite rm sping (λ 1 ), seonry enrite rm sping (λ 2 ), enrite tip rius (r ) n mushy zone epth (). Numerous stuies [1-4] hve een reporte with the im to hrterize these prmeters s funtion of growth rte n therml grient he of the mirosopi soliifition front uring iretionl soliifition for nikel-se superlloys. It must e pointe tht the enrite spings epen not only on therml grient n on growth rte, ut lso on the history of the speimen [5-9]. This hs een interprete in suh wy tht no unique sping seletion riterion opertes for λ 1 n n rry with n of the spings is stle uner given experimentl onitions. The vlue of λ 1 is ifferent with the sme finl experimentl prmeters if the soliifition proesses unerwent the vrious initil proesses, initing tht the hrteristi sle is history-epenent. Unfortuntely urrent experimentl n theoretil works re selom involving eveloping history uring iretionl soliifition for superlloys, even tht history proess to influene the resulting morphologies is not ler. The st mirostruture of superlloys lso epens on therml history of the melt. The present uthor stuie the melt superheting on the st struture of In738 superlloy [10]. Yin et l [11] investigte the effet of melt superhet histories on the grin size n MC rie morphologies of superlloys M963. No report ws foun with regrs to the reltionship etween enrite hrteristis n melt tretment histories for single rystl superlloys. The morphology evolution of liqui/soli interfe uring soliifition is mostly importnt in soliifition proess reserhing. Unfortuntely, little literture hs een foun for the morphology evolution of L/S interfe regring melt struture. The im of present presenttion is to estlish the reltionship etween mirostruture fetures n soliifition prmeters of single rystl superlloys uner high therml grient iretionl soliifition. The effets of withrwl n therml overheting histories on the enrite struture n sping re lso resse, respetively. Experimentl A first-genertion nikel-se single rystl superlloy DD3 (9.5 wt.% Cr, 5.2W, 5Co, 4.2Mo, 5.7Al, 2.3Ti, 0.006C n Ni s lne) ws use. The phse n struturl trnsformtion tempertures of the lloy were stuie y ifferentil therml nlysis y using STA NETZSCH 409CD Therml Anlyzer. Figure 1 shows the results of DD3 lloy with leftive rte 20 /min. The liquius temperture is inite to e C. At the rnge of C, there re series of enothermi or exothermi kurtosis in DTA urves whih imply ourring of severl struturl trnsformtions in the liqui. Aoring to the previous reserh [12], the pek hppening t C shoul e the issolution of MC-type ries. The mening of other peks t higher temperture is still not ler. 287

2 Figure1 Differentil therml nlysis urve of DD3 lloy The lloy ws iretionlly soliifie with Brigmn-type high therml grient pprtus evelope in the uthor s lortory. A zone intensifie overheting together with liqui metl ooling metho ws use to otin higher therml grient. As shown in Figure 2, the lloy is melte y primry heter n overhete gin to higher temperture y n intensifie heter ple t the viinity of liqui/soli interfe. Detils of the experimentl equipment re esrie elsewhere [13]. ifferent initil withrwl histories in whih vrious elertion vlues were tken efore rehing to stle growth veloity (V s ), withrw the smples t hnging rtes ruptly from one growth rte to nother, n superhet the melt to preetermine temperture ove the liquius of the lloy n holing perio n then soliify the smples. Withrwl veloity urves n e ontrolle y the frequeny justment of the servo system s input instrution. Aprt from the norml withrw, we set oth itionl withrwl moes: strt from zero n grully elerte to the onstnt spee (urve I in Figure 3), or hnge spee ruptly from one to nother (urve II n III). In the first moe, fter the vrious initil non-stle proesses, the speimen then grew no less 20mm to estlish the sme stey stte onitions. In the seon mnner, stey growth must e hieve in oth steps. During the melt superheting proess (see Figure 4), the melt ws first hete to the temperture T S, holing perio of (t 2 -t 1 ), then oole to temperture T 0,n then soliifie. The effet of therml histories, inluing superheting temperture T s n homogenistion time (t 2 -t 1 ), on the soliifition struture were exmine. Figure 3 The initil withrwl histories: I. Grul-shift seeing moe with ifferent elertion; II. Trnsition from low veloity stle growth to high veloity stle growth; III. Trnsition from high veloity stle growth to low veloity stle growth Figure 2 Zone intensifie overheting & liqui metl ooling (ZIOLMC) iretionl soliifition n otine temperture profile 1-smple, 2-ermi ruile, 3-primry heter, 4-intensifie heter, 5-therml ffle, 6- low melting liqui metl (oolnt), 7- wter ooling jket. Smples with 7mm in imeter n 115mm in length were prepre. By using omputer ontrolle withrwl system, vrious withrwl rtes, rnging from 0.1μm/s to 900μm/s, were otine. The therml grient t the soliifition front mesure using B-type thermoouples ws from 200 to 800 C/m. Single rystl ws prepre y the ottom seeing tehnique. The iretionlly soliifie mirostruture ws otine vi four ifferent wys: withrw the smples t preetermine rtes to stuy the effet of the growth rtes, withrw the smples uner Figure 4 The therml histories y superheting the melt Mirostrutures of ll smples were exmine using optil n eletron mirosopy. Quntittive nlyzing of the 288

3 mirostrutures ws performe y imge nlyzer. The onentrtions of min lloying elements of Al, Ti, T n W in interenriti res n in enrite ore were nlyze y eletron miroproe. The ooling rtes uring iretionl soliifition were etermine from primry enrite sping oring to Mehrin s eqution [14]. Results n Disussions Soliifition Morphologies uner Constnt Growth Rtes Figure 5 shows the enrite morphologies uner ifferent withrwl rtes n therml grients. At higher withrwl rtes, enrite is ontinully refine; those hrteristis n e represente s suppresse sie rnhes n shortene primry n seonry spings. Inrese in therml grient ffets the enrite morphology in the sme wy s the withrwl rte: the enrite is refine t higher therml grient uner ientil withrwl rte, see Figure 5 () n (). Figure 5 Growth morphologies uner vrious withrwl rtes n therml grients () Corse enrite, V=6.67μm/s, G=200K/m; () Corse enrite, V=13.3μm/s, G=200K/m; (e) Fine enrite, V=100μm/s, G=200K/m; () Super-fine enrite, V=100μm/s, G=800K/m. In orer to hrterize enrite morphology quntittively, we mesure the primry n seonry enrite rm spings, whih re inite with λ 1 n λ 2, respetively. The results re shown in Tle 1. It n e seen tht they re refine extensively with inresing ooling rte T, whih n e efine sgi R, where R is growth rte. The growth rte is equivlent to withrwl rte in most irumstnes. Aoringly, we n lulte the ooling rte y using withrwl rte inste of growth rte without notiele error. Referring to Figure 5, we n see tht sie rnhes egin to e suppresse t the ooling rte of 2K/se.The fine struture n e foun when the ooling rte rehes 10K/se. Its sping is 38μm in this se, reue y ftor of out 7-10 s ompre with the onventionl high rte soliifition (HRS) proess in whih typil therml grient is elow 60K/m. Tle 1 Denrite rm sping uner ifferent ooling rtes T, K/se λ 1, μm λ 2, μm N.D. The Effet of Initil Withrwl Histories The Effet of Grul-Shift Seeing Moe Aoring to the onstitutionl unerooling theory, high therml grient n effetively suppress the formtion of equixe grins he of soliifition interfe, n therefore permits mintining iretionl struture uner fst growth rte. On this sis, we n effetively inrese growth veloity sine the proution of the DS superlloy omponents is lwys time-onsuming. However, the proessing to proue single-rystl superlloys t high growth rtes, normlly lrger thn 300μm/s, y ottom seeing tehnique rings us new prolems. Owing to the hysteresis of interfe growth veloity responing to impose withrwing veloity t initil intervl, mushy zone n e move own n pulle out of the iti zone in the furne, thus prouing non-negligile trnsverse therml grients. The eprture istne of L/S interfe from nominl stle position is mximl t trnsient-shift seeing, n inreses with higher growth veloities. Uner some onitions (V>300μm/s), these trnsient-shift seeing moes my result in ening L/S interfe t initil growth perios n onsequently filure of seeing or primry enrite towr the enterline of the smples, see Figure 6(). It is proly n effetive wy to improve seeing qulity y grul-shift seeing moe, tht is to use n elerte withrwl moe t the initil perio. In this seeing moe, the initil eprture istne of interfe is reue n the growth interfe hnges to flt plne, thus uses enrites well ligne with the withrwl iretion (Figure 6 ()). It must e mentione tht for ifferent iretionl soliifition systems n interfe positions t iti zone, the initil ehvior my e ifferent. Although grul-shift seeing moe ontriutes to wellontrolle iretionl soliifition mirostruture, the imprint of initil histories on susequent stle-growth mirostruture must e tken into ount. Tle 2 shows the proessing prmeters n mirostruturl hrteristis given y ifferent initil withrwl histories. The mirostruture in the stle growth zone n e seen in Figure 7. The experimentl results inite tht the initil proessing onsierly influenes the mirostruture given y stle growth. The higher the initil elertion vlue, the smller the primry enrite rm sping 289

4 is. In ontrst, the seonry enrite rm sping eviently oes not hnge with initil proessing. Figure 6 The effet of initil seeing moes on L/S interfe shpe, growth iretion is from the left to the right. () V s =500μm/s, Trnsient-shift seeing; () V s =500μm/s, Grul-shift seeing, =50μm/s 2. Tle 2 The effet of initil withrwl prmeters on mirostruture t stey stte Frequeny ltertion moe Aelertion (μm/s 2 ) Withrwl rte (μm/s) λ 1 (μm) λ 2 (μm) 2Hz/57ms Hz/57ms See See Figure 7 The enrite morphologies t ifferent initil withrwl histories. (), () V=900μm/s, =50μm/s 2 ; (), () V=900μm/s, =10μm/s 2. The experimentl results revel tht the ontrol of the growth histories of superlloys my eome n effetive wy to hieve perfet mirostruture of single-rystl preprtion of superlloys. The primry enrite rm sping n just in response to hnging onitions y ifferent mehnisms. These justment mehnisms inlue enrite elimintion, enrite tip splitting n the growth of tertiry rms [15]. The λ 1 vlue n e ugmente y enrite elimintion n reue y enrite tip splitting n growth of tertiry rms. When enrite tip splits or tertiry rm overgrows, itionl interfe is proue n ertin mount of unerooling is neee to overome energy rrier for spontneous growth. Those phenomen my hppen in the trnsition zone where growth ours uner ifferent elertion onitions. This my give rise to the history epenene of primry enrite rm sping. Furthermore, ifferent elertion vlues n inue ifferent unerooling kinetis whih will influene non-stle evolution from orse rry to finer one. Lnger n Müller-Krumhr [16] hve rrie out etile numeril nlysis of the wvelength of instilities long the sies of enrite n hve preite the sling lw s λ 2 / r = 2, where r is enrite tip rius. Sine the enrite tip rius is ontrolle y temporl onitions, s knowlege in the moels of Kurz-Fisher [17] tht r is funtion of V n C 0. The seonry enrite rm sping is therefore not history epenent. The Effet of Arupt Growth Veloity Trnsition The effet of histories of DS proess on susequent mirostruture is essentilly the ft tht the preformtte enrite will regulte the formtion of sueeing enrite uring ontinuous growth. A trnsition whih egins with given mirostruture will e more importnt to stuy the histories-relevnt morphology evolution. Aoringly, experiments of DS proess from one ertin stle onition hnging to nother onition hve een rrie out, n the results re shown in Figure 8. The finl primry enrite sping will e shortene if it unerwent lower growth rte ut e enlrge if it unerwent higher growth rte. Tle 3 inites tht the finl primry enrite rm spings (PDAS s) re ffete gretly y the preformtte ones. For exmple, the PDAS is 71μm t norml onition n inreses to 93μm if the preformtte growth rte is 100μm/s. On the ontrry, the reue sueeing spings re foun if they unerwent the higher growth rte t whih PDAS is muh smller. In prtil soliifition proesses, there re inevitle flututions in the proessing onitions, whih my ffet the enrite growth n therey hnge the finl enriti sping. This my e the use of no unique primry ellulr/enriti sping, ut rther rnge of sping for given proessing onition in mny experimentl results. For given finl proessing onition, there re one upper n lower limits of the llowle primry enrite sping. Hunt n Lu [18] preite y using qusi-3d simultion tht the upper limit of the λ 1 istriution is out two times s the lower limit, wheres Wng [19] suggeste ftor of pproximtely three. In the present work, the results show tht the mximum vlue of the λ 1 istriution uner 100µm/s growth rte is nerly three times s tht of the minimum vlue, s seen in olumn 3 in Tle

5 e Figure 8 the effet of growth veloity trnsition on enrite morphology (), (): growth veloity trnsition from 100µm/s to 300µm/s (), (): growth veloity trnsition from 300µm/s to 100µm/s (e), (f): growth veloity trnsition from 600µm/s to 20µm/s Tle 3 The primry enrite rm spings (µm) t the stey stte when growth rte trnsits from V 1 to V 2 V 1, µm/s V 2, µm/s / / 93 / / / 600 / 62 / / f Figure 9 The effet of growth veloity trnsition on the onstitutionl segregtion n the morphology of γ-γ Thus, proper trnsition in the proess prmeters erly in the formtion of the enriti mirostruture might result in n improve prout. The Effet of Melt Superheting Tretment Figure 10 shows the effets of ifferent melt overheting tempertures on the morphology of S/L interfe. The originl interfe morphologies re plnr n ellulr-enrite t the growth veloity of 1.0μm/s n 5.0μm/s, respetively. After higher melt superheting tretment t 1780 C, the ellulr interfe is smoothene to plnr n the sie rnhes in ellulr-enrite struture ispper t ifferent growth veloities, initing tht melt superheting tretment will stilize the L/S interfe uring iretionl soliifition. The seonry enrite rm hs een moulte y ifferent PDAS. When the growth veloity hnges from 600µm/s to 20µm/s, even kin of espeil mirostruture with no visile seon enrite rm hs een foun, s seen Figure 8 (e), (f). Therefore, the onstitutionl istriution will e ltere y growth rte trnsition. As seen in Figure 9, the onstitutionl segregtion of these smples proue y rte trnsition from 600µm/s to 20µm/s is signifintly less thn tht of smple proue t the onstnt rte of 20µm/s. The segregtion rtio (SR), whih is efine s the mximum onentrtion in interenrite res over the minimum onentrtion in the enrite ore, will reue uring the trnsition. The results inite tht the previous growth veloity histories influene on the finl mirostruture fetures oviously. Figure 10 Morphology of S/L interfe uner ifferent melt overheting tempertures (), () t S =t 0 =1500 C, (), () t S =1780 C ; (), () V=1.0μm/s, (), () V=5.0μm/s. 291

6 The mirogrphs of trnsverse setion of smples in Figure 11 show the primry enrite rms t ifferent melt superheting temperture. It is lerly seen tht the enrite struture is refine with the inrese of melt superheting temperture. Figure 12 revels tht the primry enrite rm sping ereses t higher melt superheting temperture t ny growth rte. methos re effetive wys to ontrol mirostruture of single rystl superlloys. A e Figure 13 Morphology of γ prtiles uner ifferent melt overheting tempertures. (), () T S =T 0 =1500 C; (), (e) T S = 1640 C; (), (f) T S =1780 C; (), (), () enriti re (), (e), (f) interenriti re. Figure 11 The enrite morphologies t ifferent melt superheting tempertures, V=20 μm/s. () T S =T 0 =1500 C, () T S =1640 C, () T S = 1780 C. λ 1,μm T S, V=20μm /s V=50μm /s V=100μm /s Figure 12 Primry enrite sping uner ifferent melt superheting temperture The size of γ prtiles in oth enrite ore n interenriti res erese with the inrese of melt superheting temperture from 1500 to 1640 n then to Furthermore, the morphology of γ prtiles in interenriti re eome more uniform, s shown in Figure13. Sine ll struture fetures re refine fter melt superheting tretment, it is expete tht the mehnil properties will e improve for refine struture. The experiment results show tht the hnge of enrite morphologies, size of γ strengthening preipittes n mirosegregtion of the elements is fvorle uner ifferent withrwl n melt therml histories, initing tht those The mehnism of melt superheting is relte to the lterntion of physil or hemil properties of the melt, suh s iffusive oeffiient, visosity or prtition oeffiient, n even to the hnge of liqui struture. It hs een foun y some investigtions [20] tht the melt struture of Ni-se superlloy hnges from orer to isorer with melt temperture inrese. There exist Ni 3 (Al, Ti, N)-like luster, MC rie luster n other high-melting prtile in the melt when the melt temperture is lightly higher thn the liquius. The present uthor [10] foun tht MC rie will not ompletely issolve elow With the melt temperture inrese, the Ni 3 Allike n MC rie luster will erese in their sizes elow 1600 n even ispper in the lloy melt ove 1600 [20], whih my le to ovious inrese in the melt unerooling egree n the melt uniformity. All these ftors will influene the soliifition mirostrutures. Due to their unertinty n omplexity, setting up reltionship etween mirostruture feture n lloy melt hrteristis still nees to stuy. Conlusions (1) Inresing either therml grient or withrwl rte will refine the enrite. Espeilly, super-fine enrite struture with suppresse sie rnhes ws foun uner the ooling rte s high s 10K/s uring high therml iretionl soliifition. (2) The effets of initil withrwl histories on enrite morphologies re signifint. With inrese in the initil elertion withrwl vlue, the primry enrite rm sping in the stle growth zone ereses ut the seonry enrite rms re hrly ffete. (3) By growth veloity trnsition from higher one to lower one, the primry enrite sping n onstitutionl segregtion n e reue, n vie vers. A proper trnsition in the proess 292

7 prmeters erly in the formtion of the enriti mirostruture might result in n improve prout. (4) Melt superhet tretment efore soliifition will stilize soliifition interfe n refine primry enrite signifintly. The seonry enrite sping seems not sensitive to the melt superhet tretment. Aknowlegments Finnil supports from Ntionl Mjor Reserh Pln on Bsi Reserh (No.2006CB605202) n from Ntionl Nturl Siene Fountion of Chin (No ) re gretly knowlege. Referenes 1. L. Li, n R. A. Overfelt, Influene of iretionl soliifition vriles on the ellulr n primry enrite rm spings of PWA1484, J. Mter. Si., 37 (2002), A. Kermnpur, N. Vrhrm, E. Engilehei, M. Mohmmzeh, n P. Dvmi. Diretionl soliifition of Ni se superlloy IN738LC to improve reep properties, Mter. Si. Tehn., 16 (2000), A. J. Elliott, S. Tin, W. T. King, S.-C. Hung, M. F. X. Gigliotti, n T. M. Pollok. Diretionl Soliifition of Lrge Superlloy Cstings with Rition n Liqui- Metl Cooling: A Comprtive Assessement, Metll. Mter. Trns. A, 35A (2004), A. Wgner, B. A. Shollok, n M. MLen. Grin struture evelopment in iretionl soliifition of nikel-se superlloys, Mter. Si. Eng., A 374 A (2004), S. H. Hn, n R. Trivei. Primry sping seletion in iretionlly soliifie lloys, At Metll. 42 (1994), Hung Weiong, Geng Xingguo, n Zhou Yohe. Primry sping seletion of onstrine enriti growth, J. Cryst. Grow., 134 (1993), G. L. Ding, W. D. Hung, X. Hung, X. Lin, n Y. H. Zhou. On primry enriti sping uring uniiretionl soliifition, At Metll., 44 (1996), S. Z. Lu, n J. D. Hunt. A numeril nlysis of enriti n ellulr rry growth: the sping justment mehnisms, J. Cryst. Growth, 123 (1992), J. A. Wrren, n J. S. Lnger. Preition of Denriti Spings in Diretionl Soliifition Experiment, Phys.Rev. E, 47 (1993), L. Liu, B. L. Zhen, A. Bnerji, W. Reif, n F. Sommer. Effet of melt homogeniztion temperture on the st strutures of IN 738 LC superlloy, Sript Metll. Mter., 30 (1994), F. L. Yin, X.F. Sun, J.G. Li, H.R. Gun, n Z.Q. Hu. Effets of melt tretment on the st struture of M963 superlloy, Sript Mterili, 48 (2003), G. K. Bouse, n J. R. Mihlisin. Metllurgy of investment st superlloy omponent, Superlloys, Superomposites n Superermis, e. J. K. Tien, n T. Culfiel (Lonon: Aemi Press, 1989), Tiwen Hung, Lin Liu, Min Qu, Jun Zhng, n Hengzhi Fu. High therml grient iretionl soliifition n its pplition in the proessing of Nikel-se superlloys, sumitte to Intern J Mter Res. 14. R. Mehrin. Reltionship of het flow to struture in rpi soliifition proessing, Rpi Soliifition Proessing: Priniples n Tehnologies (Proeeings of the Intern. Conf. on Rpi Soliifition proessing Nov.13-16, 1977, Ruston, Virgini), e. R. Mhrin, B. H. Ker, n M. Cohen (Bton Rouge: Clitor s Pu. Division, 1977), H. Biloni, n W. J. Boettinger. Soliifition, Physil Metllurgy: 4 th revise n enhne eition, e. R. W. Chn n P. Hsen (Amsterm: Elsevier Siene BV, 1996), J.S. Lnger, n H. Müller-Krumhr. Theory of enriti growth I. Elements of stility nlysis, At Metll., 26 (1978), W. Kurz, n D.J. Fisher. Denrite growth t the limit of stility: tip rius n sping, At Metll., 29 (1981), J. D. Hunt, n S.-Z. Lu. Numeril Moeling of Cellulr/Denriti Arry Growth:Sping n Struture Preitions, Metll. Mter. Trns. A, 27(3) (1996), W. Wng, P. D. Lee, n M. MLen. A moel of soliifition mirostrutures in nikel-se superlloys: preiting primry enrite sping seletion, At Mterili, 51 (2003), F. S. Yin, X. F. Sun, H. R. Gun, n Z. Q. Hu. Effet of therml history on the liqui struture of st nikel-se superlloy M963, Journl of Alloys n Compouns, 364 (2004),