Formation mechanism of IDZ during coating of IN738 by single step gas phase aluminising

Transcription

1 Formtion mechnism of IDZ during coting of IN738 y single step gs phse luminising H. Rfiee* 1, S. Rstegri 1 nd H. Ari 2 The formtion mechnism of interdiffusion zone (IDZ) during gs phse luminising of nickel sed superlloy IN738LC nd the effects of temperture nd Al ctivity on the microstructure of IDZ were investigted in this reserch. For this purpose, single step gs phse luminising technique ws pplied for coting the surfces of IN738LC specimens t two different processing tempertures (i.e. 850 nd 1050uC) nd vrious pck compositions. Then the coting microstructures were evluted using scnning electron microscope, n energy dispersive spectrometer nd n X-ry diffrctometer. The results showed tht t the low temperture, coting ws formed y inwrd diffusion of Al, wheres t the high temperture, oth outwrd diffusion of Ni nd inwrd diffusion of Al occurred, resulting in the formtion of columnr structure or n IDZ. By incresing the Al ctivity in the powder mixture, it is shown tht the mount of Al within the inner lyer of the coting incresed. Au cours de cette recherche, on étudié le mécnisme de formtion de l zone d interdiffusion (IDZ) lors de l cloristion en phse gzeuse du superllige à se de nickel, IN738LC insi que les effets de l tempérture et de l ctivité de l Al sur l microstructure de l IDZ. Dns ce dessein, on ppliqué une technique de cloristion en phse gzeuse à étpe unique pour revêtir les surfces des échntillons d IN738LC à deux tempértures différentes de tritement (i.e. 850 et 1050uC) et vec diverses compositions de poudre. Ensuite, on évlué les microstructures du revêtement en utilisnt les techniques du microscope électronique à lyge, de l EDS et de l XRD. Les résultts ont montré qu à sse tempérture, le revêtement étit formé pr diffusion de l Al vers l intérieur lors qu à hute tempérture, tnt l diffusion du Ni vers l extérieur que l diffusion de l Al vers l intérieur se produisient, résultnt en l formtion d une structure en colonnes ou IDZ. En ugmentnt l ctivité de l Al dns le mélnge en poudre, on montre que l quntité d Al à l intérieur de l couche interne du revêtement ugmentit. Keywords: Gs phse luminising, Interdiffusion zone, Al ctivity, Formtion mechnism Introduction Coting of long nd nrrow internl ir chnnels of vnes nd ldes is necessry for enhncing the life of turine ldes nd vnes. 1,2 Owing to the smll nd complex geometry of these chnnels, nd lso the difficulties tht exist in removing the pck powder mixture from these chnnels, pck cementtion process 1 School of Metllurgy & Mterils Engineering, Irn University of Science & Technology, Nrmk, Tehrn, Irn 2 Center of Excellence for Advnced Mterils & Processings, School of Metllurgy & Mterils Engineering, Irn University of Science & Technology, Nrmk, Tehrn, Irn *Corresponding uthor, emil h_rfiee@metleng.iust.c.ir is not recommended for coting such ldes nd vnes. 1,3 So luminising through gs phse diffusion ws introduced for this purpose. 2,3 Gs phse luminising is similr to pck cementtion, ut unlike the pck cementtion in gs phse process, powder mixture is not physiclly in touch with the specimens. 3 5 From the viewpoint of luminium ctivity, there re two diffusion luminising processes. If the Al concentrtion in the Al source used in the initil powder mixture is greter thn 60 t-%, the process is known s high ctivity luminising, otherwise it is clled low ctivity. 3,6 When high ctivity luminising t low temperture is used, unlike using low ctivity luminising t high temperture, the top lyer of the coting ecomes rich in luminium. Phses such s Ni 2 Al 3 or ß 2011 Cndin Institute of Mining, Metllurgy nd Petroleum Pulished y Mney on ehlf of the Institute Received 10 April 2010; ccepted 6 August 2010 DOI / X Cndin Metllurgicl Qurterly 2011 VOL 50 NO 1 85

2 1 Schemtic showing coting equipment used for gs phse luminising process NiAl rich in Al re not desirle phses to e used for coting nd for trnsforming them to NiAl rich in Ni, therefore, n dditionl het tretment is required. 6 8 So mny luminising processes re crried out in single step process ove 1000uC using high ctivity pck powder. 6,9 This is n ttrctive technique for eliminting one step in coting process. Also, this process finlly leds to the formtion of Ni rich NiAl coting structure in single tretment, s in the cse of usul low ctivity coting. 6 Eslmi et l. 9 reported tht the coting otined y high temperture hydrogen ttck gs phse luminising is similr to tht otined y the high temperture low ctivity pck cementtion process. It hs een reported tht luminide coting usully forms y inwrd diffusion of Al, produced through Al suhlides in the pck, nd outwrd diffusion of Ni, through the mtrix. 10 Becuse of the outwrd diffusion of Ni nd susequent enrichment of the sustrte y lloying elements, precipittion of these elements occurs within the mtrix nd n interdiffusion zone (IDZ) with columnr structure usully forms under the top coting lyer. 8 It is worth mentioning tht the temperture for effective diffusion of Ni is y950uc nd only t tempertures higher thn this does outwrd diffusion of Ni hve n effective influence on the coting formtion mechnism. 3,11 In this reserch, the formtion mechnism of the IDZ ws studied, nd the effects of temperture nd Al ctivity on the coting were evluted. Mterils nd methods Rectngulr pltes of IN738LC with dimensions of mm nd men chemicl composition of Ni 8?5Co 16Cr 3?4Al 3?4Ti 2?6W 1?7T 1?7Mo 0?9N 0?05Zr 0?11C 0?01B (wt-%) were used s the sustrte mteril. These smples were ground with grde SiC emery ppers, then clened in n cetone th nd dried in ir efore the coting process. Two powder mixtures were prepred y weighing the pproprite mounts of filler powder of Al 2 O 3, Al source (i.e. pure Al or NiAl powder) for production of Al ions nd NH 4 Cl s n ctivtor. The Al source used in pck H ws pure Al powder nd in pck L ws NiAl powder, s shown in Tle 1. These powder mixtures were lelled either s pck H or pck L depending on the source of Al supply. The mount of luminium in oth pcks ws 4 wt-% nd the weight of ech pck ws 200 g of the powder mixtures. Eight grms of pure Al powder were used in pck H nd 25?6 g NiAl powder in pck L. The mount of ctivtor in oth pcks ws 2 wt-% (4 g). The specimens were plced inside the chmer of the coting pprtus s shown schemticlly in Fig. 1. Gs phse luminising ws crried out t 850 nd 1050uC for period of 4 h for oth L nd H pcks. The smples were mintined in the furnce fter coting until the temperture dropped to room temperture. It should e mentioned tht rgon gs ws circulted in the chmer from the strt of the coting opertion until the chmer temperture ecme 300uC, to keep n inert tmosphere for the coting process, nd then the flow of rgon gs ws cut. After the coting process ws ended, rgon gs ws gin circulted in the chmer in order to empty the chmer from the coting hlides. Ech specimen ws lelled with code consisting of n initil letter referring to the kind of pck used nd numer which indictes the coting temperture. The coted smples were exmined using scnning electron microscope (SEM; Veg II T-Scn) coupled with n energy dispersive spectrometer nd n X-ry diffrctometer (Cu K : l51?5418 Å; JDX-8030; JEOL, Tokyo, Jpn). Etchnt consisting of 61 vol.-% lctic cid, 36?5 vol.-% nitric cid nd 2?5 vol.-% hydrofluoric cid ws used to revel the microstructure detils. Results nd discussion Low temperture process Typicl cross-sections of the cotings formed y pcks L nd H vi the single step gs phse luminising process t 850uC re shown in Fig. 2. These cross-sections indicte tht the cotings re uniform nd consist of two min lyers (inner nd outer lyers). X-ry diffrction results shown in Fig. 3 indicte tht Al concentrtion in the coting formed on H-850 smple is higher thn tht of L-850 smple. Since the prtil pressure of Al in the high ctivity pck is higher thn tht in the low ctivity pck, 9 the kinetics of rections occurring in the high ctivity pck is fster thn tht in the low ctivity pck. By incresing the prtil pressure of Al in the pck, the Tle 1 Pck Chemicl composition of powder mixtures used in single step gs phse luminising process, wt-% Al source Composition Al NH 4 Cl Al 2 O 3 H Pure Al 4 2 Bl. L NiAl 4 2 Bl. 86 Cndin Metllurgicl Qurterly 2011 VOL 50 NO 1

3 2 Imges (SEM) of cotings formed y pck H nd pck L t 850uC, showing two lyers of coting formed t surfce of smple mount of Al ions relesed on the surfce of the smple cn e incresed. 12 Thus, the Al concentrtion in the coting of the high ctivity pck ecomes higher thn tht of the low ctivity pck. The inner lyers of the cotings shown in Fig. 4 indicte tht the trnsformtion of sustrte structure to coting structure strted to occur y receiving the incoming Al from the outer lyer of the coting. When the mount of the Al toms in this lyer increses to sufficient level, trnsformtions occur. Thus, those prts of the sustrte tht received enough Al toms grdully ecome prt of the coting. The lck of columnr structure in these specimens confirms tht no efficient outwrd diffusion of Ni from sustrte to outer lyer of cotings occurred. As mentioned erlier, effective diffusion of Ni in nickel sed lloys strts t y950uc ccording to Xing et l. 11 Therefore, one my conclude tht the results of this reserch conform with those of Ref. 11. Figure 2 shows tht the mount of sustrte trnsforming to NiAl in the coting lyer under the sme processing condition is more in the H-850 smple thn in L-850, so the thickness of the outer lyer of the coting is lrger in H-850 smple thn in L-850. However, s shown in Fig. 4, the thickness of the inner lyer in L-850 is somewht lrger thn tht in H- 850 ut the totl thickness of the coting in H-850 is greter thn tht of L-850. This is due to the vrition in the mount of Al toms entering into the sustrte of the two smples. The higher the rte of receiving Al toms y the sustrte, the sooner it trnsforms to the coting structure, so the thickness of the coting increses more. However, when the rte of incoming Al toms is low, there will not e enough of these toms within the sustrte to cuse trnsformtion completely, so the inner lyer ecomes thicker, not the outer one. 3 X-ry diffrction results from coting surfces of smples coted under vrious conditions Cndin Metllurgicl Qurterly 2011 VOL 50 NO 1 87

4 4 Imges (SEM) of cotings formed y pck H nd pck L t 850uC, showing structures of inner lyers in detil High temperture process Typicl cross-sections of the cotings formed y pcks L nd H vi the single step gs phse luminising process t 1050uC re shown in Fig. 5. The cotings otined t 1050uC consist of two mjor lyers. Compring Fig. 5 with Fig. 2, one cn see tht the inner lyers of these smples re somehow different from those of the smples coted t 850uC. The inner lyers (IDZ) of the coted smples t 1050uC hve columnr structure. This is due to the outwrd diffusion of Ni occurring sufficiently t this temperture. Susequently, the mtrix ecme rich in lloying elements which ccompnied y the precipittion of these elements within the mtrix. It is worth mentioning tht the re rich in hevier elements ppers in righter colour in cksctter imges (see Fig. 5). So, y incresing the coting temperture from 850 to 1050uC, outwrd diffusion of Ni occurred in lrge scle in the smples. Figure 5 lso indictes tht the IDZ lyer of H-1050 smple consists of two zones (i.e. zones III nd IV), wheres tht of L-1050 smple consists of only one zone. Figure 6 shows higher mgnifiction imges of IDZ lyer of oth specimens. These imges show the microstructurl detils of IDZ of the smples coted t 1050uC. As mentioned erlier, the mount of Al toms relesed t the surfce of H-1050 smple is more thn tht of L-1050 smple. X-ry diffrction results shown in Fig. 3 confirm tht the luminium concentrtion in the surfce of the coting of H-1050 smple is more thn tht of L-1050 smple. Therefore, more Al toms cn enter into the sustrte of H-1050 smple thn L-1050 nd thus they cn trnsform the top lyers of the sustrte to NiAl phse rich in Al. Susequently, s the temperture is dequte for Ni diffusion, this element strts diffusing upwrd towrds the surfce. However, due to the more Al toms received in zone III thn zone IV, zone III strts trnsforming to NiAl phse sooner thn zone IV. Thus, s shown in Fig. 6, the columns tht hve lredy lost some of its Ni toms, y receiving Al toms from the top coting lyer trnsform to the coting structure. 5 Imges (SEM) of cotings formed y pck H nd pck L t 1050uC: cotings formed y oth pcks hve two mjor lyers nd there is difference in IDZ lyer of them 88 Cndin Metllurgicl Qurterly 2011 VOL 50 NO 1

5 6 Imges (SEM) of IDZ lyer of cotings formed y pck H nd pck L t 1050uC, showing existence of refrctory elements precipittes nd Kirkendll voids in IDZ By trnsforming the sustrte to the coting structure, the soluility of lloying elements such s W, Cr nd Mo decreses nd suitle condition for precipittion of these elements will e provided. So in IDZ, in the interfce of IDZ/outer lyer nd occsionlly in the coting outer lyer, precipittion will occur. A similr type of nlysis hs een reported in Refs. 3, 6 nd 13. It hs lso een reported tht ecuse of the low soluility of refrctory elements in -NiAl, they hrdly prticipte in the formtion of -NiAl; insted they precipitte s secondry phses t the interfce etween the IDZ nd the outer lyer of the coting. 14 As mentioned erlier, the precipittion of these refrctory elements cused higher contrst t some points to pper within these zones in cksctter imges (see Figs. 6 nd 7). Another phenomenon occurring within the inner lyer of the coting is shown in Figs. 6 nd 7, nd tht is the existence of mny voids in the vicinities of columnr structures. These voids re proly Kirkendll voids tht formed y lrge mount of outwrd diffusion of Ni from this region to the top lyers of the specimen, in such wy tht could not e compensted with lesser mount of inwrdly diffused luminium (i.e. Kirkendll effect). 3,15 17 It hs een reported tht refrctory elements primrily exist in the mtrix c phse of the sustrte. 13 Their presence my result in the formtion of refrctory phses rich in these elements hving topologiclly close pcked (TCP) structure, during high temperture exposure. Figure 6 shows the presence of these phses elow the IDZ of the coting. A region elow the IDZ of the coting tht consists of these TCP phses, c nd c9,is clled secondry rection zone SRZ. These TCP phses re lso shown in Fig. 6, which is relted to smple L The mount of these phses in this smple is lesser thn tht in smple H This might e ecuse of the lower rte of trnsformtion tht occurred in L-1050 smple. In fct, these phses cn 7 Imges (SEM) of cotings formed y pck H nd pck L t 1050uC, showing existence of refrctory elements precipittes nd Kirkendll voids in coting structure Cndin Metllurgicl Qurterly 2011 VOL 50 NO 1 89

6 e formed y depletion of Ni when outwrd diffusion of this element occurs nd susequently n increse in the concentrtion of lloying elements hppens. 17 It is worth mentioning tht, t this stge of coting, luminium toms did not penetrte yet into this region. The TCP phses formed in the SRZ hve primrily needle-like or rod-like morphology. 13 As shown in Fig. 6, TCP phses re grown from IDZ columnr structure. It hs een reported tht the precipittion of the TCP phses occurs from the supersturted c phse of the sustrte. 13 It hs lso een reported tht the SRZ growth is comprtively greter within the dendrite cores thn in the interdendritic regions. 13 This cn result in wvy ppernce of the SRZ growth front s shown in Fig. 6. The difference of Ni chemicl potentil etween the outer lyer nd the inner lyer of the coting in the H-1050 smple is igger thn tht of the L-1050 specimen, so the rte nd the mount of outwrdly diffused Ni re greter in the H-1050 smple. Thus, the mount of TCP phses nd lso Kirkendll voids in the H-1050 specimen cn e more thn those of the L-1050 specimen (see Fig. 6). Conclusions 1. At the low temperture process, the inner lyer of the coting ws formed y inwrd diffusion of Al nd outwrd diffusion of Ni did not occur t this temperture, wheres t the high temperture process, inwrd diffusion of Al occurred initilly, followed y n outwrd diffusion of Ni, resulting in columnr structure in the inner coting lyer of these smples. 2. By incresing the Al ctivity in the powder mixture, the mount of Al entering into the inner lyers incresed, so the sustrte trnsformed sooner to the coting structure nd more chnges occurred in the inner lyer of smples coted y high ctivity powder compred to those coted y low ctivity powder. Also, the totl thickness nd Al concentrtion of the coting in smples coted y high ctivity powder were higher thn those coted y low ctivity powder. 3. There re so mny Kirkendll voids in the vicinities of columnr structures in IDZ nd t the interfce etween the IDZ nd the outer lyer of the L-1050 smple, indicting tht the rtes of outwrd diffusion of Ni from these zones were higher thn those of inwrd diffusion of Al. 4. Owing to the fster outwrd diffusion of Ni, the mount of Kirkendll voids nd TCP phses within the SRZ of the smples coted t 1050uC y high ctivity powder is more thn tht of the smples coted y low ctivity powder under similr processing conditions. References 1. A. B. Smith, A. Kempster nd J. Smith: Surf. Cot. Technol., 1999, , J. Kohlscheen nd H. R. Stock: Surf. Cot. Technol., 2008, 203, H. Rfiee, H. Ari nd S. Rstegri: J. Alloys Compd, 2010, to e pulished. 4. G. W. Gowrd: Surf. Cot. Technol., 1998, , M. M. El-Wkil: Powerplnt technology, ; 1984, New York, McGrw-Hill. 6. D. K. Ds, V. Singh nd S. V. Joshi: Metll. Mter. Trns. A, 1998, 29A, N. Voudouris, Ch. Christoglou nd G. N. Angelopoulos: Surf. Cot. Technol., 2001, 141, J. Kohlscheen nd H.-R. Stock: Surf. Cot. Technol., 2007, 202, A. Eslmi, S. Rstegri nd H. Ari: Cn. Metll. Q., 2009, 48, (1), A. Squillce, R. Bonetti, N. J. Archer nd J. A. Yetmn: Surf. Cot. Technol., 1999, , Z. D. Xing, J. S. Burneell-Gry nd P. K. Dtt: J. Mter. Sci., 2001, 36, Z. D. Xing nd P. K. Dtt: Act Mter., 2006, 54, D. K. Ds, K. S. Murphy, S. M nd T. M. Pollock: Metll. Mter. Trns. A, 2008, 39A, H. Wei, H. Y. Zhng, G. C. Hou, X. F. Sun, M. S. Drgusch, X. Yo nd Z. Q. Hu: J. Alloys Compd, 2009, 481, Z. D. Xing nd P. K. Dtt: J. Mter. Sci., 2003, 38, A. Chien, D. Gn nd P. Shen: Mter. Sci. Eng. A, 1999, A272, D. A. Porter nd K. E. Esterling: Phse trnsformtions in metls nd lloys, 2nd edn, 89 91,, ; 1992, London, Chpmn & Hll. 90 Cndin Metllurgicl Qurterly 2011 VOL 50 NO 1