EFFECTS OF SEGREGATION IN NICKEL-BASE SUPERALLOYS: DENDRITIC STRESSES

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1 Superlloys 004 Edited y K.A. Green,.M. Pollock, H. Hrd,.E. Howson, R.C. Reed, J.J. Schirr, nd S, Wlston MS (he Minerls, Metls & Mterils Society), 004 EFFECS OF SEGREGAION IN NICKEL-BASE SUPERALLOYS: DENDRIIC SRESSES Alexnder Epishin 1, homs Link, Udo Brückner 1, Bernrd Fedelich 1 nd Pedro Portell 1 1 Federl Institute for Mterils Reserch nd esting, Unter den Eichen 87, 105 Berlin, Germny echnicl University Berlin, Institute for Mteril Science nd Engineering, BH 18, Ernst Reuter Pltz 1, Berlin, Germny Keywords: Nickel-se superlloys, Dendritic structure, Segregtion, Residul stresses Astrct Residul stresses in the dendritic structure of single-crystl nickelse superlloys were investigted. Diltometric mesurements showed tht the therml contrction of the dendrite rms (DAs) is lrger thn tht of the interdendritic regions (IRs). he reson is, tht the phse, precipitting during cooling, hs smller lttice prmeter thn the -mtrix nd tht this misfit is higher in the DA thn in the IR. In the DA-IR-compound the different therml contrctions cuse residul stresses. he stress distriution within the dendritic cell ws nlyzed theoreticlly y finite element (FE) modeling. he stresses were proved directly y X-ry diffrction (XRD) nd indirectly y the trnsformtion of the morphology oserved fter lod free nneling. It ws found tht dendritic stresses influence the structurl nd mechnicl ehvior of superlloys during high temperture creep: rfting nd plstic deformtion strt in the primry dendrite rms (PDA). tungsten in CMSX-4 results in n inhomogeneity of the misfit: PDA: 0.19 %, SDA: 0.1 % nd IR: +0.0 % (unconstrined misfit). Accordingly two effects re expected: inhomogeneity of the coherency stresses nd inhomogeneity of the therml contrction during -precipittion. he second effect is the min interest of this investigtion. It ws checked experimentlly y diltometric nlysis. Experimentl dt out the inhomogeneity of the misfit nd the rhenium distriution llowed the modelling of the distriution of the resulting residul stresses within the dendritic cell. he theoreticl results were vlidted y XRD. he effect of dendritic stresses on rft formtion ws investigted using scnning electron microscopy (SEM), their effect on disloction moility y trnsmission electron microscopy (EM). Introduction Dendritic segregtion in nickel-se superlloys is side effect of solidifiction. It is especilly pronounced in superlloys of the newest genertions with high levels of slowly diffusing refrctory elements. Becuse the segregtion of these elements (especilly rhenium) cn not e fully removed within cceptle homogeniztion time, superlloys re used with significnt residul segregtion. herefore, it is importnt to know, how segregtion ffects the mechnicl nd structurl ehvior of superlloys. wo effects, inhomogeneity of the microstructure nd phse instility of superlloys (precipittion of topologiclly closed pcked phses) re widely investigted. his pper presents nother segregtion effect, nmely residul stresses within the dendritic cell, dendritic stresses, which re therml stresses rising during cooling of superlloys due to the different therml contrction of DAs nd IRs. Mterils nd Methods he dendritic stresses were investigted in [001] single-crystls of CMSX-4 solidified nd het treted y Doncsters Precision Cstings (DPC), Bochum, Germny. Anlysis of the mcrostructure of CMSX-4 shows, tht fter dissolving of the eutectics significnt residul segregtion of the slowly diffusing elements rhenium nd tungsten still remins. Figure 1 shows the inhomogeneous distriution of rhenium in het treted CMSX-4. he rhenium concentrtion in the PDA is lmost two times higher thn in the IR. It ws shown in [1] tht segregtion of rhenium nd Figure 1. Segregtion of Rhenium in the dendritic structure of fully het treted CMSX-4. Rhenium concentrtion in PDA is out 4.5 wt%, in IR out.4 wt%. Results Investigtion of herml Contrction he influence of the misfit inhomogeneity on the therml contrction ws checked experimentlly y csting two 537

2 specimens, one with the composition of the PDA of het treted CMSX-4, the other with tht of the IR. he diltometric nlysis of these specimens showed tht during cooling elow the solvus in the temperture rnge of precipittion, the PDA contrcts more thn the IR (see Figure ). After the precipittion is finished, the diltometric curves of the PDA nd the IR specimens re equidistnt. he totl difference in the therml contrction of these specimens during cooling from solvus to room temperture ws found to e out 0.1%. his effect is directly relted with the dendritic inhomogeneity of the misfit. During cooling, the precipittion cuses contrction of the superlloy ecuse the phse hs smller lttice spcing thn the mtrix. he contrction is lrger in the PDA where is higher thn in the IRs. he difference in therml contrction found etween the PDA nd the IR specimens fits well with the difference in misfit 0.15% mesured in these specimens y X-ry diffrction: estimting V, with volume frction V 0.7 gives 0.1%. hus the dendritic inhomogeneity of the misfit results in different therml contrctions within the dendritic cell, cusing residul stresses. PDA sides with the length l 1, where PDA is the re density of dendrites in (001) section. he prism hs the height (thickness) t eing equl to hlf the secondry dendrite rm (SDA) spcing SDA /. Becuse the scle does not influence the results of 1 SDA PDA the stress nlysis, just the rtio l t is importnt. he quntittive metllogrphic investigtion of the dendritic structure of CMSX-4 gve SDA =0.16 mm nd PDA =1.5 mm -, i.e. l/t=.5. he clcultions were performed using the FE code Aqus [], with oundry conditions ccording to the structure symmetry. he inhomogeneity of the therml contrction x ws introduced s x x V, where x descries the misfit distriution within the dendrite cell. he function x ws designed on the sis of results of the misfit [1] nd element [3] distriution in CMSX-4. It ws ssumed tht ove the solvus ll stresses in the dendritic structure relx during homogeniztion, i.e. the stress free stte t the solvus ws tken s reference for FE modeling. herml contrction, % 0,0-0,5-1,0-1,5 -,0 V ' IR PDA solidus '-solvus '-solvus '- precipittion -, [001] [010] [100] t l emperture, C Figure. herml contrction of PDA nd IR specimens during cooling elow the solvus. Insert: therml contrction of CMSX-4. Modelling of Dendritic Stresses Modelling of dendritic stresses ws performed using geometricl model of the dendritic structure shown in Figure 3. he idelized single dendrite shows in cross section fourfold symmetry nd in the longitudinl section n equidistnt rrngement of the secondry dendrite rms. In [001] single-crystls such dendrite is tken s [001] columnr sugrin with {110} oundries. he sugrins re rrnged in qudrtic lttice. From the symmetry it follows, tht the periodic cell of the dendritic structure is prism sed on equilterl right tringle with <110> orientted short Figure 3. Computtionl domin schemticlly shown with respect to the dendritic structure. Figures 4-c show the distriution of the dendritic stresses t 0 C. It is seen, tht the components of the stress tensor 11, nd 33 re positive in the dendrite rms (the indices 1,, 3 correspond to the <100> xes). In the PDA the lrgest component is MP (Figure 4c), in the SDA it is MP (Figure 4). It mens tht the DAs re in three dimensionl (3D) tension with the strongest tensile component long the rm xis. 538

3 c Figure 4. Distriution of the components of the dendritic stresses 11, nd 33 t 0 C. SDA PDA Figure 5. microstructure of CMSX-4 rfted during 300 h nneling t 1100 C under denritic stresses. Doule side rrows: dendritic stresses estimted y FE modeling. Squres: positions of the SEM frmes within the dendrite. In the IR the stresses re negtive, so the IR is under 3D compression. Clcultion of the dendritic stresses t 850 C nd 1100 C gve similr distriutions ut smller solute vlues of the stresses. At 850 C the stress vlues re out 80% of those t 0 C, while t 1100 C they re out 45%. his decrese of the stress vlues is mostly cused y the decrese of the elstic stiffness. At 1100 C the reduction of the volume frction mkes n dditionl contriution in the decrese of the stress level. Rfting during Lod Free Anneling he occurrence of dendritic stresses hs een proved indirectly y the trnsformtion of the morphology oserved fter lod free nneling. From rfting, it is known [4, 5] tht rfts form perpendiculr to the stress xis under tension nd prllel under compression (for negtive misfit). If no externl lod is cting, the residul stresses determine the rft formtion, i.e. the rft morphology gives qulittive informtion out direction nd sptil distriution of residul stresses. o revel the dendritic stresses, specimens of fully het treted CMSX-10, CMSX-4, SRR99 nd SC16 were dditionlly nneled t 1100 C for 300 h. After nneling the microstructure of ech specimen ws nlyzed in SEM. he most pronounced rfting ws found in CMSX-4, less in SRR99. In CMSX-10 rfting ws oserved only in the PDA nd in SC16 no rfting t ll. his result fits with the inhomogeneity of the misfit in these superlloys [1, 6]: for CMSX-4, SRR99, CMSX-10 nd SC16 the difference of the misfit etween PDA nd IR is out 0.1, 0.09, 0.07 nd 0% respectively. In ll rfted superlloys the rft orienttion in the DAs is perpendiculr to the rm xis, i.e. perpendiculr to the direction of the strongest tensile component of the dendritic stress (see Figure 5). Rfting in superlloys fter lod free nneling ws oserved lso in [7, 8]. 539

4 Mesurement of the Lttice Strin y X-Ry Diffrction he occurrence of dendritic stresses hs lso een proved directly y locl X-ry diffrction with sptil resolution of out 100 m. he mesurements were performed on longitudinl cut, which ws slightly tilted from the (100) plne round the [010] xis. So the dendrite xis is cut under smll ngle, which mkes it possile to oserve the center of dendrite nd to position the X-ry spot within the dendritic structure (see Figure 6). he tetrgonl strin of the lttice unit cell, i.e. the difference 3-1 etween the elstic strins long the PDA xis [001] ( 3 ) nd long the perpendiculr xis [100] ( 1 ) ws nlyzed. his tetrgonl strin cn e determined y mesuring the lttice spcings d h0l of different {h0l} plnes elonging to the [010] zone xis. For smll elstic strins it is vlid: eff 1 sin (1) where eff d h0l h l is the effective lttice spcing, the lttice prmeter in [100] direction nd the ngle etween [100] nd [h0l]. h nd l re Miller indices. hus mesuring d h0l s function of sin one cn get. A detiled description of this method is presented in [9]. Figure 6. Anlysis of the tetrgonl strin in the PDA of CMSX-4 using tilted longitudinl cut.. - Imge of the electropolished dendritic structure oserved in X-ry diffrctometer y CCD cmer.. - Geometry of the X-ry mesurements: y rottion of the specimen round the [010] xis nd dpttion of the Brgg ngle the different lttice plnes come into reflection position. 3,588 3,587 CMSX-4, het treted =0.05±0.01% 3,586 3,585 CMSX-4, s cst =0.09±0.01% eff, nm/110 3,586 3,585 (600) (60±) (40±) (40±4) (40±6) 3,584 0,0 0,1 0, 0,3 0,4 0,5 0,6 0,7 eff, nm/110 3,584 3,583 (600) (60±) (40±) (60±4) (40±4) (40±6) 3,58 0,0 0,1 0, 0,3 0,4 0,5 0,6 0,7 sin sin Figure 7. Effective lttice spcing eff for plne orienttions chnging from prllel to the PDA xis (=0 ) towrds perpendiculr (=90 ), mesured in the PDA. he dshed lines show the 95% confidentil intervls.. - Fully het treted CMSX As cst CMSX

5 When mesuring it hs to e tken into ccount tht not only the dendritic stresses cuse such lttice distortion, ut misfit stresses s well. In undeformed mteril the coherency stresses cuse n isotropic tension in the precipittes. herefore for undeformed mteril only the -position of the pek of the h0l MoK 1 reflection ws mesured to determine eff. In crept mteril the nd lttices hve opposite tetrgonl strins. For the verge / lttice spcing however this effect is compensted ecuse the elstic constnts of oth phses re very close. herefore for crept mteril the totl -profile of the h0l MoK1 reflection ws mesured nd eff ws clculted y the position of the center of grvity of the profile. he results for undeformed fully heted treted CMSX-4 re shown in Figure 7. he slope of the grph gives for out 0.05%, FE modelling 0.1%. he reson for this difference is, tht for X-ry mesurements is the verge within the irrdited mteril (out m 3 ) wheres FE modelling gives the locl mximum. Figure 7 shows results for s cst CMSX-4, which give of out 0.09%, i.e. nerly two times lrger thn in heted treted condition. hus the higher inhomogeneity results in stronger dendritic stresses. he nlogous mesurements where performed for [001] singlecrystl of CMSX-4 crept t 1100 C nd 10 MP for 150 h (the middle of stedy creep). he mesurements gve out 0.03%, which mens tht dendritic stresses prtilly relx during creep. Figure 8. Distriution the RSS in the (001) oriented mtrix chnnels in CMSX-4 t 1100 C immeditely fter loding y 10 MP stress in [001] direction. Figure 9. CMSX-4 fter 4 h creep t 1100 C, 10MP, =0.10 %, = %/h, EM, (001) cross section. Disloctions in the interfce indicting mtrix glide in the PDA (), in the IR () disloctions re rre. Inhomogeneity of Rfting nd Plstic Deformtion during Creep FE modelling of the stress distriution within the dendritic cell under unixil stress in [001] direction showed tht it is strongly inhomogeneous. Figure 8 shows the distriution of the resolved sher stress (RSS) for the <011> {111} glide system in the (001) mtrix chnnels of CMSX-4 during creep t 1100 C nd 10 MP immeditely fter loding in [001] direction. he model considered the two phse microstructure of the superlloy, i.e. the cuoidl precipittes re coherently emedded in the mtrix. It is seen tht the RSS in the PDA is out.5 times higher thn in the IR. his effect is cused y two resons: first, the 541

6 coherency stresses re stronger in the PDA ecuse is lrger there, nd second, in the PDA the pplied tensile stress ccumultes with the mximum tensile component of the dendritic stress. he result of this RSS concentrtion should e tht plstic deformtion strts in the mtrix chnnels of the PDA. his supposition ws checked y EM investigtions of specimens t n erly stge of deformtion. Figure 9 shows the (001) interfce in the PDA () nd the IR () of CMSX-4 crept t 1100 C nd 10 MP for 4 h (primry creep). In the PDA mny interfcil disloctions deposited y mtrix glide re found wheres in the IR they re quite rre, mening tht there is no remrkle mtrix glide. his result ws confirmed y X-ry K 1 PDA c = 0.14% 1 10 K 1 IR c = 0 Intensity, 1000 cts K Intensity, 1000 cts K 0 7, 7,4 7,6 7,8 73,0 73, 73,4 73,6, 0 7, 7,4 7,6 7,8 73,0 73, 73,4 73,6, Figure 10: profiles of the 600 reflection of MoK X-ry rdition mesured in the PDA () nd the IR () fter 4 h creep t 1100 C nd 10 MP. Squres re experimentl dt; dshed lines re nd suprofiles nd ckground; solid lines re totl fitted curves; c is the constrined misfit. Splitting of the pek in the PDA () indictes loss of coherency nd no splitting in the IR () coherent interfces. Figure 11. CMSX-4 fter 4 h creep t 1100 C, 10MP, =0.10%, = %/h, SEM, longitudinl section (100). Rfting of the -microstructure in the PDA (), in the IR () the microstructure remins cuoidl. 54

7 diffrction: cler splitting of the pek in the PDA (Figure 10), which mens loss of coherency nd no splitting in the IR (Figure 10) mening, tht the interfces re still coherent. By SEM lso structurl chnges, correlted to the stress inhomogeneity, were oserved. After 4 h creep in the PDA rfting is nerly complete, while in the IR the -microstructure is still cuoidl (compre Figures 11 nd ). he results otined for CMSX-4 re typicl for such superlloys showing significnt residul segregtion. he sme differences in kinetics of disloction glide, loss of coherency nd rfting were found in the PDA nd IR of CMSX-10 nd SRR99 deformed during primry creep (CMSX-10: 1100 C, 10 MP, 5 h nd 60 h; SRR99: 980 C, 00 MP, 0 h) It follows from the ove results tht segregtion cuses the misfit inhomogeneity nd herey it influences the primry creep of superlloys. his influence is quite complex (see Figure 1): the inhomogeneity of the coherency stresses nd the dendritic stresses results in erlier plstic deformtion in the PDA, which then expnds over the entire dendritic cell. Exct sttements out the influence of the segregtion on the kinetics of creep process re difficult, ut two effects should e expected: n initition of the primry creep y erlier plstic deformtion in the PDA nd non-monotonic chnge of the strin rte due to step-like development of the plstic deformtion within the dendritic cell (PDA SDAIR). Such primry creep ehvior ws oserved in [3, 10, 11]. Oviously FEM modelling is necessry for detiled understnding of the segregtion effects. inhomogeneity of coherency stresses segregtion inhomogeneity of misfit dendritic stresses higher RSS in the PDA plstic deformtion in the PDA initition of primry creep Figure 1. Influence of segregtion on primry creep in superlloys Summry Residul stresses in the dendritic structure of superlloys were proved y vrious methods. hey re cused y the different therml contrction of DA nd IR hving different composition. he dendritic stresses result in n inhomogeneous 3D stress field which influences the mechnicl nd structurl ehviour of superlloys. It is expected tht this influence ecomes relevnt t low stress levels comprle with the dendritic stresses. Acknowledgements he uthors re grteful to the Deutsche Forschungsgemeinschft for finncil support of this work (Project Po 405/4, Li 494/3). Rerefences 1. U. Brückner et l., Locl X-ry diffrction nlysis of the structure of dendrites in single-crystl nickel-se superlloys, Act mter., 45 (1997), Aqus, Version 6.3, (00). Hiitt, Krlsson & Sorensen, Inc Min Street, Pwtucket, RI , 3. A. Epishin et l., Influence of dendritic inhomogeneity on creep ehvior of single-crystl superlloy of the third genertion (Finl report, Project Po 405/4, Li 494/3, Federl Institute of Mterils Reserch nd esting - echnicl University Berlin, 004). 4. J. K. ien nd S. M. Copley, he effect of unixil stress on the periodic morphology of coherent gmm prime precipittes in nickel-se superlloy crystls, Metll. rns., (1971), J. K. ien nd S. M. Copley, he effect of orienttion nd sense of pplied stress on the morphology of coherent gmm prime precipittes in stress nneled nickel-se superlloy crystls, Metll. rns., (1971), A. Epishin et l., Influence of dendritic inhomogeneity on creep ehvior of single-crystl superlloy of the third genertion (Interim report, Project Po 405/4, Li 494/3, Federl Institute of Mterils Reserch nd esting - echnicl University Berlin, 00). 7. A. Hzotte nd J. Lcze, Chemiclly oriented plte development in nickel se superlloy, Script Metll., 3 (1989), A. Hzotte nd A. Simon, Quntittive nlysis of the structurl chnges during ging of single crystl nickel-sed superlloys, Act Stereol., 8 (1989), U. Brückner et l., Dendritic stresses in nickel-se superlloys (Pper to e presented t the 7th Conference on Residul Stresses, Xin, Chin, June 004). 10. W. Schneider, High temperture ehvior nd microstructure of single-crystl nickel-se superlloy CMSX-4 t tempertures from 800 C up to 1100 C, PhD thesis, Universität Erlngen-Nürnerg, F. Diologent et l., Creep ehvior t 1050 C of new genertion single crystl superlloy, Creep Deformtion: Fundmentls nd Applictions, Ed. y R. S. Mishr, J. C. Erthmn nd S. V. Rj, MS (he Minerls, Metls & Mterils Society), 00, pp

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