ÚNAVOVÁ ŽIVOTNOST A LOM LITÉ GAMA TIAL INTERMETALICKÉ SLITINY PŘI POKOJOVÉ A ZVÝŠENÉ TEPLOTĚ

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1 ÚNAVOVÁ ŽIVOTNOST A LOM LITÉ GAMA TIAL INTERMETALICKÉ SLITINY PŘI POKOJOVÉ A ZVÝŠENÉ TEPLOTĚ FATIGUE LIFE AND FRACTURE OF CAST GAMMA TIAL INTERMETALLIC ALLOY AT ROOM AND ELEVATED TEMPERATURES Martin PETRENEC, Miroslav ŠMÍD, Jaroslav POLÁK Institute o Physics o Materials AS CR, Žižkova 22, Brno, Czech Republic, petrenec@ipm.cz Abstrakt Byla studována nízkocyklová únava, povrchový relie a lomové plochy při pokojové a zvýšené teplotě lité gama TiAl intermetalické slitiny legované 2%at% Nb. Byly naměřeny, cyklické deormační křivky a únavové křivky života pro obě teploty. Na zkoušených tělesech byly pozorovány lomové plochy a povrchový relié za účelem studia iniciace a šíření únavových trhlin. Při pokojové teplotě bylo detekováno výrazné cyklické zpevnění zatímco při 750 C byla cyklická napěťová odezva stabilní. Byly stanoveny parametry cyklických deormačních křivek, Coin-Mansonových a Basquinových křivek života. Hlavní místa iniciace a poté i počátečního šíření únavových trhlin při obou teplotách byly persistentní skluzové stopy podél lamelárního rozhraní. Trhliny iniciované na povrchu nebo těsně pod povrchem vedly ke vzniku hladkých oblasti na lomových plochách, které odpovídají persistentním skluzovým pásům. Klíčová slova: nízkocyklová únava, litá gama slitina TiAl, lomové plochy, PSS, vysoká teplota Abstract The low-cycle atigue properties, surace relie and racture suraces o cast TiAl alloys with 2 at.% Nb having nearly lamellar microstructure were studied at room temperature and at 750 C. Cyclic stress-strain cur ves (CSSC) and atigue lie curves were obtained at both temperatures. The surace relie and the racture suraces o ractured specimens were observed using scanning electron microscopy. At room temperature, signiicant cyclic hardening is observed whereas at 750 C cyclic response was stable. Parameters o th e CSSC, the Manson-Coin law and the Basquin law were determined. Persistent slip markings ormed along interlamellar interaces were predominant locations or atigue cracks at both temperatures. The cracks initiated at surace or in sub-surace region led to the ormation o smooth lat areas on the racture surace corresponding to the persistent slip bands. Key words: low cycle atigue, cast gamma TiAl alloy, racture suraces, PSMs. elevated temperature 1. INTRODUCTION The TiAl alloys are a serious candidate material or applications demanding low density, good corrosion resistance and high strength at elevated temperatures. Recently, applications using TiAl alloys appeared e.g. as parts o vehicle's engines or in aeronautics. As an example, the turbine wheel in car turbochargers is orced to rotate by exhaustion gasses at very high temperature [1.]. The material is thus subjected to cyclic loading at high temperatures. However, the occasional large mechanical or thermal transients during service could also give rise to a signiicant amount o plastic deormation. Under these conditions, commonly reerred to as low cycle atigue (LCF), the total design lietime may involve only a ew hundred or a ew thousand o these large strain cycles. Turbine engine blades or discs are prime examples o components subjected to this type o strain-controlled atigue. Local areas o these components can undergo high stresses and strains due to external load transer, abrupt changes in geometry, temperature gradients, and material imperections [2.]. Studies o the low cycle atigue properties o the low and high Nb alloing in TiAl

2 alloys are rare [2.,3.]. Recently Gloanec et al. [3.] studied the relation between dislocation microstructures and cyclic stress-strain response at room and 750 C temperatures in polycrystalline o TiAl. Some LCF data on TiAl alloy with 2 at. % o Nb have been already reported [1.,4.,5.]. Fatigue parameters at room temperature and 750 C were evaluated [1.,4.], racture suraces were described [4] and the cyclic plasticity has been analyzed using the generalized statistical theory o the hysteresis loop [5.]. In this paper, complementary new results on 2 at.% Nb alloy are reported on the relation between atigue crack initiation locations and resulting atigue lie measured in low-cycle atigue experiments at room temperature and 750 C in air. 2. EXPERIMENTAL DETAILS 2.1 Experimental material The experimental material Ti 48Al 2Nb 2Cr 0.82B (TiAl 2Nb alloy in the ollowing) was prepared by casting in the GE Metalle und Materialien GmbH Company in Nürnberg in the orm o a cylindrical ingot o 220 mm in length and 90 mm in diameter. The ingot was subjected to hot isostatic pressing (HIP) at 1280 C an d 140 MPa or 4 hours. The alloy shows nearly lamellar microstructure with /α2 laths (with average thickness o 1.95 µm), variable grains size ( mm) and some smaller areas o single phase on the grain boundaries (see Fig. 1). The tensile properties at both temperatures are shown in Table 1. At room temperature tensile properties were measured up to the racture. At 750 C, tensile curve was derived rom the irst quarter cycle during the cyclic test. Some other material parameters are given in [1.,4.]. α2 Fig. 1. Structure o the cast TiAl 2Nb alloy: etched to reveal the heterogeneous grain size in a section perpendicular to the ingot axis (optical micrographs); electron micrographs o lamellar microstructures. Obr. 1. Struktura lité TiAl 2Nb slitiny: leptaný kolmý řez na osu ingotu ukazující heterogenní velikost zrn (optický mikroskop); detail lamelární mikrostruktury (elektronový snímek). Table 1. Tensile properties at both temperatures o studied cast TiAl 2Nb alloy [1.]. Tabulka 1. Tahové charakteristiky pro obě teploty studované lité TiAl 2Nb slitiny [1.]. Temp.[ C] E [GPa] 0.1 % yield stress [MPa] racture stress [MPa] racture εp [%] 2.2 racture ε [%] ± ± Low cycle atigue tests Cylindrical specimens [1.] used or atigue tests were careully mechanically and then electrolytically polished in the gauge length. Fatigue tests were perormed under strain control using MTS servohydraulic machine in -3-1 symmetric tension-compression cycle (Rε = -1). The total strain amplitude (εa) and strain rate o 2x s

3 were kept constant in all tests. The tests were perormed at room temperature (RT) and 750 C. During cyclic loading, hysteresis loops o selected cycles were recorded or urther analysis. The stress amplitude, mean stress and total strain amplitude were recorded. Ater the test termination, a special program was used to evaluate plastic strain amplitude (ε ap ) rom the hal-width o the hysteresis loop. The cumulative plastic strain ε c [6.] was calculated according to the equation N ε = 4ε, c i= 1 ap i where N is number o cycles. 2.3 Surace relie and racture observation The surace relie and racture surace observations were studied on the gauge length o specimens atigued to ailure using scanning electron microscope (SEM) JEOL 6460 and high resolution ield emission scanning electron microscope MIRA 3 FEG-SEM rom TESCAN Corporation. 3. RESULT S AND DISCUSION 3.1 Cyclic plasticity The shape o the hysteresis loop (stress-strain response) signiicantly changes during cycling or all strain amplitudes at room temperature (RT) contrary to 750 C where stable cyclic response was observed. Durin g cyclic straining at RT the stress range increases and the width o the hysteresis loop decreases as shown on the cyclic hardening whereas at 750 C the changes o hysteresis loop were negligible [1.] TiAl_2Nb_rt_ε c =4 TiAl_2Nb_rt_ε c =0.08 TiAl_2Nb_ σ a (MPa) 400 σ a (MPa) TiAl_2Nb_rt_ε c =4 TiAl_2Nb_rt_ε c =0.08 TiAl_2Nb_750 Gloanec et al._2nb_rt Gloanec et al._2nb_ ε ap N Fig. 2. Cyclic stress-strain curves and derived Wöhler curves o TiAl alloys at two temperatures. Open symbols were measured in [3.] or the alloy with 2 %Nb. Obr. 2. Cyklické deormační křivky a odvozné Wöhlerovy křivky TiAl 2Nb slitiny pro obě teploty. Otevřené symboly jsou převzaté z práce [3.] studující slitinu TiAl s 2at. % Nb. The basic cyclic stress-strain curves (CSSCs) o TiAl 2Nb alloys or both temperatures are shown in Fig. 2b. Stress amplitude σ a is plotted vs. plastic strain amplitude ε ap. The continuous cyclic hardening at RT does not allow to evaluate saturated σ a and ε ap. Thereore, our values o cumulative plastic strain ε c were chosen and respective σ a and ε ap values were determined. With increasing ε c the CSSC o the TiAl 2Nb alloy at room temperature is shited to higher stresses. For ε c values higher than about 4 the changes o σ a and ε ap are small. The CSSCs were approximated by the power law [6.]

4 σ n a = K ε ap where K is atigue hardening coeicient and n is atigue hardening exponent. These material parameters were evaluated by linear regression analysis and their values are listed in Table 2. Since the slopes o both CSSCs (or ε c = 4) at both temperatures are practically identical it can be deduced that the cyclic deormation mechanisms are similar. Table 2. Parameters o cyclic plasticity and low cycle atigue lie o TiAl 2Nb alloy or both temperatures. Tabulka 2. Parametry cyklické plasticity a nízko-cyklových únavových křivek TiAl 2Nb slitiny pro obě teploty. Temp. [ C] K [MPa] n [-] ε [-] c [-] σ [MPa] b [-] 23 ε c = ε c = Fatigue lie curves The derived Wöhler curves, i.e. the relation between stress amplitude σ a at hal lie or at a given cumulative plastic strain vs. number o cycles to racture N F o TiAl 2Nb alloy are shown or both temperatures in Fig. 2b. Moreover, data published in [3.] or a TiAl lamellar alloy with 2 at.% Nb are added or comparison. Experimental data were approximated by the Basquin equation [6.] a, ( 2N ) b σ = σ, where σ is atigue strength coeicient and b is atigue strength exponent. All material parameters were evaluated by linear regression analysis and their values are given in Table 2. Inspection o Fig. 2b and also o the parameters in Table 2 reveals that at both temperatures atigue lie curves o TiAl 2Nb and o the material tested in earlier [3.] are the same within the experimental scatter. The position o the curve or 2Nb is inluenced by the criterion or the choice o σ a. For ε c.= 4, the data correspond very well to those published in [3.]. Increase o the testing temperature to 750 C decreases the strength o the materials which i s relected by the shit o Basquin curves to lower N F as compared with RT curves. At 750 C, the slopes o the derived Wöhler curves and consequently the atigue strength exponents b o TiAl 2Nb or both temperatures are very similar. In Fig. 3a, plastic strain amplitude ε ap is plotted vs. N F in bilogarithmic representation. The values o ε ap were chosen similarly as in the construction o CSSC. Experimental data were itted by the Coin-Manson law [6.] ap, ( 2N ) c ε = ε, where ε is atigue ductility coeicient and c is atigue ductility exponent. Fitted parameters are presented in Table 2. RT atigue data are characterized by signiicant scatter, which relects low ductility and thus high sensitivity to the presence o material deects. At 750 C and at RT or ε c.= 4, atigue lietimes and atigue ductility exponents at both temperatures are equal within the experimental scatter. It indicates that atigue degradation mechanism at both temperatures is similar. 3.3 Surace relie and racture surace observations The suraces o gauge length and racture suraces were observed in SEM ater low cycle atigue tests at both temperatures The surace relie o the TiAl 2Nb alloy cycled at room temperature is ormed by the numerous persistent slip markings (PSMs) (see Fig. 3b) consisting rom very thin extrusions (20 nm in thickness) along lamellae and/or α 2 phases. Consequently, many ine and short atigue cracks initiated in these regions (lower part o Fig. 3b). This is in agreement with earlier observations [7.] where surace cracks initiated at lamellae interaces ater cyclic loading at RT and 600 C. The racture suraces reveal r ather brittle behavior o TiAl 2Nb even at elevated temperature (see Figs 4, 5). All the atigue cracks initiated at

5 surace or subsurace (Figs 4, 5) creating smooth lat areas with the size µm corresponding to the persistent slip bands. The rest o the racture surace represents static racture due to the low racture toughness o -TiAl based alloys [7.]. -3 εap TiAl_2Nb_rt_εc=0.08 TiAl_2Nb_rt_εc=4 TiAl_2Nb_ N Fig. 3. Coin-Manson atigue lie curves o TiAl 2Nb alloy at two temperatures; Fatigue crack initiated at PSMs o TiAl 2Nb alloy in the surace o specimen (RT, εa = %, N = 1794). Obr. 3. Coinovy-Mansonovy křivky únavového života TiAl 2Nb slitiny pro obě teploty místa iniciace únavové trhliny podél PSS na povrchu vzorku cyklovaného při RT s εa = % ( N = 1794). Fig. 4 Fatigue racture surace o TiAl 2Nb alloy with crack nucleation region (RT, εa = 0.31 %, N = 376); rotated detail o atigue crack initiation smooth lat area. Obr. 4 Lomová plocha TiAl 2Nb slitiny s vyznačením oblasti iniciace únavové trhliny vzorku cyklovaného při RT s εa = 0.31 % (N = 376); otočený detail oblasti iniciace trhliny.

6 smooth lat areas Fig. 5 Fatigue racture surace o TiAl 2Nb alloy with crack nucleation region - smooth lat area (750 C, εa = 0.34 %, N = 85). Obr. 5 Lomová plocha TiAl 2Nb slitiny s vyznačením oblasti iniciace únavové trhliny hladká plochá oblast vzorku cyklovaného při 750 C s εa = 0.34 % (N = 85). CONCLUSIONS Results o low cycle atigue tests at constant strain amplitude at RT and 750 C and surace relie an d racture surace observations o TiAl 2Nb alloy lead to ollowing conclusions: (i) The atigue lie curves obtained on the TiAl 2Nb alloy agree well with the literature data measured on the material with same chemical composition and similar microstructure. (ii) Parameters o the cyclic stress-strain curve, o the Basquin law and o the Coin-Manson law were evaluated or both temperatures and discussed. (iv) The Manson-Coin plot or εc.= 4, atigue lietimes and atigue ductility exponents or both temperatures are equal within the experimental scatter. It indicates that the atigue degradation mechanism at both temperatures is similar. (v) Crack initiation sites were ound in persistent slip markings on the surace running along interlamellar interaces at both temperatures. The cracks initiated in these markings produce smooth lat racture areas on the racture surace. ACKNOWLEDGEMENTS This research was supported by the grant No. P7/11/0704 o the Czech Science Foundation. The authors are also obliged to Mr. Jiří Dluhoš or SEM observations using Tescan MIRA 3 scanning electron microscope. REFERENCES [1.] KRUML, T. et al. Inluence o niobium alloying on the low cycle atigue o cast. Procedia Engineering, 20, Vol. 2, Issue 1, p [2.] APPEL, F., HECKEL, T. K., CHRIST, H. J. Electron microscope characterization o low cycle atigue in a highstrength multiphase titanium aluminide alloy. International Journal o Fatigue, 20, Vol. 32, Issue 5, p [3.] GLOANEC, A. et al. Low-cycle atigue and deormation substructures in an engineering TiAl alloy, Intermetallics, 2007, Vol. 15, Issue 4, p [4.] PETRENEC, M. et al. Low Cycle Fatigue o Cast -TiAl Based Alloys at High Temperature, Key Engineering Materials, 2011, Vols , p

7 [5.] PETRENEC, M. et al. Eect o Temperature on the Cyclic Stress Components o Gamma - TiAl Based Alloy with Niobium Alloying, Key Engineering Materials, 2011, Vol. 465, p [6.] POLÁK, J. Cyclic Plasticity and low Cycle atigue Lie o Metals. Elsevier, Amsterdam, 1991, 315 s. [7.] JHA, S.K., LARSEN, J.M., ROSENBERGER, A.H. The role o competing mechanisms in the atigue lie variability o a nearly ully-lamellar -TiAl based alkou, Acta Materialia, 2005, Vol. 53, Issue 5, p