Monotonic and cyclic properties of TiAl alloys Monotic and cyclic properties of TiAl alloys doped with Nb, Mo and C

P r o c e d i a E n g i n e e r i n g 7 4 ( 2 0 1 4 ) 4 0 5 – 4 0 8 Available online at www.sciencedirect.com 1877-7058 © 2014 Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of the Politecnico di Milano, Dipartimento di Meccanica doi: 10.1016/j.proeng.2014.06.289 ScienceDirect XVII International Colloquium on Mechanical Fatigue of Metals (ICMFM17) Monotonic and cyclic properties of TiAl alloys doped with Nb, Mo and C A. Chlupová a *, K. Obrtlík a , P. Beran b , M. Heczko a , J. Polák c , T. Kruml c a Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Zizkova 22, 61662 Brno, Czech republic b Nuclear Physics Institute, Academy of Sciences of the Czech Republic, Rez 130, 25068 Czech Republic c CEITEC, Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Zizkova 22, Brno, 61662 Czech Republic Abstract Six grades of TiAl alloys doped with 7 at.% Nb and variable content of C and/or Mo were studied. The as-received microstructure and phase composition of the alloys was characterized using neutron diffraction experiments and electron microscopy. They are substantially different in individual alloys. In Mo containing alloys, ordered β 0 phase is present while carbon suppresses the occurrence of this phase. Alternating lamellae of γ and α 2 phases are characteristic for all specimens. The specimens have been subjected to monotonic testing (tensile and compression) and to low cycle fatigue testing at ambient and at elevated temperatures. The differences in microstructure result in scatter of mechanical behaviour of the alloys, e.g. the yield stress at 750°C varies from 445 to 626 MPa. The mechanical properties of the materials are discussed in relation to the initial microstructure and phase composition. © 2014 The Authors. Published by Elsevier Ltd. Selection and peer-review under responsibility of the Politecnico di Milano, Dipartimento di Meccanica. Keywords: TiAl alloys, microstructure, neutron diffraction, fatigue 1. Introduction Titanium aluminides or TiAl alloys are low density materials with good mechanical properties up to ~800°C and satisfactory oxidation resistance. Since their density is about a half of that of superalloys or steels, TiAl alloys are particularly attractive in high temperature applications where fast rotation of components appear. Low mass of TiAl parts results in lower centrifugal forces. First running applications thus currently involve blades of aircraft engines or car turbochargers [1]. However, these alloys in general suffer from low ductility at room temperature which induces further complications: difficult machining, strong notch effect and consequent importance of high quality surface finish, high scatter of fracture stress and fatigue life, etc. [2] Development of this class of alloys is thus far from being finished. Efforts of engineers and researchers to improve the properties of TiAl alloys are currently intense in two directions: i) control of microstructure with regard to a particular application, ii) experimenting with additional elements and understanding of their influence on mechanical properties. In this paper, six TiAl +7at%. Nb alloys doped with Mo and/or C are under investigation. * Corresponding author. Tel.: +420-532290344; fax: +420-541218657. E-mail address: [email protected] © 2014 Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of the Politecnico di Milano, Dipartimento di Meccanica 406 A. Chlupová et al. / Procedia Engineering 74 ( 2014 ) 405 – 408 Nomenclature σ, σ a engineering stress, stress amplitude ε, ε a engineering strain, strain amplitude R p 0.2 stress at 0.2% of plastic strain in tensile test ε pf plastic strain at fracture in tensile test σ f fracture stress in tensile test 2. Experimental details Six cylindrical ingots of TiAl alloys alloyed with Nb, Mo or C were prepared by casting. HIP treatment was applied to reduce the porosity. Chemical composition of the six materials together with phase content determined from neutron diffraction and measured tensile characteristics are given in Tab. 1. Table 1. Chemical composition, phase content and tensile properties of investigated materials. Chemical composition [at.%] Phases [vol.%] Tensile properties at 750°C Label Al Nb Ni Si Mo C Ti α 2 β 0 γ R p 0.2[MPa] ε pf [%] σ f [MPa] TiAl7Nb 46 7 0.2 0.1 - - bal. 3.1 0.6 96.3 - - - TiAl7Nb 2Mo 46 7 0.2 0.1 2 - bal. 3.4 14.6 82.0 443 0.382 487 TiAl7Nb 0.2C 46 7 0.2 0.1 - 0.2 bal. 15.0 - 85.0 - 0.125 439 TiAl7Nb 0.5C 46 7 0.2 0.1 - 0.5 bal. 20.0 - 80.0 628 0.231 636 TiAl7Nb 2Mo0.2C 46 7 0.2 0.1 2 0.2 bal. 9.2 8.7 82.1 476 0.310 504 TiAl7Nb 2Mo0.5C 46 7 0.2 0.1 2 0.5 bal. 12.1 6.8 81.1 - 0.170 524 Cylindrical specimens having diameter of 6 mm and gauge length of 12 mm were machined from the cylindrical ingots close to the circumference in order to avoid casting defects present often in the centre. Gauge length of specimens was mechanically and electrolytically polished. Fatigue and tensile tests were performed using MTS servo-hydraulic machine and hydraulic collet grips. Loading was symmetrical (tension-compression) under total strain control at strain rate 1x10 -4 s -1 . Strain was measured using an extensometer. Compression tests were performed in universal test system INSTRON 8862 with electromechanical actuator. Tests were performed at RT, 700, 750 and 800°C. Microstructure of all materials was studied using light and electron microscopy. Lamellae of γ and α 2 phase are present in all six materials. In materials containing Mo as an alloying element the ordered β 0 phase is also present. The alloying with C results in higher content of ordered hexagonal α 2 phase (see Table 1). 3. Results and discussion The microstructure of the alloys differs substantially. TiAl7Nb 2Mo alloy is shown in Fig. 1a. Mo containing alloys contain 15% of β 0 phase (white regions in Fig. 1a), the rest are lamellar colonies of α 2 +γ phase and pure γ phase islands. a) b) Fig. 1 Microstructure of (a) TiAlNb 2Mo and (b) TiAlNb 02C. 407 A. Chlupová et al. / Procedia Engineering 74 ( 2014 ) 405 – 408 The alloys doped only with C developed fully lamellar structure with the highest content of α 2 phase (Fig. 1b, Table 1). Monotonic and cyclic mechanical tests are underway. Data measured at 750 °C only will be reported here. Tensile testing showed limited capability of tensile plastic straining of the materials. Maximum plastic deformation at fracture was 0.4 % (Fig. 2a). Yield stress R p 0.2, fracture stress σ f and plastic strain at fracture ε pf are given in Table 1. In two cases, the specimen fractured even before ε p = 0.2%. The fully lamellar microstructure (TiAl7Nb 0.5C) possesses the highest strength while the presence of β 0 phase increases a little bit the ductility. Compression tests were also performed. Higher plastic strain at fracture is reached in compression test, as illustrated in Fig. 2b. a) b) Fig. 2 (a) Tensile curve at 750 °C of TiAl7Nb 2Mo, (b) specimen after compression at 750 °C of TiAl7Nb 2Mo0.5C. Cyclic stress-strain curves for all materials were measured using the short-cut multiple step test procedure [3]. The cycling started at the lowest chosen level of ε a and selected number of cycles was performed. Strain amplitude was then step-wise increased. Number of cycles for a given ε a was chosen so that the cumulative plastic strain at every loading level was approximately constant. The cycling with the lowest ε a was an exception, because the plastic strain amplitude was negligible and number of cycles was chosen arbitrary. Figure 3a shows plot of the stress amplitude during such a test for TiAl7Nb 2Mo0.5C. Small cyclic hardening appears during cycling with ε a < 0.27%. Cycling with higher ε a resulted in initial cyclic softening. Its amount increased with increasing ε a , stabilization of cyclic response followed. The reason for the cyclic softening is probably the local destruction of lamellar microstructure, as discussed in details in [4,5]. Cyclic stress-strain curves obtained by the short-cut procedure are shown in Fig. 3b. Stress amplitudes at the end of each loading level are plotted vs. total strain amplitude in linear coordinates. It is visible that data points for individual material are not scattered and can be fitted by a smoothing curve. Last data point on each curve corresponds to the cycling block before fatigue fracture. Highest cyclic stresses are measured in materials with 0.5 at.% C. Mo addition has no significant effect since the difference between the curves of TiAl7Nb 2Mo0.5C and TiAl7Nb 0.5C is small. Further, the presence of β 0 phase in Mo containing alloys does not increase visibly the fatigue life, in spite of the good plasticity of this phase at high temperatures and observed positive effect on fracture toughness [6]. However, the properties of cast/HIPed alloys containing Mo can be strongly altered by subsequent heat treatment. Coarse grained lamellar colonies present are not the best microstructure to reach the good fracture properties. Schwaighofer et al. [7] discuss and compare various thermomechanical treatments of β 0 containing alloys and propose that the ductility of the material can be optimized using cyclic heat treatment. None of tested material is able to support cycling with total strain amplitude higher than 0.5%. It is due to low ductility of the material in tension in spite of good plastic deformability in compression (see Fig. 2b). 408 A. Chlupová et al. / Procedia Engineering 74 ( 2014 ) 405 – 408 a) b) Fig. 3 (a) Short-cut procedure for determination the cyclic stress-strain curve in TiAl7Nb 2Mo0.5C at 750°C. (b) Cyclic stress-strain curves for TiAl7Nb alloys doped with Mo and C at 750°C. Conclusions •TiAl alloy with 7at%. of Nb was prepared by casting and HIP treated. Doping with Mo and C results in significantly different microstructure and phase composition. •Addition of Mo in TiAl7Nb results in β 0 phase formation. •Addition of C increases the volume fraction of α 2 phase. •Tensile elongation at 750 °C is limited for all alloys; in compression the material can be deformed plastically. •Higher volume content of α 2 phase increases both monotonic and cyclic strength. Yield stress of such materials at 750°C is high, over 600MPa. •Cyclic stress-strain curves at 750°C were determined by short-cut procedure. Notable cyclic softening is observed in cycling with ε a > 0.3%. • Alloys with 0.5 at. % of C show the highest cyclic stresses. None of the alloys is able to support cycling with ε a > 0.5%. Acknowledgements Support of the project of the Czech Science Foundation 107/11/0704 is acknowledged. The research was partially conducted in CEITEC research infrastructure supported by the project CZ.1.05/1.1.00/02.0068 financed from European Regional Development Fund. References [1] F. Appel, J. Paul, M. Oehring, Gamma titanium aluminide alloys, Wiley-VCH, Weinheim, 2011. [2] T. Kruml, K. Obrtlik, M. Petrenec, J. Polak: "Low-cycle fatigue properties of TiAl alloy with high Nb content", International Journal of Materials Research (Zeitschrift fur Metallkunde) 100 (2009), 349-352 [3] J. Polák, Cyclic plasticity and low cycle fatigue life of metals, Elsevier-Academia, Amsterdam, 1991, p. 35 [4] T. Kruml, K. Obrtlík, Microstructure degradation in high temperature fatigue of TiAl alloy, Int. J. Fatigue in press, available online [5] F. Appel, T.K. Heckel, H.-J. Christ, Electron microscope characterization of low cycle fatigue in a high-strength multiphase titanium aluminide alloy, Int. J. Fatigue. 32 (2010) 792–798 [6] Y. Chen, H. Niu, F. Kong, S. Xiao, Microstructure and fracture toughness of a β phase containing TiAl alloy, Intermetallics 19 (2011), 1405-1410 [7] E. Schwaighofer, H. Clemens, S. Mayer, J. Lindemann, J. Klose, W. Smarsly, V. Güther, Microstructural design and mechanical properties of a cast and heattreated intermetallic multi-phase g-TiAl based alloy, Intermetallics 44 (2014), 128-140