X ray and Neutron Diffraction of TiAl Alloys

Transcription

1 Faculty of Mathematics & Natural Sciences FMNS 2015 X ray and Neutron Diffraction of TiAl Alloys Stefan Valkov 1,2, Dimitar Neov 2, Anatoly Beskrovny 2, Denis Kozlenko 2, Peter Petrov 1 1 Institute of Electronics, Bulgarian Academy of Science, 72 Tzarigradsko Chaussee blvd., 1784 Sofia, Bulgaria 2 Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research, Dubna, Moscow Region, Russia Abstract: TiAl alloys were prepared by electron beam hybrid method. Composite Ti-Al film, from composite target, was deposited on Ti substrate by electron beam evaporation, followed by electron beam treatment with scanning electron beam. Experiments were made using Leybold Heraus (EWS 300/ 15-60) with the following technological parameters : accelerating voltage U = 60kV; beam current I=40 ma, speed of movement of specimens V=5 cm/s, current of the focusing lens If =512mA, specimen distance D 0 = 38cm. X- ray and neutron diffraction methods were used to determine the phase composition on the surface and at the volume, respectively. Time of flight neutron diffraction study of TiAl specimens was performed on DN-2 diffractometer at fast pulsed IBR-2 reactor in FLNP JINR (Dubna, Russia).We found that intermetallic TiAl phases were successfully obtained on the surface, as well as in the volume. Keywords: Titanium, Aluminium, Alloys, Electron Beam Evaporation, XRD, Neutron Diffraction 1. INTRODUCTION TiAl alloys are attractive materials for the industry, because of their combination of light weight and mechanical properties. This alloys are advanced in the field of high temperature applications, in aerospace and automotive industries etc. [1,2]. γ TiAl alloys were introduced for turbine blades in General Electric s GENx engine [3]. Due to the specific requirements for different applications, the alloys must cover a number of properties: ductility, toughness, oxidation protection, etc. The authors [4,5] found that the ductility and toughness are greatly improved, when the TiAl alloy consist of two phases (α 2 Ti 3Al and γ TiAl), than the single phase structure of γ TiAl. 80

2 Physics and Technology In order to use TiAl based alloys in environments with a high temperature, oxidation protection is required. Independent of the protected material, the forming of surface oxide layers, such as Al 2O 3, act as a protection barrier to the other reactive agents. In this case, the bulk material is protected from the environmental degradation [6]. TiAl 3 is another possible phase at TiAl based alloys. Due to its high aluminium content, the presence of this phase is able to cause the formation of protective Al 2O 3 film on the surface. Thus, TiAl 3 rich alloys are attractive material for high temperature applications [9]. The authors [10] have conducted a comparative research on the oxidation resistance of the TiAl and TiAl 3 phases. Their results show that TiAl 3, has higher oxidation susceptibility than TiAl, under the same conditions. This study is concentrated on the phase composition analyzes, of TiAl alloys, on the surface, as well as in the volume of the obtained samples. The specimens were produced by Physical Vapor Deposition (PVD) Electron Beam (EB) hybrid method. X ray diffraction methods were used to study the phase composition on the surface. Time of flight neutron diffraction investigation has been conducted on DN-2 diffractometer at Frank Laboratory of Neutron Physics, JINR (Dubna, Russia), in order to explore the phase composition in the volume. The obtained phases on the surface, as well as in the volume, are then compared. 2. EXPERIMENTAL TiAl alloys were prepared by electron beam hybrid method. Composite Ti-Al film, from composite target, was deposited on Ti substrate by electron beam evaporation, followed by electron beam treatment with scanning electron beam. Experiments were made using Leybold Heraus (EWS 300/ 15-60) in the following technological parameter : accelerating voltage U = 60kV; beam current I=40 ma, speed of movement of specimens V=5 cm/s, current of the focusing lens If =512mA, specimen distance D 0 = 38cm. The sample was first characterised by X ray diffraction. The pattern was obtained within the range from 20 o to 80 o at 2θ scale with step of 0.02 o. The measurements were performed on a Bruker D8 Advance diffractometer with Cu Kα radiation and LynxEye detector. Time of flight neutron diffraction measurements were performed on DN- 2 diffractometer at fast pulsed IBR 2 reactor at Frank Laboratory of Neutron Physics (FLNP), JINR (Dubna, Russia) [8]. IBR-2 is high-flux pulsed reactor with peak neutron thermal flux of neutrons/cm 2 s at the moderator. The reactor operates in time of flight mode, which implies time-dispersive measurements instead of most common angle-dispersive diffraction. The initial velocity of each neutron is determined by the time of arrival of the neutron into 81

3 Faculty of Mathematics & Natural Sciences FMNS 2015 detector. Since the neutron flux is formed after passing through the moderator, it has velocity (and hence, wavelength) distribution defined by Maxwell thermal distribution and the resultant neutron spectra are blend of this distribution and sample diffraction pattern. The scattering angle and direction are fixed by the position of the detector during the experiment. Diffraction maxima in the scattered intensity appear when Bragg conditions are fulfilled. The relation between the time of the flight and the interplanar distance d hkl is the following [7] (modified Bragg law): (1) ht t d = = 2mLsinθ Lsinθ d Interplanar Distance, Å ; t Time of the Neutron Flight, µs ; L The Distance from the Neutron Source to the Detector, m ; θ The Angle Position of the Detector; h Plank Constant; m Mass of the Neutron; Fig.1 represents schematically DN-2 diffractometer. The neutrons are transported from the neutron source to the sample position in a bent neutron guide constructed from straight mirror sections with length of 0.5m. The guide together with specially designed neutron moderator enables utilization of the long-wavelength neutrons in the range Å. The time of flight neutron experiments were performed for 13.5 hours with SNM17 neutron detector, filled with 3 He. The detector was positioned at 63.8 o and the distance between the moderator and the detector was m. The neutron flux at the sample was ~ 5x10 6. Fig. 1: DN - 2 diffractometer at IBR 2 pulsed reactor. 1 Moderator; 2 Background Chopper; 3 Neutron Guide Tube; 4 One Coordinate Position Sensitive Detector; 5 3 He Detectors; 6 - Goniometer; 7 Two Coordinate Position Sensitive Detector; 8 - Mechanical Part with a Moveable Arm; 9 - Control and Operative Visualization/Analysis; 10 - Data Acquisition; 11 - EtherNet Data Transfer; [8] 82

4 Physics and Technology 3. RESULTS AND DISCUSSION Figure 2 represents X ray diffraction pattern of TiAl alloy. The pattern exhibits several diffraction peaks belonging to α Ti 3Al and γ TiAl phases. In addition, there are some peaks corresponding to Al 2O 3 and TiO oxide phases. Significant degree of oxidation state of the surface has been reached, because the sample was retrieved from the vacuum chamber at a high temperature, immediately after the electron beam treatment. Fig. 2: X ray diffraction pattern of TiAl allow. Figure 3 shows the time of flight neutron spectrum from the bulk of the sample. The presence of various diffraction peaks implies existence of mixture of at least two (Al,Ti) intermetallic phases. The interplanar distance was calculated according to Eq.1. The pattern has been shown by the interplanar distance distribution, contrary to the XRD results, presented at the angle distribution. There are several diffraction peaks, corresponding to polycrystalline 83

5 Faculty of Mathematics & Natural Sciences FMNS 2015 Ti, and some peaks, corresponding to γ TiAl. The only peak, corresponding to both α Ti 3Al and γ TiAl phases was indicated. Fig. 3: Time of flight neutron diffraction pattern of TiAl alloy The obtained XRD result, from the surface of the sample, shows that mixture of alloys (γ TiAl and α Ti 3Al phases) can be obtained successfully by electron beam hybrid method. The presence of the oxide phases means that, the oxidation requirements for the high temperature applications are fulfilled. In the volume, Ti as well as γ TiAl phase are readily observable. There is only one common peak corresponding to both α and γ phases, which means that the amount of α phase is very small, or the volume of Ti 3Al fraction is negligible. This suggests that on the surface, the mechanical properties should be more suitable than in the volume. 4. CONCLUSIONS The TiAl based alloys have been successfully produced by electron beam hybrid method with the discussed technological parameters. The comparison between the phase compositions at the surface and in the volume was carried out. α Ti 3Al, γ TiAl, as well as, Al 2O 3 and TiO phases have been observed on the surface. The formation of the oxide phases shows a good oxidation behavior of the obtained specimen. In the volume, Ti and γ TiAl phases are visible. The presence of only one common peak, corresponding to both α and γ phases means that the amount of α phase is very small, or the volume of 84

6 Physics and Technology Ti 3Al is negligible. It means, the mechanical properties on the surface should be more suitable than in the volume. 5. ACKNOWLEDGEMENTS The authors would like to thank the Bulgarian Nuclear Regulatory Agency for providing financial support, and also to Mr. Stanislav Sheverev for the neutron diffraction experiments support. 6. REFERENCES [1] Wu, X. (2006) Review of alloy and process development of TiAl alloys, Intermetallics 14(10-11), [2] Dimiduk, M. (1999) Gamma titanium aluminide alloys - an assessment within the competition of aerospace structural materials, Material Science and Engineering A 263(2), [3] Schafrik, R. (2001) A Perspective on Intermetallic Commercialization for Aero-Turbine Applications, Structural Intermetallics, edited by Hemker, K. et al, TMS, [4] Lipsitt, H., Shechtman, D., Schafrik, R. (1975) The deformation and fracture of TiAl at elevated temperatures, Metall. Trans 6A, [5] Sastry, S. and Lipsitt, H., (1977) Fatigue deformation of TiAl based alloys, Metall. Trans 8A, [6] Pflumm, R., Friedle, S., Schütze, M. (2014) Oxidation protection of γ- TiAl-based alloys - A review, Intermetallics 56(2015) [7] Windsor, C., (1981) Pulsed Neutron Scattering, New York; USA: Taylor & Francis Ltd., Halstead Press [8] [9] Eckert, J., Gao, W., Golovin, I., Imai, Y., Li, Z., Rennhofer, M., Roth, S., Russel, A., Stoica, M., Vanghan, G., Zavrazhnov, A., Zlomanov, V. (2008) Intermetallics Research Progress, New York, USA, Publisher: Nova Science Publishers [10] Umakoshi, Y., Yamaguchi, M., Sakagami, T., Yamene, T. (1989) Oxidation resistance of intermetallic compounds Al 3Ti and TiAl, Journal of Material Science 24,