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Effect of long-term ageing on microstructure stability and lattice parameters of coexisting phases in intermetallic Ti–46Al–8Ta alloy
Intermetallics 19 (2011) 121e124
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Short communication
Effect of long-term ageing on microstructure stability and lattice parameters of coexisting phases in intermetallic Tie46Ale8Ta alloy J. Lapin a, *, T. Pelachová a, V.T. Witusiewicz b, E. Dobro cka c a
Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Racianska 75, 831 02 Bratislava, Slovak Republic ACCESS e.V., Intzestr. 5, D-52072 Aachen, Germany c Institute of Electrical Engineering, Slovak Academy of Sciences, Dubravska cesta 9, 841 04 Bratislava, Slovak Republic b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 April 2010 Received in revised form 22 September 2010 Accepted 27 September 2010 Available online 23 October 2010
The lamellar a2(Ti3Al) þ g(TiAl) microstructure of intermetallic Tie46Ale8Ta (at.%) alloy is thermodynamically unstable and transforms to a2 þ g þ s type during long-term ageing at 750 C. A new ternary s-phase with B82 type structure (space group P63/mmc, Pearson symbol hP6) is identified by XRD analysis and its occurence is included to thermodynamic modelling of TieAleTa phase diagram by CALPHAD approach. The lattice parameters of the coexisting g, a2 and s phases change during ageing. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Titanium aluminides, based on TiAl B. Phase identification B. Phase diagrams D. Microstructure F. Diffraction (X-ray)
1. Introduction
gM(TiAl) through a diffusionless transformation initiated by cooling at required rate from single a-phase field [8e10]. In many TiAl-
Cast TiAl-based alloys are attractive for high-temperature structural applications due to their high specific strength at elevated temperatures and acceptable price/properties ratio when compared to counterparts from Ni-based superalloys [1,2]. However, the formation of coarse-grained microstructures during solidification [3] and significant chemical inhomogeneities of the TiAl-based castings [4] deteriorate the already low ductility and poor damage tolerance at ambient temperature. These mechanical properties result mainly from the fact that the as-cast microstructures consist of g(TiAl) and a2(Ti3Al) lamellae with a length defined by the size of as-cast a (Ti-based solid solution with hexagonal crystal structure) grains [5]. Reducing the grain size in cast samples has been shown to improve significantly room-temperature ductility without degradation of the creep resistance [6]. Two main approaches are applied to refine coarse-grained structure of cast TiAl-based alloys: (i) alloying with minor additions, mainly with boron, which usually leads to the formation of long boride particles that can act as failure initiation sites [3,7] and (ii) formation of highly faulted massive
based alloys, the critical cooling rates to achieve the formation of massive gM are relatively high which may result in shape distortion, cracking or even fracture of complex shaped castings (blades, turbocharger wheels, etc.) during cooling [11]. Therefore, development of new TiAl-based alloys requiring slow cooling rates for the formation of massive gM is of a great industrial and scientific interest. Hu et al. [11] showed that elements such as Nb and Ta have low diffusion coefficients in both a and g phases and favour the massive transformation over the lamellar formation at low cooling rates. These elements partition between a and g phases when forming the a þ g lamellar microstructure and this requirement for their diffusion slows down the lamellar transformation. Based on this concept, a new “airhardenable” Tie46Ale8Ta (at.%) alloy requiring only air cooling from single a-phase field to form massive gM was designed [12,13]. This alloy shows promising castability, mechanical properties, and relatively large processing window to achieve a2 þ g microstructure during heat treatments [13]. However, potential industrial applications are conditioned by more complex understanding of its behaviour and properties at room and high temperatures. The aim of the present work is to study the effect of long-term ageing on microstructure stability and lattice parameters of coexisting phases in Tie46Ale8Ta (at.%) alloy. This alloy
* Corresponding author. Tel.: þ421 2 49268290; fax: þ421 2 44253301. E-mail address: [email protected] (J. Lapin). 0966-9795/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2010.09.016
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J. Lapin et al. / Intermetallics 19 (2011) 121e124
Fig. 1. Backscattered SEM micrographs showing microstructure of Tie46Ale8Ta (at.%) alloy: (a) as-received; (b) after ageing at 750 C for 10,000 h.
belongs to the latest 4th generation of TiAl-based alloys [12,13] and was developed for turbine blade applications within the European integrated project IMPRESS [14]. 2. Experimental setup The studied Tie46Ale8Ta (at.%) alloy was provided in the form of centrifugally cast and heat treated cylindrical bars with a diameter of 13 mm and length of 120 mm by ACCESS [15]. Heat treatments consisted of hot isostatic pressing (HIP) at an applied pressure of 200 MPa, temperature of 1260 C for 4 h, which was followed by solution annealing at 1360 C for 1 h and air cooling at a rate of about 40 Ks1 to form massive gM. The heat treatment was finalized by HIP ageing at an applied pressure of 150 MPa, temperature of 1260 C for 2 h followed by cooling at a rate of 0.083 C s1 to form convoluted type of microstructure. The asreceived bars were cut to cylindrical samples with a diameter of 13 mm and thickness of 5 mm and isothermally aged in air. Longterm ageing experiments were performed at a temperature of 750 C for various times ranging from 500 to 10,000 h. After each ageing step, the microstructure evaluation was performed by backscattered scanning electron microscopy (BSEM), X-ray diffraction (XRD) analysis and energy-dispersive X-ray (EDX) spectroscopy. SEM samples were prepared using standard
metallographic techniques and etched in a solution of 150 ml H2O, 25 ml HNO3 and 10 ml HF. XRD analysis was carried out using Bruker D8 DISCOVER diffractometer equipped with X-ray tube with rotating Cu anode operating at 12 kW. All measurements were performed in parallel beam geometry with parabolic Goebel mirror in the primary beam. Diffraction patterns were measured within an angular range 20 e110 of 2q with an exposition time of 6 s and step size of 0.01. The obtained diffraction data were analyzed quantitatively using the program EVA 11.0. The further refinement of the patterns to identify the crystal lattice parameters of coexisting phases was performed using TOPAS 3.0. The instrumental line profile was determined from the measurements of polycrystalline texture of free corundum with sufficiently large crystallite size at the same angular range. Volume fractions of coexisting phases were determined from digitalised micrographs using a computer image analyser. 3. Results and discussion Fig. 1 shows the typical microstructure of Tie46Ale8Ta (at.%) alloy. The convoluted a2 þ g microstructure of the as-received material consists mostly of plate-like a2-phase (bright grey contrast) which forms small colonies within the g-phase, as seen in Fig. 1a. A mean length and average volume fraction of the a2 laths
Fig. 2. X-ray diffraction patterns of the as-received and aged Tie46Ale8Ta (at.%) alloy. The as-received state and ageing time at 750 C are indicated in the figure.
J. Lapin et al. / Intermetallics 19 (2011) 121e124
123
Fig. 3. Calculated isopleth for 8 at.% Ta of the TieAleTa systems covering the composition in the range of 20e70 at.% Al. The dashed line shows the A2/B2 orderedisorder transformation of the bcc-phase.
were measured to be 8.5 mm and (29.8 2.3) vol.%, respectively. Some single-phase a2 and g areas have also been seen in the microstructure. As shown by Saage et al. [12], the convoluted microstructure is formed by precipitation of a and/or a2 phases on four equivalent {111} planes of massively transformed gM during the second HIP-ing at 1260 C and cooling from two phase a þ g field. The ageing has a negligible effect on the stability and mean length of the a2 laths which was measured to vary between 8.5 and 9.2 mm at all ageing times. On the other hand, formation of two types of particles is observed along the grain boundaries, as seen in Fig. 1b. EDX analysis showed that some of these particles belong to the a2-phase and some to a new s-phase with the chemical composition Tie(36e40)Ale(12e15)Ta (at.%). Quantitative metallographic analysis of the aged samples indicates that the s-phase forms at the expense of the a2-phase. The volume fraction of the a2phase decreases from 29.8 vol.% in the as-received alloy to 25.3 vol.% while the volume fraction of the s-phase increases to 2.2 vol.% during ageing for 10,000 h. Fig. 2 shows the evolution of XRD patterns of the studied alloy during ageing. The XRD pattern of the as-received alloy shows twophase microstructure composed of a2 and g, which is in agreement with the observations reported recently by Jiang et al. [13]. However, the present experiments revealed that the lamellar a2 þ g microstructure is thermodynamically unstable and transforms to a2 þ g þ s type during long-term ageing at 750 C. The clear evidence of the s-phase formation is found in the XRD patterns after ageing for 1000 h. Crystallographically, the s-phase with B82 type structure (space group P63/mmc, Pearson symbol hP6) and Ni2In symmetry is similar to a ternary hexagonal Ti4Al3Nb phase reported recently by Witusiewicz et al. [16] for Tie46Ale8Nb (at.%) alloy. The identified s-phase is different from s(Ta2Al)-phase with tetragonal crystal structure resulting from the ternary TieAleTa phase diagram reported by Kubaschewski [17], which is illustrated schematically by the positions of diffraction peaks for the s-phase in Fig. 2. The s-phase is also not present in the ternary TieAleTa phase diagram thermodynamically modelled recently by Witusiewicz and published by Jiang et al. [13], where only two-phase a2 þ g microstructure is predicted to be stable below 1050 C. Fig. 3 shows the isopleth for 8 at.% Ta of the TieAleTa systems covering the Al composition in the range of 20e70 at.% calculated
Fig. 4. Dependence of the lattice parameters on the ageing time at 750 C: (a) lattice parameters a and c of g-TiAl, (b) lattice parameters a and c of a2-Ti3Al and (c) aspect ratio c/a for g-TiAl and a2-Ti3Al.
using new thermodynamic description and Thermo-Calc software. The description based on CALPHAD modelling of the Gibbs energy of all accepted individual phases [16,18,19] takes into account experimental evidence of the s-phase as well as new phase equilibria observed in the aged Tie46Ale8Ta (at.%) alloy and CALPHAD modelling of the Gibbs energy of all accepted individual phases [16,18,19]. According to the phase diagram, it is expected that the microstructure of the studied Tie46Ale8Ta (at.%) alloy should transform to the equilibrium g þ s type during long-term ageing below 870 C. However, further experiments are still required to refine the position of the boundary between a2 þ g þ s and g þ s phase regions. Fig. 4 shows dependence of measured lattice parameters of the a2 and g phases on the ageing time. The lattice parameter a of the g-phase decreases with increasing ageing time up to 5000 h and
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J. Lapin et al. / Intermetallics 19 (2011) 121e124
Table 1 Measured lattice parameters of s-phase after ageing at 750 C. Ageing time (h)
a (1010 m)
c (1010 m)
c/a
5000 8000 10,000
4.5660 0.0018 4.5692 0.0019 4.5771 0.0016
5.5162 0.0007 5.5170 0.0006 5.5180 0.0007
1.2081 0.0006 1.2074 0.0006 1.2056 0.0006
remains constant within the experimental error for a longer time, as shown in Fig. 4a. On the other hand, the lattice parameter c and aspect ratio c/a increase with increasing ageing time up to 10,000 h, as seen in Fig. 4a and c. The decrease of a and increase of both c and c/a can be attributed to the redistribution of alloying elements during ageing, mainly to an increase of Al content in the g-phase [20e22]. For the g-phase, a linear dependence is found between the aspect ratio c/a and the ageing time t the in the form
c a g
¼ 1:1070 þ 7:9169 108 t
(1)
The correlation coefficient of this fit is r2 ¼ 0.970. Fig. 4b shows dependence of the lattice parameters of the a2-phase on the ageing time. Both lattice parameters a and c increase with increasing ageing time. For the a2-phase, the aspect ratio c/a linearly decreases with increasing ageing time according to a relationship in the form
c a a2
¼ 0:8059 1:585 107 t
(2)
The correlation coefficient of this fit is r2 ¼ 0.975. Table 1 summarises measured lattice parameters of the s-phase. The calculated lattice parameters a and c increase and c/a ratio decreases with increasing ageing time. The change of the lattice parameters of the s-phase is connected with redistribution of alloying elements among the coexisting a2, g and s phases during ageing. 4. Conclusions The lamellar a2 þ g microstructure of the Tie46Ale8Ta (at.%) alloy is thermodynamically unstable and transforms to a2 þ g þ s type during long-term ageing at 750 C. The clear evidence of the new s-phase is found in the XRD patterns after 1000 h ageing. The ternary TieAleTa phase diagram, which takes into account occurrence of the s-phase and new phase equilibria, is modelled by CALPHAD approach. The measured lattice parameters of the g, a2 and s phases change during ageing. The lattice parameter a decreases and the lattice parameter c and the aspect ratio c/a increase with increasing ageing time for the g-phase. The lattice parameters a and c increase and the aspect ratio c/a decreases with increasing ageing time for both s- and a2-phases. Acknowledgments This work was financially supported by the Slovak Research and Development Agency under the contract APVV-0009-07 and EU
Integrated Project IMPRESS “Intermetallic Materials Processing in Relation to Earth and Space Solidification” under the contract No. NMP3-CT-2004-500635. The authors would like to thank to U. Hecht from ACCESS (Germany) for providing the experimental material.
References [1] Harding RA. Recent developments in the induction skull melting and investment casting of titanium aluminides. Kovove Mater 2004;42:225e41. [2] Shouren W, Peiquan G, Liying Y. Centrifugal precision cast TiAl turbocharger wheel using ceramic mold. J Mater Process Tech 2008;204:492e7. [3] Hecht U, Witusiewicz V, Drevermann A, Zollinger J. Grain refinement by low boron additions in niobium-rich TiAl-based alloys. Intermetallics 2008;16:969e79. [4] Zollinger J, Gabalcova Z, Daloz D, Lapin J, Combeau H. Microsegregation induced microstructures in intermetallic Tie46Ale8Nb alloy. Kovove Mater 2008;462:291e6. [5] Lapin J, Gabalcova Z, Bajana O. The effect of microstructure on mechanical properties of directionally solidified intermetallic Tie46Ale8Nb alloy. Kovove Mater 2009;47:159e67. [6] Zhang WJ, Deevi SC. Analysis of the minimum creep rates of TiAl alloys. Mater Sci Eng A 2003;362:280e91. [7] Hu D, Wu X, Loretto MH. Advances in optimisation of mechanical properties in cast TiAl alloys. Intermetallics 2005;13:914e9. [8] Hu D. Effect of composition on grain refinement in TiAl-based alloys. Intermetallics 2001;9:1037e43. [9] Clemens H, Bartels A, Bystrzanowski S, Chladil H, Leitner H, Dehm G, et al. Grain refinement in g-TiAl-based alloys by solid state phase transformations. Intermetallics 2006;14:1380e5. [10] Lapin J, Gabalcová Z. The effect of oxygen content and cooling rate on phase transformations in directionally solidified intermetallic Tie46Ale8Nb alloy. Kovove Mater 2008;46:185e95. [11] Hu D, Huang AJ, Wu X. On the massive phase transformation regime in TiAl alloys: the alloying effect on massive/lamellar competition. Intermetallics 2007;15:327e32. [12] Saage H, Huang AJ, Hu D, Loretto MH, Wu X. Microstructures and tensile properties of massively transformed and aged Ti46Al8Nb and Ti46Al8Ta alloys. Intermetallics 2009;17:32e8. [13] Jiang H, Zhang K, Hao XJ, Saage H, Wain N, Hu D, et al. Nucleation of massive gamma during air cooling of Ti46Al8Ta. Intermetallics 2010;18:938e44. [14] Jarvis DJ, Voss D. IMPRESS Integrated Project e an overview paper. Mater Sci Eng A 2005;413e414:583e91. [15] Aguilar J, Hecht U, Schievenbusch A. Qualification of an investment casting process for production of titanium aluminide components for aerospace and automotive applications. Mater Sci Forum 2010;638e642:1275e80. [16] Witusiewicz VT, Bondar AA, Hecht U, Velikanova TYa. The AleBeNbeTi system: IV. Experimental study and thermodynamic re-evaluation of the binary AleNb and ternary AleNbeTi systems. J Alloys Compd 2009;472:133e61. [17] Kubaschewski O. Aluminiumetantalumetitanium, ternary alloys, vol. 8. VCH; 1993. 406e408. [18] Witusiewicz VT, Bondar AA, Hecht U, Zollinger J, Petyukh VM, Fomichov OS, et al. Experimental study and thermodynamic re-assessment of the binary AleTa system. Intermetallics 2010;18:92e106. [19] Witusiewicz VT, Bondar AA, Hecht U, Rex S, Velikanova TYa. The AleBeNbeTi system: III. Thermodynamic re-evaluation of the constituent binary system AleTi. J Alloys Compd 2008;465:64e77. [20] Pfullmann T, Beaven PA. On the relationship between lattice parameters and composition of the g-TiAl phase. Scripta Metall Mater 1993;28:275e80. [21] Kawabata T, Fukai H, Izumi O. Effect of ternary additions on mechanical properties of TiAl. Acta Mater 1998;46:2185e94. [22] Perdrix F, Trichet MF, Bonnentien JL, Cornet M, Bigot J. Relationships between interstitial content, microstructure and mechanical properties in fully lamellar Tie48Al alloys, with special reference to carbon. Intermetallics 2001;9:807e15.
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Short communication
Effect of long-term ageing on microstructure stability and lattice parameters of coexisting phases in intermetallic Tie46Ale8Ta alloy J. Lapin a, *, T. Pelachová a, V.T. Witusiewicz b, E. Dobro cka c a
Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Racianska 75, 831 02 Bratislava, Slovak Republic ACCESS e.V., Intzestr. 5, D-52072 Aachen, Germany c Institute of Electrical Engineering, Slovak Academy of Sciences, Dubravska cesta 9, 841 04 Bratislava, Slovak Republic b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 April 2010 Received in revised form 22 September 2010 Accepted 27 September 2010 Available online 23 October 2010
The lamellar a2(Ti3Al) þ g(TiAl) microstructure of intermetallic Tie46Ale8Ta (at.%) alloy is thermodynamically unstable and transforms to a2 þ g þ s type during long-term ageing at 750 C. A new ternary s-phase with B82 type structure (space group P63/mmc, Pearson symbol hP6) is identified by XRD analysis and its occurence is included to thermodynamic modelling of TieAleTa phase diagram by CALPHAD approach. The lattice parameters of the coexisting g, a2 and s phases change during ageing. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Titanium aluminides, based on TiAl B. Phase identification B. Phase diagrams D. Microstructure F. Diffraction (X-ray)
1. Introduction
gM(TiAl) through a diffusionless transformation initiated by cooling at required rate from single a-phase field [8e10]. In many TiAl-
Cast TiAl-based alloys are attractive for high-temperature structural applications due to their high specific strength at elevated temperatures and acceptable price/properties ratio when compared to counterparts from Ni-based superalloys [1,2]. However, the formation of coarse-grained microstructures during solidification [3] and significant chemical inhomogeneities of the TiAl-based castings [4] deteriorate the already low ductility and poor damage tolerance at ambient temperature. These mechanical properties result mainly from the fact that the as-cast microstructures consist of g(TiAl) and a2(Ti3Al) lamellae with a length defined by the size of as-cast a (Ti-based solid solution with hexagonal crystal structure) grains [5]. Reducing the grain size in cast samples has been shown to improve significantly room-temperature ductility without degradation of the creep resistance [6]. Two main approaches are applied to refine coarse-grained structure of cast TiAl-based alloys: (i) alloying with minor additions, mainly with boron, which usually leads to the formation of long boride particles that can act as failure initiation sites [3,7] and (ii) formation of highly faulted massive
based alloys, the critical cooling rates to achieve the formation of massive gM are relatively high which may result in shape distortion, cracking or even fracture of complex shaped castings (blades, turbocharger wheels, etc.) during cooling [11]. Therefore, development of new TiAl-based alloys requiring slow cooling rates for the formation of massive gM is of a great industrial and scientific interest. Hu et al. [11] showed that elements such as Nb and Ta have low diffusion coefficients in both a and g phases and favour the massive transformation over the lamellar formation at low cooling rates. These elements partition between a and g phases when forming the a þ g lamellar microstructure and this requirement for their diffusion slows down the lamellar transformation. Based on this concept, a new “airhardenable” Tie46Ale8Ta (at.%) alloy requiring only air cooling from single a-phase field to form massive gM was designed [12,13]. This alloy shows promising castability, mechanical properties, and relatively large processing window to achieve a2 þ g microstructure during heat treatments [13]. However, potential industrial applications are conditioned by more complex understanding of its behaviour and properties at room and high temperatures. The aim of the present work is to study the effect of long-term ageing on microstructure stability and lattice parameters of coexisting phases in Tie46Ale8Ta (at.%) alloy. This alloy
* Corresponding author. Tel.: þ421 2 49268290; fax: þ421 2 44253301. E-mail address: [email protected] (J. Lapin). 0966-9795/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2010.09.016
122
J. Lapin et al. / Intermetallics 19 (2011) 121e124
Fig. 1. Backscattered SEM micrographs showing microstructure of Tie46Ale8Ta (at.%) alloy: (a) as-received; (b) after ageing at 750 C for 10,000 h.
belongs to the latest 4th generation of TiAl-based alloys [12,13] and was developed for turbine blade applications within the European integrated project IMPRESS [14]. 2. Experimental setup The studied Tie46Ale8Ta (at.%) alloy was provided in the form of centrifugally cast and heat treated cylindrical bars with a diameter of 13 mm and length of 120 mm by ACCESS [15]. Heat treatments consisted of hot isostatic pressing (HIP) at an applied pressure of 200 MPa, temperature of 1260 C for 4 h, which was followed by solution annealing at 1360 C for 1 h and air cooling at a rate of about 40 Ks1 to form massive gM. The heat treatment was finalized by HIP ageing at an applied pressure of 150 MPa, temperature of 1260 C for 2 h followed by cooling at a rate of 0.083 C s1 to form convoluted type of microstructure. The asreceived bars were cut to cylindrical samples with a diameter of 13 mm and thickness of 5 mm and isothermally aged in air. Longterm ageing experiments were performed at a temperature of 750 C for various times ranging from 500 to 10,000 h. After each ageing step, the microstructure evaluation was performed by backscattered scanning electron microscopy (BSEM), X-ray diffraction (XRD) analysis and energy-dispersive X-ray (EDX) spectroscopy. SEM samples were prepared using standard
metallographic techniques and etched in a solution of 150 ml H2O, 25 ml HNO3 and 10 ml HF. XRD analysis was carried out using Bruker D8 DISCOVER diffractometer equipped with X-ray tube with rotating Cu anode operating at 12 kW. All measurements were performed in parallel beam geometry with parabolic Goebel mirror in the primary beam. Diffraction patterns were measured within an angular range 20 e110 of 2q with an exposition time of 6 s and step size of 0.01. The obtained diffraction data were analyzed quantitatively using the program EVA 11.0. The further refinement of the patterns to identify the crystal lattice parameters of coexisting phases was performed using TOPAS 3.0. The instrumental line profile was determined from the measurements of polycrystalline texture of free corundum with sufficiently large crystallite size at the same angular range. Volume fractions of coexisting phases were determined from digitalised micrographs using a computer image analyser. 3. Results and discussion Fig. 1 shows the typical microstructure of Tie46Ale8Ta (at.%) alloy. The convoluted a2 þ g microstructure of the as-received material consists mostly of plate-like a2-phase (bright grey contrast) which forms small colonies within the g-phase, as seen in Fig. 1a. A mean length and average volume fraction of the a2 laths
Fig. 2. X-ray diffraction patterns of the as-received and aged Tie46Ale8Ta (at.%) alloy. The as-received state and ageing time at 750 C are indicated in the figure.
J. Lapin et al. / Intermetallics 19 (2011) 121e124
123
Fig. 3. Calculated isopleth for 8 at.% Ta of the TieAleTa systems covering the composition in the range of 20e70 at.% Al. The dashed line shows the A2/B2 orderedisorder transformation of the bcc-phase.
were measured to be 8.5 mm and (29.8 2.3) vol.%, respectively. Some single-phase a2 and g areas have also been seen in the microstructure. As shown by Saage et al. [12], the convoluted microstructure is formed by precipitation of a and/or a2 phases on four equivalent {111} planes of massively transformed gM during the second HIP-ing at 1260 C and cooling from two phase a þ g field. The ageing has a negligible effect on the stability and mean length of the a2 laths which was measured to vary between 8.5 and 9.2 mm at all ageing times. On the other hand, formation of two types of particles is observed along the grain boundaries, as seen in Fig. 1b. EDX analysis showed that some of these particles belong to the a2-phase and some to a new s-phase with the chemical composition Tie(36e40)Ale(12e15)Ta (at.%). Quantitative metallographic analysis of the aged samples indicates that the s-phase forms at the expense of the a2-phase. The volume fraction of the a2phase decreases from 29.8 vol.% in the as-received alloy to 25.3 vol.% while the volume fraction of the s-phase increases to 2.2 vol.% during ageing for 10,000 h. Fig. 2 shows the evolution of XRD patterns of the studied alloy during ageing. The XRD pattern of the as-received alloy shows twophase microstructure composed of a2 and g, which is in agreement with the observations reported recently by Jiang et al. [13]. However, the present experiments revealed that the lamellar a2 þ g microstructure is thermodynamically unstable and transforms to a2 þ g þ s type during long-term ageing at 750 C. The clear evidence of the s-phase formation is found in the XRD patterns after ageing for 1000 h. Crystallographically, the s-phase with B82 type structure (space group P63/mmc, Pearson symbol hP6) and Ni2In symmetry is similar to a ternary hexagonal Ti4Al3Nb phase reported recently by Witusiewicz et al. [16] for Tie46Ale8Nb (at.%) alloy. The identified s-phase is different from s(Ta2Al)-phase with tetragonal crystal structure resulting from the ternary TieAleTa phase diagram reported by Kubaschewski [17], which is illustrated schematically by the positions of diffraction peaks for the s-phase in Fig. 2. The s-phase is also not present in the ternary TieAleTa phase diagram thermodynamically modelled recently by Witusiewicz and published by Jiang et al. [13], where only two-phase a2 þ g microstructure is predicted to be stable below 1050 C. Fig. 3 shows the isopleth for 8 at.% Ta of the TieAleTa systems covering the Al composition in the range of 20e70 at.% calculated
Fig. 4. Dependence of the lattice parameters on the ageing time at 750 C: (a) lattice parameters a and c of g-TiAl, (b) lattice parameters a and c of a2-Ti3Al and (c) aspect ratio c/a for g-TiAl and a2-Ti3Al.
using new thermodynamic description and Thermo-Calc software. The description based on CALPHAD modelling of the Gibbs energy of all accepted individual phases [16,18,19] takes into account experimental evidence of the s-phase as well as new phase equilibria observed in the aged Tie46Ale8Ta (at.%) alloy and CALPHAD modelling of the Gibbs energy of all accepted individual phases [16,18,19]. According to the phase diagram, it is expected that the microstructure of the studied Tie46Ale8Ta (at.%) alloy should transform to the equilibrium g þ s type during long-term ageing below 870 C. However, further experiments are still required to refine the position of the boundary between a2 þ g þ s and g þ s phase regions. Fig. 4 shows dependence of measured lattice parameters of the a2 and g phases on the ageing time. The lattice parameter a of the g-phase decreases with increasing ageing time up to 5000 h and
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J. Lapin et al. / Intermetallics 19 (2011) 121e124
Table 1 Measured lattice parameters of s-phase after ageing at 750 C. Ageing time (h)
a (1010 m)
c (1010 m)
c/a
5000 8000 10,000
4.5660 0.0018 4.5692 0.0019 4.5771 0.0016
5.5162 0.0007 5.5170 0.0006 5.5180 0.0007
1.2081 0.0006 1.2074 0.0006 1.2056 0.0006
remains constant within the experimental error for a longer time, as shown in Fig. 4a. On the other hand, the lattice parameter c and aspect ratio c/a increase with increasing ageing time up to 10,000 h, as seen in Fig. 4a and c. The decrease of a and increase of both c and c/a can be attributed to the redistribution of alloying elements during ageing, mainly to an increase of Al content in the g-phase [20e22]. For the g-phase, a linear dependence is found between the aspect ratio c/a and the ageing time t the in the form
c a g
¼ 1:1070 þ 7:9169 108 t
(1)
The correlation coefficient of this fit is r2 ¼ 0.970. Fig. 4b shows dependence of the lattice parameters of the a2-phase on the ageing time. Both lattice parameters a and c increase with increasing ageing time. For the a2-phase, the aspect ratio c/a linearly decreases with increasing ageing time according to a relationship in the form
c a a2
¼ 0:8059 1:585 107 t
(2)
The correlation coefficient of this fit is r2 ¼ 0.975. Table 1 summarises measured lattice parameters of the s-phase. The calculated lattice parameters a and c increase and c/a ratio decreases with increasing ageing time. The change of the lattice parameters of the s-phase is connected with redistribution of alloying elements among the coexisting a2, g and s phases during ageing. 4. Conclusions The lamellar a2 þ g microstructure of the Tie46Ale8Ta (at.%) alloy is thermodynamically unstable and transforms to a2 þ g þ s type during long-term ageing at 750 C. The clear evidence of the new s-phase is found in the XRD patterns after 1000 h ageing. The ternary TieAleTa phase diagram, which takes into account occurrence of the s-phase and new phase equilibria, is modelled by CALPHAD approach. The measured lattice parameters of the g, a2 and s phases change during ageing. The lattice parameter a decreases and the lattice parameter c and the aspect ratio c/a increase with increasing ageing time for the g-phase. The lattice parameters a and c increase and the aspect ratio c/a decreases with increasing ageing time for both s- and a2-phases. Acknowledgments This work was financially supported by the Slovak Research and Development Agency under the contract APVV-0009-07 and EU
Integrated Project IMPRESS “Intermetallic Materials Processing in Relation to Earth and Space Solidification” under the contract No. NMP3-CT-2004-500635. The authors would like to thank to U. Hecht from ACCESS (Germany) for providing the experimental material.
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