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Microstructure and chemical composition of phases in Ti–48Al–2Cr–2Nb intermetallic alloy
Materials Chemistry and Physics 81 (2003) 438–442
Microstructure and chemical composition of phases in Ti–48Al–2Cr–2Nb intermetallic alloy J. Chrapo´nski∗ , W. Szkliniarz, A. Ko´scielna, B. Serek Department of Materials Science, Silesian University of Technology, Krasi´nskiego 8, Pl-40-019 Katowice, Poland
Abstract The microstructure of the alloy and chemical composition of phases in as-cast and heat-treated conditions were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopy. It has been shown that at high heating and cooling rates the thickness of the ␣2 and ␥ lamellae decreases. Also, under these conditions the surface area of the grain boundaries decreases. Higher annealing temperatures in the two-phase (␥ + ␣) range and low cooling rates led to a lamellar microstructure. Annealing near the eutectoid temperature resulted in degeneration of the initial lamellar microstructure. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Intermetallics; Titanium aluminides; Scanning electron microscopy; Heat treatment; Energy-dispersive X-ray spectroscopy
1. Introduction Titanium aluminide-based intermetallic compounds have a variety of interesting properties such as low density, high specific strength, substantial mechanical strength at high temperatures and good creep resistance. These properties predestinate Ti–Al alloys for high-temperature applications as potential replacements for superalloys. However, they have a limited plasticity at room temperature and the tendency to brittle fracture. An improved plasticity of the two-phase (␥ + ␣2 ) alloys is received by adding alloying elements and by microstructure modification. The basic way of forming the microstructure is heat treatment and thermomechanical working [1–3]. From the view point of the properties and possibilities of their modification by microstructure changes, most interesting are alloys with 44–48 at.% Al. In these alloys, the eutectoid reaction takes place, which is advantegous in thermal treatment processes. After various heat treatments it is possible to obtain four types of microstructure [4,5]: • near ␥: with equiaxed grains and ␣2 particles at the grain boundaries and triple points, • duplex: consisting of ␥ grains and lamellar colonies of alternating layers of ␥ and ␣2 phase, • nearly lamellar: the size of the lamellar (␣2 + ␥) grains is larger than that of the ␥ grains, • fully lamellar: with coarse lamellar (␣2 + ␥) grains. ∗ Corresponding author. Tel./fax: +48-32-256-31-97. E-mail address: [email protected] (J. Chrapo´nski).
Generally, a duplex-type microstructure is characterized by a better plasticity, but it is inferior to a lamellar microstructure with respect to crack resistance, fatigue strength and high-temperature creep resistance. The best properties are characteristic of an alloy with a structure where the ␣2 phase volume amounts to 5–20%. Therefore, a properly designed heat treatment should lead to an appropriate alloy phase composition and a high degree of grain refinement. Crucial, from the view point of alloy properties, is the morphology of the ␣2 and ␥ lamellae. 2. Experimental procedure An alloy of two-phase structure, with the nominal chemical composition (at.%) Ti–48Al–2Cr–2Nb, was investigated. The alloy was melted in a vacuum induction furnace, in a crucible made of ZrO2 -stabilized Y2 O3 . The melted alloys were vacuum cast into preheated moulds. The casts had shapes of a bar 10–15 mm in diameter and 60–150 mm in length. The ingots were annealed at 1523 K for 24 h and furnacecooled for homogenization. To determine the structure development during heating the specimens were annealed for 1 h at various temperatures in the range 1353–1603 K and furnace-cooled or water-quenched. For the same alloy, to determine the structural changes during cooling, the specimens were annealed at 1623 K for 1 h, then cooled down to various temperatures in the range 1523–1323 K with 5 K min−1 and finally water-quenched. The microsections for the metallographic research were prepared in a standard way. They were etched in a reagent of
0254-0584/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0254-0584(03)00042-7
J. Chrapo´nski et al. / Materials Chemistry and Physics 81 (2003) 438–442
the following composition: 5 ml HF, 15 ml HNO3 and 30 ml lactic acid. The microstructure observation was carried out by means of a light microscope and a Hitachi S4200 scanning electron microscope equipped with an EDX spectrometer attached with Noran’s Voyager analysis system.
439
3. Results The microstructure of the alloy after casting was of a typical dendritic character. Only after homogenization a two-phase lamellar microstrucutre was observed, composed
Fig. 1. (a) Microstructure of Ti–48Al–2Cr–2Nb alloy after homogenization at 1523 K for 24 h and furnace-cooled. (b and c) X-ray spectra obtained from marked places of image (a).
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J. Chrapo´nski et al. / Materials Chemistry and Physics 81 (2003) 438–442
of regions of alternately arranged ␣2 and ␥ phase lamellae as well as of regions free of a lamellar structure or in a form of thick lamellae (Fig. 1a). Microanalytic study has shown that the regions free of lamellar microstructure or those in the form of thick lamellae have an increased Al content (above 50 at.%) (Fig. 1b), which shows that it is the ␥-TiAl phase. The Al content in the regions consisting of fine lamellae is lower than its average content in the alloy (Fig. 1c). Thus, the predominating component in the lamellar structure is the ␣2 -Ti3 Al phase poor in Al, whereas the lamellar structure, according to the equilibrium system, is composed of a mixture of alternating (␣2 + ␥) lamellae. The microstructure changes after annealing at different temperatures and after cooling at different rates show that an increase of the annealing temperature and a decrease of the cooling rate are result in the formation of classical lamellar microstructures (Fig. 2). The annealing at the lower temper-
atures of the two-phase (␣ + ␥) range leads to a degradation of the initial lamellar microstructure (Fig. 3). A coagulated ␥-TiAl phase also appears in the microstructure, whereas along the grain boundaries processes start which lead to the formation of duplex-type regions (Fig. 2c and d). As the annealing temperature rises, especially after fast cooling, a degradation of the lamellar microstructure is observed, whereas annealing at a temperature close to the ␣ + ␥ → ␣ transition temperature causes only single, not dissolved ␥ phase lamellae (Fig. 2a) to be left in the microstructure. During slow cooling of the alloy from the ␣ phase field, an ␣ → ␣+␥ transition takes place. The transition proceeds via nucleation and growth. For the investigated alloy it begins at about 1473 K through the precipitation of a ␥ phase network along the grain boundaries (Fig. 4a). After completion of the ␥ phase nucleation, a process of growth deep into the grains begins (Fig. 4b). Simultaneously, the process of lamellae
Fig. 2. Microstructure of specimens annealed at 1553 K (a and b) and 1403 K (c and d), then water-quenched (a and c) or furnace-cooled (b and d).
Fig. 3. Examples of lamellar microstructure degradation of the specimen annealed at 1403 K for 1 h and water-quenched.
Fig. 4. Microstructure of specimens annealed at 1623 K for 1 h and then cooled down to 1473 K (a and b), 1423 K (c), 1373 K (d) with 5 K min−1 and finally water-quenched.
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J. Chrapo´nski et al. / Materials Chemistry and Physics 81 (2003) 438–442
Fig. 5. X-ray spectrum of a specimen after homogenization obtained at 25 kV.
nucleation is activated along the boundaries of other lamellar colonies (Fig. 4c). The transition is completed at 1373 K, when the all grains have a lamellar microstructure (Fig. 4d). During the examination of the chemical composition of the phases in standard conditions (at accelerating voltage of 15 kV), a broadening of the niobium L␣ line occurs in the spectrograms (Fig. 1b and c). The reason for this may be an overlapping line of another chemical element. In the case of the investigated alloy it is zirconium, which has turned up in the alloy as a result of a reaction between liquid metal and the metal of the crucible. This is confirmed by an analysis at a higher accelerating voltage (25 kV), which is enough to excite both chemical elements, for distinctly separated K lines to appear (Fig. 5).
4. Conclusions Different microstructure types can be obtained. It is, therefore, purposeful to use the eutectoid reaction for the
modification of the lamellar microstructure in order to obtain optimum properties of the examined alloy.
Acknowledgements This work was supported by Grant No. 7-T08A-003-21 and PBZ/KBN-041/T08/11-02 from the Polish Committee of Sciences. References [1] H. Clemens, H. Kestler, Adv. Eng. Mater. 9 (2000) 551–570. [2] H. Clemens, A. Lorich, N. Eberhardt, W. Glatz, W. Knabl, H. Kestler, Z. Metallkd. 90 (1999) 569–593. [3] N.S. Stoloff, V.K. Sikka (Eds.), Physical Metallurgy and Processing of Intermetallic Compounds, Chapman & Hall, London, 1995. [4] M. Yamaguchi, H. Inui, K. Ito, Acta Mater. 48 (2000) 307–322. [5] W. Szkliniarz, J. Chrapo´nski, T. Mikuszewski, A. Ko´scielna, B. Serek, Nowe materiały i technologie, X Seminarium Naukowe, Katowice, 10 maja 2002, pp. 95–98 (in Polish).
Microstructure and chemical composition of phases in Ti–48Al–2Cr–2Nb intermetallic alloy J. Chrapo´nski∗ , W. Szkliniarz, A. Ko´scielna, B. Serek Department of Materials Science, Silesian University of Technology, Krasi´nskiego 8, Pl-40-019 Katowice, Poland
Abstract The microstructure of the alloy and chemical composition of phases in as-cast and heat-treated conditions were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopy. It has been shown that at high heating and cooling rates the thickness of the ␣2 and ␥ lamellae decreases. Also, under these conditions the surface area of the grain boundaries decreases. Higher annealing temperatures in the two-phase (␥ + ␣) range and low cooling rates led to a lamellar microstructure. Annealing near the eutectoid temperature resulted in degeneration of the initial lamellar microstructure. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Intermetallics; Titanium aluminides; Scanning electron microscopy; Heat treatment; Energy-dispersive X-ray spectroscopy
1. Introduction Titanium aluminide-based intermetallic compounds have a variety of interesting properties such as low density, high specific strength, substantial mechanical strength at high temperatures and good creep resistance. These properties predestinate Ti–Al alloys for high-temperature applications as potential replacements for superalloys. However, they have a limited plasticity at room temperature and the tendency to brittle fracture. An improved plasticity of the two-phase (␥ + ␣2 ) alloys is received by adding alloying elements and by microstructure modification. The basic way of forming the microstructure is heat treatment and thermomechanical working [1–3]. From the view point of the properties and possibilities of their modification by microstructure changes, most interesting are alloys with 44–48 at.% Al. In these alloys, the eutectoid reaction takes place, which is advantegous in thermal treatment processes. After various heat treatments it is possible to obtain four types of microstructure [4,5]: • near ␥: with equiaxed grains and ␣2 particles at the grain boundaries and triple points, • duplex: consisting of ␥ grains and lamellar colonies of alternating layers of ␥ and ␣2 phase, • nearly lamellar: the size of the lamellar (␣2 + ␥) grains is larger than that of the ␥ grains, • fully lamellar: with coarse lamellar (␣2 + ␥) grains. ∗ Corresponding author. Tel./fax: +48-32-256-31-97. E-mail address: [email protected] (J. Chrapo´nski).
Generally, a duplex-type microstructure is characterized by a better plasticity, but it is inferior to a lamellar microstructure with respect to crack resistance, fatigue strength and high-temperature creep resistance. The best properties are characteristic of an alloy with a structure where the ␣2 phase volume amounts to 5–20%. Therefore, a properly designed heat treatment should lead to an appropriate alloy phase composition and a high degree of grain refinement. Crucial, from the view point of alloy properties, is the morphology of the ␣2 and ␥ lamellae. 2. Experimental procedure An alloy of two-phase structure, with the nominal chemical composition (at.%) Ti–48Al–2Cr–2Nb, was investigated. The alloy was melted in a vacuum induction furnace, in a crucible made of ZrO2 -stabilized Y2 O3 . The melted alloys were vacuum cast into preheated moulds. The casts had shapes of a bar 10–15 mm in diameter and 60–150 mm in length. The ingots were annealed at 1523 K for 24 h and furnacecooled for homogenization. To determine the structure development during heating the specimens were annealed for 1 h at various temperatures in the range 1353–1603 K and furnace-cooled or water-quenched. For the same alloy, to determine the structural changes during cooling, the specimens were annealed at 1623 K for 1 h, then cooled down to various temperatures in the range 1523–1323 K with 5 K min−1 and finally water-quenched. The microsections for the metallographic research were prepared in a standard way. They were etched in a reagent of
0254-0584/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0254-0584(03)00042-7
J. Chrapo´nski et al. / Materials Chemistry and Physics 81 (2003) 438–442
the following composition: 5 ml HF, 15 ml HNO3 and 30 ml lactic acid. The microstructure observation was carried out by means of a light microscope and a Hitachi S4200 scanning electron microscope equipped with an EDX spectrometer attached with Noran’s Voyager analysis system.
439
3. Results The microstructure of the alloy after casting was of a typical dendritic character. Only after homogenization a two-phase lamellar microstrucutre was observed, composed
Fig. 1. (a) Microstructure of Ti–48Al–2Cr–2Nb alloy after homogenization at 1523 K for 24 h and furnace-cooled. (b and c) X-ray spectra obtained from marked places of image (a).
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J. Chrapo´nski et al. / Materials Chemistry and Physics 81 (2003) 438–442
of regions of alternately arranged ␣2 and ␥ phase lamellae as well as of regions free of a lamellar structure or in a form of thick lamellae (Fig. 1a). Microanalytic study has shown that the regions free of lamellar microstructure or those in the form of thick lamellae have an increased Al content (above 50 at.%) (Fig. 1b), which shows that it is the ␥-TiAl phase. The Al content in the regions consisting of fine lamellae is lower than its average content in the alloy (Fig. 1c). Thus, the predominating component in the lamellar structure is the ␣2 -Ti3 Al phase poor in Al, whereas the lamellar structure, according to the equilibrium system, is composed of a mixture of alternating (␣2 + ␥) lamellae. The microstructure changes after annealing at different temperatures and after cooling at different rates show that an increase of the annealing temperature and a decrease of the cooling rate are result in the formation of classical lamellar microstructures (Fig. 2). The annealing at the lower temper-
atures of the two-phase (␣ + ␥) range leads to a degradation of the initial lamellar microstructure (Fig. 3). A coagulated ␥-TiAl phase also appears in the microstructure, whereas along the grain boundaries processes start which lead to the formation of duplex-type regions (Fig. 2c and d). As the annealing temperature rises, especially after fast cooling, a degradation of the lamellar microstructure is observed, whereas annealing at a temperature close to the ␣ + ␥ → ␣ transition temperature causes only single, not dissolved ␥ phase lamellae (Fig. 2a) to be left in the microstructure. During slow cooling of the alloy from the ␣ phase field, an ␣ → ␣+␥ transition takes place. The transition proceeds via nucleation and growth. For the investigated alloy it begins at about 1473 K through the precipitation of a ␥ phase network along the grain boundaries (Fig. 4a). After completion of the ␥ phase nucleation, a process of growth deep into the grains begins (Fig. 4b). Simultaneously, the process of lamellae
Fig. 2. Microstructure of specimens annealed at 1553 K (a and b) and 1403 K (c and d), then water-quenched (a and c) or furnace-cooled (b and d).
Fig. 3. Examples of lamellar microstructure degradation of the specimen annealed at 1403 K for 1 h and water-quenched.
Fig. 4. Microstructure of specimens annealed at 1623 K for 1 h and then cooled down to 1473 K (a and b), 1423 K (c), 1373 K (d) with 5 K min−1 and finally water-quenched.
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J. Chrapo´nski et al. / Materials Chemistry and Physics 81 (2003) 438–442
Fig. 5. X-ray spectrum of a specimen after homogenization obtained at 25 kV.
nucleation is activated along the boundaries of other lamellar colonies (Fig. 4c). The transition is completed at 1373 K, when the all grains have a lamellar microstructure (Fig. 4d). During the examination of the chemical composition of the phases in standard conditions (at accelerating voltage of 15 kV), a broadening of the niobium L␣ line occurs in the spectrograms (Fig. 1b and c). The reason for this may be an overlapping line of another chemical element. In the case of the investigated alloy it is zirconium, which has turned up in the alloy as a result of a reaction between liquid metal and the metal of the crucible. This is confirmed by an analysis at a higher accelerating voltage (25 kV), which is enough to excite both chemical elements, for distinctly separated K lines to appear (Fig. 5).
4. Conclusions Different microstructure types can be obtained. It is, therefore, purposeful to use the eutectoid reaction for the
modification of the lamellar microstructure in order to obtain optimum properties of the examined alloy.
Acknowledgements This work was supported by Grant No. 7-T08A-003-21 and PBZ/KBN-041/T08/11-02 from the Polish Committee of Sciences. References [1] H. Clemens, H. Kestler, Adv. Eng. Mater. 9 (2000) 551–570. [2] H. Clemens, A. Lorich, N. Eberhardt, W. Glatz, W. Knabl, H. Kestler, Z. Metallkd. 90 (1999) 569–593. [3] N.S. Stoloff, V.K. Sikka (Eds.), Physical Metallurgy and Processing of Intermetallic Compounds, Chapman & Hall, London, 1995. [4] M. Yamaguchi, H. Inui, K. Ito, Acta Mater. 48 (2000) 307–322. [5] W. Szkliniarz, J. Chrapo´nski, T. Mikuszewski, A. Ko´scielna, B. Serek, Nowe materiały i technologie, X Seminarium Naukowe, Katowice, 10 maja 2002, pp. 95–98 (in Polish).