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TEM observations of grain refinement of laser surface melted γ-TiAl based alloy
Materials Letters 57 (2003) 3810 – 3814 www.elsevier.com/locate/matlet
TEM observations of grain refinement of laser surface melted g-TiAl based alloy G.Q. Wu a,b,*, Z. Huang a, Z.F. Li a, Z.J. Ruan a a
School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, 37 Xueyuan Road, Haidian District, Beijing 100083, PR China b State Key Laboratory of Laser Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China Received 5 December 2002; received in revised form 23 February 2003; accepted 26 February 2003
Abstract Laser surface remelting followed by annealing treatment was carried out on g-TiAl based alloy, and the phenomenon of grain size refinement was observed by transmission electron (TEM). The results show that after laser surface remelting and subsequent annealing treatment, a series of changes occur in the microstructure of g-TiAl based alloy: L (liquid phase) ! a2 (dendritic structure) + g ! g (equiaxed grain) + a2 (small a2 particles). Finally, an equiaxed, ultrafine, uniform and stable microstructure is formed. D 2003 Elsevier Science B.V. All rights reserved. Keywords: TiAl based alloy; Microstructure; Laser surface treatment; Heat treatment; TEM
1. Introduction Because of superior strength-to-weight ratio, good oxidation and burn resistance, high modulus of elasticity and high-temperature strength retention, the gTiAl based alloys are of growing interest in the aerospace, automotive and power generation industries for high performance applications [1,2]. Successful joining of these materials will increase their utility in engineering application. Imayev et al. [3] reported that grain size has a great influence on superplasticity of intermetallic structure, for example, as decreasing the * Corresponding author. School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, 37 Xueyuan Road, Haidian District, Beijing 100083, PR China. Tel.: +86-10-82313240; fax: +86-10-82328153. E-mail address: [email protected] (G.Q. Wu).
grain size from 5 to 0.4 Am, the relative elongation maximum for TiAl is shifted to low temperature from 1025 to 850 jC. So, superplastic forming/diffusion bonding (SPF/DB) of fine-grained TiAl materials became one of the focuses of research [4– 7]. Generally, it is difficult to obtain ultrafine microstructure by using thermal mechanical processing, power metallurgy and so on. Furthermore, the grain size refinement of the whole material, obtained by above-mentioned conventional ways, would lead to low creep resistance at high temperatures. Wu and Huang [7] and Wu et al. [8] reported a new method to join TiAl alloys by superplastic bonding without sacrificing the high-temperature properties of the whole materials. With significant microstructural refinement achieved by laser treatment of the surface and subsequent heat treatment, SPF/DB at a lower temperature and in a shorter time than conventional
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00183-6
G.Q. Wu et al. / Materials Letters 57 (2003) 3810–3814
3811
SPF/DB was achieved. Fully understanding of the microstructure refinement of laser surface remelted TiAl alloy will be important to verify SPF/DB. As well known, ultrafine microstructure can be produced by laser surface remelting, which is followed by rapid self-quenching of the melted layer. The metastable microstructure can be transformed by subsequent heat treatment [9]. In this paper, the difference between conventional heat treatment and laser surface treatment was studied, and the mechanisms of grain refinement and phase transformation were discussed by TEM observation.
2. Experimental procedure A Ti-46.5Al-2Cr-1.5Nb-1V (Ti-46.5Al) titanium aluminide was used to study the influence of laser surface remelting and annealing treatment on grain size refinement. Fully lamellar microstructure was
Fig. 2. Microstructure of laser surface remelted TiAl alloy; (a) TEM photograph and (b) the diffraction pattern in dendritic structure.
Fig. 1. Full lamellar microstructure of g-based TiAl alloy: (a) Optical photograph, (b) transmission electron micrograph.
obtained by heating at 1350 jC for 10 min and cooling in the furnace. An HGL-84 CW CO2 laser equipped with a three-axis computer numerical controlled worktable was used to remelt the TiAl alloy in as machined metal surface at room temperature. The processing parameters used were laser power 1.1 – 1.2 kW, scanning rate 10 mm/s and facular diameter of 3 mm, respectively. During remelting, a shielding gas of argon was used to prevent oxidation. After laser surface remelting, the specimens were annealed at 1000 jC for 1 h, at 1100 jC for 1 h and at 1000 jC for 4 h, respectively. Metallographic samples were prepared using standard techniques and etched by Kroll’s reagent (7 vol.% HF; 21 vol.% HNO3; 72 vol.% H2O). Microstructures of these samples were characterized by optical (OM) and transmission electron (TEM) microscopy examination. Slices were cut from laser-melted zone for TEM examination by a spark machine. They were ground down to 50 Am in thickness, and then were ion milled at
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G.Q. Wu et al. / Materials Letters 57 (2003) 3810–3814
liquid nitrogen temperature using argon ion beam at gun voltage of 6 kV and gun current of 0.4 mA.
3. Results and discussion After laser surface remelting, an area, which is absolutely different from the original structure (Fig.
Fig. 4. Ti – Al phase diagram.
Fig. 3. Microstructure of laser surface remelted TiAl alloy; (a), (b) TEM photograph and (c) the diffraction pattern in grain boundary.
1), is developed on the surface of the Ti-46.5 Al sample [6]. The microstructure consists mainly of dendritic structure (Fig. 2a). The diffraction pattern (Fig. 2b) indicates that there is single a2 phase inside the dendritic structure, while there is a small quality of anomalistic, fine g/a2 plates (Fig. 3a) or granular g phase (Fig. 3b) at the grain boundary. Fig. 3c presents the diffraction pattern corresponding to this kind of structure. With the aid of the Ti – Al binary phase diagram in Fig. 4, we can see that solid a phase first forms during the cooling of the liquid phase of TiAl alloy. Because the heating and cooling speeds are extremely rapid introduced by laser surface remelting, the reaction of a ! a2 + g is restrained though the ordering reaction of a ! a2 is carried. Consequently, oversaturated metastable a2 phase, which would be obtained by quenching treatment, is formed. During this process, the nucleuses of grains form, but have no time to grow. Therefore, fine dendritic structure is formed with a dendritic spacing of 2 – 10 Am. In addition, because the temperature gradient from the sample surface to inside is very confined to a very short distance and a very short time, the growth of dendritic structure has some distinct direction and distance. Annealing treatment of laser surface remelted TiAl alloy at 1000 jC for 1 h shows that the dendritic structure in laser-melted zone, varying at certain heating temperature, is transformed and refined into fine and equiaxial g grain structure with a grain size of
G.Q. Wu et al. / Materials Letters 57 (2003) 3810–3814
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Fig. 5. Microstructures of the laser surface remelted TiAl alloy after different annealing treatment; (a) and (b) 1000 jC/1 h, (c) 1000 jC/4 h, (d) 1100 jC/1 h.
1 Am (Fig. 5) via the reaction a2 ! equiaxial grained structure (g + a2), in contrast to conventional transformation via the reaction a2 ! lamellar structure (g + a2). Also, granular a2 phase is separated out from the g grains (Fig. 5b). Heat treatment of laser surface remelted TiAl alloy at 1000 jC for 4 h (Fig. 5c) or at 1100 jC for 1 h (Fig. 5d) shows that the refined equiaxial grain structure is stable and does not change by extending the heating time or by increasing the heating temperature under eutectoid temperature. This kind of structure possesses equiaxed, fine, uniform and stable microstructure, which offers a good structural basis to superplastic/forming diffusion bonding at a low temperature in a short time [7,10,11].
subsequent annealing treatment. The main reason is that the structure formed by laser surface remelting is dendritic a2 phase structure. After the subsequent annealing treatment, the reaction a2 ! equiaxial grained structure (g + a2) occurs in the structure formed by laser surface remelting. After laser surface remelting and subsequent annealing treatment, an equiaxed, fine, uniform and stable microstructure with a grain size of 1 Am is formed, and it offers a good structural basis to superplastic/forming diffusion bonding at a low temperature in a short time.
Acknowledgements 4. Summary Fine-grained structure of g-TiAl based alloy can be readily obtained by laser surface remelting and
The authors would like to acknowledge the National Natural Science Foundation (No. 59971004) and Aeronautical Fundamental Science Foundation (No. 01H51004) of China for financial support.
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G.Q. Wu et al. / Materials Letters 57 (2003) 3810–3814
References [1] Y.-W. Kim, D.M. Dimiduk, JOM 8 (1991) 40. [2] S. Deevi, Adv. Mater. Process. 9 (1999) 44. [3] R.M. Imayev, G.A. Salishchev, et al., Mat. Sci. Forum 170 – 172 (1994) 453. [4] G. Cam, JOM 11 (1996) 66. [5] G. Cam, H. Clemens, R. Gerling, M. Kocak, Intermetallics 7 (1999) 1025. [6] Y.h. He, B.Y. Huang, Acta Metall. Sin. 34 (11) (1998) 1162.
[7] G.Q. Wu, Z. Huang, Scr. Mater. 45 (8) (2001) 895. [8] G.Q. Wu, Z. Huang, J.G. Lin, Mater. Lett. 56 (4) (2002) 606. [9] A. Tiziant, L. Giorddano, E. Ramous, Lasers in Materials Processing, American Society for Metals, Metals Park, OH, USA, 1983, p. 108. [10] R.M. Imayev, O.A. Kaybyshev, G.A. Salishchev, Phys. Met. Metallorg. 70 (3) (1990) 179. [11] W.B. Lee, H.S. Yang, A.K. Mukherjee, Mater. Sci. Eng., A 192 – 193 (1995) 733.
TEM observations of grain refinement of laser surface melted g-TiAl based alloy G.Q. Wu a,b,*, Z. Huang a, Z.F. Li a, Z.J. Ruan a a
School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, 37 Xueyuan Road, Haidian District, Beijing 100083, PR China b State Key Laboratory of Laser Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China Received 5 December 2002; received in revised form 23 February 2003; accepted 26 February 2003
Abstract Laser surface remelting followed by annealing treatment was carried out on g-TiAl based alloy, and the phenomenon of grain size refinement was observed by transmission electron (TEM). The results show that after laser surface remelting and subsequent annealing treatment, a series of changes occur in the microstructure of g-TiAl based alloy: L (liquid phase) ! a2 (dendritic structure) + g ! g (equiaxed grain) + a2 (small a2 particles). Finally, an equiaxed, ultrafine, uniform and stable microstructure is formed. D 2003 Elsevier Science B.V. All rights reserved. Keywords: TiAl based alloy; Microstructure; Laser surface treatment; Heat treatment; TEM
1. Introduction Because of superior strength-to-weight ratio, good oxidation and burn resistance, high modulus of elasticity and high-temperature strength retention, the gTiAl based alloys are of growing interest in the aerospace, automotive and power generation industries for high performance applications [1,2]. Successful joining of these materials will increase their utility in engineering application. Imayev et al. [3] reported that grain size has a great influence on superplasticity of intermetallic structure, for example, as decreasing the * Corresponding author. School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, 37 Xueyuan Road, Haidian District, Beijing 100083, PR China. Tel.: +86-10-82313240; fax: +86-10-82328153. E-mail address: [email protected] (G.Q. Wu).
grain size from 5 to 0.4 Am, the relative elongation maximum for TiAl is shifted to low temperature from 1025 to 850 jC. So, superplastic forming/diffusion bonding (SPF/DB) of fine-grained TiAl materials became one of the focuses of research [4– 7]. Generally, it is difficult to obtain ultrafine microstructure by using thermal mechanical processing, power metallurgy and so on. Furthermore, the grain size refinement of the whole material, obtained by above-mentioned conventional ways, would lead to low creep resistance at high temperatures. Wu and Huang [7] and Wu et al. [8] reported a new method to join TiAl alloys by superplastic bonding without sacrificing the high-temperature properties of the whole materials. With significant microstructural refinement achieved by laser treatment of the surface and subsequent heat treatment, SPF/DB at a lower temperature and in a shorter time than conventional
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00183-6
G.Q. Wu et al. / Materials Letters 57 (2003) 3810–3814
3811
SPF/DB was achieved. Fully understanding of the microstructure refinement of laser surface remelted TiAl alloy will be important to verify SPF/DB. As well known, ultrafine microstructure can be produced by laser surface remelting, which is followed by rapid self-quenching of the melted layer. The metastable microstructure can be transformed by subsequent heat treatment [9]. In this paper, the difference between conventional heat treatment and laser surface treatment was studied, and the mechanisms of grain refinement and phase transformation were discussed by TEM observation.
2. Experimental procedure A Ti-46.5Al-2Cr-1.5Nb-1V (Ti-46.5Al) titanium aluminide was used to study the influence of laser surface remelting and annealing treatment on grain size refinement. Fully lamellar microstructure was
Fig. 2. Microstructure of laser surface remelted TiAl alloy; (a) TEM photograph and (b) the diffraction pattern in dendritic structure.
Fig. 1. Full lamellar microstructure of g-based TiAl alloy: (a) Optical photograph, (b) transmission electron micrograph.
obtained by heating at 1350 jC for 10 min and cooling in the furnace. An HGL-84 CW CO2 laser equipped with a three-axis computer numerical controlled worktable was used to remelt the TiAl alloy in as machined metal surface at room temperature. The processing parameters used were laser power 1.1 – 1.2 kW, scanning rate 10 mm/s and facular diameter of 3 mm, respectively. During remelting, a shielding gas of argon was used to prevent oxidation. After laser surface remelting, the specimens were annealed at 1000 jC for 1 h, at 1100 jC for 1 h and at 1000 jC for 4 h, respectively. Metallographic samples were prepared using standard techniques and etched by Kroll’s reagent (7 vol.% HF; 21 vol.% HNO3; 72 vol.% H2O). Microstructures of these samples were characterized by optical (OM) and transmission electron (TEM) microscopy examination. Slices were cut from laser-melted zone for TEM examination by a spark machine. They were ground down to 50 Am in thickness, and then were ion milled at
3812
G.Q. Wu et al. / Materials Letters 57 (2003) 3810–3814
liquid nitrogen temperature using argon ion beam at gun voltage of 6 kV and gun current of 0.4 mA.
3. Results and discussion After laser surface remelting, an area, which is absolutely different from the original structure (Fig.
Fig. 4. Ti – Al phase diagram.
Fig. 3. Microstructure of laser surface remelted TiAl alloy; (a), (b) TEM photograph and (c) the diffraction pattern in grain boundary.
1), is developed on the surface of the Ti-46.5 Al sample [6]. The microstructure consists mainly of dendritic structure (Fig. 2a). The diffraction pattern (Fig. 2b) indicates that there is single a2 phase inside the dendritic structure, while there is a small quality of anomalistic, fine g/a2 plates (Fig. 3a) or granular g phase (Fig. 3b) at the grain boundary. Fig. 3c presents the diffraction pattern corresponding to this kind of structure. With the aid of the Ti – Al binary phase diagram in Fig. 4, we can see that solid a phase first forms during the cooling of the liquid phase of TiAl alloy. Because the heating and cooling speeds are extremely rapid introduced by laser surface remelting, the reaction of a ! a2 + g is restrained though the ordering reaction of a ! a2 is carried. Consequently, oversaturated metastable a2 phase, which would be obtained by quenching treatment, is formed. During this process, the nucleuses of grains form, but have no time to grow. Therefore, fine dendritic structure is formed with a dendritic spacing of 2 – 10 Am. In addition, because the temperature gradient from the sample surface to inside is very confined to a very short distance and a very short time, the growth of dendritic structure has some distinct direction and distance. Annealing treatment of laser surface remelted TiAl alloy at 1000 jC for 1 h shows that the dendritic structure in laser-melted zone, varying at certain heating temperature, is transformed and refined into fine and equiaxial g grain structure with a grain size of
G.Q. Wu et al. / Materials Letters 57 (2003) 3810–3814
3813
Fig. 5. Microstructures of the laser surface remelted TiAl alloy after different annealing treatment; (a) and (b) 1000 jC/1 h, (c) 1000 jC/4 h, (d) 1100 jC/1 h.
1 Am (Fig. 5) via the reaction a2 ! equiaxial grained structure (g + a2), in contrast to conventional transformation via the reaction a2 ! lamellar structure (g + a2). Also, granular a2 phase is separated out from the g grains (Fig. 5b). Heat treatment of laser surface remelted TiAl alloy at 1000 jC for 4 h (Fig. 5c) or at 1100 jC for 1 h (Fig. 5d) shows that the refined equiaxial grain structure is stable and does not change by extending the heating time or by increasing the heating temperature under eutectoid temperature. This kind of structure possesses equiaxed, fine, uniform and stable microstructure, which offers a good structural basis to superplastic/forming diffusion bonding at a low temperature in a short time [7,10,11].
subsequent annealing treatment. The main reason is that the structure formed by laser surface remelting is dendritic a2 phase structure. After the subsequent annealing treatment, the reaction a2 ! equiaxial grained structure (g + a2) occurs in the structure formed by laser surface remelting. After laser surface remelting and subsequent annealing treatment, an equiaxed, fine, uniform and stable microstructure with a grain size of 1 Am is formed, and it offers a good structural basis to superplastic/forming diffusion bonding at a low temperature in a short time.
Acknowledgements 4. Summary Fine-grained structure of g-TiAl based alloy can be readily obtained by laser surface remelting and
The authors would like to acknowledge the National Natural Science Foundation (No. 59971004) and Aeronautical Fundamental Science Foundation (No. 01H51004) of China for financial support.
3814
G.Q. Wu et al. / Materials Letters 57 (2003) 3810–3814
References [1] Y.-W. Kim, D.M. Dimiduk, JOM 8 (1991) 40. [2] S. Deevi, Adv. Mater. Process. 9 (1999) 44. [3] R.M. Imayev, G.A. Salishchev, et al., Mat. Sci. Forum 170 – 172 (1994) 453. [4] G. Cam, JOM 11 (1996) 66. [5] G. Cam, H. Clemens, R. Gerling, M. Kocak, Intermetallics 7 (1999) 1025. [6] Y.h. He, B.Y. Huang, Acta Metall. Sin. 34 (11) (1998) 1162.
[7] G.Q. Wu, Z. Huang, Scr. Mater. 45 (8) (2001) 895. [8] G.Q. Wu, Z. Huang, J.G. Lin, Mater. Lett. 56 (4) (2002) 606. [9] A. Tiziant, L. Giorddano, E. Ramous, Lasers in Materials Processing, American Society for Metals, Metals Park, OH, USA, 1983, p. 108. [10] R.M. Imayev, O.A. Kaybyshev, G.A. Salishchev, Phys. Met. Metallorg. 70 (3) (1990) 179. [11] W.B. Lee, H.S. Yang, A.K. Mukherjee, Mater. Sci. Eng., A 192 – 193 (1995) 733.