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Rapidly solidified MC carbide morphologies of a pulsed laser surface alloyed γ-TiAl intermetallic with carbon
Scripta Materialia 50 (2004) 507–510 www.actamat-journals.com
Rapidly solidified MC carbide morphologies of a pulsed laser surface alloyed c-TiAl intermetallic with carbon Y. Chen, H.M. Wang
*
Laboratory of Laser Materials Processing and Surface Engineering, School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, 37 Xue Yuan Road, Beijing 100083, PR China Received 18 November 2002; received in revised form 20 October 2003; accepted 22 October 2003
Abstract Rapidly solidified TiC type MC carbide morphologies were studied as a function of cooling rate using a Nd–YAG pulsed laser surface alloying with carbon. At a low cooling rate, the carbide morphology was well-developed dendrite. As the solidification cooling rate increased, the growth morphology of MC carbide changed to cross-petal-like with symmetrical arms and irregular block or undeveloped dendrite. 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: MC carbide; Growth morphology; Laser surface alloying
1. Introduction c-TiAl intermetallic alloys are considered as one of the promising candidate high-temperature materials due to their high specific strength and elevated temperature creep resistance. However, the tribological properties need to be improved when applied as tribological components. Therefore, improving the wear resistance of c-TiAl intermetallic alloy has attracted increasing attention in recent years [1–4]. As an interstitial phase with high hardness, excellent high-temperature stability and low density, MC carbides such as TiC and VC, etc. are anticipated to be an important strengthening phase in c-TiAl intermetallic matrix composites. The growth morphology and the size of strengthening MC carbide phases have a strong effect on the their mechanical properties. Thus, it is important to study the carbide growth morphologies in c-TiAl intermetallic matrix composites. As to the growth morphology of MC carbide, considerable investigation has been carried out in nickel-base superalloys [5–9,12]. It is well known that the near-equilibrium growth morphologies of TiC type MC carbide in nickel-base superalloys were octahedral
*
Corresponding author. Tel.: +86-10-82317102; fax: +86-1082328041/82328549. E-mail address: [email protected] (H.M. Wang).
blocks and these would transform gradually to Chinesescript morphology with increasing solidification cooling rate [5,7,9]. Under quasi-rapid solidification conditions with a cooling rate on the order of 102 K/s, TiC type MC carbide was found to have a faceted dendritic morphology having zig–zag platelet or three-dimensional faceted-platelet networks on the growing surface of dendritic arms [10,11]. Under rapid solidification conditions with a cooling rate ranging from approximately 9.0 · 103 to 1.4 · 105 K/s, carbide growth morphologies in laser-glazed single-crystal nickel-base superalloys were in flower-like and radially branched colonies, and the growth mechanism was confirmed to be lateral spreading growth mode [8,12]. In this paper, to eliminate the influence of melt inheritance on the growth morphology of MC carbide and to flexibly adjust the solidification cooling rate, laser surface in situ alloying with carbon was utilized. The carbide growth morphologies in the laser surface alloyed composite coatings were investigated.
2. Experimental procedures Carbon powders with an average particle size of 45 lm were preplaced on the surface of a cast c-titanium aluminide alloy, Ti–48Al–2Cr–2Nb, having a fully lamellar microstructure, in a thickness of approximate
1359-6462/$ - see front matter 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2003.10.028
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Y. Chen, H.M. Wang / Scripta Materialia 50 (2004) 507–510
0.1 mm. During laser surface melting, the coated carbon powders dissolved into the laser induced surface melt bath, leading to the in situ alloying of the surface with carbon. Subsequently, the carbon in situ alloyed melt bath solidified after the movement of the laser beam. The single-spot laser surface alloying parameters were: laser output power 17 J, beam diameter 1.0 mm, pulse time 1.5–4.0 ms. Average cooling rate of the laser surface alloyed composite coatings was estimated numerically using the relationship of the cooling rate and the secondary dendrite arm spacing [13]. Metallurgical cross-sections of the laser surface alloyed coatings were prepared using standard mechanical polishing procedures. The samples were deep etched in a solution of HF, HNO3 and H2 O in volume of 1:6:7 in order to reveal the stero-growth morphologies of TiC using a KYKY-2800 scanning electron microscope (SEM). The element present in the laser surface alloyed coating were identified by X-ray diffraction (XRD) using a Rigaku D/max 2200 type with CuKa radiation operated at a voltage of 40 kV, a current of 40 mA and a scanning rate of 5/min.
3. Results and discussion Figs. 1–3 show the growth morphologies of TiC type MC carbide in pulsed laser surface alloyed coatings solidified at different cooling rates. It is clear that the size of TiC particles decreases with increasing cooling rate. Furthermore, the carbide morphology in the alloyed
coating is directly dependent upon the solidification cooling rate. TiC solidified at a cooling rate of approximately 7.1 · 105 K/s has a well-developed dendrite morphology (Fig. 1b and c). As solidification cooling rate increases, the growth morphology of TiC carbide solidified at 8.3 · 105 K/s is found to be cross-petal-like, and these dendrite arms grow symmetrically (Fig. 2b and c). The carbide growth morphologies are different from those presented in nickel-base superalloys [8,12], though the orders of solidification cooling rate are the same. As illustrated in Refs. [6,12], MC carbide in nickel-base superalloy is an eutectic carbide. Thus the growth kinetics of the eutectic reaction and intrinsic growth characteristics of MC carbide have a decisive effect on the morphological selection of MC carbide. In the present study, TiC type MC carbide preferably precipitates from the laser induced melt bath due to the high melting point (3200 C) and large negative free energy of formation. Therefore, TiC is the primary phase in laser surface in situ alloying coating. The growth environment significantly affects the carbide growth morphology. As solidification cooling rate increases to 1.5 · 106 K/s, the carbide morphology is irregular blocky or undeveloped dendrite (Fig. 3b and c). Moreover, it is evident from Fig. 3b that the TiC distribution was inhomogeneous. We think that the pulse time is as short as 1.5 ms, leading to very high temperature-gradient and very drastic convection in the laser-generated melt pool. Therefore, the solute distribution in the melt pool is very inhomogeneous, and microscale inhomogeneous precipitation of TiC occurs.
Fig. 1. SEM micrographs showing the macrograph of laser surface alloyed coating (a) and growth morphology of MC carbide (b), (c) solidified with a cooling rate of approximately 7.1 · 105 K/s.
Y. Chen, H.M. Wang / Scripta Materialia 50 (2004) 507–510
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Fig. 2. SEM micrographs showing macrograph of laser surface alloyed coating (a) and the growth morphology of MC carbide (b), (c) solidified with a cooling rate of approximately 8.3 · 105 K/s.
Fig. 3. SEM micrographs showing the macrograph of laser surface alloyed coating (a) and growth morphology of MC carbide (b), (c) solidified with a cooling rate of approximately 1.5 · 106 K/s.
Additionally, comparing Fig. 1a with Figs. 2a and 3a, it is clearly seen that the carbide mainly precipitates at the
top of the laser surface alloyed coating. It is reported that the titanium often segregates at the final solidification
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Y. Chen, H.M. Wang / Scripta Materialia 50 (2004) 507–510
zone because it is the positive segregation element [14]. Moreover, the S/L interface advances from the bottom of melt pool to the free surface during the rapid solidification. As a result, the concentration of titanium in the residual melt increases gradually with advancement of the S/L interface, leading to the distribution of carbide at the top of the coatings. The different growth morphology of TiC in this study may be attributed to the influence of cooling rate on the growth interface structure. As an interstitial phase with high melting entropy and large Jackson’s factor of approximately 5–7 [15], TiC type MC carbide is considered as a typical faceted crystal, and it grows by lateral growth mode under equilibrium solidification conditions. The growth velocity of a faceted crystal is mainly dependent upon the difference between the velocity of atom attachment on the growing interface and that of atom escaping from growing interface [15]. Moreover, velocity of atom attachment on the growing interface is strongly affected by the atomic diffusion in the melt, and the velocity of atom escaping from growing interface is strongly dependent upon the atomic structure of the growing interface. In general, the high crystallographic index planes are atomically rough and have many growth steps, and the low crystallographic index planes are atomically smooth. As a result, different crystallographic planes present anisotropy of growth velocity under equilibrium solidification conditions. As solidification cooling rate or undercooling increases, the original atomically smooth growth interface would change gradually to atomically rough [16]. In the current case, as the solidification cooling rate increased further, it is thought that the original atomically smooth interface of TiC solidified at 1.5 · 106 K/s transform to atomically rough to some extent. As a result, the anisotropy in growth velocity between different crystallographic planes decreases, i.e., the difference in growth velocity between high crystallographic index planes and low crystallographic index planes of TiC is faint, leading to fine irregular block or undeveloped dendrite.
4. Conclusions In the present study, TiC reinforced composite coatings were fabricated by Nd–YAG pulsed laser surface alloying on the substrate of c-TiAl intermetallic alloy. MC carbide morphology was well-developed dendrite at low cooling rate. As cooling rate increased, carbide morphology changed to cross-petal with symmetrical arms and irregular block or undeveloped dendrite.
Acknowledgements The research was supported by the National Natural Science Foundation of China (grant no. 59971003). References [1] Wang Y, Qian Z, Li XY, Tandon KN. Surf Coat Technol 1997;91:37–42. [2] Noda T, Okabe M, Isobe S. Mater Sci Eng A 1996;213:157–61. [3] Yu LD, Thongtem S, Vilaithong T, McNallan MJ. Surf Coat Technol 2000;128:410–7. [4] Wang HM. Acta Metall Sinica 1997;33:917–20. [5] Fernandez R, Lecomte JC, Kattamis TZ. Metal Trans A 1978;9:1381–6. [6] Sun WR, Lee JH, Seo SM, Choe SJ, Hu ZQ. Mater Sci Eng A 1999;271:143–9. [7] Liu L, Fu HZ, Shi XZ. Acta Areonaut Astronaut Sinica 1986;7:181–5. [8] Wang HM. Acta Metall Sinica 1997;33:917–20. [9] Liu L, Sommer F, Fu HZ. Scrip Metall Mater 1994;30:584–91. [10] Chen Y, Wang HM. J Alloy Comp 2003;351:304–8. [11] Wang HM, Yu LG, Li XX, Jiang P. Sci Technol Adv Mater 2001;2:173–6. [12] Wang HM, Zhang JH, Tang YJ, Hu ZQ, et al. Mater Sci Eng A 1992;156:109–16. [13] Strutt PR, Lewis BG, Kear HH. In: Kear BH, Giessen BC, Cohen M, editors. Rapidly solidification amorphous and crystalline alloys. New York: Elsevier; 1980. p. 485. [14] Sun LL, Dong LK, Zhang JH, Hu ZQ. Acta Metall Sinica A 1993;29:388–91. [15] Fu HZ, Liu L, Fang XH. J North Polytech Univer 1987;5:279–87. [16] Kurz W. Fundamental of solidification. USA: Trans Tech Publications; 1984. p. 34.
Rapidly solidified MC carbide morphologies of a pulsed laser surface alloyed c-TiAl intermetallic with carbon Y. Chen, H.M. Wang
*
Laboratory of Laser Materials Processing and Surface Engineering, School of Material Science and Engineering, Beijing University of Aeronautics and Astronautics, 37 Xue Yuan Road, Beijing 100083, PR China Received 18 November 2002; received in revised form 20 October 2003; accepted 22 October 2003
Abstract Rapidly solidified TiC type MC carbide morphologies were studied as a function of cooling rate using a Nd–YAG pulsed laser surface alloying with carbon. At a low cooling rate, the carbide morphology was well-developed dendrite. As the solidification cooling rate increased, the growth morphology of MC carbide changed to cross-petal-like with symmetrical arms and irregular block or undeveloped dendrite. 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: MC carbide; Growth morphology; Laser surface alloying
1. Introduction c-TiAl intermetallic alloys are considered as one of the promising candidate high-temperature materials due to their high specific strength and elevated temperature creep resistance. However, the tribological properties need to be improved when applied as tribological components. Therefore, improving the wear resistance of c-TiAl intermetallic alloy has attracted increasing attention in recent years [1–4]. As an interstitial phase with high hardness, excellent high-temperature stability and low density, MC carbides such as TiC and VC, etc. are anticipated to be an important strengthening phase in c-TiAl intermetallic matrix composites. The growth morphology and the size of strengthening MC carbide phases have a strong effect on the their mechanical properties. Thus, it is important to study the carbide growth morphologies in c-TiAl intermetallic matrix composites. As to the growth morphology of MC carbide, considerable investigation has been carried out in nickel-base superalloys [5–9,12]. It is well known that the near-equilibrium growth morphologies of TiC type MC carbide in nickel-base superalloys were octahedral
*
Corresponding author. Tel.: +86-10-82317102; fax: +86-1082328041/82328549. E-mail address: [email protected] (H.M. Wang).
blocks and these would transform gradually to Chinesescript morphology with increasing solidification cooling rate [5,7,9]. Under quasi-rapid solidification conditions with a cooling rate on the order of 102 K/s, TiC type MC carbide was found to have a faceted dendritic morphology having zig–zag platelet or three-dimensional faceted-platelet networks on the growing surface of dendritic arms [10,11]. Under rapid solidification conditions with a cooling rate ranging from approximately 9.0 · 103 to 1.4 · 105 K/s, carbide growth morphologies in laser-glazed single-crystal nickel-base superalloys were in flower-like and radially branched colonies, and the growth mechanism was confirmed to be lateral spreading growth mode [8,12]. In this paper, to eliminate the influence of melt inheritance on the growth morphology of MC carbide and to flexibly adjust the solidification cooling rate, laser surface in situ alloying with carbon was utilized. The carbide growth morphologies in the laser surface alloyed composite coatings were investigated.
2. Experimental procedures Carbon powders with an average particle size of 45 lm were preplaced on the surface of a cast c-titanium aluminide alloy, Ti–48Al–2Cr–2Nb, having a fully lamellar microstructure, in a thickness of approximate
1359-6462/$ - see front matter 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2003.10.028
508
Y. Chen, H.M. Wang / Scripta Materialia 50 (2004) 507–510
0.1 mm. During laser surface melting, the coated carbon powders dissolved into the laser induced surface melt bath, leading to the in situ alloying of the surface with carbon. Subsequently, the carbon in situ alloyed melt bath solidified after the movement of the laser beam. The single-spot laser surface alloying parameters were: laser output power 17 J, beam diameter 1.0 mm, pulse time 1.5–4.0 ms. Average cooling rate of the laser surface alloyed composite coatings was estimated numerically using the relationship of the cooling rate and the secondary dendrite arm spacing [13]. Metallurgical cross-sections of the laser surface alloyed coatings were prepared using standard mechanical polishing procedures. The samples were deep etched in a solution of HF, HNO3 and H2 O in volume of 1:6:7 in order to reveal the stero-growth morphologies of TiC using a KYKY-2800 scanning electron microscope (SEM). The element present in the laser surface alloyed coating were identified by X-ray diffraction (XRD) using a Rigaku D/max 2200 type with CuKa radiation operated at a voltage of 40 kV, a current of 40 mA and a scanning rate of 5/min.
3. Results and discussion Figs. 1–3 show the growth morphologies of TiC type MC carbide in pulsed laser surface alloyed coatings solidified at different cooling rates. It is clear that the size of TiC particles decreases with increasing cooling rate. Furthermore, the carbide morphology in the alloyed
coating is directly dependent upon the solidification cooling rate. TiC solidified at a cooling rate of approximately 7.1 · 105 K/s has a well-developed dendrite morphology (Fig. 1b and c). As solidification cooling rate increases, the growth morphology of TiC carbide solidified at 8.3 · 105 K/s is found to be cross-petal-like, and these dendrite arms grow symmetrically (Fig. 2b and c). The carbide growth morphologies are different from those presented in nickel-base superalloys [8,12], though the orders of solidification cooling rate are the same. As illustrated in Refs. [6,12], MC carbide in nickel-base superalloy is an eutectic carbide. Thus the growth kinetics of the eutectic reaction and intrinsic growth characteristics of MC carbide have a decisive effect on the morphological selection of MC carbide. In the present study, TiC type MC carbide preferably precipitates from the laser induced melt bath due to the high melting point (3200 C) and large negative free energy of formation. Therefore, TiC is the primary phase in laser surface in situ alloying coating. The growth environment significantly affects the carbide growth morphology. As solidification cooling rate increases to 1.5 · 106 K/s, the carbide morphology is irregular blocky or undeveloped dendrite (Fig. 3b and c). Moreover, it is evident from Fig. 3b that the TiC distribution was inhomogeneous. We think that the pulse time is as short as 1.5 ms, leading to very high temperature-gradient and very drastic convection in the laser-generated melt pool. Therefore, the solute distribution in the melt pool is very inhomogeneous, and microscale inhomogeneous precipitation of TiC occurs.
Fig. 1. SEM micrographs showing the macrograph of laser surface alloyed coating (a) and growth morphology of MC carbide (b), (c) solidified with a cooling rate of approximately 7.1 · 105 K/s.
Y. Chen, H.M. Wang / Scripta Materialia 50 (2004) 507–510
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Fig. 2. SEM micrographs showing macrograph of laser surface alloyed coating (a) and the growth morphology of MC carbide (b), (c) solidified with a cooling rate of approximately 8.3 · 105 K/s.
Fig. 3. SEM micrographs showing the macrograph of laser surface alloyed coating (a) and growth morphology of MC carbide (b), (c) solidified with a cooling rate of approximately 1.5 · 106 K/s.
Additionally, comparing Fig. 1a with Figs. 2a and 3a, it is clearly seen that the carbide mainly precipitates at the
top of the laser surface alloyed coating. It is reported that the titanium often segregates at the final solidification
510
Y. Chen, H.M. Wang / Scripta Materialia 50 (2004) 507–510
zone because it is the positive segregation element [14]. Moreover, the S/L interface advances from the bottom of melt pool to the free surface during the rapid solidification. As a result, the concentration of titanium in the residual melt increases gradually with advancement of the S/L interface, leading to the distribution of carbide at the top of the coatings. The different growth morphology of TiC in this study may be attributed to the influence of cooling rate on the growth interface structure. As an interstitial phase with high melting entropy and large Jackson’s factor of approximately 5–7 [15], TiC type MC carbide is considered as a typical faceted crystal, and it grows by lateral growth mode under equilibrium solidification conditions. The growth velocity of a faceted crystal is mainly dependent upon the difference between the velocity of atom attachment on the growing interface and that of atom escaping from growing interface [15]. Moreover, velocity of atom attachment on the growing interface is strongly affected by the atomic diffusion in the melt, and the velocity of atom escaping from growing interface is strongly dependent upon the atomic structure of the growing interface. In general, the high crystallographic index planes are atomically rough and have many growth steps, and the low crystallographic index planes are atomically smooth. As a result, different crystallographic planes present anisotropy of growth velocity under equilibrium solidification conditions. As solidification cooling rate or undercooling increases, the original atomically smooth growth interface would change gradually to atomically rough [16]. In the current case, as the solidification cooling rate increased further, it is thought that the original atomically smooth interface of TiC solidified at 1.5 · 106 K/s transform to atomically rough to some extent. As a result, the anisotropy in growth velocity between different crystallographic planes decreases, i.e., the difference in growth velocity between high crystallographic index planes and low crystallographic index planes of TiC is faint, leading to fine irregular block or undeveloped dendrite.
4. Conclusions In the present study, TiC reinforced composite coatings were fabricated by Nd–YAG pulsed laser surface alloying on the substrate of c-TiAl intermetallic alloy. MC carbide morphology was well-developed dendrite at low cooling rate. As cooling rate increased, carbide morphology changed to cross-petal with symmetrical arms and irregular block or undeveloped dendrite.
Acknowledgements The research was supported by the National Natural Science Foundation of China (grant no. 59971003). References [1] Wang Y, Qian Z, Li XY, Tandon KN. Surf Coat Technol 1997;91:37–42. [2] Noda T, Okabe M, Isobe S. Mater Sci Eng A 1996;213:157–61. [3] Yu LD, Thongtem S, Vilaithong T, McNallan MJ. Surf Coat Technol 2000;128:410–7. [4] Wang HM. Acta Metall Sinica 1997;33:917–20. [5] Fernandez R, Lecomte JC, Kattamis TZ. Metal Trans A 1978;9:1381–6. [6] Sun WR, Lee JH, Seo SM, Choe SJ, Hu ZQ. Mater Sci Eng A 1999;271:143–9. [7] Liu L, Fu HZ, Shi XZ. Acta Areonaut Astronaut Sinica 1986;7:181–5. [8] Wang HM. Acta Metall Sinica 1997;33:917–20. [9] Liu L, Sommer F, Fu HZ. Scrip Metall Mater 1994;30:584–91. [10] Chen Y, Wang HM. J Alloy Comp 2003;351:304–8. [11] Wang HM, Yu LG, Li XX, Jiang P. Sci Technol Adv Mater 2001;2:173–6. [12] Wang HM, Zhang JH, Tang YJ, Hu ZQ, et al. Mater Sci Eng A 1992;156:109–16. [13] Strutt PR, Lewis BG, Kear HH. In: Kear BH, Giessen BC, Cohen M, editors. Rapidly solidification amorphous and crystalline alloys. New York: Elsevier; 1980. p. 485. [14] Sun LL, Dong LK, Zhang JH, Hu ZQ. Acta Metall Sinica A 1993;29:388–91. [15] Fu HZ, Liu L, Fang XH. J North Polytech Univer 1987;5:279–87. [16] Kurz W. Fundamental of solidification. USA: Trans Tech Publications; 1984. p. 34.