Growth Mechanism of In Situ TiB Whiskers in Spark Plasma Sintered

ABSTRACT: In situ TiB whisker reinforced Ti-4.0Fe-7.3Mo matrix composites have been fabricated ... used to fabricate metal matrix composites (MMCs), i...
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CRYSTAL GROWTH & DESIGN

Growth Mechanism of In Situ TiB Whiskers in Spark Plasma Sintered TiB/Ti Metal Matrix Composites Haibo Feng,* Yu Zhou, Dechang Jia, Qingchang Meng, and Jiancun Rao School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China

2006 VOL. 6, NO. 7 1626-1630

ReceiVed August 26, 2005; ReVised Manuscript ReceiVed May 12, 2006

ABSTRACT: In situ TiB whisker reinforced Ti-4.0Fe-7.3Mo matrix composites have been fabricated by spark plasma sintering (SPS). The microstructure of the in situ TiB whiskers and interfacial structures in the composites were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution electron microscopy (HREM). The interfaces between in situ TiB and the Ti matrix are clean and faceted along the (100)TiB plane. However, small lateral growth steps are observed along the (101) and (101h) planes of TiB. Because of the characteristics of the TiB crystal structure, the growing velocity of the (100)TiB plane along the [010]TiB direction is much faster than that of the [001]TiB direction, which leads to TiB growth as a whisker shape along the [010]TiB direction with a hexagonal transverse section. The growth mechanism of TiB whiskers is a stacking process of the (100)TiB plane. 1. Introduction In recent years, in situ reaction synthesis techniques have been used to fabricate metal matrix composites (MMCs), in which reinforcements can be synthesized in situ in the metal matrix, utilizing either the exothermic nature of reactions or the crystallization during solidification processing.1-4 TiB has been used extensively as an in situ synthesized reinforcement for titanium MMCs due to its excellent chemical stability and high stiffness.3-6 Researchers have made TiB/Ti composites using various in situ synthesizing techniques such as combustion synthesis,7 rapid solidification,1,8 and powder metallurgy.2,3,5,9 Various boron source powders, such as TiB2, CrB, MoB, B4C, and purity B powder have been used to react with Ti in situ to synthesize TiB reinforcement.5-8,10,11 A high volume fraction (about 80 vol %) of the TiB phase in Ti-TiB composites can be synthesized by powder metallurgy methods.12,13 Mechanical properties of the composites with in situ synthesized TiB (or TiB + TiC) reinforcements have been widely evaluated.14-17 It is well-known that the mechanical properties of composites are greatly influenced by the morphology of the reinforcements and the reinforcement/matrix interfacial structure, which is related to the growth of in situ TiB reinforcements. Therefore, it is of great significance to reveal the growth mechanism of the in situ synthesized TiB. The microstructure of in situ TiB and TiB/R-Ti interfaces were systematically investigated by Lu et al.,18 who confirmed that the TiB is well-bonded with the R-Ti matrix and the interface is clean and flat. However, studies on the interfaces between TiB and the titanium matrix by highresolution electron microscopy (HREM) are still very limited. Different morphologies of the in situ synthesized TiB, such as needle shape, plate shape, and so on, were investigated by Fan1,8 and Kooi.19 Our previous studies indicated that the microstructure and growth mechanism of different TiB morphologies are the same as needle-shaped TiB.13 The in situ TiB nucleated and grew up through the diffusion of boron atoms during the synthesis processing.20 Microstructural characterization of in situ TiB via various synthesis methods has been reported by many researchers.1,5-8,18,19 However, the growth mechanism of in situ TiB remains unclear. Stacking faults have * Corresponding author. Tel: +86-0451-86418792, E-mail: Haibo Feng: [email protected].

been observed in in situ TiB crystals, whenever it was reactionsynthesized by laser cladding, common casting, rapid solidification, or spark plasma sintering (SPS).1,18,19,21 An interesting observation is that all the stacking faults were formed in the (100) plane of TiB and throughout the entire width of a fine TiB needle. On the basis of analysis of these previous results and the structural characteristics of the TiB crystal, a theoretical model of the formation mechanism of stacking faults has been proposed and confirmed by HREM investigations. The stacking faults in TiB whiskers typically involve a stacking faults plane of (100)TiB. The stacking faults in the inner of TiB and TiB/ matrix interfaces are formed during the growth processing. So, we can explore the growth mechanism of in situ TiB by the microstructural characterization of TiB and the TiB/matrix interfaces. In the present work, in situ TiB whiskers reinforced Ti-4.0Fe7.3Mo matrix composites have been fabricated by SPS. The microstructure of in situ TiB whiskers and the interfacial structure of TiB/R-Ti and TiB/β-Ti in the composites were investigated in detail by scanning electron microscopy (SEM) and HREM. The present paper focuses on the morphologies of different TiB/matrix interfaces and stacking faults to explore the growth mechanism of in situ TiB whiskers in TiB/Ti MMCs synthesized by SPS. 2. Experimental Section Commercial powders of Ti, Fe-65wt %Mo, TiB2, and B powders were used as starting materials. The predesigned volume fractions of in situ reaction-synthesized TiB in Ti matrix composites are 5, 10, 15, and 20%, respectively. The powder mixture was first ball-milled at a rotational speed of 500 rpm for 10 h in an argon atmosphere and then sintered at 800, 1000, and 1200 °C with a pressure of 20 MPa for 5 min in a vacuum, respectively. Previous studies showed that the original Ti and B (or TiB2) powders were able to react to completion and formed a lot of in situ TiB whiskers during the SPS process.21,22 The samples for SEM observations were prepared by standard metallographic methods. A deep etching process was performed on polished samples with a solution of water, nitric acid, and hydrofluoric acid in a molar ratio of 80:15:5. The morphology of in situ synthesized TiB whiskers was examined using a JSM-5600LV type SEM. Samples for transmission electron microscopy (TEM) and HREM observations were first ground to a thickness of 20 µm using SiC abrasive papers and then punched into disks with a diameter of 3.0 mm. The further thinning of the disk samples were performed by argon ion milling with an incident angle of 10° until perforation occurred. Microstructural

10.1021/cg050443k CCC: $33.50 © 2006 American Chemical Society Published on Web 06/15/2006

Growth Mechanism of In Situ TiB Whiskers

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Figure 1. SEM image of the TiB whiskers in the composite sintered at 1200 °C.

Figure 2. TEM images of an in situ TiB whisker: (a) transverse section and (b) longitudinal section. The insets are SAED patterns of (a) and (b), respectively. observations were performed with a Philips CM12 type TEM operated at 120 kV and a JEM-2010F type HREM operated at 200 kV, respectively.

3. Results The TiB reinforcement can be synthesized in situ during the SPS process. Figure 1 shows a typical SEM morphology of the in situ TiB sintered at 1200 °C. The in situ TiB exhibits a whisker shape with a high aspect ratio and is uniformly distributed in the titanium matrix. TEM investigations show that transverse section of the in situ TiB is a hexagonal shape, which always consists of (100), (101), and (101h) planes, as shown in Figure 2a. A TEM image of a TiB whisker along the [001] zone axis is shown in Figure 2b. It indicates that the TiB whiskers always grow along the [010] direction. Stacking faults are also observed in these TiB whiskers, and the stacking faults plane is always (100)TiB. Moreover, some small steps are found along the (101)TiB plane and the titanium matrix, but the TiB/ Ti interface along the (100)TiB plane is straight as shown in Figure 2a. So, the interfacial structures of the (100), (101), (101h) planes and end surface of TiB whiskers were investigated in detail by HREM. Figure 3 shows the morphology of the irregular transverse section of a TiB whisker along the [010]TiB zone axis and the HREM interfacial structure between the TiB transverse section and the Ti matrix. A lot of stacking faults in the (100)TiB plane can be observed, and all these stacking faults go through the TiB whisker along the [001]TiB direction, as shown in Figure 3a. The streaking along the [100]TiB direction in the selected area electron diffraction (SAED) pattern, as shown in Figure 3b, indicates that plane faults exist in the (100)TiB plane in the

Figure 3. TEM images of TiB in the R-Ti phase: (a) transverse view of TiB and (b) the SAED pattern; (c, d) HREM images of TiB/Ti interfaces along the (101), (101h) planes and (100)TiB plane, respectively.

in situ TiB whiskers. The HREM image of the TiB/Ti interface along the (101) and (101h) planes are irregular as shown in Figure 3c. There are many small facets between the two stacking faults in the (101) and (101h) planes. The interfaces of TiB/Ti along the (100) plane are straight and perfect, as shown in Figure 3d.

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Figure 4. TEM images of TiB near the R-Ti and β-Ti phase boundary: (a) transverse section of TiB; (b) SAED of (a); (c) HREM image of the TiB/β-Ti interfacial structure.

Figure 5. TEM images of TiB in the R-Ti phase: (a) longitudinal section of TiB; (b) SAED of (a); and (c) HREM image of TiB/R-Ti interface.

The stacking faults usually have a width of about several atomic layers. So, the interfaces of TiB/β-Ti and TiB/R-Ti along the (100)TiB plane were further investigated by HREM. Figure 4 shows the TEM image of TiB along the [010] zone axis near the R-Ti and β-Ti phase boundary. The SAED pattern in Figure 4b indicates that the orientation relationship between the TiB and the β-Ti matrix is as follows: (001)TiB//(110)β-Ti, [010]TiB//[1h11]β-Ti. On the basis of the HREM image of the TiB/β-Ti interface in Figure 4c, a well-bonded semi-coherent interface between (001)TiB and (110)β-Ti was observed. Moreover, a well-bonded interface between the TiB and the R-Ti matrix was also observed in a longitudinal view of the TiB whisker, as shown in Figure 5. The following orientation relationships can be determined from Figure 5b: (100)TiB// (100)R-Ti, [01h1]TiB//[011]R-Ti. The interface between TiB and R-Ti is coherent as shown in Figure 5c. As shown in Figure 5b, only streaking along the [100]TiB direction was observed in the SAED pattern, indicating that there are plane faults in the (100)TiB plane in the TiB whisker. The interfacial structure between the end of the TiB whisker and the titanium matrix is shown in Figure 6. As no crystal plane was present in the [011]TiB direction between TiB and the titanium matrix, the interface is irregular. Figure 6 also shows that the stacking faults extended throughout the width of the TiB whisker. So, the formation of stacking faults is related to the in situ growth process of the TiB whisker.

4. Discussion From HREM observations, the straight (100)TiB crystal plane has coherent or semi-coherent interfaces with the titanium matrix. The in situ synthesized TiB with an orthorhombic structure has lattice parameters of a ) 0.612 nm, b ) 0.306 nm, and c ) 0.456 nm. As shown in Figure 4c, the interplanar spacing of the (001)TiB plane is 0.4560 nm, which is nearly twice as large as that of the (110)β-Ti plane of 0.2338 nm. The lattice mismatch between the above-mentioned two interplanar spacing is 2.52%, and then the lattice mismatch along the (101)TiB plane is about 10%. Figure 5c shows that the (100)TiB plane has a coherent interface along the whisker axis when the (001)TiB plane is parallel to the (12h2)R-Ti plane. The interplanar spacing of the (001)TiB and (12h2)R-Ti planes are 0.2541 and 0.1248 nm, respectively, and their lattice mismatch is about -1.77%. So, the (100)TiB planes are well bonded with both the R-Ti and the β-Ti matrix. The formation of TiB is controlled by nucleation and growth mechanisms. The in situ synthesized TiB is prone to nucleate and grow along the lattice-matched planes to minimize the interface energy. So, the original nucleus of TiB should be one crystal plane of (100)TiB. The growth of TiB along the [100]TiB crystallographic direction can be taken as the stacking process of the (100)TiB planes with the diffusion of boron atoms, shown by the schematic diagrams in Figure 7. The formation of stacking faults in TiB depends on the sites of B atoms during the growth process. The distance between the sites of a perfect

Growth Mechanism of In Situ TiB Whiskers

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Figure 8. Schematic diagrams of atoms projection of TiB along the (a) [010]TiB and (b) [001]TiB directions.

Figure 6. HREM image showing the TiB/Ti interface along the [011]TiB direction.

Figure 9. Morphology of deep-etched TiB whiskers having a hexagonal cross section.

Figure 7. Schematic diagrams of the in situ growth of a TiB whisker.

and a fault B atom is only 0.01824 nm (0.04c) along the [001]TiB direction. Stacking faults formed in the (100)TiB planes have a minimum energy as contrasted to those formed in the (001)TiB and (010)TiB planes. Thus, only stacking faults in (100)TiB formed in in situ TiB whiskers. The crystal structure of TiB shows high asymmetry, and the atomic projection of TiB along the [010]TiB and [001]TiB directions are shown in Figure 8. The arrangements of Ti and B atoms are different in the (100), (001), and (010) planes of TiB. In the (100)TiB plane, Ti and B atoms occupy the sites of alternating layers. Although both of the atomic ratios between Ti and B are 1:1, the arrangements of Ti and B atoms in the (001)TiB plane are different from those in the (010)TiB plane. In the (001)TiB plane, the center of the B atom has a shift of 0.02c from that of the Ti atom at the same layer along the [001]TiB direction as shown in Figure 8a, while in the (010) plane, the centers of the Ti and B atoms are same in the [010]TiB direction. The growth normal to planes containing both Ti and B atoms in the same stoichiometry as the crystal should be faster than growth along directions involving alternating planes of Ti and B atoms.18,20 So the growth normal to the (010)TiB and (001)TiB planes should be faster than that of the (100)TiB plane. Moreover, the stacking period of atomic layers in the [010]TiB direction is

shorter than that in the [001]TiB direction, the growth along the [010]TiB direction should be faster than along the [001]TiB direction. Thus, TiB always forms a whisker shape with the axis of the [010]TiB direction. As shown in Figure 7, the growth of TiB along the [100]TiB direction is a stacking process of the (100)TiB planes one by one, and the growth of TiB along the [001]TiB direction is faster than along [100]TiB, and the transverse section of TiB should be a hexagonal shape. So, perfect TiB should be a whisker shape with a hexagonal transverse section, where the long faces are usually the (100)TiB planes and the shorter planes forming a wedge shape are (101)TiB and (101h)TiB. As the (101)TiB and (101h)TiB planes are formed during the stacking process of (100) planes, small steps were formed along the (101)TiB and (101h)TiB planes of TiB. These steps and stacking faults in (100)TiB plane will reduce the lattice strain between the (101)TiB and (101h)TiB planes and the titanium matrix. Figure 9 shows a typical SEM image of deep-etched in situ TiB whiskers synthesized at 1200 °C. It can be seen that the whisker shape with side steps conforms to the schematic diagram as shown in Figure 7. The TiB whisker is prone to nucleate along the titanium plane, which has Ti atomic arrangements similar to the (100)TiB plane, such as the (112)β-Ti plane and so on. This can explain why similar orientation relationships have been observed by us and others.23-26 5. Conclusions The microstructure of in situ synthesized TiB and interfacial structures were investigated by TEM and HREM. In situ TiB exhibits a whisker shape with a high aspect ratio along the [010]TiB direction. The transverse section of in situ TiB whisker is a hexagonal shape that always consists of (100), (101), and (101h) planes. HREM investigations indicated that the straight

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(100)TiB has semi-coherent interfaces with both the R-Ti and the β-Ti matrix. The lattice mismatch along (101)TiB and (101h)TiB is great. The following orientation relationships between the TiB and the Ti matrix were observed: (001)TiB//(110)β-Ti, [010]TiB//[1h11]β-Ti and (100)TiB//(100)R-Ti, [01h1]TiB//[011]R-Ti. The growth mechanism of TiB whiskers is a stacking process of the (100)TiB plane. Because of the characteristics of the TiB crystal structure, the growing velocity of the (100)TiB plane along the [010]TiB direction is much faster than that of the [001]TiB direction, which leads to TiB growing as a whisker shape along the [010]TiB direction with a hexagonal transverse section. Mass stacking faults in the (100)TiB plane were observed in TiB whiskers. TiB is prone to nucleate along the titanium plane, which has a Ti atom arrangement similar to the (100)TiB plane. Both the stacking faults and the steps can reduce the lattice strain between the (101)TiB and (101h)TiB planes and the titanium matrix. References (1) Fan, Z.; Miodownik, A. P. Microstructural evolution in rapidly solidified Ti-7.5Mn-0.5B alloy. Acta Mater. 1996, 44, 93-100. (2) Gorsse, S.; Chaminade, J. P.; Le Petitcrops, Y. In situ preparation of titanium base composites reinforced by TiB single crystals using a powder metallurgy technique. Composites, Part A 1998, 29, 12291234. (3) Saito, T.; Takamiya, H.; Furuta, T. Thermomechanical properties of P/M β titanium metal matrix composite. Mater. Sci. Eng. 1998, A243, 273-278. (4) Sahay, S. S.; Ravichandran, K. S.; Atri, R.; Chen, B.; Rubin, J. Evolution of microstructure and phases in in-situ processed Ti-TiB composites containing high volume fractions of TiB whiskers. J. Mater. Res. 1999, 14, 4214-4223. (5) Feng, H. B.; Jia, D. C.; Zhou, Y.; Meng, Q. C. Microstructure and mechanical properties of in situ TiB reinforced titanium matrix composites based on Ti-FeMo-B prepared by spark plasma sintering. Compos. Sci. Technol. 2004, 64 (16), 2495-2500. (6) Li, B. S.; Shang, J. L.; Guo, J. J.; Fu, H. Z. Formation of TiBw reinforcement in in-situ titanium matrix composites. J. Mater. Sci. 2004, 39, 1131-1133. (7) Zhang, X. H.; Xu, Q.; Han, J. C.; Kvanin, V. L. Self-propagation high-temperature combustion synthesis of TiB/Ti composites. Mater. Sci. Eng. 2003, A348, 41-46. (8) Fan, Z.; Chandrasekaran, L.; Ward-Close, C. M.; Miodownik, A. P. The effect of pre-consolidation heat treatment on TiB morphology and mechanical properties of rapidly solidified Ti-6Al-4V-XB alloys. Scr. Metall. Mater. 1995, 32 (6), 833-838. (9) Godfrey, T. M. T.; Wisbey, A.; Goodwin, P. S.; Bagnall, K.; WardClose, C. M. Microstructure and tensile properties of mechanically alloyed Ti-6Al-4V with boron additions. Mater. Sci. Eng. 2000, A282, 240-250.

Feng et al. (10) Kobayashi, M.; Funami, K.; Suzuki, S.; Ouchi, C. Manufacturing process and mechanical properties of fine TiB dispersed Ti-6Al-4V alloy composites obtained by reaction sintering. Mater. Sci. Eng. 1998, A243, 279-284. (11) Ma, Z. Y.; Tiong, S. C.; Gen, L. In-situ Ti-TiB metal-matrix composite prepared by a reactive pressing process. Scr. Mater. 2000, 42, 367-373. (12) Atri, R. R.; Ravichandran, K. S.; Jha, S. K. Elastic properties of in situ processed Ti-TiB composites measured by impulse excitation of vibration. Mater. Sci. Eng. 1999, A271, 150-159. (13) Feng, H. B.; Meng, Q. C.; Zhou, Y.; Jia, D. C. Spark plasma sintering of functionally graded material in the Ti-TiB2-B system. Mater. Sci. Eng. 2005, A397, 92-97. (14) Panda, K. B.; Ravichandran, K. S. Synthesis of ductile titaniumtitanium boride composites with a beta-titanium matrix: the nature of TiB formation and composites properties. Metall. Mater. Trans. A 2003, 34, 1371-1385. (15) Weiss, I.; Semiatin, S. L. Thermomechanical processing of beta titanium alloys-an overview. Mater. Sci. Eng. 1998, A243, 46-65. (16) Ranganath, S. A review on particle-reinforced titanium matrix composites. J. Mater. Sci. 1997, 32 (1), 1-16. (17) Tjong, S. C.; Ma, Z. Y. Microstructural and mechanical characteristics of in situ metal matrix composites. Mater. Sci. Eng. Rep. 2000, 29, 49-113. (18) Lu, W. J.; Zhang, X. N.; Wu, R. J.; Sakata, T.; Mori, H. Microstructural characterization of TiB in in situ synthesized titanium matrix composites prepared by common casting technique. J. Alloys Compd. 2001, 327, 240-247. (19) Kooi, B. J.; Pei, Y. T.; De Hosson, J. T. M. The evolution of microstructure in a laser clad TiB-Ti composite coating. Acta Mater. 2003, 51, 831-845. (20) Fan, Z.; Guo, Z. X.; Cantor, B. The kinetics and mechanism of interfacial reaction in sigma fibre-reinforced Ti MMCs. Composites, Part A 1997, 28A, 131-140. (21) Feng, H. B.; Zhou, Y.; Jia, D. C.; Meng, Q. C. Rapid synthesis of Ti alloy with B addition by spark plasma sintering. Mater. Sci. Eng. 2005, A390, 344-349. (22) Feng, H. B.; Jia, D. C.; Zhou, Y. Spark plasma sintering reaction synthesized TiB reinforced titanium matrix composites. Composites, Part A 2005, 36, 558-563. (23) Hyman, M. E.; McCullough, C.; Valencia, J. J.; Levi, C. G.; Mehrabian, R. Microstructure evolution in TiAl with B additions: conventional solidification. Metall. Trans. A 1989, 20A, 1847-1859. (24) Li, D. X.; Ping, D. H.; Lu, Y. X.; Ye, H. Q. HREM Study of the microstructure in nanocrystalline materials. Mater. Lett. 1993, 16, 322-326. (25) Mogilevsky, P.; Werner, A.; Dudek, H. J. Application of diffusion barriers in composite materials. Mater. Sci. Eng. 1998, A242, 235247. (26) Feng, H. B.; Zhou, Y.; Jia, D. C.; Meng, Q. C. Microstructural characterization of spark plasma sintered in situ TiB reinforced Ti matrix composite by EBSD and TEM. Mater. Trans. 2005, 3 (46), 575-580.

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