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Effect of Nb2O5 on the microstructure and mechanical properties of TiAl based composites produced by hot pressing
Materials Science and Engineering A 528 (2011) 6642–6646
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Rapid communication
Effect of Nb2 O5 on the microstructure and mechanical properties of TiAl based composites produced by hot pressing Jianfeng Zhu ∗ , Wenwen Yang, Haibo Yang, Fen Wang Key Laboratory of Auxiliary Chemistry & Technology for Chemical Industry, Ministry of Education, Shaanxi University of Science & Technology, Xi’an, Shaanxi, 710021, China
a r t i c l e
i n f o
Article history: Received 26 December 2010 Received in revised form 9 March 2011 Accepted 19 April 2011 Available online 27 April 2011 Keywords: TiAl Al2 O3 Composite Microstructure Mechanical properties
a b s t r a c t In situ composites of TiAl reinforced with Al2 O3 particles are successfully synthesized from an elemental powder mixture of Ti, Al and Nb2 O5 by the hot-press-assisted reaction synthesis (HPRS) method. The as-prepared composites are mainly composed of TiAl, Al2 O3 , NbAl3 , as well as small amounts of the Ti3 Al phase. The in situ formed fine Al2 O3 particles tend to disperse on the matrix grain boundaries of TiAl resulting in an excellent combination of matrix grain refinement and uniform Al2 O3 distribution in the composites. The Rockwell hardness and densities of TiAl based composites increase gradually with increasing Nb2 O5 content, and the flexural strength and fracture toughness of the composites have the maximum values of 634 MPa and 9.78 MPa m1/2 , respectively, when the Nb2 O5 content reaches 6.62 wt.%. The strengthening mechanism was also discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The Ti–Al binary phase diagram contains four stable intermetallic compounds at room temperature, Ti3 Al, TiAl, Al2 Ti and Al3 Ti. Among them, TiAl exhibits attractive properties, such as low density, high modules, and acceptable flexural toughness at room temperature [1,2]. Because of this, it has great potential for specific applications in the aerospace, automotive and turbine power generation markets [3,4], but its practical use as a light-weight structure material is still hindered by brittleness at both ambient temperature and high temperature. Recent investigations demonstrated that the mechanical properties of TiAl intermetallics are strongly dependent on their microstructure and that micro-alloying and optimizing the processing parameters can control the microstructure to form the expected lamellar or duplex microstructure. Therefore, some elements, such as Nb, Cr, W, Si, and B, have been introduced to TiAl intermetallics to improve their mechanical properties [5–10]. On the other hand, intermetallic matrix composites (IMCs) have also been shown to have a beneficial combination property and to solve the problem of poor ductility and toughness at ambient temperature [11–17]. Among them, Al2 O3 has been chosen as reinforcement for TiAl matrix because of its excellent thermomechanical behavior, high hardness, high modulus and excellent chemical stability, close match between their thermal expansion
∗ Corresponding author. Tel.: +86 29 86168688; fax: +86 29 86168688. E-mail addresses: [email protected], [email protected] (J. Zhu), [email protected] (W. Yang). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.04.062
coefficients, and it exhibited significant progress [13–17]. Their results revealed that the existence of second-phase Al2 O3 particles can hinder the movement of the grain boundaries, restrain the growth of grains, and enhance the mechanical properties. It was also reported that the addition of particles could exert influences on the oxidation resistance of the TiAl composites [18], which would extend its potential for engine component applications such as high pressure compressor, turbine, and some combustor applications. Recently, the present authors were successful in fabricating a wide range of dense Al2 O3 reinforced TiAl composites by reaction sintering of attrition milled powder blends consisting of cheap raw materials of Ti, Al, and TiO2 , which is based on the exothermic displacement reaction 3TiO2 + 7Al → 2Al2 O3 + 3TiAl. This processing technology is an energetically efficient means of in situ materials processing because the reaction is sufficiently exothermic to become self-sustained so that the system is amenable for self propagating high-temperature synthesis [13–15]. And it was found that the TiAl based composites can be fabricated at relatively low temperature and the in situ formed Al2 O3 increased the strength and the fracture toughness of the TiAl composites greatly. In the present paper, considering the idea of both micro-alloying with Nb and composite strengthening by Al2 O3 , TiAl based composites were fabricated by the hot-press-assisted in situ reaction synthesis method with elemental powder mixtures of Ti, Al, and Nb2 O5 . In comparison, nominal Ti–Al monolithic compound was also prepared using the same processing route. The phase compositions of the as-synthesized composite are reported, and the effect of the Nb2 O5 addition on microstructure and mechanical properties is investigated in detail.
J. Zhu et al. / Materials Science and Engineering A 528 (2011) 6642–6646
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Table 1 The component of the samples (wt.%). Specimens No.
Ti
Al
Nb2 O5
Theoretical target Al2 O3 content
(a) (b) (c) (d) (e)
64.00 62.05 56.20 48.40 40.60
36.00 36.39 37.54 39.08 40.62
0 1.56 6.26 12.52 18.78
0 1 4 8 12
2. Experimental Ti (280 mesh, 99.3% purity), Al (200 mesh, 99.5% purity), and Nb2 O5 (500 mesh, 99.5% purity) powders were used as the starting materials. The powders were weighed with compositions according to the stoichiometry of the following reaction. The detailed component of the samples was listed in Table 1. XTi + (28 + X) Al + 3 Nb2 O5 → 5 Al2 O3 +6 NbAl3 + XTiAl
(1)
The blend of powders was ball milled in alcohol for 1 h and then dried at 40 ◦ C for several hours. Then the milled powders were compacted uniaxially under 10 MPa in a graphite mold coated with BN inside. The compacted samples were hot press sintered in vacuum (∼10−2 Pa) at a rate of 5 ◦ C/min to 1300 ◦ C, and held for 2 h under a pressure of 17.6 MPa. Finally, the samples were cooled down to room temperature in the furnace. The surface layer of the samples was machined off to remove contaminants prior to characterization. Phase identification was conducted via X-ray diffraction (XRD, D/max-2200PC X, Cu K␣ radiation (40 kV and 30 mA)), and microstructure observation was carried out by scanning electron microscopy (SEM, JEOL JSM-6700F) and energy dispersive spectroscopy (EDS). Hardness measurements were performed at room temperature on an HXD-1000 tester with a diamond indenter under a 300 N load for 15 s. The fracture toughness and flexural strength tests were conducted at room temperature by the PC-1036PC material tester at a testing rate of 0.05 and 0.5 mm/min, respectively. At least five samples were measured for each kind of test. The flexural strength was calculated according to the following equation. =
3pl 2bh2
(2)
where p is the fracture load, and l, b, and h denote the span, width, and height, respectively. The fracture toughness was measured by the single edge precracked beam (SEPB) method with a notch depth of 0.4W (W is the width of specimen) with a loading span of 30 mm. The variations between load, deflectometer and time were automatically recorded by the data acquisition program. The fracture toughness, KIC , was calculated using the following equation. KIC =
Y × 3PLa1/2 2BW 2
(3)
where a is the notch length, and Y a geometrical factor, given by the following equation. Y =1.93 − 3.07
a w
a 2
+ 14.53
w
a 3
− 25.07
w
a 4
+ 25.08
w (4)
3. Results and discussion The X-ray diffraction patterns of monolithic TiAl and TiAl/Al2 O3 composites hot pressed at 1300 ◦ C for 2 h are shown in Fig. 1.
Fig. 1. XRD patterns of in situ TiAl composites hot pressed at 1300 ◦ C with various Al2 O3 content.
Fig. 1(a) shows that the monolithic TiAl from the sample of Ti:Al = 1:1 (in mole ratio) is composed of single-phase TiAl, implying that the expected reaction between the Ti and Al has been completed. This is consistent with the phase relationship demonstrated in the Ti–Al binary phase diagram through the reaction of Ti + Al → TiAl. When the Nb2 O5 content was 1.56 wt.%, a small amount of Ti3 Al phase was also found in addition to the TiAl phase. The formation of Ti3 Al was due to the ratio change between the Ti and Al because of the formation of the solid solution by replacement of Ti with Nb, which resulted from the reaction between Al and Nb2 O5 . Al2 O3 , the accompanying product of the reaction, could not be clearly identified, likely due to the quantity being below the XRD detecting limit. The solid solution reaction could easily take place because Ti and Nb have similar atomic radii of 0.145 and 0.182 nm, respectively. However, as the Nb2 O5 amount increased to 18.78 wt.%, a small amount of the Al2 O3 phase could be found. It was also found that not only does the Al2 O3 content increase gradually with increasing Nb2 O5 content, but the Ti3 Al phase is converted to NbAl3 . This result is due to that the Nb and Ti could only form the limited solid solution, hence the NbAl3 phase would produce when the Nb content was over the required level for solid solution formation according to the following equation: 28Al + 3Nb2 O5 → 6NbAl3 + 5Al2 O3
(5)
Fig. 2 shows the SEM micrograph of the fracture surface and the EDS analysis of the composite with Al2 O3 of 8.0 wt.%. The material consists of a dark matrix phase and a bright second phase. Although the content of each phase in the product is hard to indentify precisely, the EDS analysis results indicate that the bright phase is enriched with Al and O and their atomic compositions are 36.29% and 63.33% in mole ratio, respectively, which is close to stoichiometric Al2 O3 . Conversely, the dark phase was very rich with Al, Nb and Ti. Combined with the XRD analysis in Fig. 1, it was determined that the dark phase is made up of TiAl and Nb containing phases and that the Al2 O3 ceramic particles tended to reside on the grain boundaries of the TiAl matrix. Fig. 3 shows the SEM micrographs of the in situ hot pressed TiAl based composites with different Nb2 O5 concentrations. The matrix grain size of the products decreased significantly as the Nb2 O5 content increased, and the bright area corresponding to the Al2 O3 particles changed from an agglomerating state to an interpenetrating network structure, which was beneficial for the toughening effect in the composite material. However, when the Nb2 O5 content was reached 18.78 wt.% (Fig. 3d), the Al2 O3 particles tended to
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J. Zhu et al. / Materials Science and Engineering A 528 (2011) 6642–6646
Fig. 2. EDS spectra for the in situ TiAl composite (with Al2 O3 of 8 wt.%) hot pressed at 1300 ◦ C for 2 h.
be agglomerate again, which were detrimental to the mechanical properties of the in situ TiAl composites. Fig. 4 shows that the density and hardness of the TiAl composites increase from 3.59 to 4.02 g/cm3 and from 66.0 to 88.6 HRA with increasing the Nb2 O5 content of from 0 to 18.78 wt.%, respectively. Although TiAl and Al2 O3 have similar densities of 3.8 g/cm3 and 3.97 g/cm3 , respectively, the densities of the Nb containing phase of NbAl3 is much higher than that of TiAl and Al2 O3 and the amount of Nb containing phase increased obviously with increasing the Nb2 O5 content. This resulted in the increase in the density of the TiAl/Al2 O3 composite. The increase of hardness is mainly attributed to the higher hardness of the in situ formed Al2 O3 . In addition, with increasing the Nb2 O5 content, the microstructures
of the composites are more fine and compact (Fig. 3), which also leads to an increase in the density and hardness. The flexural strength and fracture toughness (KIC ) of the TiAl in situ composites hot pressed at 1300 ◦ C for 2 h as a function of Nb2 O5 content are plotted in Fig. 5. Both the flexural strength and fracture toughness peaked at 634 MPa and 9.78 MPa m1/2 , respectively, when the Nb2 O5 concentration was 6.62 wt.%. These values were increases of 99.8% for the flexural strength and 96.4% for the fracture toughness. Typical properties of the as-prepared TiAl based composite (with 4 wt.% Al2 O3 ) are summarized in Table 2. For comparison, those values of TiAl/Al2 O3 (with ∼12 wt.% Al2 O3 ) [15] and TiAl–Al2 Ti4 C2 –Al2 O3 –TiC [17] from the previously reported results
Fig. 3. SEM microstructures of the hot-pressed TiAl composites with different Al2 O3 content. (a) 0 wt.%; (b) 1 wt.%; (c) 4 wt.%; (d) 8 wt.%.
J. Zhu et al. / Materials Science and Engineering A 528 (2011) 6642–6646
Fig. 4. The density and hardness of the TiAl in situ composites with different Al2 O3 contents.
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Fig. 6. Typical crack propagation patterns of the TiAl composite (with Al2 O3 8.0 wt.%).
The zigzag crack propagation in the in situ composites was caused by the presence of dispersed fine Al2 O3 particles and refined precipitated particles. The as-prepared composite exhibited a significant increase of the flexural strength and fracture toughness due to a combination of several kinds of strengthening effects. Firstly, the strengthening effect resulted from the high hardness (18 GPa) and high content of in situ formed Al2 O3 with small particle size, which will result in higher deformation resistance and residual stress toughening. The toughening of the composite is a result of diminishing the residual compressive stress intensity at the tip of the deflected crack because ˛p < ˛m (˛p is 8.3 × 10−6 K−1 for Al2 O3 and ˛m is 10.8 × 10−6 K−1 for TiAl). Secondly, the fine precipitated Al2 O3 particles and fine colony grain strengthening play an important role in increasing the strength. The fine colony grain strengthening can be estimated by the Hall–Petch equation: Fig. 5. The flexural strength and fracture toughness of TiAl in situ composites with various Al2 O3 content. Table 2 Summary of typical properties of the as-prepared TiAl based composite (with 4 wt.% Al2 O3 ), together with those from Ai [15] and Peng et al. [17] for comparison. Properties
TiAl/Al2 O3
TiAl–Al2 Ti4 C2 –Al2 O3 –TiC
TiAl/Al2 O3
Source Density (g cm−3 ) Hardness (HRA) Bending strength (MPa) KIC (MPa m1/2 )
[15] – 72.4 398.5 6.99
[17] – – 236 5.9
This work 3.7 66 634 9.78
are also included. It can be seen that the flexural strength and fracture toughness of the present samples are much higher than those reported in Refs. [15,17]. The previously tested samples contained more excess Al2 O3 content (around 12 wt.%), and agglomeration of Al2 O3 becomes serious in TiAl/Al2 O3 composite and eventually results in a decrease in the flexural strength and fracture toughness. Second, some residual porosity exists in the previously tested samples due to the low external pressure, which accounts for the strength reduction of the composites. So, it is reasonable that present samples possess much higher mechanical strengths. Fig. 6 shows the typical crack propagation patterns of the as synthesized TiAl composites, which were a mixed complex fracture mode of intergranular, transgranular, and delaminating fractures with crack-bridging, branching and deflection also being observed.
= i + Ks d−1/2
(6)
where i and Ks are both material constants, and and d denote the flexural strength and grain diameter. With increasing Nb2 O5 content, both the matrix grain size and the reinforcement size decrease obviously, and the volume fraction increases gradually, resulting in the strength increasing. This is associated with the fine Al2 O3 particles hindering dislocation slips. These conclusions are in agreement with the results drawn from the SEM microstructures (Fig. 3) and mechanical properties (Fig. 5). Thirdly, the Nb (formed by reducing Nb2 O5 with Al) microalloying also played an important role in enhancing the TiAl matrix. Previous investigations revealed that Nb could modify the microstructure and increase the mechanical properties of TiAl alloy at both room temperature and high temperatures [19]. The investigation results indicated that the Nb addition increased the c axis and slightly influenced the a axis, resulting in a beneficial increase in the axial c/a ratio, which improved the mechanical properties of the TiAl alloy. In this study, Nb was produced by the reaction between Nb2 O5 and Al, some of which would react with Al to form Nb containing phase of NbAl3 that would act as the microalloying element (confirmed by XRD analysis in Fig. 1), which also improved the properties of TiAl/Al2 O3 composite. 4. Conclusions TiAl in situ composites were fabricated by a hot-press-assisted reaction synthesis (HPRS) method based on the Ti–Al–Nb2 O5
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system. The phases of the as-sintered samples were mainly made of TiAl, Al2 O3 , and NbAl3 , and the in situ formed fine Al2 O3 particles tended to disperse on the TiAl matrix grain boundaries forming an interpenetrating structure. With increasing Nb2 O5 content, the grains of the composites were better refined and the dispersion of Al2 O3 particles became more uniform. When the Nb2 O5 content was 6.62 wt.% (Al2 O3 4 wt.%), the flexural strength and fracture toughness of the in situ composites reached the maximum values of 634 MPa and 9.78 MPa m1/2 , which were increased by 99.8% and 96.4%, respectively. Acknowledgements This work was supported by the National Foundation of Natural Science, China (50802057), the Natural Foundation of Shaanxi Province, China (2010JM6014), Scientific and Technological Project of Wenzhou (H20100079 and H20100087), and the Graduate Innovation Fund of Shaanxi University of Science and Technology.
References [1] G. Chen, L. Zhang, Mater. Sci. Eng. A 329–331 (2002) 163–170. [2] Y.Q. Yan, Z.Q. Zhang, Z.G. Zhen, K.G. Wang, L. Zhou, Mater. Sci. Eng. A 280 (2000) 187–191. [3] M. Yamaguchi, H. Inui, K. Ito, Acta Mater. 48 (2000) 307–322. [4] X. Wu, Intermetallics 14 (2006) 1114–1122. [5] I. Gil, M.A. Munoz-Morris, D.G. Morris, Intermetallics 9 (2001) 373–385. [6] D. Hu, Intermetallics 10 (2002) 851–858. [7] M.A. Munoz-Morris, I. Gil, D.G. Morris, Intermetallics 13 (2005) 929– 936. [8] D. Hu, A.J. Huang, D. Novovic, X. Wu, Intermetallics 14 (2006) 818–825. [9] J. Lapin, T. Pelachova, Intermetallics 14 (2006) 1175–1180. [10] W. Lu, C. Chen, Y. Xi, F. Wang, L. He, Intermetallics 15 (2007) 989–998. [11] M. Wood, M. Ward-Close, Mater. Sci. Eng. A 192/193 (1995) 590–596. [12] C.M. Ward-Close, R. Minor, P.J. Doorbar, Intermetallics 4 (1996) 217–219. [13] J.F. Zhu, J.Q. Gao, F. Wang, Key Eng. Mater. 336–338 (2007) 1397–1399. [14] F. Wang, Z.K. Fan, J.F. Zhu, Acta Metall. Sin. 19 (2006) 432–440. [15] T.T. Ai, Chin. J. Aeronaut. 21 (2008) 559–564. [16] Z.H. Cai, D.L. Zhang, Mater. Sci. Eng. A 19 (2006) 310–317. [17] L.M. Peng, Z. Li, H. Li, J.H. Wang, M. Gong, J. Alloys Compd. 414 (2006) 100– 106. [18] Z.W. Li, W. Gao, D.L. Zhang, Z.H. Cai, Corros. Sci. 46 (2004) 1997–2007. [19] Y. Song, D.S. Xu, R. Yang, D. Li, Z.Q. Hu, Intermetallics 6 (1998) 157–165.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Rapid communication
Effect of Nb2 O5 on the microstructure and mechanical properties of TiAl based composites produced by hot pressing Jianfeng Zhu ∗ , Wenwen Yang, Haibo Yang, Fen Wang Key Laboratory of Auxiliary Chemistry & Technology for Chemical Industry, Ministry of Education, Shaanxi University of Science & Technology, Xi’an, Shaanxi, 710021, China
a r t i c l e
i n f o
Article history: Received 26 December 2010 Received in revised form 9 March 2011 Accepted 19 April 2011 Available online 27 April 2011 Keywords: TiAl Al2 O3 Composite Microstructure Mechanical properties
a b s t r a c t In situ composites of TiAl reinforced with Al2 O3 particles are successfully synthesized from an elemental powder mixture of Ti, Al and Nb2 O5 by the hot-press-assisted reaction synthesis (HPRS) method. The as-prepared composites are mainly composed of TiAl, Al2 O3 , NbAl3 , as well as small amounts of the Ti3 Al phase. The in situ formed fine Al2 O3 particles tend to disperse on the matrix grain boundaries of TiAl resulting in an excellent combination of matrix grain refinement and uniform Al2 O3 distribution in the composites. The Rockwell hardness and densities of TiAl based composites increase gradually with increasing Nb2 O5 content, and the flexural strength and fracture toughness of the composites have the maximum values of 634 MPa and 9.78 MPa m1/2 , respectively, when the Nb2 O5 content reaches 6.62 wt.%. The strengthening mechanism was also discussed. © 2011 Elsevier B.V. All rights reserved.
1. Introduction The Ti–Al binary phase diagram contains four stable intermetallic compounds at room temperature, Ti3 Al, TiAl, Al2 Ti and Al3 Ti. Among them, TiAl exhibits attractive properties, such as low density, high modules, and acceptable flexural toughness at room temperature [1,2]. Because of this, it has great potential for specific applications in the aerospace, automotive and turbine power generation markets [3,4], but its practical use as a light-weight structure material is still hindered by brittleness at both ambient temperature and high temperature. Recent investigations demonstrated that the mechanical properties of TiAl intermetallics are strongly dependent on their microstructure and that micro-alloying and optimizing the processing parameters can control the microstructure to form the expected lamellar or duplex microstructure. Therefore, some elements, such as Nb, Cr, W, Si, and B, have been introduced to TiAl intermetallics to improve their mechanical properties [5–10]. On the other hand, intermetallic matrix composites (IMCs) have also been shown to have a beneficial combination property and to solve the problem of poor ductility and toughness at ambient temperature [11–17]. Among them, Al2 O3 has been chosen as reinforcement for TiAl matrix because of its excellent thermomechanical behavior, high hardness, high modulus and excellent chemical stability, close match between their thermal expansion
∗ Corresponding author. Tel.: +86 29 86168688; fax: +86 29 86168688. E-mail addresses: [email protected], [email protected] (J. Zhu), [email protected] (W. Yang). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.04.062
coefficients, and it exhibited significant progress [13–17]. Their results revealed that the existence of second-phase Al2 O3 particles can hinder the movement of the grain boundaries, restrain the growth of grains, and enhance the mechanical properties. It was also reported that the addition of particles could exert influences on the oxidation resistance of the TiAl composites [18], which would extend its potential for engine component applications such as high pressure compressor, turbine, and some combustor applications. Recently, the present authors were successful in fabricating a wide range of dense Al2 O3 reinforced TiAl composites by reaction sintering of attrition milled powder blends consisting of cheap raw materials of Ti, Al, and TiO2 , which is based on the exothermic displacement reaction 3TiO2 + 7Al → 2Al2 O3 + 3TiAl. This processing technology is an energetically efficient means of in situ materials processing because the reaction is sufficiently exothermic to become self-sustained so that the system is amenable for self propagating high-temperature synthesis [13–15]. And it was found that the TiAl based composites can be fabricated at relatively low temperature and the in situ formed Al2 O3 increased the strength and the fracture toughness of the TiAl composites greatly. In the present paper, considering the idea of both micro-alloying with Nb and composite strengthening by Al2 O3 , TiAl based composites were fabricated by the hot-press-assisted in situ reaction synthesis method with elemental powder mixtures of Ti, Al, and Nb2 O5 . In comparison, nominal Ti–Al monolithic compound was also prepared using the same processing route. The phase compositions of the as-synthesized composite are reported, and the effect of the Nb2 O5 addition on microstructure and mechanical properties is investigated in detail.
J. Zhu et al. / Materials Science and Engineering A 528 (2011) 6642–6646
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Table 1 The component of the samples (wt.%). Specimens No.
Ti
Al
Nb2 O5
Theoretical target Al2 O3 content
(a) (b) (c) (d) (e)
64.00 62.05 56.20 48.40 40.60
36.00 36.39 37.54 39.08 40.62
0 1.56 6.26 12.52 18.78
0 1 4 8 12
2. Experimental Ti (280 mesh, 99.3% purity), Al (200 mesh, 99.5% purity), and Nb2 O5 (500 mesh, 99.5% purity) powders were used as the starting materials. The powders were weighed with compositions according to the stoichiometry of the following reaction. The detailed component of the samples was listed in Table 1. XTi + (28 + X) Al + 3 Nb2 O5 → 5 Al2 O3 +6 NbAl3 + XTiAl
(1)
The blend of powders was ball milled in alcohol for 1 h and then dried at 40 ◦ C for several hours. Then the milled powders were compacted uniaxially under 10 MPa in a graphite mold coated with BN inside. The compacted samples were hot press sintered in vacuum (∼10−2 Pa) at a rate of 5 ◦ C/min to 1300 ◦ C, and held for 2 h under a pressure of 17.6 MPa. Finally, the samples were cooled down to room temperature in the furnace. The surface layer of the samples was machined off to remove contaminants prior to characterization. Phase identification was conducted via X-ray diffraction (XRD, D/max-2200PC X, Cu K␣ radiation (40 kV and 30 mA)), and microstructure observation was carried out by scanning electron microscopy (SEM, JEOL JSM-6700F) and energy dispersive spectroscopy (EDS). Hardness measurements were performed at room temperature on an HXD-1000 tester with a diamond indenter under a 300 N load for 15 s. The fracture toughness and flexural strength tests were conducted at room temperature by the PC-1036PC material tester at a testing rate of 0.05 and 0.5 mm/min, respectively. At least five samples were measured for each kind of test. The flexural strength was calculated according to the following equation. =
3pl 2bh2
(2)
where p is the fracture load, and l, b, and h denote the span, width, and height, respectively. The fracture toughness was measured by the single edge precracked beam (SEPB) method with a notch depth of 0.4W (W is the width of specimen) with a loading span of 30 mm. The variations between load, deflectometer and time were automatically recorded by the data acquisition program. The fracture toughness, KIC , was calculated using the following equation. KIC =
Y × 3PLa1/2 2BW 2
(3)
where a is the notch length, and Y a geometrical factor, given by the following equation. Y =1.93 − 3.07
a w
a 2
+ 14.53
w
a 3
− 25.07
w
a 4
+ 25.08
w (4)
3. Results and discussion The X-ray diffraction patterns of monolithic TiAl and TiAl/Al2 O3 composites hot pressed at 1300 ◦ C for 2 h are shown in Fig. 1.
Fig. 1. XRD patterns of in situ TiAl composites hot pressed at 1300 ◦ C with various Al2 O3 content.
Fig. 1(a) shows that the monolithic TiAl from the sample of Ti:Al = 1:1 (in mole ratio) is composed of single-phase TiAl, implying that the expected reaction between the Ti and Al has been completed. This is consistent with the phase relationship demonstrated in the Ti–Al binary phase diagram through the reaction of Ti + Al → TiAl. When the Nb2 O5 content was 1.56 wt.%, a small amount of Ti3 Al phase was also found in addition to the TiAl phase. The formation of Ti3 Al was due to the ratio change between the Ti and Al because of the formation of the solid solution by replacement of Ti with Nb, which resulted from the reaction between Al and Nb2 O5 . Al2 O3 , the accompanying product of the reaction, could not be clearly identified, likely due to the quantity being below the XRD detecting limit. The solid solution reaction could easily take place because Ti and Nb have similar atomic radii of 0.145 and 0.182 nm, respectively. However, as the Nb2 O5 amount increased to 18.78 wt.%, a small amount of the Al2 O3 phase could be found. It was also found that not only does the Al2 O3 content increase gradually with increasing Nb2 O5 content, but the Ti3 Al phase is converted to NbAl3 . This result is due to that the Nb and Ti could only form the limited solid solution, hence the NbAl3 phase would produce when the Nb content was over the required level for solid solution formation according to the following equation: 28Al + 3Nb2 O5 → 6NbAl3 + 5Al2 O3
(5)
Fig. 2 shows the SEM micrograph of the fracture surface and the EDS analysis of the composite with Al2 O3 of 8.0 wt.%. The material consists of a dark matrix phase and a bright second phase. Although the content of each phase in the product is hard to indentify precisely, the EDS analysis results indicate that the bright phase is enriched with Al and O and their atomic compositions are 36.29% and 63.33% in mole ratio, respectively, which is close to stoichiometric Al2 O3 . Conversely, the dark phase was very rich with Al, Nb and Ti. Combined with the XRD analysis in Fig. 1, it was determined that the dark phase is made up of TiAl and Nb containing phases and that the Al2 O3 ceramic particles tended to reside on the grain boundaries of the TiAl matrix. Fig. 3 shows the SEM micrographs of the in situ hot pressed TiAl based composites with different Nb2 O5 concentrations. The matrix grain size of the products decreased significantly as the Nb2 O5 content increased, and the bright area corresponding to the Al2 O3 particles changed from an agglomerating state to an interpenetrating network structure, which was beneficial for the toughening effect in the composite material. However, when the Nb2 O5 content was reached 18.78 wt.% (Fig. 3d), the Al2 O3 particles tended to
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Fig. 2. EDS spectra for the in situ TiAl composite (with Al2 O3 of 8 wt.%) hot pressed at 1300 ◦ C for 2 h.
be agglomerate again, which were detrimental to the mechanical properties of the in situ TiAl composites. Fig. 4 shows that the density and hardness of the TiAl composites increase from 3.59 to 4.02 g/cm3 and from 66.0 to 88.6 HRA with increasing the Nb2 O5 content of from 0 to 18.78 wt.%, respectively. Although TiAl and Al2 O3 have similar densities of 3.8 g/cm3 and 3.97 g/cm3 , respectively, the densities of the Nb containing phase of NbAl3 is much higher than that of TiAl and Al2 O3 and the amount of Nb containing phase increased obviously with increasing the Nb2 O5 content. This resulted in the increase in the density of the TiAl/Al2 O3 composite. The increase of hardness is mainly attributed to the higher hardness of the in situ formed Al2 O3 . In addition, with increasing the Nb2 O5 content, the microstructures
of the composites are more fine and compact (Fig. 3), which also leads to an increase in the density and hardness. The flexural strength and fracture toughness (KIC ) of the TiAl in situ composites hot pressed at 1300 ◦ C for 2 h as a function of Nb2 O5 content are plotted in Fig. 5. Both the flexural strength and fracture toughness peaked at 634 MPa and 9.78 MPa m1/2 , respectively, when the Nb2 O5 concentration was 6.62 wt.%. These values were increases of 99.8% for the flexural strength and 96.4% for the fracture toughness. Typical properties of the as-prepared TiAl based composite (with 4 wt.% Al2 O3 ) are summarized in Table 2. For comparison, those values of TiAl/Al2 O3 (with ∼12 wt.% Al2 O3 ) [15] and TiAl–Al2 Ti4 C2 –Al2 O3 –TiC [17] from the previously reported results
Fig. 3. SEM microstructures of the hot-pressed TiAl composites with different Al2 O3 content. (a) 0 wt.%; (b) 1 wt.%; (c) 4 wt.%; (d) 8 wt.%.
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Fig. 4. The density and hardness of the TiAl in situ composites with different Al2 O3 contents.
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Fig. 6. Typical crack propagation patterns of the TiAl composite (with Al2 O3 8.0 wt.%).
The zigzag crack propagation in the in situ composites was caused by the presence of dispersed fine Al2 O3 particles and refined precipitated particles. The as-prepared composite exhibited a significant increase of the flexural strength and fracture toughness due to a combination of several kinds of strengthening effects. Firstly, the strengthening effect resulted from the high hardness (18 GPa) and high content of in situ formed Al2 O3 with small particle size, which will result in higher deformation resistance and residual stress toughening. The toughening of the composite is a result of diminishing the residual compressive stress intensity at the tip of the deflected crack because ˛p < ˛m (˛p is 8.3 × 10−6 K−1 for Al2 O3 and ˛m is 10.8 × 10−6 K−1 for TiAl). Secondly, the fine precipitated Al2 O3 particles and fine colony grain strengthening play an important role in increasing the strength. The fine colony grain strengthening can be estimated by the Hall–Petch equation: Fig. 5. The flexural strength and fracture toughness of TiAl in situ composites with various Al2 O3 content. Table 2 Summary of typical properties of the as-prepared TiAl based composite (with 4 wt.% Al2 O3 ), together with those from Ai [15] and Peng et al. [17] for comparison. Properties
TiAl/Al2 O3
TiAl–Al2 Ti4 C2 –Al2 O3 –TiC
TiAl/Al2 O3
Source Density (g cm−3 ) Hardness (HRA) Bending strength (MPa) KIC (MPa m1/2 )
[15] – 72.4 398.5 6.99
[17] – – 236 5.9
This work 3.7 66 634 9.78
are also included. It can be seen that the flexural strength and fracture toughness of the present samples are much higher than those reported in Refs. [15,17]. The previously tested samples contained more excess Al2 O3 content (around 12 wt.%), and agglomeration of Al2 O3 becomes serious in TiAl/Al2 O3 composite and eventually results in a decrease in the flexural strength and fracture toughness. Second, some residual porosity exists in the previously tested samples due to the low external pressure, which accounts for the strength reduction of the composites. So, it is reasonable that present samples possess much higher mechanical strengths. Fig. 6 shows the typical crack propagation patterns of the as synthesized TiAl composites, which were a mixed complex fracture mode of intergranular, transgranular, and delaminating fractures with crack-bridging, branching and deflection also being observed.
= i + Ks d−1/2
(6)
where i and Ks are both material constants, and and d denote the flexural strength and grain diameter. With increasing Nb2 O5 content, both the matrix grain size and the reinforcement size decrease obviously, and the volume fraction increases gradually, resulting in the strength increasing. This is associated with the fine Al2 O3 particles hindering dislocation slips. These conclusions are in agreement with the results drawn from the SEM microstructures (Fig. 3) and mechanical properties (Fig. 5). Thirdly, the Nb (formed by reducing Nb2 O5 with Al) microalloying also played an important role in enhancing the TiAl matrix. Previous investigations revealed that Nb could modify the microstructure and increase the mechanical properties of TiAl alloy at both room temperature and high temperatures [19]. The investigation results indicated that the Nb addition increased the c axis and slightly influenced the a axis, resulting in a beneficial increase in the axial c/a ratio, which improved the mechanical properties of the TiAl alloy. In this study, Nb was produced by the reaction between Nb2 O5 and Al, some of which would react with Al to form Nb containing phase of NbAl3 that would act as the microalloying element (confirmed by XRD analysis in Fig. 1), which also improved the properties of TiAl/Al2 O3 composite. 4. Conclusions TiAl in situ composites were fabricated by a hot-press-assisted reaction synthesis (HPRS) method based on the Ti–Al–Nb2 O5
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system. The phases of the as-sintered samples were mainly made of TiAl, Al2 O3 , and NbAl3 , and the in situ formed fine Al2 O3 particles tended to disperse on the TiAl matrix grain boundaries forming an interpenetrating structure. With increasing Nb2 O5 content, the grains of the composites were better refined and the dispersion of Al2 O3 particles became more uniform. When the Nb2 O5 content was 6.62 wt.% (Al2 O3 4 wt.%), the flexural strength and fracture toughness of the in situ composites reached the maximum values of 634 MPa and 9.78 MPa m1/2 , which were increased by 99.8% and 96.4%, respectively. Acknowledgements This work was supported by the National Foundation of Natural Science, China (50802057), the Natural Foundation of Shaanxi Province, China (2010JM6014), Scientific and Technological Project of Wenzhou (H20100079 and H20100087), and the Graduate Innovation Fund of Shaanxi University of Science and Technology.
References [1] G. Chen, L. Zhang, Mater. Sci. Eng. A 329–331 (2002) 163–170. [2] Y.Q. Yan, Z.Q. Zhang, Z.G. Zhen, K.G. Wang, L. Zhou, Mater. Sci. Eng. A 280 (2000) 187–191. [3] M. Yamaguchi, H. Inui, K. Ito, Acta Mater. 48 (2000) 307–322. [4] X. Wu, Intermetallics 14 (2006) 1114–1122. [5] I. Gil, M.A. Munoz-Morris, D.G. Morris, Intermetallics 9 (2001) 373–385. [6] D. Hu, Intermetallics 10 (2002) 851–858. [7] M.A. Munoz-Morris, I. Gil, D.G. Morris, Intermetallics 13 (2005) 929– 936. [8] D. Hu, A.J. Huang, D. Novovic, X. Wu, Intermetallics 14 (2006) 818–825. [9] J. Lapin, T. Pelachova, Intermetallics 14 (2006) 1175–1180. [10] W. Lu, C. Chen, Y. Xi, F. Wang, L. He, Intermetallics 15 (2007) 989–998. [11] M. Wood, M. Ward-Close, Mater. Sci. Eng. A 192/193 (1995) 590–596. [12] C.M. Ward-Close, R. Minor, P.J. Doorbar, Intermetallics 4 (1996) 217–219. [13] J.F. Zhu, J.Q. Gao, F. Wang, Key Eng. Mater. 336–338 (2007) 1397–1399. [14] F. Wang, Z.K. Fan, J.F. Zhu, Acta Metall. Sin. 19 (2006) 432–440. [15] T.T. Ai, Chin. J. Aeronaut. 21 (2008) 559–564. [16] Z.H. Cai, D.L. Zhang, Mater. Sci. Eng. A 19 (2006) 310–317. [17] L.M. Peng, Z. Li, H. Li, J.H. Wang, M. Gong, J. Alloys Compd. 414 (2006) 100– 106. [18] Z.W. Li, W. Gao, D.L. Zhang, Z.H. Cai, Corros. Sci. 46 (2004) 1997–2007. [19] Y. Song, D.S. Xu, R. Yang, D. Li, Z.Q. Hu, Intermetallics 6 (1998) 157–165.