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TiB2 particulates using SHS reactions of Ni–Ti–B4C and Ni–Ti–B4C–C systems during casting
Materials Science and Engineering A 445–446 (2007) 398–404
Fabrication of steel matrix composites locally reinforced with different ratios of TiC/TiB2 particulates using SHS reactions of Ni–Ti–B4C and Ni–Ti–B4C–C systems during casting Ya-feng Yang, Hui-yuan Wang, Yun-hong Liang, Ru-yi Zhao, Qi-chuan Jiang ∗ Key Laboratory of Automobile Materials of Ministry of Education and Department of Materials Science and Engineering, Jilin University, Nanling Campus, No. 142 Renmin Street, Changchun 130025, PR China Received 15 July 2006; received in revised form 16 September 2006; accepted 19 September 2006
Abstract Steel matrix composites locally reinforced with different molar ratios of in situ TiC/TiB2 particulates (2:1, 1:1 and 1:2, respectively) have been fabricated successfully utilizing the self-propagating high-temperature synthesis (SHS) reactions of Ni–Ti–B4 C and Ni–Ti–B4 C–C systems during casting. Differential thermal analysis (DTA) and X-ray diffraction (XRD) results reveal that the exothermic reactions of the Ni–Ti–B4 C and Ni–Ti–B4 C–C systems proceed in such a way that Ni initially reacts with B4 C and Ti to form Ni2 B and Ti2 Ni compounds, respectively, with heat evolution at 1037 ◦ C; Subsequently, the external heat and the evolved heat from these exothermic reactions promote the reactions forming TiC and TiB2 at 1133 ◦ C. In the composites reinforced with 1:2 molar ratio of TiC/TiB2 , almost all TiB2 grains have clubbed structures, while TiC grains exhibit near-spherical morphologies. Furthermore, TiB2 grain sizes decrease, with the increase of TiC content. In particular, in the composites reinforced with 2:1 molar ratio of TiC/TiB2 , it is difficult to find the clubbed TiB2 grains. Macro-pores and blowholes are absent in the local reinforcing region of the composites reinforced with 1:1 and 1:2 molar ratios of TiC/TiB2 , while a few macro-pores can be observed in the composite reinforced with 2:1 molar ratio of TiC/TiB2 . Moreover, the densities of the composites reinforced with 1:1 and 1:2 molar ratios of TiC/TiB2 are higher than that of the composite reinforced with 2:1 molar ratio of TiC/TiB2 . The composite reinforced with 1:2 molar ratio of TiC/TiB2 has the highest hardness and the best wear resistance. © 2006 Elsevier B.V. All rights reserved. Keywords: Steel matrix composites; SHS; In situ; Casting; Ceramic
1. Introduction Metal-matrix composites (MMCs) have received much attention as potential structural materials for their high specific strength and stiffness [1,2]. Although most of the work on MMCs is directed towards novel and lightweight engineering materials, there is also considerable interest in developing iron and steel matrix composites due to their potential for good wear resistance. In particular, iron and steel matrix composites reinforced with in situ synthesized ceramic particulates have been the subject of intensive investigation, due to their ease of fabrication, low costs and isotropic properties [3], in which the reinforcements are formed in situ by exothermic reactions between reactants [1,2]. Because of the formation of clean, ultrafine and
∗
Corresponding author. Tel.: +86 431 509 4699; fax: +86 431 509 4699. E-mail address: [email protected] (Q.-c. Jiang).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.09.062
stable ceramic reinforcements, the in situ MMCs exhibit excellent mechanical properties [4]. Currently, among several fabrication techniques to synthesize in situ ceramic particulates reinforced iron and steel matrix composites, self-propagating high temperature synthesis (SHS) technology has attached much attention, due to its low energy consumption, high time efficiency and highly product purity [5]. However, one of the initial drawbacks of the SHS reactions for industrial application has been that the final products can be highly porous [6]. Therefore, this process must be combined with a densification step, such as hot-pressing, extrusion, quasi-isostatic pressing (QIP) or shock-wave compaction [7]. Unfortunately, such techniques are too expensive or complex to be accepted by the engineering community for general application. In our previous work [3,8], we have demonstrated that the use of the SHS plus traditional casting routes provided a promising process to produce ceramic particulate reinforced iron and steel
Y.-f. Yang et al. / Materials Science and Engineering A 445–446 (2007) 398–404
matrix composites. The general procedure is that the molten steel was poured into a sand mould where the reactant preforms were preplaced and fixed using locating pins. The surface region of the preform was rapidly heated by the high-temperature steel melt and the reactants were ignited to form ceramics. Meanwhile, the steel melt infiltrated into the porosity of reacted preform. Using this method, the ceramic particulates locally reinforced steel matrix composites can be prepared successfully, in which local regions of the casting rather than the entire casting are reinforced by the relative costly particulates to achieve high wear resistance, while high toughness and strength are retained in the bulk casting, which is different from the monolithic composites, i.e., the ceramic reinforcement is homogeneously distributed in the entire body of the composites [8–15]. Moreover, the formation of clean, ultrafine and stable in situ ceramic reinforcements in the steel matrix also makes for the improvement of mechanical properties. Among various ceramic particulates, titanium carbide and titanium diboride are potential materials because of their outstanding properties, such as high hardness, low density, high melting temperature, high modulus, good wear and corrosion resistance [16–18] as well as good wettability and stability in the steel melt compared to other ceramics [19]. Therefore, they are widely used as the reinforcements in steel and iron matrix composites. However, several limitations associated with the use of pure TiC and TiB2 were found [20]. The high thermal expansion coefficient of TiC could lead to significant stresses across the preform in the local reinforcing region, while the very high reaction heat released during the formation of pure TiB2 could cause the appearance of large numbers of pores in the local reinforcing region. It is expected that the simultaneous formation of TiB2 and TiC in a TiB2 /TiC locally reinforced composite would mitigate the problems associated with the monolithic TiC and TiB2 , and improve the mechanical properties of the composites. The principal objective of the present study is to study the feasibility of the fabrication of the steel matrix composites locally reinforced with different molar ratios of TiC and TiB2 (2:1, 1:1 and 1:2, respectively) particulates synthesized by utilizing the heat of the liquid steel during casting to induce the SHS reaction of Ni–Ti–B4 C, Ni–Ti–B4 C–C systems. Furthermore, the effects of the formation of TiC and TiB2 with different molar ratios on the microstructure, hardness and wear resistance of the locally reinforced composites are investigated. It is expected that these preliminary results could be significant in promoting the development and practical application of in situ particulate locally reinforced steel matrix composites.
399
Table 1 Target compositions and formulations Sample
Material
Target composition (molar ratio)
Composition
Mass (g)
Ni content (wt.%)
1
Ni Ti B4 C
6 6.48 2.52
40
TiC:TiB2 = 1:2
2
Ni Ti B4 C C
6 6.65 1.93 0.42
40
TiC:TiB2 = 1:1
3
Ni Ti B4 C C
6 6.82 1.33 0.85
40
TiC:TiB2 = 2:1
using a stainless steel die to obtain green densities of 75 ± 2% theoretical density. After being dried in a vacuum oven at about 300 ◦ C for 3 h to remove any trace of moisture, the preforms were placed and fixed on the bottom of the sand mold under the air, as schematically illustrated in Fig. 1 in Ref. [3]. Subsequently, an austenite manganese steel (0.8C–1.4Si–9.1Mn–Fe balance, all in wt.%) melt prepared in a 5 kg medium-frequency induction furnace with a temperature of about 1500 ◦ C was poured into the mold to ignite the SHS reaction of the preforms. After solidification and cooling, composite castings were removed and sectioned in the side position. The SHS reaction temperatures were studied by differential thermal analysis (DTA) (Model Rigaku-8150, Japan) under argon. Sliding abrasive wear was tested under a load of 25N using a pin-on-disc apparatus. Both the steel matrix and the reinforcing regions of the composites machined into specimens of 6 mm diameter and 15 mm height were used as pin materials, and 600 mesh SiC abrasive papers were used as the counterface. Archimedes’ principle was used to determine the density of each specimen. The relative volumetric wear rates of the steel matrix and composites against the SiC abrasive paper were used
2. Experimental The starting materials were made of commercial powders of Ni (99.5% purity, ∼45 m), Ti (99.5% purity, ∼25 m), B4 C (98.0% purity, ∼25 m) and graphite (99.5% purity, ∼38 m), respectively, and the designed molar ratios of the products as well as the compositions of materials are shown in Table 1. The powder blends were mixed sufficiently by ball milling for 8 h and then pressed into cylindrical preforms (20 mm diameter)
Fig. 1. Change of standard Gibbs free energy as a temperature for Eqs. (1)–(3).
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to evaluate the wear resistance. Microstructures were examined using scanning electron microscopy (SEM) (Model Jeol JSM-5310, Japan) together with energy-dispersive spectrometry (EDS) (Model Link-Isis, Britain) and phases were identified using X–ray diffraction (XRD) (Model D/Max 2500PC Rigaku, Japan) with CuK␣ radiation. 3. Results and discussion 3.1. Theory background Several groups of TiC/TiB2 ceramics can be synthesized with different chemical molar ratios between Ti and B4 C through following chemical reactions: 3/4Ti + 1/4B4 C = 1/2TiB2 + 1/4TiC
(1)
2/3Ti + 1/6B4 C + 1/6C = 1/3TiB2 + 1/3TiC
(2)
3/5Ti + 1/10B4 C + 3/10C = 2/5TiC + 1/5TiB2
(3)
The Gibbs free energy G0 of reactions (1), (2) and (3) have been calculated using the thermodynamic data from Ref. [21], and the results are shown in Fig. 1. It can be seen that the G0 of the above three reactions are all negative, which indicates that these reactions all can take place. The G0 of reaction (1) is less than those of reactions (2) and (3), which indicates that reaction (1) is most likely to occur. In the SHS process, the possible maximum temperature during the exothermic reaction is attained under adiabatic conditions without heat loss. The theoretical limit, designated as the adiabatic temperature Tad , can be calculated from the heat capacities and enthalpies of formation and transformation [22]. Tad is an important thermodynamic parameter of combustion synthesis reactions. It can be used in a semi-quantitative way to ascertain whether or not the synthesis of a given material can be accomplished by a self-propagating method. It has been empirically suggested that combustion reactions will not become self-sustaining unless Tad ≥1800 K [23]. The adiabatic temperature of SHS reaction at room temperature can be theoretically calculated using the thermodynamic data from the reference [21] according to the equation: Tad H(298) + nj Cp (Pj )dT + nj L(Pj ) = 0 (4) 298
298−Tad
Where, H (298) is the reaction enthalpy at 298 K, Cp (Pj ) and L(Pj ) are the heat capacity and latent heat of the products, Pj and nj refer to the products and the mole fractions, respectively. The calculated adiabatic temperatures Tad of samples 1–3 in Table 1 are 2902, 2827 and 2995 K, respectively. The adiabatic temperatures are all greater than 1800 K, which indicated that the reactions can be self-sustained, namely SHS can occur. 3.2. Differential thermal analysis Because the liquid Ni formed with solid TiC and TiB2 for the low wetting angle under vacuum at 1450 ◦ C [24], nickel was selected as the additive metal, which not only decreased
Fig. 2. DTA curves for various compositions heated in argon environments at a heating rate of 30 ◦ C/min to a 1200 ◦ C.
the ignition temperature, but also improved the abrasive and structural applications. To study the SHS reaction temperatures of the Ni–Ti–B4 C and Ni–Ti–B4 C–C systems, 40 ± 1 mg reactant mixtures of samples 1, 2 and 3 were heated at 30 ◦ C/min to about 1200 ◦ C under an argon atmosphere in the DTA apparatus and the results were shown in Fig. 2. The DTA curves indicate that two significant exothermic peaks with maxima at about 1037 ◦ C and 1133 ◦ C appeared in three samples, respectively. However, it should be noticed that the heat rate is fast and will lead to a slight shift of the reaction peaks to higher temperatures. In order to develop an understanding of the SHS reactions of the three samples, experiments quenched from about 1037 ◦ C were carried out in the three designed mixtures, respectively, and the results are shown in Fig. 3(a). The XRD results indicate that a large quantity of Ti2 Ni, Ni2 B, and some TiB2 and TiC were detected in the three samples, which indicated that the exothermic peak with the maximum at 1037 ◦ C mainly corresponded to the formation of Ti2 Ni and Ni2 B compounds. Fig. 3(b) shows the XRD results of the DTA products heated to 1200 ◦ C. According to the XRD results, Ni, TiB2 and TiC phases as well as the intermediate phases Ni3 Ti, Ni2 B and C were detected in three samples. Therefore, it can be deduced that external heat and the evolved heat from the above exothermic reactions at 1037 ◦ C could promote the reaction of TiC and TiB2 formation, which corresponds to the exothermic peak with the maximum at 1133 ◦ C. Based on the results, it is believed that nickel serves not only as a diluent but also as a reactant and participates in the SHS reaction. TiC and TiB2 are the thermodynamically stable phase in the final products. According to [22], TiC and TiB2 can be formed at 1600 ◦ C from the Ti and B4 C mixtures. Therefore, the addition of nickel provides an easier route for the TiC and TiB2 formation in the Ni–Ti–B4 C and Ni–Ti–B4 C–C systems. However, the reactions in the DTA apparatus were incomplete and some Ni3 Ti, C and Ni3 B phases were detected in the final products. This may be due to the smaller volume size of the DTA
Y.-f. Yang et al. / Materials Science and Engineering A 445–446 (2007) 398–404
401
Fig. 3. XRD patterns (CuK␣ radiation) of DTA products from samples 1, 2 and 3: (a) quenched at 1037 ◦ C and (b) at 1200 ◦ C.
sample, and thus, the heat released by these reactions may dissipate into the surroundings more easily, resulting in an incomplete reaction [12]. 3.3. Phase constituents and microstructures In the present study, the smelting temperature of matrix steel is selected to be about 1500 ◦ C, significantly higher than 1130 ◦ C, which corresponds to the TiC and TiB2 formation temperature in the DTA. When the molten steel was poured into the sand mold, the SHS reactions of the Ni–Ti–B4 C and Ni–Ti–B4 C–C systems were ignited by the heat release of the steel melts. Generally, the SHS reaction products were extremely porous, and therefore, the molten steel will spontaneously infiltrate into the pores of the reacting preform driven by capillary forces during the SHS reaction. Figs. 4 and 5 show the XRD patterns and SEM microstructures of the locally reinforced steel composites fabricated by using sample 1, 2 and 3, respectively. XRD results reveal that the composites mainly consist of TiC, TiB2, austenite and [Fe, Ni]. However, it is worth noting that no Ni3 Ti and Ni2 B compounds are found in the XRD results, which indicate that the reactions in the steel melt are complete. Moreover, the TiC peaks in the composites fabricated by using sample 3 are more intense than those of TiB2 , whereas the TiC peaks in the composites fabricated by using sample 1 are weaker than those of TiB2 , and the intensities of TiC and TiB2 in the composites fabricated by using sample 2 are similar. This was expected given the designed reactant compositions. It is worth noting that the Ni element can sufficiently disssolve into the infiltrated steel melt; therefore, [Fe, Ni] can be formed around ceramics particulates. Fig. 5 revealed a relatively uniform distribution of TiC and TiB2 in the composites. The elongated or rectangular particulates are TiB2 , whereas the nearly spherical particulates are TiC. These particulates morphologies were similar to those observed for monolithic TiB2 and TiC [3]. In the SEM of the composites synthesized by using sample 1, almost all TiB2 particulates have a “clubbed” structure because TiB2 particulates have enough
space to grow amply between the TiC particulates, as shown in Fig. 5(a). From Fig. 5(b), TiB2 particulates exhibit elongated morphologies, while the morphologies of TiC particulates are almost invariant. Moreover, compared with that in Fig. 5(a), TiB2 sizes decrease. With the increase of TiC content, near–spherical TiC particulates connect with each other to form a framework structure, as shown in Fig. 5(c). Thus, the growth of TiB2 was kept within limits and TiB2 only forms in the channel or gap between the TiC particulates and it is difficult to find the clubbed TiB2 particulates. 3.4. Porosity and density Microstructural characterization shows that the presence of significant macro-pores can be observed in the composite syn-
Fig. 4. XRD patterns (CuK␣ radiation) of the locally reinforced steel matrix composites fabricated by using samples 1, 2 and 3.
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Fig. 5. Typical microstructures of the locally reinforced steel matrix composites fabricated by using (a) sample 1, (b) sample 2 and (3) sample 3.
thesized by using sample 3 (as shown in Fig. 6), while macropores and blowholes are absent in the local reinforcing region in the composites synthesized by using samples 1 and 2, which indicates that the different molar ratios of TiC/TiB2 have a significant effect on the fabrication of dense in situ composites. In fact, it is easy to understand the results since the adiabatic temperature Tad (2995 K) of sample 3 is the highest in the three samples. As a result, the evaporation of gas and impurity generated by the SHS reaction is increased, resulting in the formation of the composite with higher porosity. Moreover, it can be seen from Fig. 6 that most of fine TiC and TiB2 particulates are present in the local reinforcing region, although some large strip shape particulates are entrapped inside the Fe-rich region located in
the reinforcing region, as found in our previous work [12]. The formation of fine ceramic particulates in the local reinforcing region is assisted by the reaction-solution-precipitation mechanism during the SHS reaction, while large strip shape ceramic particulates precipitate out of the melts by the nucleation-growth mechanism during solidification [12]. A detailed discussion of this has been given in our previous work [12]. Table 2 shows the densities of the reinforcing region of the composites synthesized by using samples 1, 2 and 3. It can be seen that the densities of the reinforcing region of the composites synthesized by using samples 1 and 2 are higher than that using sample 3. Differences between densities of the locally reinforced composites may have been a consequence of the different thermal expansion coefficients of TiC and TiB2 . The mean thermal expansion coefficients for TiB2 and TiC are 4.6 and 7.95 × 10−6 K−1 , respectively, at 300–1300 K [20]. The higher expansion coefficients associated with the TiC-rich composites may have resulted in cracking as the composite cooled. This would cause the density of the composite synthesized by using sample 3 to be significantly lower than the other two values. Moreover, the higher Tad of sample 3 may have created more porosity, which would contribute to the decrease of density. 3.5. Hardness and wear resistance
Fig. 6. The distribution of porosity of the locally reinforced steel matrix composite fabricated by using sample 3.
Hardness values and wear resistance of the reinforcing region in the composites synthesized by using samples 1, 2 and 3 are listed in Table 2. Generally, the near-spherical morphology of the ceramic particulates made for the improvement of mechanical
Y.-f. Yang et al. / Materials Science and Engineering A 445–446 (2007) 398–404
403
Table 2 The densities, hardness values and wear rates of the steel matrix and the composites Materials (g/cm3 )
Density Hardness (HRC) Volumetric wear loss (10−10 m3 /m)
Steel matrix
Sample 1
Sample 2
Sample 3
7.82 <20 2.281
6.64 46.3 0.5463
6.47 42.6 0.7713
6.11 39.8 1.1406
property of the composites. However, the composite synthesized by using sample 1 has the highest hardness value and the lowest volumetric wear loss. It has previously been reported that the clubbed structure of TiB2 was very useful to improve the mechanical properties of TiC/TiB2 ceramics [22]. Apparently, the current results also demonstrate that the introduction of more clubbed TiB2 grains into TiC/TiB2 ceramics resulted in an increase in the hardness and wear resistance of the composites. This can be understood as follows: in the composites synthesized by using sample 1, the 1:2 molar ratio of TiC/TiB2 can weaken the high thermal expansion effect of excessive TiC content and reduce the thermal stresses; however, compared with the formation of pure TiB2 , this can decrease the high reaction heat, which can reduce the porosity caused by the gas volatilization. Therefore, compared with the two composites synthesized by using samples 2 and 3, lower thermal stresses and reduced porosity were achieved in the composites synthesized by using sample 1, which led to a strong interfacial bonding between particulates and matrix. This improves both hardness and wear resistance. 4. Conclusion (1) DTA results reveal that the exothermic reactions of the Ni–Ti–B4 C and Ni–Ti–B4 C–C systems proceed in such a way that Ni initially reacts with B4 C and Ti to form Ni2 B and Ti2 Ni, respectively, with heat evolution at 1037 ◦ C; Subsequently, external heat and the evolved heat from these exothermic reactions promote the reactions forming TiC and TiB2 at 1130 ◦ C. Nickel serves not only as a diluent but also as a reactant and participates in the SHS reaction; TiC and TiB2 are the thermodynamically stable phase in the final products. In addition to the Ni, TiB2 and TiC phases, Ni3 Ti, Ni–B compounds and residual C are also detected by XRD in the products, which indicate the DTA exotherm is recording an incomplete reaction. (2) Steel matrix composites locally reinforced with different molar ratios of in situ TiC and TiB2 (2:1, 1:1 and 1:2, respectively) particulates are fabricated successfully utilizing the SHS reactions of Ni–Ti–B4 C and Ni–Ti–B4 C–C systems during casting. As-cast microstructures of the in situ processed composites reveal a relatively uniform distribution of TiC/TiB2 particulates in the local reinforcing regions. In the composites reinforced with 1:2 molar ratio of TiC/TiB2 , almost all TiB2 grains have clubbed structures, while TiC grains exhibit near-spherical morphologies. With the increase of TiC content, TiB2 sizes decrease. In particular, in the composites reinforced with 2:1 molar ratio of
TiC/TiB2 , the growth of TiB2 was kept within limits, thereby restricting the formation of clubbed TiB2 grains. (3) Macro-pores and blowholes are absent in the local reinforcing region of the composites reinforced with 1:1 and 1:2 molar ratios of TiC/TiB2 , while a few macro-pores can be observed in the composite reinforced with 2:1 molar ratios of TiC/TiB2 . Moreover, the densities of composites reinforced with 1:1 and 1:2 molar ratios of TiC/TiB2 are higher than that of the composite reinforced with 2:1 molar ratio of TiC/TiB2 . (4) The introduction of more clubbed TiB2 grains into TiC/TiB2 ceramics resulted in an increase in the hardness and wear resistance of the local reinforcing region in the composites; and the composite reinforced with 1:2 molar ratio of TiC/TiB2 has the highest hardness and the best wear resistance. Acknowledgements This work is supported by The National Natural Science Foundation of China (No. 50531030) and The Ministry of Science and Technology of the People’s Republic of China (No. 2005CCA00300) as well as The Project 985-Automotive Engineering of Jilin University. References [1] S.Y. Chang, S.J. Cho, S.K. Hong, D.H. Shin, J. Alloys Compd. 316 (2001) 275–279. [2] E. Pagounis, V.K. Lindroos, Mater. Sci. Eng. A 246 (1998) 221–234. [3] Q.C. Jiang, B.X. Ma, H.Y. Wang, Y. Wang, Y.P. Dong, Composites Part A37 (2005) 133–138. [4] H.Y. Wang, Q.C. Jiang, X.L. Li, F. Zhao, J. Alloys Compd. 366 (2004) 9–12. [5] S.C. Tjong, Z.Y. Ma, Mater. Sci. Eng. R29 (2000) 49–113. [6] J.J. Moore, H.J. Feng, Prog. Mater. Sci. 39 (1995) 243–273. [7] J.C. Han, X.H. Zhang, J.V. Wood, Mater. Sci. Eng. A280 (2000) 328–333. [8] H.Y. Wang, Q.C. Jiang, B.X. Ma, Y. Wang, F. Zhao, Adv. Eng. Mater. 7 (2005) 58–63. [9] Y.S. Wang, X.Y. Zhang, G.T. Zeng, F.C. Li, Composites Part A 32 (2001) 281–286. [10] Y.S. Wang, X.Y. Zhang, F.C. Li, G.T. Zeng, Mater. Des. 20 (1999) 233–236. [11] C. Tassin, F. Laroudie, M. Pons, L. Lelait, Surf. Coat. Technol. 80 (1996) 207–210. [12] H.Y. Wang, L. Huang, Q.C. Jiang, Mater. Sci. Eng. A407 (2005) 98–104. [13] D. Suh, S. Lee, S.J. Kwon, Y.M. Koo, Metall. Mater. Trans. A 28A (1997) 1499–1508. [14] S.J. Kwon, S.H. Choo, S. Lee, Scr. Mater. 40 (1998) 235–240. [15] S. Tondu, T. Schnick, L. Pawlowski, B. Wielage, S. Steinhauser, L. Sabatier, Surf. Coat. Technol. 123 (2000) 247–251. [16] C.C. Degnan, P.H. Shipway, Wear 252 (2002) 832–841.
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Fabrication of steel matrix composites locally reinforced with different ratios of TiC/TiB2 particulates using SHS reactions of Ni–Ti–B4C and Ni–Ti–B4C–C systems during casting Ya-feng Yang, Hui-yuan Wang, Yun-hong Liang, Ru-yi Zhao, Qi-chuan Jiang ∗ Key Laboratory of Automobile Materials of Ministry of Education and Department of Materials Science and Engineering, Jilin University, Nanling Campus, No. 142 Renmin Street, Changchun 130025, PR China Received 15 July 2006; received in revised form 16 September 2006; accepted 19 September 2006
Abstract Steel matrix composites locally reinforced with different molar ratios of in situ TiC/TiB2 particulates (2:1, 1:1 and 1:2, respectively) have been fabricated successfully utilizing the self-propagating high-temperature synthesis (SHS) reactions of Ni–Ti–B4 C and Ni–Ti–B4 C–C systems during casting. Differential thermal analysis (DTA) and X-ray diffraction (XRD) results reveal that the exothermic reactions of the Ni–Ti–B4 C and Ni–Ti–B4 C–C systems proceed in such a way that Ni initially reacts with B4 C and Ti to form Ni2 B and Ti2 Ni compounds, respectively, with heat evolution at 1037 ◦ C; Subsequently, the external heat and the evolved heat from these exothermic reactions promote the reactions forming TiC and TiB2 at 1133 ◦ C. In the composites reinforced with 1:2 molar ratio of TiC/TiB2 , almost all TiB2 grains have clubbed structures, while TiC grains exhibit near-spherical morphologies. Furthermore, TiB2 grain sizes decrease, with the increase of TiC content. In particular, in the composites reinforced with 2:1 molar ratio of TiC/TiB2 , it is difficult to find the clubbed TiB2 grains. Macro-pores and blowholes are absent in the local reinforcing region of the composites reinforced with 1:1 and 1:2 molar ratios of TiC/TiB2 , while a few macro-pores can be observed in the composite reinforced with 2:1 molar ratio of TiC/TiB2 . Moreover, the densities of the composites reinforced with 1:1 and 1:2 molar ratios of TiC/TiB2 are higher than that of the composite reinforced with 2:1 molar ratio of TiC/TiB2 . The composite reinforced with 1:2 molar ratio of TiC/TiB2 has the highest hardness and the best wear resistance. © 2006 Elsevier B.V. All rights reserved. Keywords: Steel matrix composites; SHS; In situ; Casting; Ceramic
1. Introduction Metal-matrix composites (MMCs) have received much attention as potential structural materials for their high specific strength and stiffness [1,2]. Although most of the work on MMCs is directed towards novel and lightweight engineering materials, there is also considerable interest in developing iron and steel matrix composites due to their potential for good wear resistance. In particular, iron and steel matrix composites reinforced with in situ synthesized ceramic particulates have been the subject of intensive investigation, due to their ease of fabrication, low costs and isotropic properties [3], in which the reinforcements are formed in situ by exothermic reactions between reactants [1,2]. Because of the formation of clean, ultrafine and
∗
Corresponding author. Tel.: +86 431 509 4699; fax: +86 431 509 4699. E-mail address: [email protected] (Q.-c. Jiang).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.09.062
stable ceramic reinforcements, the in situ MMCs exhibit excellent mechanical properties [4]. Currently, among several fabrication techniques to synthesize in situ ceramic particulates reinforced iron and steel matrix composites, self-propagating high temperature synthesis (SHS) technology has attached much attention, due to its low energy consumption, high time efficiency and highly product purity [5]. However, one of the initial drawbacks of the SHS reactions for industrial application has been that the final products can be highly porous [6]. Therefore, this process must be combined with a densification step, such as hot-pressing, extrusion, quasi-isostatic pressing (QIP) or shock-wave compaction [7]. Unfortunately, such techniques are too expensive or complex to be accepted by the engineering community for general application. In our previous work [3,8], we have demonstrated that the use of the SHS plus traditional casting routes provided a promising process to produce ceramic particulate reinforced iron and steel
Y.-f. Yang et al. / Materials Science and Engineering A 445–446 (2007) 398–404
matrix composites. The general procedure is that the molten steel was poured into a sand mould where the reactant preforms were preplaced and fixed using locating pins. The surface region of the preform was rapidly heated by the high-temperature steel melt and the reactants were ignited to form ceramics. Meanwhile, the steel melt infiltrated into the porosity of reacted preform. Using this method, the ceramic particulates locally reinforced steel matrix composites can be prepared successfully, in which local regions of the casting rather than the entire casting are reinforced by the relative costly particulates to achieve high wear resistance, while high toughness and strength are retained in the bulk casting, which is different from the monolithic composites, i.e., the ceramic reinforcement is homogeneously distributed in the entire body of the composites [8–15]. Moreover, the formation of clean, ultrafine and stable in situ ceramic reinforcements in the steel matrix also makes for the improvement of mechanical properties. Among various ceramic particulates, titanium carbide and titanium diboride are potential materials because of their outstanding properties, such as high hardness, low density, high melting temperature, high modulus, good wear and corrosion resistance [16–18] as well as good wettability and stability in the steel melt compared to other ceramics [19]. Therefore, they are widely used as the reinforcements in steel and iron matrix composites. However, several limitations associated with the use of pure TiC and TiB2 were found [20]. The high thermal expansion coefficient of TiC could lead to significant stresses across the preform in the local reinforcing region, while the very high reaction heat released during the formation of pure TiB2 could cause the appearance of large numbers of pores in the local reinforcing region. It is expected that the simultaneous formation of TiB2 and TiC in a TiB2 /TiC locally reinforced composite would mitigate the problems associated with the monolithic TiC and TiB2 , and improve the mechanical properties of the composites. The principal objective of the present study is to study the feasibility of the fabrication of the steel matrix composites locally reinforced with different molar ratios of TiC and TiB2 (2:1, 1:1 and 1:2, respectively) particulates synthesized by utilizing the heat of the liquid steel during casting to induce the SHS reaction of Ni–Ti–B4 C, Ni–Ti–B4 C–C systems. Furthermore, the effects of the formation of TiC and TiB2 with different molar ratios on the microstructure, hardness and wear resistance of the locally reinforced composites are investigated. It is expected that these preliminary results could be significant in promoting the development and practical application of in situ particulate locally reinforced steel matrix composites.
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Table 1 Target compositions and formulations Sample
Material
Target composition (molar ratio)
Composition
Mass (g)
Ni content (wt.%)
1
Ni Ti B4 C
6 6.48 2.52
40
TiC:TiB2 = 1:2
2
Ni Ti B4 C C
6 6.65 1.93 0.42
40
TiC:TiB2 = 1:1
3
Ni Ti B4 C C
6 6.82 1.33 0.85
40
TiC:TiB2 = 2:1
using a stainless steel die to obtain green densities of 75 ± 2% theoretical density. After being dried in a vacuum oven at about 300 ◦ C for 3 h to remove any trace of moisture, the preforms were placed and fixed on the bottom of the sand mold under the air, as schematically illustrated in Fig. 1 in Ref. [3]. Subsequently, an austenite manganese steel (0.8C–1.4Si–9.1Mn–Fe balance, all in wt.%) melt prepared in a 5 kg medium-frequency induction furnace with a temperature of about 1500 ◦ C was poured into the mold to ignite the SHS reaction of the preforms. After solidification and cooling, composite castings were removed and sectioned in the side position. The SHS reaction temperatures were studied by differential thermal analysis (DTA) (Model Rigaku-8150, Japan) under argon. Sliding abrasive wear was tested under a load of 25N using a pin-on-disc apparatus. Both the steel matrix and the reinforcing regions of the composites machined into specimens of 6 mm diameter and 15 mm height were used as pin materials, and 600 mesh SiC abrasive papers were used as the counterface. Archimedes’ principle was used to determine the density of each specimen. The relative volumetric wear rates of the steel matrix and composites against the SiC abrasive paper were used
2. Experimental The starting materials were made of commercial powders of Ni (99.5% purity, ∼45 m), Ti (99.5% purity, ∼25 m), B4 C (98.0% purity, ∼25 m) and graphite (99.5% purity, ∼38 m), respectively, and the designed molar ratios of the products as well as the compositions of materials are shown in Table 1. The powder blends were mixed sufficiently by ball milling for 8 h and then pressed into cylindrical preforms (20 mm diameter)
Fig. 1. Change of standard Gibbs free energy as a temperature for Eqs. (1)–(3).
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to evaluate the wear resistance. Microstructures were examined using scanning electron microscopy (SEM) (Model Jeol JSM-5310, Japan) together with energy-dispersive spectrometry (EDS) (Model Link-Isis, Britain) and phases were identified using X–ray diffraction (XRD) (Model D/Max 2500PC Rigaku, Japan) with CuK␣ radiation. 3. Results and discussion 3.1. Theory background Several groups of TiC/TiB2 ceramics can be synthesized with different chemical molar ratios between Ti and B4 C through following chemical reactions: 3/4Ti + 1/4B4 C = 1/2TiB2 + 1/4TiC
(1)
2/3Ti + 1/6B4 C + 1/6C = 1/3TiB2 + 1/3TiC
(2)
3/5Ti + 1/10B4 C + 3/10C = 2/5TiC + 1/5TiB2
(3)
The Gibbs free energy G0 of reactions (1), (2) and (3) have been calculated using the thermodynamic data from Ref. [21], and the results are shown in Fig. 1. It can be seen that the G0 of the above three reactions are all negative, which indicates that these reactions all can take place. The G0 of reaction (1) is less than those of reactions (2) and (3), which indicates that reaction (1) is most likely to occur. In the SHS process, the possible maximum temperature during the exothermic reaction is attained under adiabatic conditions without heat loss. The theoretical limit, designated as the adiabatic temperature Tad , can be calculated from the heat capacities and enthalpies of formation and transformation [22]. Tad is an important thermodynamic parameter of combustion synthesis reactions. It can be used in a semi-quantitative way to ascertain whether or not the synthesis of a given material can be accomplished by a self-propagating method. It has been empirically suggested that combustion reactions will not become self-sustaining unless Tad ≥1800 K [23]. The adiabatic temperature of SHS reaction at room temperature can be theoretically calculated using the thermodynamic data from the reference [21] according to the equation: Tad H(298) + nj Cp (Pj )dT + nj L(Pj ) = 0 (4) 298
298−Tad
Where, H (298) is the reaction enthalpy at 298 K, Cp (Pj ) and L(Pj ) are the heat capacity and latent heat of the products, Pj and nj refer to the products and the mole fractions, respectively. The calculated adiabatic temperatures Tad of samples 1–3 in Table 1 are 2902, 2827 and 2995 K, respectively. The adiabatic temperatures are all greater than 1800 K, which indicated that the reactions can be self-sustained, namely SHS can occur. 3.2. Differential thermal analysis Because the liquid Ni formed with solid TiC and TiB2 for the low wetting angle under vacuum at 1450 ◦ C [24], nickel was selected as the additive metal, which not only decreased
Fig. 2. DTA curves for various compositions heated in argon environments at a heating rate of 30 ◦ C/min to a 1200 ◦ C.
the ignition temperature, but also improved the abrasive and structural applications. To study the SHS reaction temperatures of the Ni–Ti–B4 C and Ni–Ti–B4 C–C systems, 40 ± 1 mg reactant mixtures of samples 1, 2 and 3 were heated at 30 ◦ C/min to about 1200 ◦ C under an argon atmosphere in the DTA apparatus and the results were shown in Fig. 2. The DTA curves indicate that two significant exothermic peaks with maxima at about 1037 ◦ C and 1133 ◦ C appeared in three samples, respectively. However, it should be noticed that the heat rate is fast and will lead to a slight shift of the reaction peaks to higher temperatures. In order to develop an understanding of the SHS reactions of the three samples, experiments quenched from about 1037 ◦ C were carried out in the three designed mixtures, respectively, and the results are shown in Fig. 3(a). The XRD results indicate that a large quantity of Ti2 Ni, Ni2 B, and some TiB2 and TiC were detected in the three samples, which indicated that the exothermic peak with the maximum at 1037 ◦ C mainly corresponded to the formation of Ti2 Ni and Ni2 B compounds. Fig. 3(b) shows the XRD results of the DTA products heated to 1200 ◦ C. According to the XRD results, Ni, TiB2 and TiC phases as well as the intermediate phases Ni3 Ti, Ni2 B and C were detected in three samples. Therefore, it can be deduced that external heat and the evolved heat from the above exothermic reactions at 1037 ◦ C could promote the reaction of TiC and TiB2 formation, which corresponds to the exothermic peak with the maximum at 1133 ◦ C. Based on the results, it is believed that nickel serves not only as a diluent but also as a reactant and participates in the SHS reaction. TiC and TiB2 are the thermodynamically stable phase in the final products. According to [22], TiC and TiB2 can be formed at 1600 ◦ C from the Ti and B4 C mixtures. Therefore, the addition of nickel provides an easier route for the TiC and TiB2 formation in the Ni–Ti–B4 C and Ni–Ti–B4 C–C systems. However, the reactions in the DTA apparatus were incomplete and some Ni3 Ti, C and Ni3 B phases were detected in the final products. This may be due to the smaller volume size of the DTA
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401
Fig. 3. XRD patterns (CuK␣ radiation) of DTA products from samples 1, 2 and 3: (a) quenched at 1037 ◦ C and (b) at 1200 ◦ C.
sample, and thus, the heat released by these reactions may dissipate into the surroundings more easily, resulting in an incomplete reaction [12]. 3.3. Phase constituents and microstructures In the present study, the smelting temperature of matrix steel is selected to be about 1500 ◦ C, significantly higher than 1130 ◦ C, which corresponds to the TiC and TiB2 formation temperature in the DTA. When the molten steel was poured into the sand mold, the SHS reactions of the Ni–Ti–B4 C and Ni–Ti–B4 C–C systems were ignited by the heat release of the steel melts. Generally, the SHS reaction products were extremely porous, and therefore, the molten steel will spontaneously infiltrate into the pores of the reacting preform driven by capillary forces during the SHS reaction. Figs. 4 and 5 show the XRD patterns and SEM microstructures of the locally reinforced steel composites fabricated by using sample 1, 2 and 3, respectively. XRD results reveal that the composites mainly consist of TiC, TiB2, austenite and [Fe, Ni]. However, it is worth noting that no Ni3 Ti and Ni2 B compounds are found in the XRD results, which indicate that the reactions in the steel melt are complete. Moreover, the TiC peaks in the composites fabricated by using sample 3 are more intense than those of TiB2 , whereas the TiC peaks in the composites fabricated by using sample 1 are weaker than those of TiB2 , and the intensities of TiC and TiB2 in the composites fabricated by using sample 2 are similar. This was expected given the designed reactant compositions. It is worth noting that the Ni element can sufficiently disssolve into the infiltrated steel melt; therefore, [Fe, Ni] can be formed around ceramics particulates. Fig. 5 revealed a relatively uniform distribution of TiC and TiB2 in the composites. The elongated or rectangular particulates are TiB2 , whereas the nearly spherical particulates are TiC. These particulates morphologies were similar to those observed for monolithic TiB2 and TiC [3]. In the SEM of the composites synthesized by using sample 1, almost all TiB2 particulates have a “clubbed” structure because TiB2 particulates have enough
space to grow amply between the TiC particulates, as shown in Fig. 5(a). From Fig. 5(b), TiB2 particulates exhibit elongated morphologies, while the morphologies of TiC particulates are almost invariant. Moreover, compared with that in Fig. 5(a), TiB2 sizes decrease. With the increase of TiC content, near–spherical TiC particulates connect with each other to form a framework structure, as shown in Fig. 5(c). Thus, the growth of TiB2 was kept within limits and TiB2 only forms in the channel or gap between the TiC particulates and it is difficult to find the clubbed TiB2 particulates. 3.4. Porosity and density Microstructural characterization shows that the presence of significant macro-pores can be observed in the composite syn-
Fig. 4. XRD patterns (CuK␣ radiation) of the locally reinforced steel matrix composites fabricated by using samples 1, 2 and 3.
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Fig. 5. Typical microstructures of the locally reinforced steel matrix composites fabricated by using (a) sample 1, (b) sample 2 and (3) sample 3.
thesized by using sample 3 (as shown in Fig. 6), while macropores and blowholes are absent in the local reinforcing region in the composites synthesized by using samples 1 and 2, which indicates that the different molar ratios of TiC/TiB2 have a significant effect on the fabrication of dense in situ composites. In fact, it is easy to understand the results since the adiabatic temperature Tad (2995 K) of sample 3 is the highest in the three samples. As a result, the evaporation of gas and impurity generated by the SHS reaction is increased, resulting in the formation of the composite with higher porosity. Moreover, it can be seen from Fig. 6 that most of fine TiC and TiB2 particulates are present in the local reinforcing region, although some large strip shape particulates are entrapped inside the Fe-rich region located in
the reinforcing region, as found in our previous work [12]. The formation of fine ceramic particulates in the local reinforcing region is assisted by the reaction-solution-precipitation mechanism during the SHS reaction, while large strip shape ceramic particulates precipitate out of the melts by the nucleation-growth mechanism during solidification [12]. A detailed discussion of this has been given in our previous work [12]. Table 2 shows the densities of the reinforcing region of the composites synthesized by using samples 1, 2 and 3. It can be seen that the densities of the reinforcing region of the composites synthesized by using samples 1 and 2 are higher than that using sample 3. Differences between densities of the locally reinforced composites may have been a consequence of the different thermal expansion coefficients of TiC and TiB2 . The mean thermal expansion coefficients for TiB2 and TiC are 4.6 and 7.95 × 10−6 K−1 , respectively, at 300–1300 K [20]. The higher expansion coefficients associated with the TiC-rich composites may have resulted in cracking as the composite cooled. This would cause the density of the composite synthesized by using sample 3 to be significantly lower than the other two values. Moreover, the higher Tad of sample 3 may have created more porosity, which would contribute to the decrease of density. 3.5. Hardness and wear resistance
Fig. 6. The distribution of porosity of the locally reinforced steel matrix composite fabricated by using sample 3.
Hardness values and wear resistance of the reinforcing region in the composites synthesized by using samples 1, 2 and 3 are listed in Table 2. Generally, the near-spherical morphology of the ceramic particulates made for the improvement of mechanical
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403
Table 2 The densities, hardness values and wear rates of the steel matrix and the composites Materials (g/cm3 )
Density Hardness (HRC) Volumetric wear loss (10−10 m3 /m)
Steel matrix
Sample 1
Sample 2
Sample 3
7.82 <20 2.281
6.64 46.3 0.5463
6.47 42.6 0.7713
6.11 39.8 1.1406
property of the composites. However, the composite synthesized by using sample 1 has the highest hardness value and the lowest volumetric wear loss. It has previously been reported that the clubbed structure of TiB2 was very useful to improve the mechanical properties of TiC/TiB2 ceramics [22]. Apparently, the current results also demonstrate that the introduction of more clubbed TiB2 grains into TiC/TiB2 ceramics resulted in an increase in the hardness and wear resistance of the composites. This can be understood as follows: in the composites synthesized by using sample 1, the 1:2 molar ratio of TiC/TiB2 can weaken the high thermal expansion effect of excessive TiC content and reduce the thermal stresses; however, compared with the formation of pure TiB2 , this can decrease the high reaction heat, which can reduce the porosity caused by the gas volatilization. Therefore, compared with the two composites synthesized by using samples 2 and 3, lower thermal stresses and reduced porosity were achieved in the composites synthesized by using sample 1, which led to a strong interfacial bonding between particulates and matrix. This improves both hardness and wear resistance. 4. Conclusion (1) DTA results reveal that the exothermic reactions of the Ni–Ti–B4 C and Ni–Ti–B4 C–C systems proceed in such a way that Ni initially reacts with B4 C and Ti to form Ni2 B and Ti2 Ni, respectively, with heat evolution at 1037 ◦ C; Subsequently, external heat and the evolved heat from these exothermic reactions promote the reactions forming TiC and TiB2 at 1130 ◦ C. Nickel serves not only as a diluent but also as a reactant and participates in the SHS reaction; TiC and TiB2 are the thermodynamically stable phase in the final products. In addition to the Ni, TiB2 and TiC phases, Ni3 Ti, Ni–B compounds and residual C are also detected by XRD in the products, which indicate the DTA exotherm is recording an incomplete reaction. (2) Steel matrix composites locally reinforced with different molar ratios of in situ TiC and TiB2 (2:1, 1:1 and 1:2, respectively) particulates are fabricated successfully utilizing the SHS reactions of Ni–Ti–B4 C and Ni–Ti–B4 C–C systems during casting. As-cast microstructures of the in situ processed composites reveal a relatively uniform distribution of TiC/TiB2 particulates in the local reinforcing regions. In the composites reinforced with 1:2 molar ratio of TiC/TiB2 , almost all TiB2 grains have clubbed structures, while TiC grains exhibit near-spherical morphologies. With the increase of TiC content, TiB2 sizes decrease. In particular, in the composites reinforced with 2:1 molar ratio of
TiC/TiB2 , the growth of TiB2 was kept within limits, thereby restricting the formation of clubbed TiB2 grains. (3) Macro-pores and blowholes are absent in the local reinforcing region of the composites reinforced with 1:1 and 1:2 molar ratios of TiC/TiB2 , while a few macro-pores can be observed in the composite reinforced with 2:1 molar ratios of TiC/TiB2 . Moreover, the densities of composites reinforced with 1:1 and 1:2 molar ratios of TiC/TiB2 are higher than that of the composite reinforced with 2:1 molar ratio of TiC/TiB2 . (4) The introduction of more clubbed TiB2 grains into TiC/TiB2 ceramics resulted in an increase in the hardness and wear resistance of the local reinforcing region in the composites; and the composite reinforced with 1:2 molar ratio of TiC/TiB2 has the highest hardness and the best wear resistance. Acknowledgements This work is supported by The National Natural Science Foundation of China (No. 50531030) and The Ministry of Science and Technology of the People’s Republic of China (No. 2005CCA00300) as well as The Project 985-Automotive Engineering of Jilin University. References [1] S.Y. Chang, S.J. Cho, S.K. Hong, D.H. Shin, J. Alloys Compd. 316 (2001) 275–279. [2] E. Pagounis, V.K. Lindroos, Mater. Sci. Eng. A 246 (1998) 221–234. [3] Q.C. Jiang, B.X. Ma, H.Y. Wang, Y. Wang, Y.P. Dong, Composites Part A37 (2005) 133–138. [4] H.Y. Wang, Q.C. Jiang, X.L. Li, F. Zhao, J. Alloys Compd. 366 (2004) 9–12. [5] S.C. Tjong, Z.Y. Ma, Mater. Sci. Eng. R29 (2000) 49–113. [6] J.J. Moore, H.J. Feng, Prog. Mater. Sci. 39 (1995) 243–273. [7] J.C. Han, X.H. Zhang, J.V. Wood, Mater. Sci. Eng. A280 (2000) 328–333. [8] H.Y. Wang, Q.C. Jiang, B.X. Ma, Y. Wang, F. Zhao, Adv. Eng. Mater. 7 (2005) 58–63. [9] Y.S. Wang, X.Y. Zhang, G.T. Zeng, F.C. Li, Composites Part A 32 (2001) 281–286. [10] Y.S. Wang, X.Y. Zhang, F.C. Li, G.T. Zeng, Mater. Des. 20 (1999) 233–236. [11] C. Tassin, F. Laroudie, M. Pons, L. Lelait, Surf. Coat. Technol. 80 (1996) 207–210. [12] H.Y. Wang, L. Huang, Q.C. Jiang, Mater. Sci. Eng. A407 (2005) 98–104. [13] D. Suh, S. Lee, S.J. Kwon, Y.M. Koo, Metall. Mater. Trans. A 28A (1997) 1499–1508. [14] S.J. Kwon, S.H. Choo, S. Lee, Scr. Mater. 40 (1998) 235–240. [15] S. Tondu, T. Schnick, L. Pawlowski, B. Wielage, S. Steinhauser, L. Sabatier, Surf. Coat. Technol. 123 (2000) 247–251. [16] C.C. Degnan, P.H. Shipway, Wear 252 (2002) 832–841.
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