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TiB2 particulates locally reinforced steel matrix composites
Materials and Design 40 (2012) 64–69
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effect of Cu content in Cu–Ti–B4C system on fabricating TiC/TiB2 particulates locally reinforced steel matrix composites Yunhong Liang, Zhiwu Han, Zhihui Zhang, Xiujuan Li, Luquan Ren ⇑ The Key Laboratory of Bionic Engineering, Ministry of Education, Jilin University, Changchun 130025, PR China
a r t i c l e
i n f o
Article history: Received 15 January 2012 Accepted 14 March 2012 Available online 23 March 2012 Keywords: A. Metal matrix composites C. Casting E. Wear
a b s t r a c t The austenite manganese steel–matrix composites locally reinforced with in situ TiC/TiB2 particulates were successfully fabricated using self-propagating high-temperature synthesis (SHS) reactions in a Cu–Ti–B4C system and the effect of the Cu content during casting has been investigated. The local reinforcing regions in the steel matrix composites mainly consist of TiC, TiB2, Cu and austenite, without any intermediate phases. With the increase in the Cu content, the size of the ceramic particulates in the local reinforcing regions decreases, the interface bonding between reinforcing region and matrix becomes poor and the macro-pores and blowholes in the local reinforcing regions become fewer. Moreover, with the increase in the Cu content, the hardness value decreases monotonically, while the wear resistance of the reinforced region increases first then decreases. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Ceramic particle reinforced metal matrix composite has higher strength and wear resistance than its matrix material because the ceramic phases can strongly resist abrasive wear [1–4]. Gopalakrishnan et al. [5] reported that the TiCp/Al composites fabricated by the stir casting method could significantly enhance the specific strength and wear resistance of the Al alloys. Wang et al. [6] investigated the effect of the boron content on the wear resistance of the TiC/Ti6Al4 V composites, the two phases TiC–TiB reinforced Ti6Al4 V matrix composite possesses the best wear resistance. Under above discussion, it can be seen that most of the work on MMC is centered on light metal alloys to improve upon their strength and stiffness. Recently, there is also considerable interest in developing iron-and steel-based MMCs [7,8]. Zhong et al. [9] reported that the addition of in situ synthesized TiC particulates in the iron matrix could largely improved the hardness and wear resistance of the gray cast iron. In the present work, we attempt to introduce some TiC/TiB2 particulates into the steel matrix by Cu–Ti–B4C system to improve the wear resistance of the steel matrix. Recently, there has been an interest in the liquid-based routes involving the in situ formation of the ceramic particles in casting process by a liquid reaction [10,11]. The composites containing ceramic reinforcements generated in situ are likely to have virgin uncontaminated interfaces and can be further tailored during ⇑ Corresponding author. Tel./fax: +86 431 8509 5760. E-mail address: [email protected] (L. Ren). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.03.023
the solidification process to promote an adequate bonding between the matrix and the reinforcements. Self-propagating high-temperature synthesis (SHS) and traditional casting routes provide an easy process to produce ceramic particulate reinforced steel matrix composites [12,13]. Among these composites, in situ ceramic particulate locally reinforced steel matrix composite may be one of the most promising composites which can be widely used in the practical application [14–16]. It combines the advantages of the SHS reaction with the easiness and high efficiency [17–19] in the casting process. On the other hand, compared with the monolithic composites, the locally reinforced composites are promising type of materials due to reducing processing cost and improvement on poor castability [20]. It is well known that a casting process is the most effective way to obtain low cost MMCs in conventional process. Therefore, in the present work, we highlight a potential in situ process in which SHS plus traditional casting techniques to produce steel matrix composites. Austenite manganese steel as a wear-resistant material has excellent strength and toughness. However, due to no particle phases in the matrix, its wear resistance is poor [21]. In our previous study [22], austenite manganese steel matrix composite locally reinforced with in situ synthesized TiB2/TiC particulate was fabricated successfully using the SHS reaction in a Ni–Ti–B4C–C system during casting. The wear resistance of the reinforced region is significantly higher than that of the matrix. The Cu is a well-known alloying element, which is used to improve the diffusion activation of carbon in austenite manganese steel. In the present study, the feasibility of the fabrication of in situ TiB2/TiC particulates locally
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reinforced steel matrix composites by the SHS reaction in a Cu–Ti– B4C system with different Cu contents during casting was investigated. Furthermore, the effects of the Cu content on the microstructures, hardness and wear resistance of the reinforcing regions were also discussed in detail. The purpose of the use of SHS and casting routes is to provide a new and promising process for the production of steel matrix composites because of its significant inherent simplicity and potential cost-effectiveness in scale-up manufacturing.
2. Experimental details The starting materials were made from commercial powders of Cu (99.0%, 45 lm), Ti (99.5%, 38 lm) and B4C (99.9%, 3.5 lm). The Ti and B4C powders with a ratio corresponding to that of stoichiometric 2TiB2–TiC mixed with 10–60 wt.% Cu content were used for the powder blends. The reactant powders were mixed by a ball-milling with a rotation of 35 rpm for 8 h to ensure homogeneity, and then pressed into cylindrical compacts (about 22 mm in diameter and 12 ± 2 mm in length) using a stainless steel die to obtain densities of 65 ± 2% theoretical density. The medium carbon steel (0.45C–0.42Si–0.30Mn–Fe balance, all in wt.%) melt was prepared in a 5 kg medium-frequency induction furnace in air environment. After being dried in a vacuum oven at about 300 °C for 3 h to remove any trace of moisture, the compacts with various contents of Cu were placed on the bottom of the sand mold, as schematically illustrated in Ref. [23]. Subsequently, the steel melts with a temperature of about 1500 °C was poured into the sand mold to ignite the SHS reactions of these compacts. After solidification and cooling, composite castings were removed and sectioned in the side position. The microstructure and mechanical properties of the locally reinforced steel matrix composites are significantly influenced by the phase components and purity degree of the products in the locally reinforcing region. Due to the high combustion temperature and extremely fast velocity of the SHS reaction of the Cu–Ti–B4C system in the liquid steel, knowledge of the microscopic reaction mechanisms involved in these processes is still quite limited. Thus, the SHS experiments were conducted first in a self-made vacuum vessel in order to offer some guidance to the fabrication and investigation of the locally reinforced steel matrix composites. The SHS experiments were conducted in a self-made vacuum vessel filled with Ar at 1 atm. The compacts were ignited on the graphite flat which was placed at the top of tungsten electrode and heated by the heat of arc. A small hole (2 mm in diameter and 2 mm in depth) was drilled at the top of the compact, and a thermocouple pair of W/Re5–W/Re26 (0.5 mm in diameter) was inserted into the hole and linked up with an temperature acquisition recorder by means of which a temperature–time curve could be recorded. It is worth noting that the current in the present experiment is selected as 90A. The wear experiments were conducted on a pin-on-disk machine under an applied load of 35 N. The commercial SiC abrasive papers with the abrasive particle size of 20 lm (600 grit) were employed as the counterface. The samples were thoroughly cleaned with ethanol in ultrasonic vibrator before and after the wear test. The wear rate is defined as volume loss divided by sliding distance, and the volume loss is obtained from the ratio of weight loss to the density of the sample. The weight loss was measured using an electronic balance having a resolution of 0.0001 g. The densities of the samples were measured by Archimede’s water-immersion method. Microstructures were examined using scanning electron microscopy (SEM) (Model JSM-5310, Japan) together with energy-dispersive spectrometry (EDS) (Model LinkIsis, Britain). The phases were identified using X-ray diffraction (XRD) (Model D/Max 2500PC Rigaku, Japan).
Fig. 1. Variation of the Tc and ignition time in the Cu–Ti–B4C system with various Cu contents.
3. Results and discussion 3.1. SHS process The SHS reactions in the Ti–B4C system are somewhat difficult to initiate due to the high melting temperatures of the reactants and lack of a pre-activation reaction [24]. The Cu is selected as the additive metal in the present study, which not only decreases the ignition temperature, but also improves the abrasive and structural applications. Fig. 1 shows the variation of the combustion temperature (Tc) and ignition time for the Cu–Ti–B4C compacts with various Cu contents during the SHS process in the combustion chamber. The time from heating to the occurrence of SHS reaction is defined as the ignition time. It can be seen that with the increase in the Cu content, the combustion temperature (Tc) decreases and the ignition time decreases first then increases.
Fig. 2. XRD patterns of the SHS reaction products with various Cu contents.
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Fig. 2 shows the XRD results of the SHS products. It can be seen that the products consist of TiC, TiB2 and Cu without any intermediate phases, indicating that the reaction is complete. According to the literatures [4,22,25], the intermediate phases (MexTiy and MexBy) would be formed during the reaction process in the Me-Ti-B4C systems. In our previous study, the existence of the intermediate phases is largely related to the content of metal and the size of the B4C particle. In the present study, the Cu content is relatively low and the size of the B4C particle is finer, which are beneficial to the complete reaction in the Cu–Ti–B4C system. According to the XRD results, the pure TiC/TiB2 ceramic particulates are formed during the SHS process. Moreover, under the above discussion, it can be confirmed that the parameters (Cu content and B4C particle size) are proper to fabricate the locally reinforced steel matrix composites. Fig. 3a and b shows the microstructures of the SHS products with 10 and 30 wt.% Cu content, respectively. As indicated, the elongated or rectangular particles are TiB2, while the nearly spherical particles are TiC. With the increase in the Cu content, the quantity of the TiC and TiB2 ceramics reduces and their size decreases considerably. The size reduction of the ceramic particles may be caused by the lower combustion temperature of the compact with higher Cu content, because crystal growth is an exponential function of the combustion temperature [26,27]. 3.2. Phase components and microstructure The DTA results of the Cu–Ti–B4C (Cu 6 3 lm) system in the previous experiment [28] shows that the initial formation temperature of the TiC and TiB2 could be prophesied at about 995 °C. In the present study, the smelting temperature of the matrix steel is about 1500 °C, which significantly higher than the formation temperature of the TiC and TiB2 (995 °C). When the molten steel was poured into the sand mold, the SHS reaction of the Cu–Ti–B4C system was ignited by the heat release of the steel melts. XRD patterns of the locally reinforced austenite manganese steel–matrix composites fabricated in the Cu–Ti–B4C systems with 10, 30 and 50 wt.% Cu content are shown in Fig. 4. The XRD results reveal that the composites mainly consist of TiC, TiB2, Cu and austenite without any intermediate phases. It indicates that all the reactions in the molten steel are complete. Fig. 5a–c shows the SEM microstructures of the local reinforcing region synthesized in the Cu–Ti–B4C systems with 10, 30 and 50 wt.% Cu content, respectively. The TiC and TiB2 particulates in the locally reinforced regions exhibit a relatively uniform distribution. The elongated or rectangular particulates are TiB2, while the nearly spherical particulates are TiC. These particulate morphologies are similar to those observed for monolithic TiB2 and TiC. It can be clearly seen that the size of the TiC and TiB2 particulates decreases with the increase in the Cu content. According to Fig. 1, with the increase in the Cu content, the combustion temperature decreases, and the growth of the
Fig. 4. XRD patterns of the local reinforcing region fabricated in the Cu–Ti–B4C system with various Cu contents.
ceramic particles is an exponential function of the combustion temperature. 3.3. Macro-interface between reinforcing region and matrix Fig. 6a–c shows the interface micrographs of the particulate locally reinforced steel matrix composites fabricated in the Cu–Ti– B4C system with 10, 30 and 50 wt.% Cu content, respectively. With the Cu content of 10 and 30 wt.%, the metallurgy-bonding between the reinforcing region and matrix in the steel matrix composite is a good. For the composite with 50 wt.% Cu content, the interface bonding becomes poor. Moreover, there are some large cracks between the reinforcing region and matrix. In fact, it is easy to understand this difference based on the kinetics of the SHS reaction. According to [29], the solidification time of the steel melt is estimated to be 62.7 s. It can be seen from Fig. 2 that the SHS reaction in the Cu–Ti–B4C system with 10, 30 and 50 wt.% Cu content are ignited after being heated for 8.95, 11.6 and 13.9 s, respectively, which are much shorter than the solidification time of the steel melt. High combustion temperature makes sure that there is sufficient time for the steel melt to infiltrate into the reacting compact,
Fig. 3. Typical microstructures of the SHS products with (a) 10 wt.% Cu and (b) 30 wt.% Cu.
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Fig. 5. Typical microstructures of the local reinforcing region fabricated in the Cu–Ti–B4C system with (a) 10 wt.% Cu, (b) 30 wt.% Cu and (c) 50 wt.% Cu.
Fig. 6. Interface micrographs between steel matrix and reinforcing region of the locally reinforced steel matrix composites fabricated in the Cu–Ti–B4C systems with (a) 10 wt.% Cu, (b) 30 wt.% Cu and (c) 50 wt.% Cu.
leading to a good interface bonding. However, with the increase in the Cu content, the combustion temperature decreases, that is, high Cu content leads to lower combustion temperature. When the SHS reaction in the compact occurs, some steel melt starts to solidify and the fluidity of melt becomes poor, leading to the insufficient infiltration of steel melt and the formation of a poor interface bonding. 3.4. Porosity, hardness and wear resistance The distribution of macro-pores and blowholes in the local reinforcing region in the steel matrix composites fabricated in the Cu–
Ti–B4C system with 10, 30 and 50 wt.% Cu content is shown in Fig. 7a–c, respectively. It is obvious that a large number of macro-pores and blowholes exist in the local reinforcing region of the composite fabricated in the Cu–Ti–B4C system with 10 wt.% Cu content, while few macro-pores and blowholes can be found in the local reinforcing region of the composites fabricated in the Cu–Ti–B4C system with 30 and 50 wt.% Cu content. It reveals that near fully dense locally reinforced composite can be successfully fabricated in the Cu–Ti–B4C system with 30 and 50 wt.% Cu content. The decrease of the porosity is directly related to the solidification time of the steel melt and the SHS reaction kinetics of the compact. Shorter ignition time and higher combustion tempera-
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Fig. 7. The distribution of porosities in the local reinforcing region fabricated in the Cu–Ti–B4C systems with (a) 10 wt.% Cu, (b) 30 wt.% Cu and (c) 50 wt.% Cu.
Table 1 The hardness values and wear rate of the steel matrix and the reinforcing regions. Steel matrix
Hardness (HRC) Wear rate (10 10 m3/m)
<20 3.42
Local reinforcing regions (Cu contents, wt.%) 10
20
30
40
50
60
50.2 1.17
48.9 1.09
48.5 0.92
45.8 1.35
40.6 1.97
38.3 2.35
ture are in favor of the sufficient infiltration of the steel melt and the gas evaporation from the melt, which can decrease the porosity. However, too low Cu content leads to the higher combustion temperature, which is high enough to cause the evolution of most impurities in the reactants. The gas evaporation and the release of these contaminants would be violent. Consequently, gas channels and open cracks can form in the reactants and in some instances the reaction may be completely disrupted, resulting in the compacts being blown apart. It can be seen from Fig. 1 that the combustion temperature decreases with the increase in the Cu content. Therefore, it can be concluded that the amount of the pores generated by the gas evaporation decreases with the increase in the Cu content in the compacts. Thus, the composites synthesized with 30 and 50 wt.% Cu content exhibit the fewest pores. The hardness values and wear rate of the steel matrix and the reinforced region are listed in Table 1. Due to the presence of high volume fraction of ceramics, the hardness values and wear resistance of the reinforced region are significantly higher than those of the matrix. With the increase in the Cu content, the hardness of the reinforced region decreases monotonically and the wear rate decreases first then increases. The reinforcing region of the composite synthesized in the Cu–Ti–B4C system with 30 wt.% Cu content possesses the highest wear resistance. The reinforcing region synthesized in the Cu–Ti–B4C system with low content of Cu exhibits the lower wear resistance, which is mainly attributed to the existence of large numbers of pores. The reinforcing region synthesized in the Cu–Ti–B4C system with relatively high Cu content also has the lower wear resistance, which is mainly attributed to reduction of the content of the TiC and TiB2 particles.
4. Conclusions The local reinforcing region in the steel matrix composites synthesized in the Cu–Ti–B4C system with various Cu contents mainly consist of TiC, TiB2, Cu and austenite without any intermediate phases. The microstructures of the local reinforcing region reveal a relatively uniform distribution of the TiC/TiB2. Moreover, the size of the ceramic particulates in the local reinforcing regions decreases with the increase in the Cu content. With the increase in Cu content, the interface bonding between the reinforcing region and matrix becomes poor, the macro-pores and blowholes in the local reinforcing region become fewer, the hardness of the reinforcing region decreases monotonically and the wear rate decreases first then increases. Acknowledgements This work is supported by Cooperative Innovation Platform of National Oil Shale Exploration Development and Research, the National Natural Science foundation for Youths (Nos. 51005097 and 51175220), the Science and Technology Development Project of Jilin Province (No. 201201025), and Fundamental Science Research Funds for the Central Universities (No. 450060445671). References [1] Liu JJ, Liu ZD. An experimental study on synthesizing TiC–TiB2–Ni composite coating using electro-thermal explosion ultra-high speed spraying method. Mater Lett 2010;64:684–7. [2] Cook BA, Peters JS, Harringa JL. Russell AM. Enhanced wear resistance in AlMgB14–TiB2 composites. Wear 2011;271:640–6.
Y. Liang et al. / Materials and Design 40 (2012) 64–69 [3] Venkataraman B, Sundararajan G. The sliding wear behaviour of Al–SiC particulate composites-II. The characterization of subsurface deformation and correlation with wear behaviour. Acta Mater 1996;44:461–73. [4] Zou BL, Shen P, Cao XQ, Jiang QC. The mechanism of thermal explosion (TE) synthesis of TiC–TiB2 particulate locally reinforced steel matrix composites from an Al–Ti–B4C system via a TE-casting route. Mater Chem Phys 2012;132:51–61. [5] Gopalakrishnan S, Murugan N. Production and wear characterisation of AA 6061 matrix titanium carbide particulate reinforced composite by enhanced stir casting method. Composites Part B 2012;43:302–8. [6] Wang X, Ma XL, Li XL, Dong LH, Wang MJ. Effect of boron addition on microstructure and mechanical properties of TiC/Ti6Al4V composites. Mater Des 2012;36:41–6. [7] Li BH, Liu Y, Li J, Gao SJ, Cao H, He L. Effect of tungsten addition on the microstructure and tensile properties of in situ TiB2/Fe composite produced by vacuum induction melting. Mater Des 2010;31:877–83. [8] Akhtar F. Microstructure evolution and wear properties of in situ synthesized TiB2 and TiC reinforced steel matrix composites. J Alloy Comp 2008;459:491–7. [9] Zhong LS, Xu YH, Hojamberdiev M, Wang JB, Wang J. In situ fabrication of titanium carbide particulates-reinforced iron matrix composites. Mater Des 2011;32:3790–5. [10] Wang YS, Ding YC, Wang J, Cheng FJ, Shi JG. In situ production of vanadium carbide particulates reinforced iron matrix surface composite by castsintering. Mater Des 2007;28:2202–6. [11] Kattamis TZ, Suganuma T. Solidification processing and tribological behavior of particulate TiC-ferrous matrix composites. Mater Sci Eng A 1990;128:241–52. [12] Das K, Bandyopadhyay TK, Das S. A review on these various synthesis routes of TiC reinforced ferrous based composites. J Mater Sci 2002;37:3881–92. [13] Wang Y, Zhang ZQ, Wang HY, Ma BX, Jiang QC. Effect of Fe content in Fe–Ti–B system on fabricating TiB2 particulate locally reinforced steel matrix composites. Mater Sci Eng A 2006;422:339–45. [14] Wang HY, Jiang QC, Ma BX, Wang Y, Zhao F. Fabrication of steel matrix composite locally reinforced with in situ TiB2 particulate using selfpropagating high-temperature synthesis reaction of Ni–Ti–B using system during casting Source. Adv Eng Mater 2005;7:58–63. [15] Wang HY, Huang L, Jiang QC. In situ synthesis of TiB2–TiC particulates locally reinforced medium carbon steel–matrix composites via the SHS reaction of Ni–Ti–B4C system during casting. Mater Sci Eng A 2005;407:98–104.
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[16] Jiang QC, Zhao F, Wang HY, Zhang ZQ. In situ TiC–reinforced steel composite fabricated via self-propagating high-temperature synthesis of Ni–Ti–C system. Mater Lett 2005;59:2043–7. [17] Tjong SC, Ma ZY. Microstructural and mechanical characteristics of in situ metal matrix composites. Mater Sci Eng R 2000;29:49–65. [18] Saidi A, Chrysanthou A, Wood JV, Kellie JLF. Characteristics of the combustion synthesis of TiC and Fe–TiC composites. J Mater Sci 1994;29:4993–8. [19] Choi Y, Lee JK, Mullins ME. Densification process of TiC–Nix composites f or med by self-propagating high-temperature synthesis reaction. J Mater Sci 1997;32:1717–24. [20] Suh DW, Lee SH, Kwon SJ, Koo YM. Surface hardening of a ductile-cast iron roll using high-energy electron beams. Metall Mater Trans A 1997;28:1499–508. [21] He ZM, Jiang QC, Fu SB, Xie JP. Improved work-hardening ability and wearresistance of austenitic manganese steel under non-severe impact-loading conditions. Wear 1987;120:305–19. [22] Yang YF, Wang HY, Liang YH, Zhao RY, Jiang QC. 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. Mater Sci Eng A 2007;445–446:398–404. [23] Jiang QC, Ma BX, Wang HY, Wang Y, Dong YP. Fabrication of steel matrix composites locally reinforced with in situ TiB2–TiC particulates using selfpropagating high-temperature synthesis reaction of Al–Ti–B4C system during casting. Composites Part A 2006;37:133–8. [24] Shen P, Zou BL, Jin CB, Jiang QC. Reaction mechanism in self-propagating high temperature synthesis of TiC–TiB2/Al composites from an Al–Ti–B4C system. Mater Sci Eng A 2007;454–455:300–9. [25] Yang YF, Wang HY, Zhao RY, Liang YH, Jiang QC. Effect of Ni content on the reaction behaviors of self-propagating high-temperature synthesis in the Ni– Ti–B4C system. Int J Refract Met H 2008;26:77–83. [26] Choi Y, Thee SW. Effect of aluminium addition on the combustion reaction of titanium and carbon to form TiC. J Mater Sci 1993;28:6669–75. [27] Benoit C, Ellen H, Nikhil K, Dominique V, Sylvain D. TiC nucleation/growth processes during SHS reactions. Powder Tech 2005;157:92–9. [28] Liang YH, Wang HY, Yang YF, Du YL, Jiang QC. Reaction path of the synthesis of TiC–TiB2 in Cu–Ti–B4C system. Int J Refract Met H 2008;26:383–8. [29] Yang YF, Wang HY, Liang YH, Zhao RY, Jiang QC. Effect of C particle size on the porous formation of TiC particulate locally reinforced steel matrix composites via the SHS reaction of Ni–Ti–C system during casting. Mater Sci Eng A 2008;474:355–62.
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effect of Cu content in Cu–Ti–B4C system on fabricating TiC/TiB2 particulates locally reinforced steel matrix composites Yunhong Liang, Zhiwu Han, Zhihui Zhang, Xiujuan Li, Luquan Ren ⇑ The Key Laboratory of Bionic Engineering, Ministry of Education, Jilin University, Changchun 130025, PR China
a r t i c l e
i n f o
Article history: Received 15 January 2012 Accepted 14 March 2012 Available online 23 March 2012 Keywords: A. Metal matrix composites C. Casting E. Wear
a b s t r a c t The austenite manganese steel–matrix composites locally reinforced with in situ TiC/TiB2 particulates were successfully fabricated using self-propagating high-temperature synthesis (SHS) reactions in a Cu–Ti–B4C system and the effect of the Cu content during casting has been investigated. The local reinforcing regions in the steel matrix composites mainly consist of TiC, TiB2, Cu and austenite, without any intermediate phases. With the increase in the Cu content, the size of the ceramic particulates in the local reinforcing regions decreases, the interface bonding between reinforcing region and matrix becomes poor and the macro-pores and blowholes in the local reinforcing regions become fewer. Moreover, with the increase in the Cu content, the hardness value decreases monotonically, while the wear resistance of the reinforced region increases first then decreases. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Ceramic particle reinforced metal matrix composite has higher strength and wear resistance than its matrix material because the ceramic phases can strongly resist abrasive wear [1–4]. Gopalakrishnan et al. [5] reported that the TiCp/Al composites fabricated by the stir casting method could significantly enhance the specific strength and wear resistance of the Al alloys. Wang et al. [6] investigated the effect of the boron content on the wear resistance of the TiC/Ti6Al4 V composites, the two phases TiC–TiB reinforced Ti6Al4 V matrix composite possesses the best wear resistance. Under above discussion, it can be seen that most of the work on MMC is centered on light metal alloys to improve upon their strength and stiffness. Recently, there is also considerable interest in developing iron-and steel-based MMCs [7,8]. Zhong et al. [9] reported that the addition of in situ synthesized TiC particulates in the iron matrix could largely improved the hardness and wear resistance of the gray cast iron. In the present work, we attempt to introduce some TiC/TiB2 particulates into the steel matrix by Cu–Ti–B4C system to improve the wear resistance of the steel matrix. Recently, there has been an interest in the liquid-based routes involving the in situ formation of the ceramic particles in casting process by a liquid reaction [10,11]. The composites containing ceramic reinforcements generated in situ are likely to have virgin uncontaminated interfaces and can be further tailored during ⇑ Corresponding author. Tel./fax: +86 431 8509 5760. E-mail address: [email protected] (L. Ren). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.03.023
the solidification process to promote an adequate bonding between the matrix and the reinforcements. Self-propagating high-temperature synthesis (SHS) and traditional casting routes provide an easy process to produce ceramic particulate reinforced steel matrix composites [12,13]. Among these composites, in situ ceramic particulate locally reinforced steel matrix composite may be one of the most promising composites which can be widely used in the practical application [14–16]. It combines the advantages of the SHS reaction with the easiness and high efficiency [17–19] in the casting process. On the other hand, compared with the monolithic composites, the locally reinforced composites are promising type of materials due to reducing processing cost and improvement on poor castability [20]. It is well known that a casting process is the most effective way to obtain low cost MMCs in conventional process. Therefore, in the present work, we highlight a potential in situ process in which SHS plus traditional casting techniques to produce steel matrix composites. Austenite manganese steel as a wear-resistant material has excellent strength and toughness. However, due to no particle phases in the matrix, its wear resistance is poor [21]. In our previous study [22], austenite manganese steel matrix composite locally reinforced with in situ synthesized TiB2/TiC particulate was fabricated successfully using the SHS reaction in a Ni–Ti–B4C–C system during casting. The wear resistance of the reinforced region is significantly higher than that of the matrix. The Cu is a well-known alloying element, which is used to improve the diffusion activation of carbon in austenite manganese steel. In the present study, the feasibility of the fabrication of in situ TiB2/TiC particulates locally
Y. Liang et al. / Materials and Design 40 (2012) 64–69
65
reinforced steel matrix composites by the SHS reaction in a Cu–Ti– B4C system with different Cu contents during casting was investigated. Furthermore, the effects of the Cu content on the microstructures, hardness and wear resistance of the reinforcing regions were also discussed in detail. The purpose of the use of SHS and casting routes is to provide a new and promising process for the production of steel matrix composites because of its significant inherent simplicity and potential cost-effectiveness in scale-up manufacturing.
2. Experimental details The starting materials were made from commercial powders of Cu (99.0%, 45 lm), Ti (99.5%, 38 lm) and B4C (99.9%, 3.5 lm). The Ti and B4C powders with a ratio corresponding to that of stoichiometric 2TiB2–TiC mixed with 10–60 wt.% Cu content were used for the powder blends. The reactant powders were mixed by a ball-milling with a rotation of 35 rpm for 8 h to ensure homogeneity, and then pressed into cylindrical compacts (about 22 mm in diameter and 12 ± 2 mm in length) using a stainless steel die to obtain densities of 65 ± 2% theoretical density. The medium carbon steel (0.45C–0.42Si–0.30Mn–Fe balance, all in wt.%) melt was prepared in a 5 kg medium-frequency induction furnace in air environment. After being dried in a vacuum oven at about 300 °C for 3 h to remove any trace of moisture, the compacts with various contents of Cu were placed on the bottom of the sand mold, as schematically illustrated in Ref. [23]. Subsequently, the steel melts with a temperature of about 1500 °C was poured into the sand mold to ignite the SHS reactions of these compacts. After solidification and cooling, composite castings were removed and sectioned in the side position. The microstructure and mechanical properties of the locally reinforced steel matrix composites are significantly influenced by the phase components and purity degree of the products in the locally reinforcing region. Due to the high combustion temperature and extremely fast velocity of the SHS reaction of the Cu–Ti–B4C system in the liquid steel, knowledge of the microscopic reaction mechanisms involved in these processes is still quite limited. Thus, the SHS experiments were conducted first in a self-made vacuum vessel in order to offer some guidance to the fabrication and investigation of the locally reinforced steel matrix composites. The SHS experiments were conducted in a self-made vacuum vessel filled with Ar at 1 atm. The compacts were ignited on the graphite flat which was placed at the top of tungsten electrode and heated by the heat of arc. A small hole (2 mm in diameter and 2 mm in depth) was drilled at the top of the compact, and a thermocouple pair of W/Re5–W/Re26 (0.5 mm in diameter) was inserted into the hole and linked up with an temperature acquisition recorder by means of which a temperature–time curve could be recorded. It is worth noting that the current in the present experiment is selected as 90A. The wear experiments were conducted on a pin-on-disk machine under an applied load of 35 N. The commercial SiC abrasive papers with the abrasive particle size of 20 lm (600 grit) were employed as the counterface. The samples were thoroughly cleaned with ethanol in ultrasonic vibrator before and after the wear test. The wear rate is defined as volume loss divided by sliding distance, and the volume loss is obtained from the ratio of weight loss to the density of the sample. The weight loss was measured using an electronic balance having a resolution of 0.0001 g. The densities of the samples were measured by Archimede’s water-immersion method. Microstructures were examined using scanning electron microscopy (SEM) (Model JSM-5310, Japan) together with energy-dispersive spectrometry (EDS) (Model LinkIsis, Britain). The phases were identified using X-ray diffraction (XRD) (Model D/Max 2500PC Rigaku, Japan).
Fig. 1. Variation of the Tc and ignition time in the Cu–Ti–B4C system with various Cu contents.
3. Results and discussion 3.1. SHS process The SHS reactions in the Ti–B4C system are somewhat difficult to initiate due to the high melting temperatures of the reactants and lack of a pre-activation reaction [24]. The Cu is selected as the additive metal in the present study, which not only decreases the ignition temperature, but also improves the abrasive and structural applications. Fig. 1 shows the variation of the combustion temperature (Tc) and ignition time for the Cu–Ti–B4C compacts with various Cu contents during the SHS process in the combustion chamber. The time from heating to the occurrence of SHS reaction is defined as the ignition time. It can be seen that with the increase in the Cu content, the combustion temperature (Tc) decreases and the ignition time decreases first then increases.
Fig. 2. XRD patterns of the SHS reaction products with various Cu contents.
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Fig. 2 shows the XRD results of the SHS products. It can be seen that the products consist of TiC, TiB2 and Cu without any intermediate phases, indicating that the reaction is complete. According to the literatures [4,22,25], the intermediate phases (MexTiy and MexBy) would be formed during the reaction process in the Me-Ti-B4C systems. In our previous study, the existence of the intermediate phases is largely related to the content of metal and the size of the B4C particle. In the present study, the Cu content is relatively low and the size of the B4C particle is finer, which are beneficial to the complete reaction in the Cu–Ti–B4C system. According to the XRD results, the pure TiC/TiB2 ceramic particulates are formed during the SHS process. Moreover, under the above discussion, it can be confirmed that the parameters (Cu content and B4C particle size) are proper to fabricate the locally reinforced steel matrix composites. Fig. 3a and b shows the microstructures of the SHS products with 10 and 30 wt.% Cu content, respectively. As indicated, the elongated or rectangular particles are TiB2, while the nearly spherical particles are TiC. With the increase in the Cu content, the quantity of the TiC and TiB2 ceramics reduces and their size decreases considerably. The size reduction of the ceramic particles may be caused by the lower combustion temperature of the compact with higher Cu content, because crystal growth is an exponential function of the combustion temperature [26,27]. 3.2. Phase components and microstructure The DTA results of the Cu–Ti–B4C (Cu 6 3 lm) system in the previous experiment [28] shows that the initial formation temperature of the TiC and TiB2 could be prophesied at about 995 °C. In the present study, the smelting temperature of the matrix steel is about 1500 °C, which significantly higher than the formation temperature of the TiC and TiB2 (995 °C). When the molten steel was poured into the sand mold, the SHS reaction of the Cu–Ti–B4C system was ignited by the heat release of the steel melts. XRD patterns of the locally reinforced austenite manganese steel–matrix composites fabricated in the Cu–Ti–B4C systems with 10, 30 and 50 wt.% Cu content are shown in Fig. 4. The XRD results reveal that the composites mainly consist of TiC, TiB2, Cu and austenite without any intermediate phases. It indicates that all the reactions in the molten steel are complete. Fig. 5a–c shows the SEM microstructures of the local reinforcing region synthesized in the Cu–Ti–B4C systems with 10, 30 and 50 wt.% Cu content, respectively. The TiC and TiB2 particulates in the locally reinforced regions exhibit a relatively uniform distribution. The elongated or rectangular particulates are TiB2, while the nearly spherical particulates are TiC. These particulate morphologies are similar to those observed for monolithic TiB2 and TiC. It can be clearly seen that the size of the TiC and TiB2 particulates decreases with the increase in the Cu content. According to Fig. 1, with the increase in the Cu content, the combustion temperature decreases, and the growth of the
Fig. 4. XRD patterns of the local reinforcing region fabricated in the Cu–Ti–B4C system with various Cu contents.
ceramic particles is an exponential function of the combustion temperature. 3.3. Macro-interface between reinforcing region and matrix Fig. 6a–c shows the interface micrographs of the particulate locally reinforced steel matrix composites fabricated in the Cu–Ti– B4C system with 10, 30 and 50 wt.% Cu content, respectively. With the Cu content of 10 and 30 wt.%, the metallurgy-bonding between the reinforcing region and matrix in the steel matrix composite is a good. For the composite with 50 wt.% Cu content, the interface bonding becomes poor. Moreover, there are some large cracks between the reinforcing region and matrix. In fact, it is easy to understand this difference based on the kinetics of the SHS reaction. According to [29], the solidification time of the steel melt is estimated to be 62.7 s. It can be seen from Fig. 2 that the SHS reaction in the Cu–Ti–B4C system with 10, 30 and 50 wt.% Cu content are ignited after being heated for 8.95, 11.6 and 13.9 s, respectively, which are much shorter than the solidification time of the steel melt. High combustion temperature makes sure that there is sufficient time for the steel melt to infiltrate into the reacting compact,
Fig. 3. Typical microstructures of the SHS products with (a) 10 wt.% Cu and (b) 30 wt.% Cu.
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Fig. 5. Typical microstructures of the local reinforcing region fabricated in the Cu–Ti–B4C system with (a) 10 wt.% Cu, (b) 30 wt.% Cu and (c) 50 wt.% Cu.
Fig. 6. Interface micrographs between steel matrix and reinforcing region of the locally reinforced steel matrix composites fabricated in the Cu–Ti–B4C systems with (a) 10 wt.% Cu, (b) 30 wt.% Cu and (c) 50 wt.% Cu.
leading to a good interface bonding. However, with the increase in the Cu content, the combustion temperature decreases, that is, high Cu content leads to lower combustion temperature. When the SHS reaction in the compact occurs, some steel melt starts to solidify and the fluidity of melt becomes poor, leading to the insufficient infiltration of steel melt and the formation of a poor interface bonding. 3.4. Porosity, hardness and wear resistance The distribution of macro-pores and blowholes in the local reinforcing region in the steel matrix composites fabricated in the Cu–
Ti–B4C system with 10, 30 and 50 wt.% Cu content is shown in Fig. 7a–c, respectively. It is obvious that a large number of macro-pores and blowholes exist in the local reinforcing region of the composite fabricated in the Cu–Ti–B4C system with 10 wt.% Cu content, while few macro-pores and blowholes can be found in the local reinforcing region of the composites fabricated in the Cu–Ti–B4C system with 30 and 50 wt.% Cu content. It reveals that near fully dense locally reinforced composite can be successfully fabricated in the Cu–Ti–B4C system with 30 and 50 wt.% Cu content. The decrease of the porosity is directly related to the solidification time of the steel melt and the SHS reaction kinetics of the compact. Shorter ignition time and higher combustion tempera-
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Fig. 7. The distribution of porosities in the local reinforcing region fabricated in the Cu–Ti–B4C systems with (a) 10 wt.% Cu, (b) 30 wt.% Cu and (c) 50 wt.% Cu.
Table 1 The hardness values and wear rate of the steel matrix and the reinforcing regions. Steel matrix
Hardness (HRC) Wear rate (10 10 m3/m)
<20 3.42
Local reinforcing regions (Cu contents, wt.%) 10
20
30
40
50
60
50.2 1.17
48.9 1.09
48.5 0.92
45.8 1.35
40.6 1.97
38.3 2.35
ture are in favor of the sufficient infiltration of the steel melt and the gas evaporation from the melt, which can decrease the porosity. However, too low Cu content leads to the higher combustion temperature, which is high enough to cause the evolution of most impurities in the reactants. The gas evaporation and the release of these contaminants would be violent. Consequently, gas channels and open cracks can form in the reactants and in some instances the reaction may be completely disrupted, resulting in the compacts being blown apart. It can be seen from Fig. 1 that the combustion temperature decreases with the increase in the Cu content. Therefore, it can be concluded that the amount of the pores generated by the gas evaporation decreases with the increase in the Cu content in the compacts. Thus, the composites synthesized with 30 and 50 wt.% Cu content exhibit the fewest pores. The hardness values and wear rate of the steel matrix and the reinforced region are listed in Table 1. Due to the presence of high volume fraction of ceramics, the hardness values and wear resistance of the reinforced region are significantly higher than those of the matrix. With the increase in the Cu content, the hardness of the reinforced region decreases monotonically and the wear rate decreases first then increases. The reinforcing region of the composite synthesized in the Cu–Ti–B4C system with 30 wt.% Cu content possesses the highest wear resistance. The reinforcing region synthesized in the Cu–Ti–B4C system with low content of Cu exhibits the lower wear resistance, which is mainly attributed to the existence of large numbers of pores. The reinforcing region synthesized in the Cu–Ti–B4C system with relatively high Cu content also has the lower wear resistance, which is mainly attributed to reduction of the content of the TiC and TiB2 particles.
4. Conclusions The local reinforcing region in the steel matrix composites synthesized in the Cu–Ti–B4C system with various Cu contents mainly consist of TiC, TiB2, Cu and austenite without any intermediate phases. The microstructures of the local reinforcing region reveal a relatively uniform distribution of the TiC/TiB2. Moreover, the size of the ceramic particulates in the local reinforcing regions decreases with the increase in the Cu content. With the increase in Cu content, the interface bonding between the reinforcing region and matrix becomes poor, the macro-pores and blowholes in the local reinforcing region become fewer, the hardness of the reinforcing region decreases monotonically and the wear rate decreases first then increases. Acknowledgements This work is supported by Cooperative Innovation Platform of National Oil Shale Exploration Development and Research, the National Natural Science foundation for Youths (Nos. 51005097 and 51175220), the Science and Technology Development Project of Jilin Province (No. 201201025), and Fundamental Science Research Funds for the Central Universities (No. 450060445671). References [1] Liu JJ, Liu ZD. An experimental study on synthesizing TiC–TiB2–Ni composite coating using electro-thermal explosion ultra-high speed spraying method. Mater Lett 2010;64:684–7. [2] Cook BA, Peters JS, Harringa JL. Russell AM. Enhanced wear resistance in AlMgB14–TiB2 composites. Wear 2011;271:640–6.
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