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Microstructure and properties of Ti–Co–Si ternary intermetallic alloys
Journal of Alloys and Compounds 464 (2008) 138–145
Microstructure and properties of Ti–Co–Si ternary intermetallic alloys Y. Xue, H.M. Wang ∗ Laboratory of Laser Materials Processing and Manufacturing, School of Materials Science and Engineering, Beihang University (formerly Beijing University of Aeronautics and Astronautics), Beijing 100083, China Received 16 September 2007; received in revised form 23 September 2007; accepted 26 September 2007 Available online 29 September 2007
Abstract Ti–Co–Si ternary intermetallic alloys with Ti5 Si3 as the main reinforcing phase and intermetallic TiCo as the toughening matrix were fabricated by the laser-melting deposition (LMD) process. Microstructure of the intermetallic alloys was characterized by OM, SEM, XRD and EDS. Hightemperature oxidation resistance of the alloys was evaluated by isothermal oxidation at 1173 K and metallic dry-sliding wear property was evaluated at room temperature. The effect of reinforcing phase Ti5 Si3 content on hardness, oxidation and wear resistance of the alloys was investigated. Results indicate that microstructure of the alloys transforms from hypoeutectic to hypereutectic, while hardness and oxidation resistance increases with the increasing Ti5 Si3 content. The alloys have good oxidation resistance at 1173 K and the oxidation kinetic curves are approximately parabolic. Wear resistance of the alloys is insensitive to the microstructure and is up to 15–19 times higher than the hardened tool steel 1.0%C–1.5%Cr under dry-sliding wear test conditions. The excellent wear resistance of alloys is attributed to the effective reinforcement of Ti5 Si3 and the excellent toughness of the intermetallic TiCo. © 2007 Published by Elsevier B.V. Keywords: Intermetallics; Laser processing; Microstructure; Oxidation
1. Introduction Tribological components working in high-temperature environments demand materials with good combinations of strength, toughness, wear and oxidation resistant properties. Traditional tribological metallic materials hardly satisfied all the aforementioned material properties. Development of new wear and oxidation resistant alloys has attracted increasing attention [1,2]. The intermetallic compound TiCo having the simple B2 crystal lattice was reported to have high strength and strong yield-anomaly. It also exhibited noticeable room- and hightemperature ductility and toughness due to its mobile 1 0 0 and 1 1 1 dislocations [3–5]. TiCo/Ti2 Co dual-phase intermetallic coatings with hard Ti2 Co as the reinforcing phase and ductile TiCo as the matrix were fabricated on titanium alloy in our previous studies. The coatings displayed excellent abrasive and adhesive wear resistance under dry-sliding wear test conditions [6]. However, hardness (about HV760), melting
∗
Corresponding author. Tel.: +86 10 8231 7102; fax: +86 10 8233 8131. E-mail addresses: [email protected], [email protected] (H.M. Wang). 0925-8388/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.jallcom.2007.09.106
point (about 1353 K) and high-temperature oxidation resistance of the Ti2 Co is not high enough as a reinforcing phase for the TiCo-based wear and high-temperature oxidation resistant materials [7]. It limits the service of the Ti2 Co/TiCo intermetallic coatings under high-temperature and aggressive wear conditions. The refractory titanium silicide Ti5 Si3 is regarded as one of the alternative high-temperature structural materials because of its high melting point, low density and excellent hightemperature strength and oxidation resistance [8,9]. Its serious room-temperature brittleness prevented Ti5 Si3 from industrial applications as structural materials. Nevertheless, the combination of good abrasive and adhesive wear resistance, which is due to its very high hardness and strong covalent atomic bonds, and excellent high-temperature oxidation resistance do make the metal silicide Ti5 Si3 a good candidate as the reinforcing phase in a wear and high-temperature oxidation resistant alloys. Majumdar et al. [10,11] reported the tribological and oxidative properties of laser surface alloyed coatings containing Ti5 Si3 reinforcement on commercial pure titanium. Results demonstrated that laser surface alloying with Si is more effective in terms of wear resistance improvement of commercial pure titanium than with Si + Al or Al, and laser surface alloying with Si or Si + Al is more effective than with Al alone in improving oxida-
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tion resistance of commercial pure titanium. The improvement of wear and oxidation resistance was attributed to the presence of uniformly distributed Ti5 Si3 in the laser surface alloyed zone. The improvement of room- and high-temperature wear resistance of titanium alloys using laser surface alloyed or laser clad Ti5 Si3 /Ti, Ti5 Si3 /NiTi2 , Ti5 Si3 /NiTi, etc., intermetallic composite coatings was also investigated by the authors’ group [12–14]. These intermetallic coatings exhibited excellent wear resistance due to the high hardness and strong intermetallic atomic bonds of metal silicide Ti5 Si3 . A series of Ti5 Si3 /TiAl composites were also synthesized by a combination of mechanical alloying and hot isostatic pressing [15]. Hardness of the TiAl matrix is about 4–7 GPa and the dispersive Ti5 Si3 particles are up to 10–12 GPa. An intermetallic alloy with the refractory Ti5 Si3 as the reinforcing phase and the ductile TiCo as the matrix would be expected to have an outstanding combination of toughness and tribological and high-temperature oxidation resistant properties, and therefore, is anticipated to be a novel wear and high-temperature oxidation resistant candidate material. In this paper, the wear and high-temperature oxidation resistant Ti–Co–Si ternary intermetallic alloys consisting of transition metal silicide Ti5 Si3 and intermetallic TiCo were fabricated by the laser-melting deposition process. Microstructure of the Ti–Co–Si intermetallic alloys was characterized by OM, SEM, XRD and EDS, and the effect of reinforcing phase Ti5 Si3 content on hardness, wear and high-temperature oxidation resistance of the Ti–Co–Si ternary intermetallic alloys was investigated and discussed. 2. Experimental procedures In order to investigate the effect of reinforcing phase Ti5 Si3 content on microstructure and properties, three Ti–Co–Si ternary intermetallic alloys (referred to as alloys #1, #2 and #3) with increasing Ti5 Si3 content were designed in Ti5 Si3 –TiCo pseudo-binary system of the Ti–Co–Si ternary alloy phase diagram at 800 ◦ C [16], as shown in Fig. 1. Monolithic TiCo alloy was also designed as reference material. The nominal chemical compositions of the designed alloys
Fig. 1. Isothermal section of the Ti–Co–Si ternary alloy at 800 ◦ C [16].
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Table 1 Chemical compositions of the Ti–Co–Si intermetallic alloys (at.%) Alloys
Ti
Co
Si
Monolithic TiCo alloy Alloy #1 Alloy #2 Alloy #3
47 51 54 56
53 39 28 20
0 10 18 24
are listed in Table 1. Titanium, cobalt and silicide elemental powders with an average particle size ranging from −300 to +100 meshes are selected as the precursor materials. The intermetallic alloys were fabricated by the laser-melting deposition process using a patented laser-melting furnace [17], as schematically illustrated in Fig. 2. The elemental powder mixtures are put into the copper-mold and then melted by a high power laser beam delivered from an 8 kW continuouswave CO2 laser. The laser processing parameters are laser beam output power 4.5 kW, beam diameter 10 mm and laser-melting time 30 ± 5 s. Short cylindershape ingots with an ingot diameter of approximately 15 mm and an ingot height of approximately 15 mm were produced. Microstructure of the Ti–Co–Si intermetallic alloys was characterized by Olympus BX51M optical microscope using a Sisc IAS6.0 image analyzing software and CAMBRIDGE S-360, JSM-5800 and KYKY-2800 SEM. XRD was conducted using the Rigaku D/max 2200 pc automatic X-ray diffractometer with Cu target K␣ radiation. Chemical compositions of the phase constituents were analyzed by EDS using Noran Ventage DSI spectrometer. Average microhardness and single-phase microhardness of the alloys were measured using a MH-6 semiautomatic Vickers microhardness tester with a test load of 1000 and 100 g, respectively, and a load-dwell time of 10 s. Density of the alloys was measured using the water immersion method. The oxidation resistant Ni-based superalloy NiCoCrAlY was selected as the reference material in order to rate the high-temperature oxidation resistance of the Ti–Co–Si ternary intermetallic alloys. Specimens for oxidation, 10 mm × 10 mm × 2 mm in size, were prepared by electro-discharge machining followed by mechanical milling and grinding. Surface of the specimens were polished with SiC paper up to 2000# and were ultrasonically cleaned using acetone, dried and weighed using an electronic balance (Sartorius BS110) with an accuracy of 0.1 mg prior to isothermal oxidation. Each specimen was placed in an alumina crucible and inserted/removed from the furnace at 1173 K (air cooling) and weighed after each interval of cumulative exposure. The less the mass gain during oxidation, the better the high-temperature oxidation resistance of the alloys. Room-temperature wear resistance of the Ti–Co–Si ternary intermetallic alloys was evaluated on a MM-200 block-on-wheel dry-sliding wear tester, where a square block-like specimen (10 mm × 10 mm × 10 mm in size) is pressed under an applied test load of 196 N against the outer periphery surface of a hardened 0.45%C steel wheel (HRc56–58) rotating at 400 rpm, resulting in
Fig. 2. Schematic on laser-melting deposition of the Ti–Co–Si ternary intermetallic alloy ingots using a laser-melting furnace [17].
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a relative sliding speed of 0.92 m/s. The wear test cycle lasted for 60 min and the total wear sliding distance is 3312 m. Quench-tempered high-carbon tool steel in nominal composition (wt.%) of 1.0%C–1.5%Cr with an average Rockwell hardness number of HRc58-62 was selected as the reference material in order to rank the wear resistance of the Ti–Co–Si ternary intermetallic alloys. Wear mass loss was measured and wear volume loss was calculated based on the wear mass loss and material density. The wear volume loss was utilized to rate the relative wear resistance of the alloys in comparison to the reference test material. The larger the ratio, the higher the wear resistance of the Ti–Co–Si ternary intermetallic alloys.
3. Results and discussion 3.1. Microstructure and hardness Figs. 3 and 4 show microstructure of the monolithic TiCo alloy and the Ti–Co–Si ternary intermetallic alloys, respectively. Fig. 5 shows the XRD patterns of aforementioned alloys. EDS analysis (Table 2) indicates that microstructure of the monolithic TiCo alloy consists of primary TiCo dendrites and small amount of interdendritic TiCo/Ti2 Co eutectics. The dark gray strips of the interdendritic eutectic are the intermetallic Ti2 Co, which is not detected by XRD because of its very low content, as shown
Table 2 EDS results of the Ti–Co–Si intermetallic alloys (at.%) Alloys
Phase
EDS results (at.%)
Monolithic TiCo alloy
Primary phase Dark phase of the eutectic structure Dark facet primary phase Light dendrite primary phase Primary phase Primary phase
51.74Ti48.26Co 66.34Ti33.66Co
Alloy #1
Alloy #2 Alloy #3
49.91Ti30.64Co19.45Si 50.26Ti46.77Co2.97Si 60.62Ti37.22Si2.17Co 65.77Ti32.92Si1.3Co
in Fig. 3(b). Volume fraction of the TiCo primary dendrites in the monolithic TiCo alloy is approximately 94% measured by quantitative metallographic analysis method on low-magnification optical photographs (200×) using a commercial contrast-based image analyzing software. Results of XRD (Fig. 5(b)–(d)) indicate that main phase constituents of the alloys #1, #2 and #3 are Ti5 Si3 and TiCo. Microstructure of the alloy #1 also contains a new ternary metal silicide Ti3 Co2 Si with different composition from the known ternary metal silicides in the Ti–Co–Si ternary alloy system, which was detected and named by Chen et al. [18]. As shown in Fig. 4(a), alloy #1 has a uniform microstructure consisting of the primary dendrite and the interdendritic eutectics. Results of EDS analysis (Table 2) indicate that the grayish coarse primary dendrites are the intermetallic TiCo and a few irregular faceted block phase coupled with the TiCo primary dendrites are the metal silicide Ti3 Co2 Si. In the interdendritic regions is the Ti5 Si3 /TiCo eutectic in which the dark black strip-like phase is Ti5 Si3 , as clearly shown in Fig. 4(b). Microstructure of the alloy #1 shows hypoeutectic characteristics and the reinforcing phase Ti5 Si3 exists in the strip-like morphologies. On the contrary, the alloys #2 and #3 all have the typical hypereutectic microstructure, as shown in Fig. 4(c)–(f). Results of EDS analysis indicate that the dark gray coarse irregular blocks are the Ti5 Si3 primary phase and the remainder is Ti5 Si3 /TiCo eutectics. The size and content of Ti5 Si3 primary phase of the alloy #2 are smaller than that of the alloy #3, as shown in Fig. 4(d) and (f). The reinforcing phase Ti5 Si3 mainly exists in the form of irregular block primary phase except for strip-like eutectic phase. The Ti5 Si3 content has significant influence on hardness of the Ti–Co–Si alloys, as listed in Table 3. The three Ti–Co–Si alloys all have higher hardness than that of the monolithic Table 3 Average and single-phase microhardness and densities of the Ti–Co–Si intermetallic alloys
Fig. 3. OM (a) and SEM (b) micrographs showing the microstructure of the monolithic TiCo alloy, low (a) and high (b) magnifications.
Alloys
Average microhardness (HV)
Single-phase microhardness (HV)
Density (g/cm3 )
Monolithic TiCo alloy Alloy #1
350
Primary dendrite TiCo: 340
5.92
550
Primary dendrite TiCo: 520 Primary phase Ti3 Co2 Si: 1000
5.62
Alloy #2 Alloy #3
700 800
Primary phase Ti5 Si3 : 920 Primary phase Ti5 Si3 : 980
5.35 5.19
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Fig. 4. OM micrographs showing microstructure of the Ti–Co–Si intermetallic alloys. Alloy #1 (a and b), alloy #2 (c and d) and alloy #3 (e and f).
TiCo alloy because of the presence of the high hardness (about HV960) metal silicide Ti5 Si3 . The hardness of the Ti–Co–Si alloys increases with the increasing Ti5 Si3 content. The microhardness of the ternary metal silicide Ti3 Co2 Si is approximately HV1000 and it is expected to be a good reinforcing phase for the alloy #1. Fig. 6 shows the average microhardness indentation morphology of the Ti–Co–Si alloys with a test load of 1000 g. Microhardness indentations of the alloys #1 and #2 are all symmetrical without any microcracking features, while serious microcracking characteristics are visible at hardness indents of alloy #3. 3.2. Isothermal oxidation Fig. 7 shows the specific mass gain of the Ti–Co–Si ternary intermetallic alloys and the superalloy NiCoCrAlY versus the exposure times at 1173 K. Although the mass gain of the Ti–Co–Si alloys is slightly higher than the reference material NiCoCrAlY, the mass gain curves of the alloys follow the
parabolic law, which is similar to the reference material. This indicates that the isothermal oxidation rate of the Ti–Co–Si intermetallic alloys is diffusion-controlled and would have good high-temperature oxidation resistance at 1173 K. The oxidation resistance of the Ti–Co–Si alloys increases slightly with the increasing of Ti5 Si3 content. 3.3. Tribological property The Ti–Co–Si alloys exhibited quite outstanding wear resistance under room-temperature dry-sliding wear test conditions in comparison to the reference material hardened tool steel 1.0%C–1.5%Cr. Wear mass loss of all the alloys is much lower than the reference material, as listed in Table 4. Although hardness of the alloys is different from each other (from HV550 to HV800), wear resistance is almost unchanged and is up to 15–19 times higher than the hardened tool steel 1.0%C–1.5%Cr, as shown in Fig. 8. The excellent wear resistance of the Ti–Co–Si alloys is attributed to the inherent high hardness and the covalent-
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Fig. 5. XRD patterns of the Ti–Co–Si alloys: (a) monolithic TiCo alloy, (b) alloy #1, (c) alloy #2 and (d) alloy #3.
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Fig. 7. Weight gain per unit surface area of the Ti–Co–Si ternary intermetallic alloys and the reference material NiCoCrAlY superalloy vs. the exposure times at 1173 K.
Table 4 Wear mass loss and wear volume loss rate of the monolithic TiCo alloy, Ti–Co–Si alloys and the hardened GCr15 tool steel under room-temperature dry-sliding wear conditions Alloys
Wear mass loss (mg)/Wear volume loss rate (×10−6 cm3 /min)
Monolithic TiCo alloy Alloy #1 Alloy #2 Alloy #3 Hardened tool steel 1.0%C–1.5%Cr
8.3/23.44 7.2/21.43 5.45/17.14 5.6/18.3 153.2/326.93
Fig. 6. OM micrographs showing typical indentations of the Ti–Co–Si intermetallic alloys made during microhardness measurement: (a) alloy #1, (b) alloy #2 and (c) alloy #3.
Fig. 8. Room-temperature dry-sliding wear test results of the Ti–Co–Si intermetallic alloys and the hardened tool steel 1.0%C–1.5%Cr.
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Fig. 9. SEM micrographs showing the worn subsurface microstructure of the monolithic TiCo alloy (a) and the Ti–Co–Si ternary intermetallic alloys: (b) alloy #1, (c) alloy #2 and (d) alloy #3.
dominant strong atomic bonds of the reinforcing phase Ti5 Si3 . The hard Ti5 Si3 phase made the alloys effective in resisting plastic deformation and abrasion as well as adhesion, while the tough and ductile TiCo having anomalous temperature dependence made the alloys free from suffering of microcracking during sliding wear process. It is remarkable that the wear resistance of the alloy #2 is higher than the alloy #3 although its hardness is significantly lower than the former. The excellent wear resistance of the alloy #2 is attributed to the optimum content and uniform distribution of the reinforcing phase Ti5 Si3 , which is effectively supported by the tough TiCo matrix. The alloy #3 has a too high volume fraction of the coarse reinforcing phase Ti5 Si3 and suffered wear by the mechanisms of microcracking during the dry-sliding wear process. This was evidenced by the worn subsurface microstructure of the alloy #3 exhibiting smashing and microfracturing characteristic, as shown in Fig. 9(d). Severe subsurface plastic deformation along the wear sliding direction occurred for the monolithic TiCo alloy and alloy #1 with minor amount of Ti5 Si3 reinforcement, as shown in Fig. 9(a) and (b). No evidence of local plastic deformation, brittle fracture and selective wear (Fig. 9(c)) occurred for the alloy #2 due to its combination of high hardness of Ti5 Si3 and good toughness of TiCo and the wear resistance of the alloy #2 is the highest among the three experimental alloys. 4. Conclusions Wear and high-temperature oxidation resistant Ti–Co–Si ternary intermetallic alloys were fabricated by the laser-melting deposition process. Microstructure of the Ti–Co–Si ternary
intermetallic alloys transforms from hypoeutectic to hypereutectic, hardness and high-temperature oxidation resistance of the alloys increases with the increasing content of the Ti5 Si3 reinforcement. The Ti–Co–Si alloys have good high-temperature oxidation resistance at 1173 K and the oxidation kinetics is of parabolic. The Ti–Co–Si alloys have excellent wear resistance under dry-sliding wear test conditions. Wear resistance of the alloys is insensitive to the microstructure and is up to 15–19 times higher than the hardened tool steel 1.0%C–1.5%Cr. The excellent wear resistance of the alloys is attributed to the effective reinforcement of metal silicide Ti5 Si3 and the excellent toughness of the intermetallic TiCo. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant nos. 50625413 and 50331010) and the Innovation Foudation of BUAA for PhD graduates. The authors would like to thank Mr. Zhang Lingyun, Mr. Li An and Ms. Yu Rongli, etc. for their invaluable assistance during the laser-melting deposition and the metallographic experiments. References [1] [2] [3] [4] [5]
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[13] H.M. Wang, Y.F. Liu, Mater. Sci. Eng. A 338 (2002) 126–132. [14] Y.F. Liu, H.Y. Zhao, H.M. Wang, Rare Met. Mater. Eng. 32 (2003) 367– 371. [15] K.P. Rao, J.B. Zhou, Mater. Sci. Eng. A 356 (2003) 208–218. [16] P. Villars, A. Prince, H. Okamoto, Handbook of Ternary Alloy Phase Diagrams, ASM, New York, 1995, p. 8676. [17] H.M. Wang, D.Y. Luan, L.Y. Zhang, Scripta Mater. 48 (2003) 1179–1184. [18] X.H. Hu, G.L. Chen, C. Ion, K.Q. Ni, J. Mater. Sci. Technol. 16 (2000) 91–93.
Microstructure and properties of Ti–Co–Si ternary intermetallic alloys Y. Xue, H.M. Wang ∗ Laboratory of Laser Materials Processing and Manufacturing, School of Materials Science and Engineering, Beihang University (formerly Beijing University of Aeronautics and Astronautics), Beijing 100083, China Received 16 September 2007; received in revised form 23 September 2007; accepted 26 September 2007 Available online 29 September 2007
Abstract Ti–Co–Si ternary intermetallic alloys with Ti5 Si3 as the main reinforcing phase and intermetallic TiCo as the toughening matrix were fabricated by the laser-melting deposition (LMD) process. Microstructure of the intermetallic alloys was characterized by OM, SEM, XRD and EDS. Hightemperature oxidation resistance of the alloys was evaluated by isothermal oxidation at 1173 K and metallic dry-sliding wear property was evaluated at room temperature. The effect of reinforcing phase Ti5 Si3 content on hardness, oxidation and wear resistance of the alloys was investigated. Results indicate that microstructure of the alloys transforms from hypoeutectic to hypereutectic, while hardness and oxidation resistance increases with the increasing Ti5 Si3 content. The alloys have good oxidation resistance at 1173 K and the oxidation kinetic curves are approximately parabolic. Wear resistance of the alloys is insensitive to the microstructure and is up to 15–19 times higher than the hardened tool steel 1.0%C–1.5%Cr under dry-sliding wear test conditions. The excellent wear resistance of alloys is attributed to the effective reinforcement of Ti5 Si3 and the excellent toughness of the intermetallic TiCo. © 2007 Published by Elsevier B.V. Keywords: Intermetallics; Laser processing; Microstructure; Oxidation
1. Introduction Tribological components working in high-temperature environments demand materials with good combinations of strength, toughness, wear and oxidation resistant properties. Traditional tribological metallic materials hardly satisfied all the aforementioned material properties. Development of new wear and oxidation resistant alloys has attracted increasing attention [1,2]. The intermetallic compound TiCo having the simple B2 crystal lattice was reported to have high strength and strong yield-anomaly. It also exhibited noticeable room- and hightemperature ductility and toughness due to its mobile 1 0 0 and 1 1 1 dislocations [3–5]. TiCo/Ti2 Co dual-phase intermetallic coatings with hard Ti2 Co as the reinforcing phase and ductile TiCo as the matrix were fabricated on titanium alloy in our previous studies. The coatings displayed excellent abrasive and adhesive wear resistance under dry-sliding wear test conditions [6]. However, hardness (about HV760), melting
∗
Corresponding author. Tel.: +86 10 8231 7102; fax: +86 10 8233 8131. E-mail addresses: [email protected], [email protected] (H.M. Wang). 0925-8388/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.jallcom.2007.09.106
point (about 1353 K) and high-temperature oxidation resistance of the Ti2 Co is not high enough as a reinforcing phase for the TiCo-based wear and high-temperature oxidation resistant materials [7]. It limits the service of the Ti2 Co/TiCo intermetallic coatings under high-temperature and aggressive wear conditions. The refractory titanium silicide Ti5 Si3 is regarded as one of the alternative high-temperature structural materials because of its high melting point, low density and excellent hightemperature strength and oxidation resistance [8,9]. Its serious room-temperature brittleness prevented Ti5 Si3 from industrial applications as structural materials. Nevertheless, the combination of good abrasive and adhesive wear resistance, which is due to its very high hardness and strong covalent atomic bonds, and excellent high-temperature oxidation resistance do make the metal silicide Ti5 Si3 a good candidate as the reinforcing phase in a wear and high-temperature oxidation resistant alloys. Majumdar et al. [10,11] reported the tribological and oxidative properties of laser surface alloyed coatings containing Ti5 Si3 reinforcement on commercial pure titanium. Results demonstrated that laser surface alloying with Si is more effective in terms of wear resistance improvement of commercial pure titanium than with Si + Al or Al, and laser surface alloying with Si or Si + Al is more effective than with Al alone in improving oxida-
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tion resistance of commercial pure titanium. The improvement of wear and oxidation resistance was attributed to the presence of uniformly distributed Ti5 Si3 in the laser surface alloyed zone. The improvement of room- and high-temperature wear resistance of titanium alloys using laser surface alloyed or laser clad Ti5 Si3 /Ti, Ti5 Si3 /NiTi2 , Ti5 Si3 /NiTi, etc., intermetallic composite coatings was also investigated by the authors’ group [12–14]. These intermetallic coatings exhibited excellent wear resistance due to the high hardness and strong intermetallic atomic bonds of metal silicide Ti5 Si3 . A series of Ti5 Si3 /TiAl composites were also synthesized by a combination of mechanical alloying and hot isostatic pressing [15]. Hardness of the TiAl matrix is about 4–7 GPa and the dispersive Ti5 Si3 particles are up to 10–12 GPa. An intermetallic alloy with the refractory Ti5 Si3 as the reinforcing phase and the ductile TiCo as the matrix would be expected to have an outstanding combination of toughness and tribological and high-temperature oxidation resistant properties, and therefore, is anticipated to be a novel wear and high-temperature oxidation resistant candidate material. In this paper, the wear and high-temperature oxidation resistant Ti–Co–Si ternary intermetallic alloys consisting of transition metal silicide Ti5 Si3 and intermetallic TiCo were fabricated by the laser-melting deposition process. Microstructure of the Ti–Co–Si intermetallic alloys was characterized by OM, SEM, XRD and EDS, and the effect of reinforcing phase Ti5 Si3 content on hardness, wear and high-temperature oxidation resistance of the Ti–Co–Si ternary intermetallic alloys was investigated and discussed. 2. Experimental procedures In order to investigate the effect of reinforcing phase Ti5 Si3 content on microstructure and properties, three Ti–Co–Si ternary intermetallic alloys (referred to as alloys #1, #2 and #3) with increasing Ti5 Si3 content were designed in Ti5 Si3 –TiCo pseudo-binary system of the Ti–Co–Si ternary alloy phase diagram at 800 ◦ C [16], as shown in Fig. 1. Monolithic TiCo alloy was also designed as reference material. The nominal chemical compositions of the designed alloys
Fig. 1. Isothermal section of the Ti–Co–Si ternary alloy at 800 ◦ C [16].
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Table 1 Chemical compositions of the Ti–Co–Si intermetallic alloys (at.%) Alloys
Ti
Co
Si
Monolithic TiCo alloy Alloy #1 Alloy #2 Alloy #3
47 51 54 56
53 39 28 20
0 10 18 24
are listed in Table 1. Titanium, cobalt and silicide elemental powders with an average particle size ranging from −300 to +100 meshes are selected as the precursor materials. The intermetallic alloys were fabricated by the laser-melting deposition process using a patented laser-melting furnace [17], as schematically illustrated in Fig. 2. The elemental powder mixtures are put into the copper-mold and then melted by a high power laser beam delivered from an 8 kW continuouswave CO2 laser. The laser processing parameters are laser beam output power 4.5 kW, beam diameter 10 mm and laser-melting time 30 ± 5 s. Short cylindershape ingots with an ingot diameter of approximately 15 mm and an ingot height of approximately 15 mm were produced. Microstructure of the Ti–Co–Si intermetallic alloys was characterized by Olympus BX51M optical microscope using a Sisc IAS6.0 image analyzing software and CAMBRIDGE S-360, JSM-5800 and KYKY-2800 SEM. XRD was conducted using the Rigaku D/max 2200 pc automatic X-ray diffractometer with Cu target K␣ radiation. Chemical compositions of the phase constituents were analyzed by EDS using Noran Ventage DSI spectrometer. Average microhardness and single-phase microhardness of the alloys were measured using a MH-6 semiautomatic Vickers microhardness tester with a test load of 1000 and 100 g, respectively, and a load-dwell time of 10 s. Density of the alloys was measured using the water immersion method. The oxidation resistant Ni-based superalloy NiCoCrAlY was selected as the reference material in order to rate the high-temperature oxidation resistance of the Ti–Co–Si ternary intermetallic alloys. Specimens for oxidation, 10 mm × 10 mm × 2 mm in size, were prepared by electro-discharge machining followed by mechanical milling and grinding. Surface of the specimens were polished with SiC paper up to 2000# and were ultrasonically cleaned using acetone, dried and weighed using an electronic balance (Sartorius BS110) with an accuracy of 0.1 mg prior to isothermal oxidation. Each specimen was placed in an alumina crucible and inserted/removed from the furnace at 1173 K (air cooling) and weighed after each interval of cumulative exposure. The less the mass gain during oxidation, the better the high-temperature oxidation resistance of the alloys. Room-temperature wear resistance of the Ti–Co–Si ternary intermetallic alloys was evaluated on a MM-200 block-on-wheel dry-sliding wear tester, where a square block-like specimen (10 mm × 10 mm × 10 mm in size) is pressed under an applied test load of 196 N against the outer periphery surface of a hardened 0.45%C steel wheel (HRc56–58) rotating at 400 rpm, resulting in
Fig. 2. Schematic on laser-melting deposition of the Ti–Co–Si ternary intermetallic alloy ingots using a laser-melting furnace [17].
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a relative sliding speed of 0.92 m/s. The wear test cycle lasted for 60 min and the total wear sliding distance is 3312 m. Quench-tempered high-carbon tool steel in nominal composition (wt.%) of 1.0%C–1.5%Cr with an average Rockwell hardness number of HRc58-62 was selected as the reference material in order to rank the wear resistance of the Ti–Co–Si ternary intermetallic alloys. Wear mass loss was measured and wear volume loss was calculated based on the wear mass loss and material density. The wear volume loss was utilized to rate the relative wear resistance of the alloys in comparison to the reference test material. The larger the ratio, the higher the wear resistance of the Ti–Co–Si ternary intermetallic alloys.
3. Results and discussion 3.1. Microstructure and hardness Figs. 3 and 4 show microstructure of the monolithic TiCo alloy and the Ti–Co–Si ternary intermetallic alloys, respectively. Fig. 5 shows the XRD patterns of aforementioned alloys. EDS analysis (Table 2) indicates that microstructure of the monolithic TiCo alloy consists of primary TiCo dendrites and small amount of interdendritic TiCo/Ti2 Co eutectics. The dark gray strips of the interdendritic eutectic are the intermetallic Ti2 Co, which is not detected by XRD because of its very low content, as shown
Table 2 EDS results of the Ti–Co–Si intermetallic alloys (at.%) Alloys
Phase
EDS results (at.%)
Monolithic TiCo alloy
Primary phase Dark phase of the eutectic structure Dark facet primary phase Light dendrite primary phase Primary phase Primary phase
51.74Ti48.26Co 66.34Ti33.66Co
Alloy #1
Alloy #2 Alloy #3
49.91Ti30.64Co19.45Si 50.26Ti46.77Co2.97Si 60.62Ti37.22Si2.17Co 65.77Ti32.92Si1.3Co
in Fig. 3(b). Volume fraction of the TiCo primary dendrites in the monolithic TiCo alloy is approximately 94% measured by quantitative metallographic analysis method on low-magnification optical photographs (200×) using a commercial contrast-based image analyzing software. Results of XRD (Fig. 5(b)–(d)) indicate that main phase constituents of the alloys #1, #2 and #3 are Ti5 Si3 and TiCo. Microstructure of the alloy #1 also contains a new ternary metal silicide Ti3 Co2 Si with different composition from the known ternary metal silicides in the Ti–Co–Si ternary alloy system, which was detected and named by Chen et al. [18]. As shown in Fig. 4(a), alloy #1 has a uniform microstructure consisting of the primary dendrite and the interdendritic eutectics. Results of EDS analysis (Table 2) indicate that the grayish coarse primary dendrites are the intermetallic TiCo and a few irregular faceted block phase coupled with the TiCo primary dendrites are the metal silicide Ti3 Co2 Si. In the interdendritic regions is the Ti5 Si3 /TiCo eutectic in which the dark black strip-like phase is Ti5 Si3 , as clearly shown in Fig. 4(b). Microstructure of the alloy #1 shows hypoeutectic characteristics and the reinforcing phase Ti5 Si3 exists in the strip-like morphologies. On the contrary, the alloys #2 and #3 all have the typical hypereutectic microstructure, as shown in Fig. 4(c)–(f). Results of EDS analysis indicate that the dark gray coarse irregular blocks are the Ti5 Si3 primary phase and the remainder is Ti5 Si3 /TiCo eutectics. The size and content of Ti5 Si3 primary phase of the alloy #2 are smaller than that of the alloy #3, as shown in Fig. 4(d) and (f). The reinforcing phase Ti5 Si3 mainly exists in the form of irregular block primary phase except for strip-like eutectic phase. The Ti5 Si3 content has significant influence on hardness of the Ti–Co–Si alloys, as listed in Table 3. The three Ti–Co–Si alloys all have higher hardness than that of the monolithic Table 3 Average and single-phase microhardness and densities of the Ti–Co–Si intermetallic alloys
Fig. 3. OM (a) and SEM (b) micrographs showing the microstructure of the monolithic TiCo alloy, low (a) and high (b) magnifications.
Alloys
Average microhardness (HV)
Single-phase microhardness (HV)
Density (g/cm3 )
Monolithic TiCo alloy Alloy #1
350
Primary dendrite TiCo: 340
5.92
550
Primary dendrite TiCo: 520 Primary phase Ti3 Co2 Si: 1000
5.62
Alloy #2 Alloy #3
700 800
Primary phase Ti5 Si3 : 920 Primary phase Ti5 Si3 : 980
5.35 5.19
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Fig. 4. OM micrographs showing microstructure of the Ti–Co–Si intermetallic alloys. Alloy #1 (a and b), alloy #2 (c and d) and alloy #3 (e and f).
TiCo alloy because of the presence of the high hardness (about HV960) metal silicide Ti5 Si3 . The hardness of the Ti–Co–Si alloys increases with the increasing Ti5 Si3 content. The microhardness of the ternary metal silicide Ti3 Co2 Si is approximately HV1000 and it is expected to be a good reinforcing phase for the alloy #1. Fig. 6 shows the average microhardness indentation morphology of the Ti–Co–Si alloys with a test load of 1000 g. Microhardness indentations of the alloys #1 and #2 are all symmetrical without any microcracking features, while serious microcracking characteristics are visible at hardness indents of alloy #3. 3.2. Isothermal oxidation Fig. 7 shows the specific mass gain of the Ti–Co–Si ternary intermetallic alloys and the superalloy NiCoCrAlY versus the exposure times at 1173 K. Although the mass gain of the Ti–Co–Si alloys is slightly higher than the reference material NiCoCrAlY, the mass gain curves of the alloys follow the
parabolic law, which is similar to the reference material. This indicates that the isothermal oxidation rate of the Ti–Co–Si intermetallic alloys is diffusion-controlled and would have good high-temperature oxidation resistance at 1173 K. The oxidation resistance of the Ti–Co–Si alloys increases slightly with the increasing of Ti5 Si3 content. 3.3. Tribological property The Ti–Co–Si alloys exhibited quite outstanding wear resistance under room-temperature dry-sliding wear test conditions in comparison to the reference material hardened tool steel 1.0%C–1.5%Cr. Wear mass loss of all the alloys is much lower than the reference material, as listed in Table 4. Although hardness of the alloys is different from each other (from HV550 to HV800), wear resistance is almost unchanged and is up to 15–19 times higher than the hardened tool steel 1.0%C–1.5%Cr, as shown in Fig. 8. The excellent wear resistance of the Ti–Co–Si alloys is attributed to the inherent high hardness and the covalent-
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Fig. 5. XRD patterns of the Ti–Co–Si alloys: (a) monolithic TiCo alloy, (b) alloy #1, (c) alloy #2 and (d) alloy #3.
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Fig. 7. Weight gain per unit surface area of the Ti–Co–Si ternary intermetallic alloys and the reference material NiCoCrAlY superalloy vs. the exposure times at 1173 K.
Table 4 Wear mass loss and wear volume loss rate of the monolithic TiCo alloy, Ti–Co–Si alloys and the hardened GCr15 tool steel under room-temperature dry-sliding wear conditions Alloys
Wear mass loss (mg)/Wear volume loss rate (×10−6 cm3 /min)
Monolithic TiCo alloy Alloy #1 Alloy #2 Alloy #3 Hardened tool steel 1.0%C–1.5%Cr
8.3/23.44 7.2/21.43 5.45/17.14 5.6/18.3 153.2/326.93
Fig. 6. OM micrographs showing typical indentations of the Ti–Co–Si intermetallic alloys made during microhardness measurement: (a) alloy #1, (b) alloy #2 and (c) alloy #3.
Fig. 8. Room-temperature dry-sliding wear test results of the Ti–Co–Si intermetallic alloys and the hardened tool steel 1.0%C–1.5%Cr.
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Fig. 9. SEM micrographs showing the worn subsurface microstructure of the monolithic TiCo alloy (a) and the Ti–Co–Si ternary intermetallic alloys: (b) alloy #1, (c) alloy #2 and (d) alloy #3.
dominant strong atomic bonds of the reinforcing phase Ti5 Si3 . The hard Ti5 Si3 phase made the alloys effective in resisting plastic deformation and abrasion as well as adhesion, while the tough and ductile TiCo having anomalous temperature dependence made the alloys free from suffering of microcracking during sliding wear process. It is remarkable that the wear resistance of the alloy #2 is higher than the alloy #3 although its hardness is significantly lower than the former. The excellent wear resistance of the alloy #2 is attributed to the optimum content and uniform distribution of the reinforcing phase Ti5 Si3 , which is effectively supported by the tough TiCo matrix. The alloy #3 has a too high volume fraction of the coarse reinforcing phase Ti5 Si3 and suffered wear by the mechanisms of microcracking during the dry-sliding wear process. This was evidenced by the worn subsurface microstructure of the alloy #3 exhibiting smashing and microfracturing characteristic, as shown in Fig. 9(d). Severe subsurface plastic deformation along the wear sliding direction occurred for the monolithic TiCo alloy and alloy #1 with minor amount of Ti5 Si3 reinforcement, as shown in Fig. 9(a) and (b). No evidence of local plastic deformation, brittle fracture and selective wear (Fig. 9(c)) occurred for the alloy #2 due to its combination of high hardness of Ti5 Si3 and good toughness of TiCo and the wear resistance of the alloy #2 is the highest among the three experimental alloys. 4. Conclusions Wear and high-temperature oxidation resistant Ti–Co–Si ternary intermetallic alloys were fabricated by the laser-melting deposition process. Microstructure of the Ti–Co–Si ternary
intermetallic alloys transforms from hypoeutectic to hypereutectic, hardness and high-temperature oxidation resistance of the alloys increases with the increasing content of the Ti5 Si3 reinforcement. The Ti–Co–Si alloys have good high-temperature oxidation resistance at 1173 K and the oxidation kinetics is of parabolic. The Ti–Co–Si alloys have excellent wear resistance under dry-sliding wear test conditions. Wear resistance of the alloys is insensitive to the microstructure and is up to 15–19 times higher than the hardened tool steel 1.0%C–1.5%Cr. The excellent wear resistance of the alloys is attributed to the effective reinforcement of metal silicide Ti5 Si3 and the excellent toughness of the intermetallic TiCo. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant nos. 50625413 and 50331010) and the Innovation Foudation of BUAA for PhD graduates. The authors would like to thank Mr. Zhang Lingyun, Mr. Li An and Ms. Yu Rongli, etc. for their invaluable assistance during the laser-melting deposition and the metallographic experiments. References [1] [2] [3] [4] [5]
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