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Microstructural characteristics of PTA-overlayed NbC on pure Ti
Surface & Coatings Technology 200 (2006) 6881 – 6887 www.elsevier.com/locate/surfcoat
Microstructural characteristics of PTA-overlayed NbC on pure Ti Fei-Yi Hung ⁎, Zao-You Yan, Li-Hui Chen, Truan-Sheng Lui Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan 701, R.O. China Received 31 May 2005; accepted in revised form 11 October 2005 Available online 5 December 2005
Abstract This study uses the technique of PTA (plasma transferred arc) to overlay NbC reinforcing particles on the surface of commercially pure Ti in order to investigate the microstructural features of the overlayer and the interface between the overlayer and base metal by changing the overlaying current. The results indicate that the matrix phase of the overlayer was α-Ti containing about 10 at.% Nb and 1 at.% C. NbC and precipitated TiC produced by dissolved NbC reacted with Ti dispersed in the matrix. The microstructure of the cross-section of the overlayer (from surface to base metal), which was composed of α-Ti, can be separated into three layers: an upper overlayer with TiC, a middle overlayer with TiC and NbC, and a lower overlayer (interfacial layer and heat affected zone, HAZ). Due to solidification beginning at the interface and the effect of dilution, the TiC in the interfacial layer was finer. Owing to faster solidification under low-current conditions, the TiC particles were finer than under high-current conditions. Also, dendritic TiC under a low-current in the upper overlayer was also finer than under a high-current. Meanwhile, TiC precipitate that resulted from heterogeneous nucleation and Gibbs' free energy was also found around NbC. This NbC diffusion layer between TiC and NbC may have been βNb2C phase. © 2005 Elsevier B.V. All rights reserved. Keywords: Plasma transferred arc; PTA; NbC; Ti; TiC; Nb2C; Gibbs' free energy
1. Introduction Surface-overlayed processes may improve the surface hardness, as well as the wear and corrosion properties of metallic surfaces without seriously affecting the properties of the substrate material. Plasma transferred arc (PTA) possesses many characteristics: (1) The PTA-overlay is thicker than a laserinduced overlay. Because the PTA-overlay is metallurgical bound, its impact-resistance is higher. (2) Since the powder can be selected at random, the composition of the overlayer can be established easily. (3) The working distance between the electrode and work-surface is more flexible. PTA processes have higher work efficiency and are commonly used for surface-overlayed manufacturing [1–3]. Relevant studies [4,5] have revealed that PTA-overlayed W2C or Cr3C2 on Ti–6Al– 4V produced a β-Ti overlayer. If NbC or TiC powders were selected to overlay, the overlayer of Ti–6Al–4V was α-Ti. Regardless of the structure of the overlayer, undissolved carbides were present in the overlayer. However, one related ⁎ Corresponding author. Tel.: +886 6 2757575x62964. E-mail address: [email protected] (F.-Y. Hung). 0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.10.021
study [5] focused only on the overlayer structure and the retained undissolved carbides indicated that the β-Ti overlayer had higher sliding wear resistance and hardness than the α-Ti one. But it did not consider these possibilities: (1) PTA heat induced the phase transformation of Ti–6Al–4V substrate. (2) The V2C (or VC) and Al4C3 reacted with melting Ti–6Al–4V.
Fig. 1. SEM micrograph of the NbC powders.
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Table 1 Chemical composition of pure Ti (wt.%)
Table 2 PTA operation parameters
Cr
Fe
C
Ti
Torch gap
10 mm
0.12
0.07
0.05
Bal.
Powder feeding rate Plasma gas (Ar) Powder delivery gas (Ar) Shielding gas (Ar) Overlay current (voltage) Travel speed Nozzle vibration
25 g/min 4 l/min 6 l/min 18 l/min 150 A (37 V), 180 A (40 V) 110 mm/min 35 times/min
(3) The metallurgical phenomenon of the undissolved carbide particles on the overlayer during solidification. For the above reasons, this study avoided the reactions of V2C (or VC) and Al4C3, the phase transformation of Ti–6Al–4V and decreased complicated metallurgy by selected pure Ti as a substrate. Therefore, PTA overlayed the NbC particles on the surface to obtain an overlayer with undissolved NbC carbides. This paper not only investigates the overlayer structures, but also discusses the effect of changing the overlaying current.
velocity was 2° min− 1. In order to understand the difference in chemical composition between the overlayer and matrices, the structures of the affected regions were also observed using EPMA/WDS. 3. Results
2. Experimental procedures NbC powder of composition 48 at.% Nb and 52 at.% C (the micrograph of NbC is shown in Fig. 1, 80–120 μm average grit size) was overlayed on the surface of commercially pure Ti (the chemical composition is shown in Table 1) using the technique of PTA. A schematic drawing of the PTA equipment used is presented in Fig. 2. Before overlaying, the surface of the specimens was polished with 400-grit SiC paper to obtain the same surface roughness. All specimens examined here were treated under the same conditions given in Table 2. Two heat input conditions were chosen for PTA (150 A: low overlaying current and 180 A: high overlaying current). After overlaying, the microstructures of the surface layers (including the overlayer, interfacial layer and heat affected zone) were determined quantitatively using an image analyzer. Moreover, BEI (backscattered electron imaging) was used to observe the cross-section of the overlayer microstructures. X-ray diffraction was also used for quantitative analysis of the overlayer microstructures. The Cu-Kα standard (λ = 1.5403 nm) was selected for X-ray diffraction. The scanning angle was varied from 30° to 80° and the scanning
The cross-section structure of the layer is shown in Fig. 3 (the thickness of the layer was about 3–4 mm). The
Upper overlayer
NbC Middle overlayer
Lower layer
Base metal
Fig. 2. Schematic illustration of the PTA process.
Fig. 3. Macroscopic optical feature of the transverse section of the overlayer (using 180 A overlaying current as an example).
F.-Y. Hung et al. / Surface & Coatings Technology 200 (2006) 6881–6887
(a)
50μm
6883
Figs. 6 and 7 show the BEI cross-section structures of the overlayer. In addition to the undissolved NbC (white particles), there are precipitated dark carbides produced by a liquid state reaction with Ti dispersed in the matrix (see arrow C). Meanwhile, a dark reacting layer can be observed around NbC particles. The precipitated carbide particles in the 180 A specimen are larger in size compared with the 150 A specimen. In addition, the precipitated dendrite carbides are clearly seen in the upper overlayer of the 180 A specimen (see arrow D of Fig. 7(a)). At the 150 A heat input condition, the precipitated carbides are mostly finer particles and the dendrite structure is not clear (see Fig. 6(a)). The EPMA/ WDS data of the overlayer cross-sections is shown in Table
(b) (a)
NbC
dendrite 100μm 100μm
(c) (b)
Interface layer NbC
HAZ
100μm
Fig. 4. Microstructures of the cross section of the 150 A overlayer specimen: (a) the upper overlayer, (b) the middle overlayer and (c) the lower overlayer (including the interfacial layer and heat affected zone, HAZ).
microstructural feature of the overlayer can be separated into three layers: upper overlayer, middle overlayer and lower overlayer (interfacial layer and heat affected zone, HAZ). Magnified overlayer structures for both 150 A and 180 A overlaying current are shown in Figs. 4 and 5. For the 150 A overlaying current specimens, very small precipitated carbide particles were produced in the upper overlayer, while the dendrites were not obvious. The dendritic precipitated carbides in the 180 A specimens were easily observable. In addition, both overlayers possessed undissolved NbC particles. The black zone in the lower overlayer is the interfacial layer. The needle-shaped structure is the heat affected zone between the matrix and the nearby interfacial layer.
100μm
(c)
interface layer HAZ
100μm Fig. 5. Microstructures of the cross section of the 180 A overlayer specimen: (a) the upper overlayer, (b) the middle overlayer and (c) the lower overlayer (including the interfacial layer and heat affected zone, HAZ).
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(a)
C
30μm
(b)
C
The X-ray data of the two overlaying current specimens is given in Fig. 8. Their upper overlayer matrices were α-Ti and TiC carbides that had dispersed in the matrix. In addition, NbC peaks were detected by XRD revealing that the cladding coatings still contained a few fine NbC particles. Comparing 150 A with 180 A by TiC diffraction intensity (2θ: 35–45°, (111) and (200)), the 180 A matrix had higher TiC content. The mapping analysis of NbC nucleus centers to matrix is given in Fig. 9 (using 180 A overlaying current as an example). The results indicate that the Nb content was highest in the NbC nucleus center, while that in the dark precipitated TiC zone was lowest. The dark precipitated TiC zone possessed a greater C content, which the C content in the matrix was lower. Ti content was highest in the matrix and
(a)
D
50μm
(c) 30μm
(b)
C C 30μm Fig. 6. Magnified BEI of the overlayer structure of the 150 A specimen: (a) the upper overlayer, (b) the middle overlayer and (c) the top of the lower overlayer.
50μm
(c) 3. The results indicate that the preceding carbides, dark reacting layer and the precipitated dendrite carbides contained about 2–9 at.% Nb (the ratio of Ti/C atoms was about 1:1, which corresponds to the composition of TiC). In addition, the compositions of dendrite phase and matrix phases obtained using EMPA analysis are given in Table 4. Therefore, it can be assumed that the dendrite phase was TiC. Meanwhile, the 180 A matrix phase had higher Nb solid solubility than the 150 A matrix. The C solid solubility of the two heat input matrices was about 1 at.%. Clearly, part of the NbC had dissolved during PTA process. The dissolved NbC not only raised Nb solid solubility in melt Ti, but also supplied the C requirement for the TiC of the precipitated carbides.
C
30μm Fig. 7. Magnified BEI of the overlayer structure of the 180 A specimen: (a) the upper overlayer, (b) the middle overlayer and (c) the top of the lower overlayer.
F.-Y. Hung et al. / Surface & Coatings Technology 200 (2006) 6881–6887
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Table 3 WDS analysis of the overlayer (at.%) Precipitated carbide (upper overlayer)
Precipitated carbide (middle overlayer)
Dark-ring reacting layer (around NbC)
Bright layer (around NbC)
Ti: 0% Nb: 48% C: 52% Ti: 0% Nb: 48% C: 52%
Ti: 90% Nb: 9% C: 1% Ti: 88% Nb: 11% C: 1%
Ti: 51% Nb: 2% C: 47% Ti: 53% Nb: 3% C: 44%
Ti: 54% Nb: 3% C: 43% Ti:55% Nb: 3% C: 42%
Ti: 41% Nb: 8% C: 51% Ti: 46% Nb: 9% C: 45%
Ti: 6% Nb: 71% C: 23% Ti: 7% Nb: 73% C: 20%
TiC phase Matrix
(101,200)
(112,222) (201)
(103,311) (311)
(101)
(200)
(102)
(110,220)
(002)
(100,111) (111)
30
40
50
60
(112) (201)
(103)
(110)
(102)
(002)
(100)
substrate
70
80
(101,200)
2θ
(b)
:α-Ti :NbC :TiC
(101)
(311) (103) (311) (112,222) (201)
(110,220)
(102)
(200)
overlayer
(112) (201)
(110)
(102)
(103)
substrate
Table 4 Analysis results of the dendritic precipitated TiC phase and matrix phase Phase (180A) upper overlayer
:α-Ti :NbC :TiC
overlayer
(100,111) (111) (002)
Two kinds of carbides containing undissolved NbC powder and precipitated TiC carbides were dispersed in the overlayer matrix. Relevant studies [6] have revealed that the size of powder and the value of the surfacing current affected the melting of NbC particles. So, the two surfacing arc currents of this system were not able to supply enough energy to fuse the NbC powder. In addition, the upper overlayer was TiC, while the middle overlayer contained both TiC and NbC. There was a dark-ring of precipitated TiC around undissolved NbC particles. Because the density of NbC (7.6 g/cm3) is greater than that of TiC (4.9 g/cm3), the NbC sank in the Ti melting pool. According to the reports of Wang Xibao et al. [7,8], the dilution of coating materials and the cooling rate of the melting pool affected the phenomena of solidification. Therefore, it can be deduced that the density, the effect of dilution and the solidification beginning at the interface are as the reasons why the great majority of undissolved NbC was observed in the middle overlayer. In Table 3, the partly dissolved NbC raised both Nb and C contents in the Ti melting pool during PTA process. According to the phase diagrams of Nb–Ti and C– Ti [9,10], the Nb solid solubility limit was ∼5 wt.% and the C solid solubility limit was very low (0.1 wt.%) in α-Ti. A
(a)
(002)
4. Discussion
related study [5] has revealed that the Nb solid solution content of Ti must be over 40 wt.% (about 25 at.%) to form β-Ti. Based on Table 3 and the X-ray analysis (see
(100)
there was no Ti in the NbC nucleus. Fig. 9 and Table 3 explain why there was no Ti to diffuse into undissolved NbC powder. Moreover, the BEI in Fig. 9 exhibits a bright thin layer between NbC particles and a dark-ring TiC zone (see arrow). The composition of this bright thin layer center is also listed in Table 3 confirming that it is probably Nb2C. Fig. 10 shows the distributive composition data of EPMA/ WDS (the analysis distance from the NbC nucleus to the bright thin layer). The results show an increasing Nb content and decreasing C content from the NbC nucleus to the bright thin layer interface. Also, a small amount of Ti existed in the bright thin layer.
Intensity
180 A
Matrix (average of zone)
Intensity
150 A
NbC nuclear center
Compositions (at.%) Ti
C
Nb
50.22 90.68
48.37 1.47
1.41 7.85
30
40
50
60
70
80
2θ Fig. 8. XRD patterns of the overlaying specimens: (a) 150 A and (b) 180A.
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Fig. 9. The EPMA analysis from the NbC nucleus centers to matrix (using 180A overlaying current as an example).
Fig. 8), the two overlay matrices with different overlaying current were α-Ti and the heat affected zone was needle αTi [11]. Fig. 11 shows the relationship between Gibbs' free energy of carbide reacting and temperature [12]. This explains how the C reacted with Ti melt to produce precipitated TiC during PTA process. Meanwhile, there was no precipitated NbC in the overlayer and the reacting Nb would be solid solution in the matrix. Also, both C and Nb in the Ti melt had a chance to react into Nb2C. But according to Fig. 11, when the temperature was over 1200 °C, TiC was the major precipitated phase. In Fig. 9, a dark-ring of precipitated TiC around undissolved NbC particles (outside of the bright thin layer) can be observed. The reason was that NbC provided a nucleation site and the TiC grew with heterogeneous nucleation. As for the Nb, C and Ti concentration variations in Fig. 10, the outside bright thin layer of NbC was a diffusion reacting layer. Table 3 reveals that the C content of the diffusion layer was ∼ 20 at.%. According to the C–Nb phase diagram [13], it is safe to say that the diffusion layer was βNb2C. This is the reason that the speed of C diffusion was greater than that of Nb. This resulted in the TiC precipitating easily in the Ti melt, leasing insufficient C to precipitate into βNb2C and causing ∼9 at.% Nb to become solid solution in Ti matrix after the solidification process (see Table 3). Comparing NbC with Nb2C in Fig. 11, C and NbC reacted to produce Nb2C which was more easily formed than NbC. Obviously, C underwent some reactions during the PAT solidification process. To begin with, C and Ti melt reacted to form precipitated TiC dispersed in the matrix.
Then, C and undissolved NbC reacted to form a dark-ring of TiC with heterogeneous nucleation. Finally, the diffusing C reacted to form Nb2C between undissolved NbC nucleus and the dark-ring TiC zone after the cooling process. Moreover, the thickness of this diffusion layer of Nb2C increased with increasing the value of the overlaying current. The reason was that the 180 A condition had a larger heat input to prolong the duration of solidification and diffusion [14]. Reportedly [3], increasing the current also results in increased heat input and a decreased solidification rate. According to one study [15], the solidification structure got finer as the solidification rate was increased during PAT process. In short, the precipitated carbide particles of 150 A were finer than those of 180 A. And the dendritic TiC carbides in the upper overlayer became more obvious as the overlaying current was increased. 5. Conclusions 1. The overlayer matrix was α-Ti containing about 10 at.% Nb and 1 at.% C. The cross-section of the overlayer microstructure from surface to base metal can be separated into three layers: an upper overlayer with TiC, a middle overlayer with TiC + NbC and a lower overlayer containing an interfacial layer and a heat affected zone. 2. Low overlaying current will increase the solidification rate resulting in a decrease in precipitated TiC particle size and making the dendritic TiC of the upper overlayer finer. In addition, two surfacing arc currents were not able to supply enough energy to fuse the NbC powders, while the undissolved NbC provides a heterogeneous nucleation site and reacted to
F.-Y. Hung et al. / Surface & Coatings Technology 200 (2006) 6881–6887
(a)
NbC
-25
TiC
Bright
6887
layer
(b)
80
C content Nb content Ti content
-35
Δ G (kcal / mole C)
content (at. %)
60
40
20
0
0.4
0.8
Nb2C -45
1.2
Distance (μm) 80
TiC
-55
C content Nb content Ti content
-60
60
content (at. %)
-40
-50
0
(c)
NbC
-30
0
300
600
900
1200 1500 1800 2100
Temperature (°C) 40
Fig. 11. Gibbs' free energy as a function of carbide reacting temperature.
References
20
0
0
0.4
0.8
1.2
Distance (μm)
[1] [2] [3] [4] [5]
Fig. 10. EPMA/WDS analysis of the distributive composition from the NbC nucleus to the bright thin layer (a) schematic structure site, (b) 150 A overlaying current specimen and (c) 180 A overlaying current specimen.
[6] [7] [8]
form a dark-ring of precipitated TiC. A bright diffusion layer of Nb2C was present between the undissolved NbC nucleus and the dark-ring TiC zone. Acknowledgements The authors are grateful to the Chinese National Science Council for its financial support (contract: NSC 94-2216-E006-034).
[9] [10] [11] [12] [13] [14] [15]
Metals Handbook, 9th ed., ASM, 13, 1988, p. 658. U. Nakamura, J. Yosettsu Gijutsu 8 (1986) 45. W.S. Dai, L.H. Chen, T.S. Lui, Wear 248 (2001) 201. A. Hirose, D. Ozamoto, R. Aoki, K.F. Kobayashi, Z. Metallkd. 86 (1995) 580. S. Takahashi, N. Okada, Z. Shida, M. Nakanishi, Tetsu to Hagane 8 (1991) 124. Xibao Wang, Hua Lui, Surf. Coat. Technol. 106 (1998) 156. Xibao Wang, Xiaofeng Wang, Zhongquan Shi, Surf. Coat. Technol. 192 (2005) 257. Xibao Wang, Liang Yong, Songlan Yang, Surf. Coat. Technol. 137 (2001) 209. ASM Handbook, 10th ed., 3, 1992, p. 2307. T.B. Massalski, ASM (1986) 333. K. Rudinger, Z. Werkstofftech. 13 (7) (1982) 229. S.R. Shatynski, Oxidation of Metals 13 (2) (1979) 105. R. Freer, The Physics and Chemistry of Carbides, Nitrides, and Borides, Kluwer Academic Publishers, Boston, 1989, p. 216. W. Jost, Diffusion in Solids, Liquids, Gases, Academic Press, New York, 1960, p. 69. W.F. Savage, E.F. Nipper, J.S. Erickson, Weld. J. (1976) 213.
Microstructural characteristics of PTA-overlayed NbC on pure Ti Fei-Yi Hung ⁎, Zao-You Yan, Li-Hui Chen, Truan-Sheng Lui Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan 701, R.O. China Received 31 May 2005; accepted in revised form 11 October 2005 Available online 5 December 2005
Abstract This study uses the technique of PTA (plasma transferred arc) to overlay NbC reinforcing particles on the surface of commercially pure Ti in order to investigate the microstructural features of the overlayer and the interface between the overlayer and base metal by changing the overlaying current. The results indicate that the matrix phase of the overlayer was α-Ti containing about 10 at.% Nb and 1 at.% C. NbC and precipitated TiC produced by dissolved NbC reacted with Ti dispersed in the matrix. The microstructure of the cross-section of the overlayer (from surface to base metal), which was composed of α-Ti, can be separated into three layers: an upper overlayer with TiC, a middle overlayer with TiC and NbC, and a lower overlayer (interfacial layer and heat affected zone, HAZ). Due to solidification beginning at the interface and the effect of dilution, the TiC in the interfacial layer was finer. Owing to faster solidification under low-current conditions, the TiC particles were finer than under high-current conditions. Also, dendritic TiC under a low-current in the upper overlayer was also finer than under a high-current. Meanwhile, TiC precipitate that resulted from heterogeneous nucleation and Gibbs' free energy was also found around NbC. This NbC diffusion layer between TiC and NbC may have been βNb2C phase. © 2005 Elsevier B.V. All rights reserved. Keywords: Plasma transferred arc; PTA; NbC; Ti; TiC; Nb2C; Gibbs' free energy
1. Introduction Surface-overlayed processes may improve the surface hardness, as well as the wear and corrosion properties of metallic surfaces without seriously affecting the properties of the substrate material. Plasma transferred arc (PTA) possesses many characteristics: (1) The PTA-overlay is thicker than a laserinduced overlay. Because the PTA-overlay is metallurgical bound, its impact-resistance is higher. (2) Since the powder can be selected at random, the composition of the overlayer can be established easily. (3) The working distance between the electrode and work-surface is more flexible. PTA processes have higher work efficiency and are commonly used for surface-overlayed manufacturing [1–3]. Relevant studies [4,5] have revealed that PTA-overlayed W2C or Cr3C2 on Ti–6Al– 4V produced a β-Ti overlayer. If NbC or TiC powders were selected to overlay, the overlayer of Ti–6Al–4V was α-Ti. Regardless of the structure of the overlayer, undissolved carbides were present in the overlayer. However, one related ⁎ Corresponding author. Tel.: +886 6 2757575x62964. E-mail address: [email protected] (F.-Y. Hung). 0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.10.021
study [5] focused only on the overlayer structure and the retained undissolved carbides indicated that the β-Ti overlayer had higher sliding wear resistance and hardness than the α-Ti one. But it did not consider these possibilities: (1) PTA heat induced the phase transformation of Ti–6Al–4V substrate. (2) The V2C (or VC) and Al4C3 reacted with melting Ti–6Al–4V.
Fig. 1. SEM micrograph of the NbC powders.
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Table 1 Chemical composition of pure Ti (wt.%)
Table 2 PTA operation parameters
Cr
Fe
C
Ti
Torch gap
10 mm
0.12
0.07
0.05
Bal.
Powder feeding rate Plasma gas (Ar) Powder delivery gas (Ar) Shielding gas (Ar) Overlay current (voltage) Travel speed Nozzle vibration
25 g/min 4 l/min 6 l/min 18 l/min 150 A (37 V), 180 A (40 V) 110 mm/min 35 times/min
(3) The metallurgical phenomenon of the undissolved carbide particles on the overlayer during solidification. For the above reasons, this study avoided the reactions of V2C (or VC) and Al4C3, the phase transformation of Ti–6Al–4V and decreased complicated metallurgy by selected pure Ti as a substrate. Therefore, PTA overlayed the NbC particles on the surface to obtain an overlayer with undissolved NbC carbides. This paper not only investigates the overlayer structures, but also discusses the effect of changing the overlaying current.
velocity was 2° min− 1. In order to understand the difference in chemical composition between the overlayer and matrices, the structures of the affected regions were also observed using EPMA/WDS. 3. Results
2. Experimental procedures NbC powder of composition 48 at.% Nb and 52 at.% C (the micrograph of NbC is shown in Fig. 1, 80–120 μm average grit size) was overlayed on the surface of commercially pure Ti (the chemical composition is shown in Table 1) using the technique of PTA. A schematic drawing of the PTA equipment used is presented in Fig. 2. Before overlaying, the surface of the specimens was polished with 400-grit SiC paper to obtain the same surface roughness. All specimens examined here were treated under the same conditions given in Table 2. Two heat input conditions were chosen for PTA (150 A: low overlaying current and 180 A: high overlaying current). After overlaying, the microstructures of the surface layers (including the overlayer, interfacial layer and heat affected zone) were determined quantitatively using an image analyzer. Moreover, BEI (backscattered electron imaging) was used to observe the cross-section of the overlayer microstructures. X-ray diffraction was also used for quantitative analysis of the overlayer microstructures. The Cu-Kα standard (λ = 1.5403 nm) was selected for X-ray diffraction. The scanning angle was varied from 30° to 80° and the scanning
The cross-section structure of the layer is shown in Fig. 3 (the thickness of the layer was about 3–4 mm). The
Upper overlayer
NbC Middle overlayer
Lower layer
Base metal
Fig. 2. Schematic illustration of the PTA process.
Fig. 3. Macroscopic optical feature of the transverse section of the overlayer (using 180 A overlaying current as an example).
F.-Y. Hung et al. / Surface & Coatings Technology 200 (2006) 6881–6887
(a)
50μm
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Figs. 6 and 7 show the BEI cross-section structures of the overlayer. In addition to the undissolved NbC (white particles), there are precipitated dark carbides produced by a liquid state reaction with Ti dispersed in the matrix (see arrow C). Meanwhile, a dark reacting layer can be observed around NbC particles. The precipitated carbide particles in the 180 A specimen are larger in size compared with the 150 A specimen. In addition, the precipitated dendrite carbides are clearly seen in the upper overlayer of the 180 A specimen (see arrow D of Fig. 7(a)). At the 150 A heat input condition, the precipitated carbides are mostly finer particles and the dendrite structure is not clear (see Fig. 6(a)). The EPMA/ WDS data of the overlayer cross-sections is shown in Table
(b) (a)
NbC
dendrite 100μm 100μm
(c) (b)
Interface layer NbC
HAZ
100μm
Fig. 4. Microstructures of the cross section of the 150 A overlayer specimen: (a) the upper overlayer, (b) the middle overlayer and (c) the lower overlayer (including the interfacial layer and heat affected zone, HAZ).
microstructural feature of the overlayer can be separated into three layers: upper overlayer, middle overlayer and lower overlayer (interfacial layer and heat affected zone, HAZ). Magnified overlayer structures for both 150 A and 180 A overlaying current are shown in Figs. 4 and 5. For the 150 A overlaying current specimens, very small precipitated carbide particles were produced in the upper overlayer, while the dendrites were not obvious. The dendritic precipitated carbides in the 180 A specimens were easily observable. In addition, both overlayers possessed undissolved NbC particles. The black zone in the lower overlayer is the interfacial layer. The needle-shaped structure is the heat affected zone between the matrix and the nearby interfacial layer.
100μm
(c)
interface layer HAZ
100μm Fig. 5. Microstructures of the cross section of the 180 A overlayer specimen: (a) the upper overlayer, (b) the middle overlayer and (c) the lower overlayer (including the interfacial layer and heat affected zone, HAZ).
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(a)
C
30μm
(b)
C
The X-ray data of the two overlaying current specimens is given in Fig. 8. Their upper overlayer matrices were α-Ti and TiC carbides that had dispersed in the matrix. In addition, NbC peaks were detected by XRD revealing that the cladding coatings still contained a few fine NbC particles. Comparing 150 A with 180 A by TiC diffraction intensity (2θ: 35–45°, (111) and (200)), the 180 A matrix had higher TiC content. The mapping analysis of NbC nucleus centers to matrix is given in Fig. 9 (using 180 A overlaying current as an example). The results indicate that the Nb content was highest in the NbC nucleus center, while that in the dark precipitated TiC zone was lowest. The dark precipitated TiC zone possessed a greater C content, which the C content in the matrix was lower. Ti content was highest in the matrix and
(a)
D
50μm
(c) 30μm
(b)
C C 30μm Fig. 6. Magnified BEI of the overlayer structure of the 150 A specimen: (a) the upper overlayer, (b) the middle overlayer and (c) the top of the lower overlayer.
50μm
(c) 3. The results indicate that the preceding carbides, dark reacting layer and the precipitated dendrite carbides contained about 2–9 at.% Nb (the ratio of Ti/C atoms was about 1:1, which corresponds to the composition of TiC). In addition, the compositions of dendrite phase and matrix phases obtained using EMPA analysis are given in Table 4. Therefore, it can be assumed that the dendrite phase was TiC. Meanwhile, the 180 A matrix phase had higher Nb solid solubility than the 150 A matrix. The C solid solubility of the two heat input matrices was about 1 at.%. Clearly, part of the NbC had dissolved during PTA process. The dissolved NbC not only raised Nb solid solubility in melt Ti, but also supplied the C requirement for the TiC of the precipitated carbides.
C
30μm Fig. 7. Magnified BEI of the overlayer structure of the 180 A specimen: (a) the upper overlayer, (b) the middle overlayer and (c) the top of the lower overlayer.
F.-Y. Hung et al. / Surface & Coatings Technology 200 (2006) 6881–6887
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Table 3 WDS analysis of the overlayer (at.%) Precipitated carbide (upper overlayer)
Precipitated carbide (middle overlayer)
Dark-ring reacting layer (around NbC)
Bright layer (around NbC)
Ti: 0% Nb: 48% C: 52% Ti: 0% Nb: 48% C: 52%
Ti: 90% Nb: 9% C: 1% Ti: 88% Nb: 11% C: 1%
Ti: 51% Nb: 2% C: 47% Ti: 53% Nb: 3% C: 44%
Ti: 54% Nb: 3% C: 43% Ti:55% Nb: 3% C: 42%
Ti: 41% Nb: 8% C: 51% Ti: 46% Nb: 9% C: 45%
Ti: 6% Nb: 71% C: 23% Ti: 7% Nb: 73% C: 20%
TiC phase Matrix
(101,200)
(112,222) (201)
(103,311) (311)
(101)
(200)
(102)
(110,220)
(002)
(100,111) (111)
30
40
50
60
(112) (201)
(103)
(110)
(102)
(002)
(100)
substrate
70
80
(101,200)
2θ
(b)
:α-Ti :NbC :TiC
(101)
(311) (103) (311) (112,222) (201)
(110,220)
(102)
(200)
overlayer
(112) (201)
(110)
(102)
(103)
substrate
Table 4 Analysis results of the dendritic precipitated TiC phase and matrix phase Phase (180A) upper overlayer
:α-Ti :NbC :TiC
overlayer
(100,111) (111) (002)
Two kinds of carbides containing undissolved NbC powder and precipitated TiC carbides were dispersed in the overlayer matrix. Relevant studies [6] have revealed that the size of powder and the value of the surfacing current affected the melting of NbC particles. So, the two surfacing arc currents of this system were not able to supply enough energy to fuse the NbC powder. In addition, the upper overlayer was TiC, while the middle overlayer contained both TiC and NbC. There was a dark-ring of precipitated TiC around undissolved NbC particles. Because the density of NbC (7.6 g/cm3) is greater than that of TiC (4.9 g/cm3), the NbC sank in the Ti melting pool. According to the reports of Wang Xibao et al. [7,8], the dilution of coating materials and the cooling rate of the melting pool affected the phenomena of solidification. Therefore, it can be deduced that the density, the effect of dilution and the solidification beginning at the interface are as the reasons why the great majority of undissolved NbC was observed in the middle overlayer. In Table 3, the partly dissolved NbC raised both Nb and C contents in the Ti melting pool during PTA process. According to the phase diagrams of Nb–Ti and C– Ti [9,10], the Nb solid solubility limit was ∼5 wt.% and the C solid solubility limit was very low (0.1 wt.%) in α-Ti. A
(a)
(002)
4. Discussion
related study [5] has revealed that the Nb solid solution content of Ti must be over 40 wt.% (about 25 at.%) to form β-Ti. Based on Table 3 and the X-ray analysis (see
(100)
there was no Ti in the NbC nucleus. Fig. 9 and Table 3 explain why there was no Ti to diffuse into undissolved NbC powder. Moreover, the BEI in Fig. 9 exhibits a bright thin layer between NbC particles and a dark-ring TiC zone (see arrow). The composition of this bright thin layer center is also listed in Table 3 confirming that it is probably Nb2C. Fig. 10 shows the distributive composition data of EPMA/ WDS (the analysis distance from the NbC nucleus to the bright thin layer). The results show an increasing Nb content and decreasing C content from the NbC nucleus to the bright thin layer interface. Also, a small amount of Ti existed in the bright thin layer.
Intensity
180 A
Matrix (average of zone)
Intensity
150 A
NbC nuclear center
Compositions (at.%) Ti
C
Nb
50.22 90.68
48.37 1.47
1.41 7.85
30
40
50
60
70
80
2θ Fig. 8. XRD patterns of the overlaying specimens: (a) 150 A and (b) 180A.
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F.-Y. Hung et al. / Surface & Coatings Technology 200 (2006) 6881–6887
Fig. 9. The EPMA analysis from the NbC nucleus centers to matrix (using 180A overlaying current as an example).
Fig. 8), the two overlay matrices with different overlaying current were α-Ti and the heat affected zone was needle αTi [11]. Fig. 11 shows the relationship between Gibbs' free energy of carbide reacting and temperature [12]. This explains how the C reacted with Ti melt to produce precipitated TiC during PTA process. Meanwhile, there was no precipitated NbC in the overlayer and the reacting Nb would be solid solution in the matrix. Also, both C and Nb in the Ti melt had a chance to react into Nb2C. But according to Fig. 11, when the temperature was over 1200 °C, TiC was the major precipitated phase. In Fig. 9, a dark-ring of precipitated TiC around undissolved NbC particles (outside of the bright thin layer) can be observed. The reason was that NbC provided a nucleation site and the TiC grew with heterogeneous nucleation. As for the Nb, C and Ti concentration variations in Fig. 10, the outside bright thin layer of NbC was a diffusion reacting layer. Table 3 reveals that the C content of the diffusion layer was ∼ 20 at.%. According to the C–Nb phase diagram [13], it is safe to say that the diffusion layer was βNb2C. This is the reason that the speed of C diffusion was greater than that of Nb. This resulted in the TiC precipitating easily in the Ti melt, leasing insufficient C to precipitate into βNb2C and causing ∼9 at.% Nb to become solid solution in Ti matrix after the solidification process (see Table 3). Comparing NbC with Nb2C in Fig. 11, C and NbC reacted to produce Nb2C which was more easily formed than NbC. Obviously, C underwent some reactions during the PAT solidification process. To begin with, C and Ti melt reacted to form precipitated TiC dispersed in the matrix.
Then, C and undissolved NbC reacted to form a dark-ring of TiC with heterogeneous nucleation. Finally, the diffusing C reacted to form Nb2C between undissolved NbC nucleus and the dark-ring TiC zone after the cooling process. Moreover, the thickness of this diffusion layer of Nb2C increased with increasing the value of the overlaying current. The reason was that the 180 A condition had a larger heat input to prolong the duration of solidification and diffusion [14]. Reportedly [3], increasing the current also results in increased heat input and a decreased solidification rate. According to one study [15], the solidification structure got finer as the solidification rate was increased during PAT process. In short, the precipitated carbide particles of 150 A were finer than those of 180 A. And the dendritic TiC carbides in the upper overlayer became more obvious as the overlaying current was increased. 5. Conclusions 1. The overlayer matrix was α-Ti containing about 10 at.% Nb and 1 at.% C. The cross-section of the overlayer microstructure from surface to base metal can be separated into three layers: an upper overlayer with TiC, a middle overlayer with TiC + NbC and a lower overlayer containing an interfacial layer and a heat affected zone. 2. Low overlaying current will increase the solidification rate resulting in a decrease in precipitated TiC particle size and making the dendritic TiC of the upper overlayer finer. In addition, two surfacing arc currents were not able to supply enough energy to fuse the NbC powders, while the undissolved NbC provides a heterogeneous nucleation site and reacted to
F.-Y. Hung et al. / Surface & Coatings Technology 200 (2006) 6881–6887
(a)
NbC
-25
TiC
Bright
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layer
(b)
80
C content Nb content Ti content
-35
Δ G (kcal / mole C)
content (at. %)
60
40
20
0
0.4
0.8
Nb2C -45
1.2
Distance (μm) 80
TiC
-55
C content Nb content Ti content
-60
60
content (at. %)
-40
-50
0
(c)
NbC
-30
0
300
600
900
1200 1500 1800 2100
Temperature (°C) 40
Fig. 11. Gibbs' free energy as a function of carbide reacting temperature.
References
20
0
0
0.4
0.8
1.2
Distance (μm)
[1] [2] [3] [4] [5]
Fig. 10. EPMA/WDS analysis of the distributive composition from the NbC nucleus to the bright thin layer (a) schematic structure site, (b) 150 A overlaying current specimen and (c) 180 A overlaying current specimen.
[6] [7] [8]
form a dark-ring of precipitated TiC. A bright diffusion layer of Nb2C was present between the undissolved NbC nucleus and the dark-ring TiC zone. Acknowledgements The authors are grateful to the Chinese National Science Council for its financial support (contract: NSC 94-2216-E006-034).
[9] [10] [11] [12] [13] [14] [15]
Metals Handbook, 9th ed., ASM, 13, 1988, p. 658. U. Nakamura, J. Yosettsu Gijutsu 8 (1986) 45. W.S. Dai, L.H. Chen, T.S. Lui, Wear 248 (2001) 201. A. Hirose, D. Ozamoto, R. Aoki, K.F. Kobayashi, Z. Metallkd. 86 (1995) 580. S. Takahashi, N. Okada, Z. Shida, M. Nakanishi, Tetsu to Hagane 8 (1991) 124. Xibao Wang, Hua Lui, Surf. Coat. Technol. 106 (1998) 156. Xibao Wang, Xiaofeng Wang, Zhongquan Shi, Surf. Coat. Technol. 192 (2005) 257. Xibao Wang, Liang Yong, Songlan Yang, Surf. Coat. Technol. 137 (2001) 209. ASM Handbook, 10th ed., 3, 1992, p. 2307. T.B. Massalski, ASM (1986) 333. K. Rudinger, Z. Werkstofftech. 13 (7) (1982) 229. S.R. Shatynski, Oxidation of Metals 13 (2) (1979) 105. R. Freer, The Physics and Chemistry of Carbides, Nitrides, and Borides, Kluwer Academic Publishers, Boston, 1989, p. 216. W. Jost, Diffusion in Solids, Liquids, Gases, Academic Press, New York, 1960, p. 69. W.F. Savage, E.F. Nipper, J.S. Erickson, Weld. J. (1976) 213.