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Elucidating the microstructure and wear behavior for multicomponent alloy clad layers by in situ synthesis
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Surface & Coatings Technology 202 (2008) 4666 – 4672 www.elsevier.com/locate/surfcoat
Elucidating the microstructure and wear behavior for multicomponent alloy clad layers by in situ synthesis Y.C. Lin ⁎, Y.H. Cho Department of Mechanical Engineering, National Taiwan University of Science and Technology, 43, Keelung Road, Section 4, Taipei 106, Taiwan, ROC Received 28 December 2007; accepted in revised form 28 March 2008 Available online 7 April 2008
Abstract High-entropy alloys with a wide range of novel microstructures and properties are under extensive development. In this study, two equimolar powder mixtures of NiCrAlCoW and NiCrAlCoSi were prepared as cladding materials. They were clad by gas tungsten arc welding on AISI 1050 medium carbon steel substrate to form an in situ synthesized multicomponent clad layer. The microstructure, chemical composition and constituent phases of the clad layers were characterized by FESEM, EPMA and XRD, respectively. A rotating tribometer was employed to evaluate the tribological properties of the clad layers. Experimental results demonstrate that the NiCrAlCoW clad layer is composed of W (precipitates and residual particles), AlNi and Cr15.58Fe7.42C6. The NiCrAlCoSi clad layer contains only a BCC phase. The two clad layers exhibit high hardness, caused by precipitation hardening with different precipitates. The NiCrAlCoW clad layer exhibits strong mechanical interlocking, which results from the complex phase and microstructure of the NiCrAlCoW clad layer. During rubbing, the in situ phase transformation of the NiCrAlCoSi clad layer induces the softening of the matrix. Therefore, the wear performance of the NiCrAlCoW clad layer is better than that of the NiCrAlCoSi clad layer. © 2008 Elsevier B.V. All rights reserved. PACS: 81.05.Bx; 81.20.Ev; 81.40.-z; 81.65.-b Keywords: Clad layer; In situ synthesized; Wear performance
1. Introduction In recent years, high-entropy alloys, an advanced alloy system that contains multiple principal elements in equimolar ratios instead of one major element, have been developed [1]. The formation of a complex microstructure in high-entropy alloys, leading to difficulties in analysis was not anticipated. Highentropy alloys have a stable simple solid solution structure, and can easily form nanoprecipitates and an amorphous phase due to the high mixing entropy [2]. Research on high-entropy alloys has found that nanoparticles are dispersed in amorphous matrices [3,4]. Additionally, exhibiting high hardness and superior resistance to wear, oxidation and corrosion, high-entropy alloys have a wide range of industrial applications. However, many studies of high-entropy alloys have adopted arc melting and
⁎ Corresponding author. Tel.: +886 2 27376498; fax: +886 2 27376460. E-mail address: [email protected] (Y.C. Lin). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.03.033
casting to obtain bulk ingots [1–5]. Relatively few investigations have focused on in situ synthesized high-entropy alloy coatings produced from multicomponent mixtures by GTAW instead of by the arc melting and casting. Surface modification can change the surface properties for particular purposes but cannot alter the bulk properties. Accordingly, surface cladding is always used for surface modification to improve the wear performance of mechanical components [6–8]. Numerous Fe-based [9,10], Ni-
Table 1 Cladding process parameters Weld current (A)
100
Weld speed (m/s) Arc voltage (V) Arc gap (m) Electrode type Electrode polarity Argon, flow rate (L/min)
1.3 × 10− 3 16 2 × 10− 3 W–2%Th DCSP 8
Y.C. Lin, Y.H. Cho / Surface & Coatings Technology 202 (2008) 4666–4672 Table 2 Wear test conditions Apparent contact stress (MPa)
262
Sliding speed (m/s) Sliding distance (m) Lubrication condition Temperature (°C)
0.603/0.905/1.206 542.87 Dry Room temperature
based [11,12], and Co-based [13,14] alloy systems have been used for surface cladding. However, these cladding materials always keep one major element and the amounts of other elements are adjusted to yield a suitable composition. The cladding of multicomponent alloy systems has seldom been addressed. Consequently, this work considers two multicomponent clad layers by in situ synthesis to analyze their microstructures and wear behaviors. Gas tungsten arc welding (GTAW) can rapidly melt various alloys onto metal substrates and produce good metallurgical bonding between the clad layer and the substrate. Furthermore, the variety of metallurgical structures and mechanical properties of the clad layers obtained using the GTAW method
Fig. 1. Micrographs of cross-sections of (a) NiCrAlCoW and (b) NiCrAlCoSi clad layers, etching in aqua regia for a few minutes.
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has motivated many researchers [15–17]. Hence, the GTAW method has for a long time been used in surface modification and surface alloying. In this work, different powder mixtures with equimolar ratios were clad on AISI 1050 medium carbon steel to yield an in situ synthesized multicomponent alloy coated by the GTAW method. Some experiments were conducted to evaluate the microstructure, hardness, chemical composition and wear performance of the clad layers formed from in situ synthesized multicomponent alloy. 2. Experimental procedures Ni, Cr, Al, Co, W and Si powders were used as the base powders. Two equimolar powder mixtures of NiCrAlCoW and NiCrAlCoSi were separately prepared as cladding materials. To increase the uniformity of the powders, steel balls were added to the ball mixing-jar during blending. After mixing for 24 h, the composite powders were blended with 4 wt.% polyvinyl alcohol
Fig. 2. Typical microstructure of NiCrAlCoW clad layer, etching in aqua regia for a few minutes: (a) microstructure of clad layer, (b) magnified image of residual particles connected to form a cluster.
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as binders. The paste was then agitated to ensure its homogeneity, and pre-placed on the AISI 1050 medium carbon steel surface. The thickness of the pre-placed paste was maintained at 2 mm and the specimens were dried in a furnace at 60 °C for 24 h. Finally, cladding was carried out by gas tungsten arc welding (GTAW). Table 1 presents the parameters of the cladding process. After cladding, the required specimens were cut from the clad layer using a wire-EDM machine. The microstructure and composition distribution were examined under a field-emission scanning electron microscope (FESEM) and an electron probe microanalyzer (EPMA), respectively. The crystalline structure of the clad layers was identified using an Xray diffractometer (XRD) with Cu Kα radiation at a working voltage of 40 kV and a current of 100 mA. The hardness distribution profile of the cladding layers was measured using a Vickers microhardness tester with a load of 300 g. The wear behavior of the clad layers and AISI 1050 medium carbon steel were determined without lubrication using a pin-on-disc rotating tribometer. A moving upper specimen (pin) was slid against the flat surface of a fixed lower specimen (disc) with hardness HRC 62. The disc was made of hardened AISI 52100 bearing steel and had a diameter of 60 mm and a thickness of 2 mm. The rubbed
surface of the disc was polished by SiC emery paper up to 600 grits and had a surface roughness Ra = 0.06 μm prior to wear testing. Then, the specimens for wear testing were cleaned ultrasonically in acetone for 30 min. During wear testing, the frictional and normal forces were monitored continuously using a personal computer connected to the data acquisition system. Table 2 lists the wear test conditions. 3. Results and discussion 3.1. Microstructure and hardness distribution of the clad layers Fig. 1 shows the micrographs of the cross-sections of the two coatings. The coatings have a good metallurgical bonding between the clad layer and the substrate without visible crack. Fig. 2 presents the typical microstructure of the NiCrAlCoW clad layer. It is composed of a dendritic reinforcement, a network structure and a granular structure (which was removed by etching), as shown in Fig. 2(a). Backscattered electron image and EPMA elemental mapping were utilized to elucidate the clad layer without etching. Fig. 3 displays typical results. EPMA analysis indicates that the dendritic reinforcement comprised
Fig. 3. Backscattered electron image and EPMA elemental mapping of NiCrAlCoW clad layer.
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Fig. 4. Typical microstructure of NiCrAlCoSi clad layer, etching in aqua regia for a few minutes: (a) microstructure of clad layer, (b) magnified image of veinlike precipitate to form a network.
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pounds. The intermetallic compound with the granular structure is AlNi. Cr combines with Fe and C from AISI 1050 medium carbon steel substrate to form Cr15.58Fe7.42C6 with the network structure. Additionally, the dendritic reinforcement is constituted by W solid solution. During the cladding process, the fast solidification of the molten pool results in the partial melting of W particles, because W has a high melting point. Furthermore, the solid phase of W is primarily precipitated in the form of dendrite, because of the high melting point of W. As the temperature of the molten pool falls, AlNi solidifies interdendritically following the solidification of W dendrites. Finally, the chromium–iron carbide is precipitated from the residual liquid. Therefore, the chromium–iron carbide exhibits a network structure. Moreover, the moving interface between solid and liquid and the expulsion of undissolvable W particles cause the undissolvable W particles to form a cluster during solidification. Accordingly, these undissolvable W particles are trapped in the final solidification region, causing them to connect to one another, as shown in Fig. 2(b). The intermetallic compounds and residual particles are present in the NiCrAlCoW clad layer, weakening the effect of the high mixing entropy of the alloy system. In contrast, the high mixing entropy of the NiCrAlCoSi alloy system tends to make it a simple solid solution composed of various elements. The XRD analysis reveals only a BCC phase with a lattice constant of a = 0.286 nm in the NiCrAlCoSi clad layer, as displayed in Fig. 6(b). The results imply that an equimolar multicomponent alloy system can also form a solid solution with a simple crystal structure. Fig. 7 presents the hardness variation with the distance from clad surface. The results indicate that the hardness of the NiCrAlCoW clad layer is slightly higher than that of the NiCrAlCoSi clad layer. The two clad layers exhibit high hardness (about 730 Hv at the top of the clad layers), caused by precipitation hardening with different precipitates. Fig. 7 also reveals that the effective depth of the clad layers is approximately 2.6 mm. 3.2. Wear behavior of the clad specimens
mostly W elements, and the network structure is rich in Cr and some Fe and C. Moreover, the granular structure, which is uniformly distributed in the matrix, consists of Al and Ni elements. Interestingly, Co is uniformly present in the NiCrAlCoW clad layer, but not in the dendritic reinforcement. The microstructural morphology of the NiCrAlCoSi clad layer differs from that of the NiCrAlCoW clad layer, as presented in Fig. 4. In the NiCrAlCoSi clad layer, the matrix phase was etched, leaving the caves, as shown in Fig. 4(a). Precipitates with vein-like shape are distributed in the matrix. Fig. 4(b) reveals that the vein-like precipitates are connected to each other, forming a network in the clad layer. EPMA analysis of the clad layer without etching shows that Cr, Si and C are the primary elements in the vein-like precipitate while Ni, Al, Fe and Co are the primary elements in the matrix, as displayed in Fig. 5. Fig. 6 presents the X-ray diffraction patterns of the NiCrAlCoW and NiCrAlCoSi clad layers. The results demonstrate that in the NiCrAlCoW clad layer, the network structure and granular structure comprise different intermetallic com-
The wear performance of the clad specimens was evaluated under various wear test conditions. Fig. 8 plots the wear volume loss of the AISI 1050 medium carbon steel and the clad specimens at various sliding speeds after sliding through 542.87 m. The results show that the wear performance of the clad specimens is better than that of AISI 1050 medium carbon steel under all test conditions. Fig. 9(a) displays the typical worn scar of AISI 1050 medium carbon steel at the highest sliding speed, 1.206 m/s. At the high sliding speed, rubbing increases the temperature of the rubbed surface, softening it; plastic flow generally accompanies adhesion wear, resulting in high wear loss. Fig. 9(a) clearly demonstrates that the worn scar exhibits plastic deformation and adhesive features, suggesting that AISI 1050 medium carbon steel underwent severe adhesive wear. On the contrary, the worn scar of the NiCrAlCoW clad specimen is quite smooth, as shown in Fig. 9(b). The dendritic reinforcements are clearly observed on the worn scar. The strong mechanical interlocking associated with the complex geometric
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Fig. 5. Backscattered electron image and EPMA elemental mapping of NiCrAlCoSi clad layer.
effect is caused by the strong bonding between the dense dendrites and the matrix. During a wear test, the dense dendrites not only strengthen the structure, but also prevent rubbing from plastic flow, resulting in low wear loss of the NiCrAlCoW clad specimen at high sliding speed. Fig. 9(c) presents the worn scar of the NiCrAlCoSi clad specimen. The major wear mechanism is abrasive wear, which indicates that the hardness of the NiCrAlCoSi clad specimen is markedly lower than that of the counter body. Hence, the wear loss of the NiCrAlCoSi clad specimen exceeds that of the NiCrAlCoW clad specimen even though their hardness values are approximately equal. Some researchers refer to multicomponent alloy systems as highentropy alloys due to their high configuration entropy. Additionally, some metastable phases are present in such an alloy system, and are responsible for its high hardness. In GTAW cladding, the high cooling rate of the weld pool and the effect of high mixing entropy increase the complexity of the alloy system, leading to the formation of a metastable phase with high internal energy. During rubbing, the metastable phase of the alloy system transforms into a stable phase when the required activation energy reaches a critical value. The atoms have high activity in the period of phase transformation, resulting in softening. The softening causes plastic deformation of the matrix and the detachment of precipitates from the
rubbing surface. The introduction of hard precipitates between the two sliding surfaces may cause three-body abrasive wear. Therefore, the abrasive wear of the NiCrAlCoSi clad layer may reasonably be caused by softening, which arises from the metastable phase transformation during rubbing. Research on wear has established that softening can occur during rubbing [18,19]. Furthermore, a comparison between Figs. 2(a) and 4(a) indicates that the NiCrAlCoW clad layer exhibits a more complex phase and microstructure than does the NiCrAlCoSi clad layer. The mechanical interlocking of the NiCrAlCoW clad layer is higher than that of the NiCrAlCoSi clad layer. Accordingly, the wear performance of the NiCrAlCoW clad layer, which is enhanced by mechanical interlocking, is better than that of the NiCrAlCoSi clad layer. 4. Conclusions Based on the results of microstructural analysis and wear testing, the following conclusions are drawn. 1. In situ synthesized multicomponent alloy clad layers were successfully generated using the GTAW method. The hardness of the clad layers of the NiCrAlCoW and NiCrAlCoSi alloy system are strengthened by the different precipitates.
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Fig. 8. Wear volume loss as a function of sliding speed under apparent contact stress = 262 MPa and sliding distance = 542.87 m.
2. The mechanical interlocking of the NiCrAlCoW clad layer is stronger than that of the NiCrAlCoSi clad layer. Hence, the wear performance of the NiCrAlCoW clad layer exceeds that of the NiCrAlCoSi clad layer. Acknowledgements The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 95-2221-E-011-007. References Fig. 6. X-ray diffraction patterns of (a) NiCrAlCoW and (b) NiCrAlCoSi clad layers.
Fig. 7. Hardness distribution of NiCrAlCoW and NiCrAlCoSi clad layers.
[1] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Adv. Eng. Mater. 6 (2004) 299. [2] J.W. Yeh, S.K. Chen, J.Y. Gan, S.J. Lin, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Metall. Mat. Trans. A Phys. Metall. Mat. Sci. 35 (2004) 2533. [3] Y.J. Zhou, Y. Zhang, Y.L. Wang, G.L. Chen, Mater. Sci. Eng. A 454–455 (2007) 260. [4] X.F. Wang, Y. Zhang, Y. Qiao, G.L. Chen, Intermet. 15 (2007) 357. [5] B. Cantor, I.T.H. Chang, P. Knight, A.J.B. Vincent, Mater. Sci. Eng. A 375–377 (2004) 213. [6] P.H. Chong, H.C. Man, T.M. Yue, Surf. Coat. Technol. 154 (2002) 268. [7] Y. Chen, H.M. Wang, Mater. Sci. Eng. A 368 (2004) 80. [8] G. Abbas, U. Ghazanfar, Wear 258 (2005) 258. [9] X. Wu, Surf. Coat. Technol. 115 (1999) 153. [10] L. Zhang, B. Liu, H. Yu, D. Sun, Surf. Coat. Technol. 201 (2007) 5931. [11] E. Fernandez, M. Cadenas, R. Gonzalez, C. Navas, R. Fernandez, J. De Damborenea, Wear 259 (2005) 870. [12] G. Xu, M. Kutsuna, Z. Liu, JSME Int. J. Ser. C. 49 (2006) 370. [13] H. So, C.T. Chen, Y.A. Chen, Wear 192 (1996) 78. [14] C. Cui, Z. Guo, Y. Liu, Q. Xie, Z. Wang, J. Hu, Y. Yao, Opt. Laser Technol. 39 (2007) 1544. [15] M. Eroglu, N. O zdemir, Surf. Coat. Technol. 154 (2002) 209. [16] X.H. Wang, M. Zhang, Z.D. Zou, S.L. Song, F. Han, S.Y. Qu, Surf. Coat. Technol. 200 (2006) 6117. [17] S. Buytoz, M. Ulutan, M.M. Yildirim, Appl. Surf. Sci. 252 (2005) 1313. [18] Y.C. Lin, S.W. Wang, T.M. Chen, J. Mater. Process. Technol. 120 (2002) 126. [19] A. Bahrami, S.H. Mousavi Anijdan, M.A. Golozar, M. Shamanian, N. Varahram, Wear 258 (2005) 846.
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Fig. 9. SEM micrographs of worn surface of (a) AISI 1050 steel (b) NiCrAlCoW clad layer (c) NiCrAlCoSi clad layer, with apparent contact stress = 262 MPa, sliding speed = 1.206 m/s, sliding distance = 542.87 m.
Surface & Coatings Technology 202 (2008) 4666 – 4672 www.elsevier.com/locate/surfcoat
Elucidating the microstructure and wear behavior for multicomponent alloy clad layers by in situ synthesis Y.C. Lin ⁎, Y.H. Cho Department of Mechanical Engineering, National Taiwan University of Science and Technology, 43, Keelung Road, Section 4, Taipei 106, Taiwan, ROC Received 28 December 2007; accepted in revised form 28 March 2008 Available online 7 April 2008
Abstract High-entropy alloys with a wide range of novel microstructures and properties are under extensive development. In this study, two equimolar powder mixtures of NiCrAlCoW and NiCrAlCoSi were prepared as cladding materials. They were clad by gas tungsten arc welding on AISI 1050 medium carbon steel substrate to form an in situ synthesized multicomponent clad layer. The microstructure, chemical composition and constituent phases of the clad layers were characterized by FESEM, EPMA and XRD, respectively. A rotating tribometer was employed to evaluate the tribological properties of the clad layers. Experimental results demonstrate that the NiCrAlCoW clad layer is composed of W (precipitates and residual particles), AlNi and Cr15.58Fe7.42C6. The NiCrAlCoSi clad layer contains only a BCC phase. The two clad layers exhibit high hardness, caused by precipitation hardening with different precipitates. The NiCrAlCoW clad layer exhibits strong mechanical interlocking, which results from the complex phase and microstructure of the NiCrAlCoW clad layer. During rubbing, the in situ phase transformation of the NiCrAlCoSi clad layer induces the softening of the matrix. Therefore, the wear performance of the NiCrAlCoW clad layer is better than that of the NiCrAlCoSi clad layer. © 2008 Elsevier B.V. All rights reserved. PACS: 81.05.Bx; 81.20.Ev; 81.40.-z; 81.65.-b Keywords: Clad layer; In situ synthesized; Wear performance
1. Introduction In recent years, high-entropy alloys, an advanced alloy system that contains multiple principal elements in equimolar ratios instead of one major element, have been developed [1]. The formation of a complex microstructure in high-entropy alloys, leading to difficulties in analysis was not anticipated. Highentropy alloys have a stable simple solid solution structure, and can easily form nanoprecipitates and an amorphous phase due to the high mixing entropy [2]. Research on high-entropy alloys has found that nanoparticles are dispersed in amorphous matrices [3,4]. Additionally, exhibiting high hardness and superior resistance to wear, oxidation and corrosion, high-entropy alloys have a wide range of industrial applications. However, many studies of high-entropy alloys have adopted arc melting and
⁎ Corresponding author. Tel.: +886 2 27376498; fax: +886 2 27376460. E-mail address: [email protected] (Y.C. Lin). 0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2008.03.033
casting to obtain bulk ingots [1–5]. Relatively few investigations have focused on in situ synthesized high-entropy alloy coatings produced from multicomponent mixtures by GTAW instead of by the arc melting and casting. Surface modification can change the surface properties for particular purposes but cannot alter the bulk properties. Accordingly, surface cladding is always used for surface modification to improve the wear performance of mechanical components [6–8]. Numerous Fe-based [9,10], Ni-
Table 1 Cladding process parameters Weld current (A)
100
Weld speed (m/s) Arc voltage (V) Arc gap (m) Electrode type Electrode polarity Argon, flow rate (L/min)
1.3 × 10− 3 16 2 × 10− 3 W–2%Th DCSP 8
Y.C. Lin, Y.H. Cho / Surface & Coatings Technology 202 (2008) 4666–4672 Table 2 Wear test conditions Apparent contact stress (MPa)
262
Sliding speed (m/s) Sliding distance (m) Lubrication condition Temperature (°C)
0.603/0.905/1.206 542.87 Dry Room temperature
based [11,12], and Co-based [13,14] alloy systems have been used for surface cladding. However, these cladding materials always keep one major element and the amounts of other elements are adjusted to yield a suitable composition. The cladding of multicomponent alloy systems has seldom been addressed. Consequently, this work considers two multicomponent clad layers by in situ synthesis to analyze their microstructures and wear behaviors. Gas tungsten arc welding (GTAW) can rapidly melt various alloys onto metal substrates and produce good metallurgical bonding between the clad layer and the substrate. Furthermore, the variety of metallurgical structures and mechanical properties of the clad layers obtained using the GTAW method
Fig. 1. Micrographs of cross-sections of (a) NiCrAlCoW and (b) NiCrAlCoSi clad layers, etching in aqua regia for a few minutes.
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has motivated many researchers [15–17]. Hence, the GTAW method has for a long time been used in surface modification and surface alloying. In this work, different powder mixtures with equimolar ratios were clad on AISI 1050 medium carbon steel to yield an in situ synthesized multicomponent alloy coated by the GTAW method. Some experiments were conducted to evaluate the microstructure, hardness, chemical composition and wear performance of the clad layers formed from in situ synthesized multicomponent alloy. 2. Experimental procedures Ni, Cr, Al, Co, W and Si powders were used as the base powders. Two equimolar powder mixtures of NiCrAlCoW and NiCrAlCoSi were separately prepared as cladding materials. To increase the uniformity of the powders, steel balls were added to the ball mixing-jar during blending. After mixing for 24 h, the composite powders were blended with 4 wt.% polyvinyl alcohol
Fig. 2. Typical microstructure of NiCrAlCoW clad layer, etching in aqua regia for a few minutes: (a) microstructure of clad layer, (b) magnified image of residual particles connected to form a cluster.
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Y.C. Lin, Y.H. Cho / Surface & Coatings Technology 202 (2008) 4666–4672
as binders. The paste was then agitated to ensure its homogeneity, and pre-placed on the AISI 1050 medium carbon steel surface. The thickness of the pre-placed paste was maintained at 2 mm and the specimens were dried in a furnace at 60 °C for 24 h. Finally, cladding was carried out by gas tungsten arc welding (GTAW). Table 1 presents the parameters of the cladding process. After cladding, the required specimens were cut from the clad layer using a wire-EDM machine. The microstructure and composition distribution were examined under a field-emission scanning electron microscope (FESEM) and an electron probe microanalyzer (EPMA), respectively. The crystalline structure of the clad layers was identified using an Xray diffractometer (XRD) with Cu Kα radiation at a working voltage of 40 kV and a current of 100 mA. The hardness distribution profile of the cladding layers was measured using a Vickers microhardness tester with a load of 300 g. The wear behavior of the clad layers and AISI 1050 medium carbon steel were determined without lubrication using a pin-on-disc rotating tribometer. A moving upper specimen (pin) was slid against the flat surface of a fixed lower specimen (disc) with hardness HRC 62. The disc was made of hardened AISI 52100 bearing steel and had a diameter of 60 mm and a thickness of 2 mm. The rubbed
surface of the disc was polished by SiC emery paper up to 600 grits and had a surface roughness Ra = 0.06 μm prior to wear testing. Then, the specimens for wear testing were cleaned ultrasonically in acetone for 30 min. During wear testing, the frictional and normal forces were monitored continuously using a personal computer connected to the data acquisition system. Table 2 lists the wear test conditions. 3. Results and discussion 3.1. Microstructure and hardness distribution of the clad layers Fig. 1 shows the micrographs of the cross-sections of the two coatings. The coatings have a good metallurgical bonding between the clad layer and the substrate without visible crack. Fig. 2 presents the typical microstructure of the NiCrAlCoW clad layer. It is composed of a dendritic reinforcement, a network structure and a granular structure (which was removed by etching), as shown in Fig. 2(a). Backscattered electron image and EPMA elemental mapping were utilized to elucidate the clad layer without etching. Fig. 3 displays typical results. EPMA analysis indicates that the dendritic reinforcement comprised
Fig. 3. Backscattered electron image and EPMA elemental mapping of NiCrAlCoW clad layer.
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Fig. 4. Typical microstructure of NiCrAlCoSi clad layer, etching in aqua regia for a few minutes: (a) microstructure of clad layer, (b) magnified image of veinlike precipitate to form a network.
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pounds. The intermetallic compound with the granular structure is AlNi. Cr combines with Fe and C from AISI 1050 medium carbon steel substrate to form Cr15.58Fe7.42C6 with the network structure. Additionally, the dendritic reinforcement is constituted by W solid solution. During the cladding process, the fast solidification of the molten pool results in the partial melting of W particles, because W has a high melting point. Furthermore, the solid phase of W is primarily precipitated in the form of dendrite, because of the high melting point of W. As the temperature of the molten pool falls, AlNi solidifies interdendritically following the solidification of W dendrites. Finally, the chromium–iron carbide is precipitated from the residual liquid. Therefore, the chromium–iron carbide exhibits a network structure. Moreover, the moving interface between solid and liquid and the expulsion of undissolvable W particles cause the undissolvable W particles to form a cluster during solidification. Accordingly, these undissolvable W particles are trapped in the final solidification region, causing them to connect to one another, as shown in Fig. 2(b). The intermetallic compounds and residual particles are present in the NiCrAlCoW clad layer, weakening the effect of the high mixing entropy of the alloy system. In contrast, the high mixing entropy of the NiCrAlCoSi alloy system tends to make it a simple solid solution composed of various elements. The XRD analysis reveals only a BCC phase with a lattice constant of a = 0.286 nm in the NiCrAlCoSi clad layer, as displayed in Fig. 6(b). The results imply that an equimolar multicomponent alloy system can also form a solid solution with a simple crystal structure. Fig. 7 presents the hardness variation with the distance from clad surface. The results indicate that the hardness of the NiCrAlCoW clad layer is slightly higher than that of the NiCrAlCoSi clad layer. The two clad layers exhibit high hardness (about 730 Hv at the top of the clad layers), caused by precipitation hardening with different precipitates. Fig. 7 also reveals that the effective depth of the clad layers is approximately 2.6 mm. 3.2. Wear behavior of the clad specimens
mostly W elements, and the network structure is rich in Cr and some Fe and C. Moreover, the granular structure, which is uniformly distributed in the matrix, consists of Al and Ni elements. Interestingly, Co is uniformly present in the NiCrAlCoW clad layer, but not in the dendritic reinforcement. The microstructural morphology of the NiCrAlCoSi clad layer differs from that of the NiCrAlCoW clad layer, as presented in Fig. 4. In the NiCrAlCoSi clad layer, the matrix phase was etched, leaving the caves, as shown in Fig. 4(a). Precipitates with vein-like shape are distributed in the matrix. Fig. 4(b) reveals that the vein-like precipitates are connected to each other, forming a network in the clad layer. EPMA analysis of the clad layer without etching shows that Cr, Si and C are the primary elements in the vein-like precipitate while Ni, Al, Fe and Co are the primary elements in the matrix, as displayed in Fig. 5. Fig. 6 presents the X-ray diffraction patterns of the NiCrAlCoW and NiCrAlCoSi clad layers. The results demonstrate that in the NiCrAlCoW clad layer, the network structure and granular structure comprise different intermetallic com-
The wear performance of the clad specimens was evaluated under various wear test conditions. Fig. 8 plots the wear volume loss of the AISI 1050 medium carbon steel and the clad specimens at various sliding speeds after sliding through 542.87 m. The results show that the wear performance of the clad specimens is better than that of AISI 1050 medium carbon steel under all test conditions. Fig. 9(a) displays the typical worn scar of AISI 1050 medium carbon steel at the highest sliding speed, 1.206 m/s. At the high sliding speed, rubbing increases the temperature of the rubbed surface, softening it; plastic flow generally accompanies adhesion wear, resulting in high wear loss. Fig. 9(a) clearly demonstrates that the worn scar exhibits plastic deformation and adhesive features, suggesting that AISI 1050 medium carbon steel underwent severe adhesive wear. On the contrary, the worn scar of the NiCrAlCoW clad specimen is quite smooth, as shown in Fig. 9(b). The dendritic reinforcements are clearly observed on the worn scar. The strong mechanical interlocking associated with the complex geometric
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Fig. 5. Backscattered electron image and EPMA elemental mapping of NiCrAlCoSi clad layer.
effect is caused by the strong bonding between the dense dendrites and the matrix. During a wear test, the dense dendrites not only strengthen the structure, but also prevent rubbing from plastic flow, resulting in low wear loss of the NiCrAlCoW clad specimen at high sliding speed. Fig. 9(c) presents the worn scar of the NiCrAlCoSi clad specimen. The major wear mechanism is abrasive wear, which indicates that the hardness of the NiCrAlCoSi clad specimen is markedly lower than that of the counter body. Hence, the wear loss of the NiCrAlCoSi clad specimen exceeds that of the NiCrAlCoW clad specimen even though their hardness values are approximately equal. Some researchers refer to multicomponent alloy systems as highentropy alloys due to their high configuration entropy. Additionally, some metastable phases are present in such an alloy system, and are responsible for its high hardness. In GTAW cladding, the high cooling rate of the weld pool and the effect of high mixing entropy increase the complexity of the alloy system, leading to the formation of a metastable phase with high internal energy. During rubbing, the metastable phase of the alloy system transforms into a stable phase when the required activation energy reaches a critical value. The atoms have high activity in the period of phase transformation, resulting in softening. The softening causes plastic deformation of the matrix and the detachment of precipitates from the
rubbing surface. The introduction of hard precipitates between the two sliding surfaces may cause three-body abrasive wear. Therefore, the abrasive wear of the NiCrAlCoSi clad layer may reasonably be caused by softening, which arises from the metastable phase transformation during rubbing. Research on wear has established that softening can occur during rubbing [18,19]. Furthermore, a comparison between Figs. 2(a) and 4(a) indicates that the NiCrAlCoW clad layer exhibits a more complex phase and microstructure than does the NiCrAlCoSi clad layer. The mechanical interlocking of the NiCrAlCoW clad layer is higher than that of the NiCrAlCoSi clad layer. Accordingly, the wear performance of the NiCrAlCoW clad layer, which is enhanced by mechanical interlocking, is better than that of the NiCrAlCoSi clad layer. 4. Conclusions Based on the results of microstructural analysis and wear testing, the following conclusions are drawn. 1. In situ synthesized multicomponent alloy clad layers were successfully generated using the GTAW method. The hardness of the clad layers of the NiCrAlCoW and NiCrAlCoSi alloy system are strengthened by the different precipitates.
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Fig. 8. Wear volume loss as a function of sliding speed under apparent contact stress = 262 MPa and sliding distance = 542.87 m.
2. The mechanical interlocking of the NiCrAlCoW clad layer is stronger than that of the NiCrAlCoSi clad layer. Hence, the wear performance of the NiCrAlCoW clad layer exceeds that of the NiCrAlCoSi clad layer. Acknowledgements The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC 95-2221-E-011-007. References Fig. 6. X-ray diffraction patterns of (a) NiCrAlCoW and (b) NiCrAlCoSi clad layers.
Fig. 7. Hardness distribution of NiCrAlCoW and NiCrAlCoSi clad layers.
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Fig. 9. SEM micrographs of worn surface of (a) AISI 1050 steel (b) NiCrAlCoW clad layer (c) NiCrAlCoSi clad layer, with apparent contact stress = 262 MPa, sliding speed = 1.206 m/s, sliding distance = 542.87 m.