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Synthesis of Fe-based amorphous composite coatings with low purity materials by laser cladding
Applied Surface Science 253 (2007) 7060–7064 www.elsevier.com/locate/apsusc
Synthesis of Fe-based amorphous composite coatings with low purity materials by laser cladding Zhu Qingjun*, Qu Shiyao, Wang Xinhong, Zou Zengda School of Materials Science and Engineering, Shandong University, Jinan 250061, PR China Received 23 December 2006; received in revised form 1 February 2007; accepted 11 February 2007 Available online 23 February 2007
Abstract Amorphous composite coatings Fe38Ni30 XSi16B14V2MX (X = 0, 1, 2) (M contains Al, Ti, Mo, and C) were prepared with low purity of raw materials by laser cladding. X-ray diffraction and transmission electron microscopy results show that the coating have an amorphous structure with a few crystalline phase on it. The amorphous phase is the primary phase. The glass forming ability as well as the microhardness of the Fe-based alloy made from low purity raw materials can be much enhanced by adding small amount of multi-components. However, the elements addition has its optimal quantity. When X is equal to 1, the microstructure of the coating contains 97.93% amorphous phase and 2.07% crystalline phase on it. As a result, the microhardness of the coating reaches maximum. With further increasing of the additions, the amorphous phase in the coating lessens instead of augment and the crystalline phase begins to accumulate, which result in the decrease of the microhardness. # 2007 Elsevier B.V. All rights reserved. Keywords: Amorphous; Composite coating; Elements addition; Laser cladding
1. Introduction Since the metallic glasses had been developed by Duwez in 1960 [1], the materials have attracted increasing attention because of their excellent properties, such as higher hardness and wear resistance [2]. Most of all, the synthesis and properties studies of multi-component Fe-based bulk metallic glasses (BMGs) have attracted more and more attention for their potential industrial application. However, it is well know that the Fe-based amorphous alloys found before 1990 require high cooling rates above 105 K/s and the resulting sample thickness is limited to less than about 50 mm [3]. Further improvement of the Fe-based amorphous alloys was restricted by their poor amorphous forming ability and brittleness or toughness. Fortunately, the research on metallic glasses gained more momentum after Inoue succeeded in finding new multicomponent alloy systems in 1990s [4,5]. After that, a series of Fe-based amorphous alloys have been developed [6]. This gives a bright future for the utilization of Fe-based amorphous alloys. Nevertheless, in order to fabricate the Fe-based BMGs, a lot of
* Corresponding author. Tel.: +86 531 8839 6145; fax: +86 531 8261 6431. E-mail address: [email protected] (Q. Zhu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.055
exceptive methods have to be used in case of eliminating heterogeneous nucleation, such as fluxing and high vacuum in rapid quenching [5,7]. Laser cladding is an effective method to prepare amorphous composite coatings on metallic substrates. It is a significant way to improve the surface properties of materials by surface modification [8]. The first report on fabricating amorphous composite coatings using laser cladding was published in 1975 and carried out by Yoshioka et al.[9]. The results exhibited that the amorphous composite coatings could be prepared only when the components of the alloys were restricted in a narrow range. The requirements of laser cladding were more restrict than the traditional quenching methods, which indicated the difficulty of amorphization by laser cladding. Nevertheless, researchers have done different works on a series of BMGs using laser cladding. For example, Wang et al. has cladded a Zr–Al–Ni–Cu amorphous alloy on Ti substrate [10]. Their results show that the coating consists of a mixture of intermetallic compound, including a nanocrystalline phase and an amorphous phase. However, the conditions of synthesis amorphous composite coatings are severely rugged, such as protective container and high purity of powders [10–12]. This is because of oxygen and other impurities would counteract the formation of amorphous phase in the coatings dramatically.
Q. Zhu et al. / Applied Surface Science 253 (2007) 7060–7064
That is, the heterogeneous nucleation would be trigger and partial elements of the alloy would be oxygenated, which causes the glass forming ability (GFA) of the amorphous alloys depressed deeply. The high purity of raw materials and the strict processing cause a high cost for producing amorphous composite coatings, and restrict their applications [13]. To our knowledge, there is no report on fabricating amorphous composite coatings by laser cladding using raw materials in low purity and without protection while the powders were mixed. In this paper, powders with the nominal composition of Fe38Ni30Si16B14V2 (at.%) are selected as a specific Fe-based alloy and an exploratory study have been carried out to prepare the Fe-based amorphous composite coatings more economically and easily by laser cladding. The processing consisted of melting the base metal with pre-coated Fe-based alloy powders and then forming a protective layer with different properties from those of the substrate. The results show that a small amount of multi-components addition is effective in improving the GFA of Fe38Ni30Si16B14V2 alloy even though the raw materials are in low purity. 2. Experiments Rectangular specimens of AISI 1045, 100 mm long, 10 mm wide and 10 mm thick were used as the base metal. A powder mixture with compositions of Fe38Ni30 XSi16B14V2MX (at.%) was selected for laser cladding, where X was 0, 1 and 2, M contained Al, Ti, Mo, and C. The size of the particles was in the range from 100 to 200 meshes. It is worthwhile to point out that the materials used in this study were industrial purity and the purity of iron, nickel and silicon is 98 wt.%. The iron boron alloy contains 79.8 wt.% of Fe, 18.4 wt.% of B and other components, such as aluminum, silicon, carbon, sulfur, phosphor and their oxides in remainder. Similarly, the iron vanadium alloy contains 59.8 wt.% of Fe, 35 wt.% of V and other components either. The content of boron and vanadium in the alloy can be adjusted by adding different amount of the iron boron alloy and iron vanadium alloy, which are much cheaper than pure elements such as pure B [14]. The powders were evenly mixed in air by a self-made mixer. After that, the mixed powders were pre-coated onto the specimen surface homogeneously using waterglass. The thicknesses of the pre-coated coatings were about 1 mm. And then, the samples were dried by electric hair dryer. A 5 kW continuous CO2 laser (HL-T5000) was used to prepare the coating. The laser processing, which was carried out under a industrial condition, was divided into three sections. Firstly, the fusion parameter was 2000 W and the scanning speed was 300 mm/min while the laser beam diameter was 3 mm. Secondly, a velocity of 600 mm/min was used to re-melt the coating under the same fusion parameter and laser beam diameter. Finally, the parameters of laser power, laser beam diameter and scanning speed were 4800 W, 2 mm, 3500 mm/ min, respectively. Argon served as protective gas in order to prohibit the alloy powders from oxidation during laser processing. The gas feed rates were 15 L/min during all of the three sections.
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After being etched with aqua regia, the coatings were characterized by Hitachi scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS) (S-2500), the detecting error of which is less than 0.5%. Rigaku X-ray diffractometry (D/max-rB, 4 kW) was used to classify amorphous coatings with Cu Ka irradiation (l = 0.154060 nm) while the scanning speed was 48 per minute and the step size was 0.028. The microstructures of the coatings were identified with Hitachi transmission electron microscopy (TEM) (H-800). The microhardness was measured using a Vickers hardness tester (MH-6) with a load of 200 g, employing a loading time of 15 s. 3. Results and discussion 3.1. Synthesis of the amorphous coating Fig. 1(a) exhibits the picture of laser cladded Fe38Ni30 X Si16B14V2MX (X = 0, 1, 2) alloys coatings. All of the layers have smooth outer surface and good metallic luster, which illustrates the argon atmosphere can prevent the alloys from being oxidized effectively. The low-magnification optical morphology at the transition region between the layer and the substrate shows the coating has no pore and incomplete fusion. The bond zone exhibits a good metallurgical bonding between the substrate and the coating. At the same time, no
Fig. 1. The low-magnification optical micrograph of the Fe38Ni30 X Si16B14V2MX (X = 0, 1, 2) alloys coatings (a) and microstructure of interface in the laser-cladding coating (b).
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crack exists both in the coating and the bond zone. The coating reveals a featureless pattern contrast with an etched substrate. Fig. 1(b) shows SEM morphology of the bonding region between the coating and the substrate. In this area, a thin film of the substrate is melted under heat conduction in the course of laser processing, and mix with the amorphous alloy in liquid stage, which is the source of dilution. The composition and cooling rate is different from other zones in the coating. Therefore, the microstructure changed obviously at the interface. It can be seen that the coating is diluted obviously by the substrate. In particular, the substrate reveals an epitaxial growth and grows into the coating in the form of columnar crystallites because of the high temperature gradient and concentration gradient. Unmelted or semi-fused grains of the substrate benefit the epitaxial growth. However, the sharp change of the composition and cooling rate in the coating restricts the growth of the columnar crystallites. Therefore, the microstructure of the coating turns to no obviously characteristic in the coating zone. 3.2. Influence of the X on the fabrication of the coatings Fig. 2 shows the XRD patterns at a distance of 0.4 mm away from the top surface of the laser cladding Fe38Ni30 X Si16B14V2MX (X = 0, 1, 2) alloys with different elements additions. Without elements addition (X = 0 in Fig. 2), whole amorphous structure could not be fabricated, as illustrated in Fig. 2 that several peaks corresponding to crystalline phase imposed on the diffused diffraction maxima, which means that crystalline phases exist in the alloy. The crystalline phase is identified as Fe2B and g-Fe,Ni. Therefore, the GFA of the alloy is rather lower without elements addition and the coating only can be synthesized with partial amorphous phase. The crystallization cannot be avoided during the laser processing. Varies experiments with different parameters obtain the same result. Compared with other alloys, in which X is equal to 1 or 2, GFA of the alloy can be considerably enhanced by addition elements. For example, when X is equal to 1, the broad halo peak
is much wider than the previous and the diffraction peaks corresponding to crystalline phase are much lower. When X is equal to 1, the crystalline peaks is feeble and the coating reveals an apparent amorphous structural characteristic within the detect limitation of the XRD. In other words, the crystallization of Fe2B and g-Fe,Ni is depressed and the amorphous phase is principal in the coating. However, when X is equal to 2, the diffraction peaks corresponding to Fe2B can be seen again. This means the amorphous phase is no more the priority one in the coatings. Although the multi-component addition can improve the GFA of the Fe38Ni30 XSi16B14V2MX alloys, superabundant additions acts the villain of the piece. On the other hand, in the alloy in which X is equal to 2, the intensity of the diffraction peaks is lower than the alloy without additions, and the crystalline phase is less either, for example, the g-Fe,Ni phase cannot be detected. In a word, the system of crystal growth in these alloys is changed with the addition of multi-component. Further addition reveals the same trends. For X is equal to 2 or more, the crystalline peaks boom again and the broad halo peak becomes smaller. The results prove that the Fe-based amorphous composite coating can be fabricated by using low purity raw Fe element through addition of multi-components. The possible reason is that the multicomponents can alter the system of crystallization during the laser cladding processing and facilitate the forming of amorphous phase. According to the theory developed by Inoue [5], the GFA of the amorphous alloys is related to the atomic structure greatly. Structurally, the effective addition of Al, Ti, Mo and C causes the sequential change of atomic size in the previous alloy. At the same time, new atomic pairs with various negative heats of mixing have been generated. This means the atomic rearrangement is difficulty in the molten pool, leading to a decrease of atomic diffusivity and an increase of viscosity. The topological and chemical short-range orders are enhanced, leading to the formation of highly dense random packed structure with low atomic diffusivity in the supercooled liquid. However, with more elements addition (X > 2), the crystalline phases become the priority phase again. This means that excess elements addition increases crystallization instead of bring further improvement to GFA. Therefore, a proper addition can improve the GFA of the Fe38Ni30 XSi16B14V2MX alloy, and too much of additions (X > 2) also lead to the boom of crystalline phase. The degree of crystallization can be calculated by MIDI-JADE5, which is shown in Table 1. 3.3. Influence of the X on the microstructure of the coatings Fig. 3 is cross-section SEM image of the laser cladded coatings, in which the X is equal to 0, 1 and 2, respectively. Although the coatings have experienced the same processing Table 1 Degree of crystallization with different multi-component additions
Fig. 2. The XRD patterns of the laser cladded Fe38Ni30 1, 2) alloys coatings.
XSi16B14V2MX
(X = 0,
X=0 X=1 X=2
Crystallization phase (%)
Amorphous phase (%)
47.53 2.07 23.11
52.47 97.93 76.89
Q. Zhu et al. / Applied Surface Science 253 (2007) 7060–7064
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Fig. 3. SEM pattern of the laser cladded coatings: (a) X = 0; (b) X = 1; (c) X = 2.
and etching, the morphologies presents various characteristics for their different contains of X. For the coating in which X is equal to 0, as shown in Fig. 3(a), lots of crystalline grains embedded in the amorphous substrate. For the one in which X is equal to 1, as we can see in Fig. 3(b), only a little crystalline grains embedded in the amorphous substrate. Even more important, the crystalline grains are much smaller and regular than in the previous alloy and distributes more homogeneously. In other words, multistage crystallization is existed in the laser processing and the homogeneous nucleation sites generates in the amorphous phase. However, the addition of the solute element with low atomic diffusivity at the nanocrystal/ amorphous interface suppressed the growth reaction. For the layer in which X is equal to 2, the grains accumulate and grow up. The microstructure of which is shown in Fig. 3(c). However, the degree of crystallization is much lower than the alloy without element addition. This is in accordance with the calculated result, which is shown in Table 1.
In order to identify the microstructure of the coating, in which X is equal to 1, TEM was carried out. Fig. 4 is TEM image and selected-area diffraction pattern (SADP) at a distance of approximately 0.4 mm away from the coating surface. Fig. 4(a) shows the TEM pattern of the laser cladded coating. The microstructure in which is the specific characteristic of the amorphous phase. Fig. 4(b) shows SADP of the coating. The pattern indicates that amorphous phase exists in the coating. However, a few nanocrystalline is also can be seen in the TEM patterns, which shows crystallization cannot be suppressed completely. This is in accordance with the XRD pattern in Fig. 2. 3.4. Influence of the X on the microhardness of the coatings Fig. 5 shows the microhardness of the amorphous composite coating from top of the coating to the substrate. Each of the data points in Fig. 5 is the average of five measurements. For alloy in which the X is equal to 0, a gradient distribution of the
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Fig. 4. TEM pattern of the laser cladded coating: (a) morphology of the amorphous alloy; (b) the halo ring diffraction pattern.
microhardness can be seen ranging from 891 to 1208 in the coating. The hardest area is in the midst of the coating. It is also seen from Fig. 5 that the value of microhardness in the coating increases from 1177 at the surface to 1208 in the middle. The same trends can be seen in the other two alloys. The reason is that the powders are mixed in air and the superficial coating is exposed to the air, which usually causes the oxidation of the pre-coated powders on the substrate. Even more important, the oxidation during the laser processing is unavoidable. All of this deviate the amorphous alloy from eutectic composition, which enhances the critical cooling rate of the amorphous alloy and decreases the GFA. On the other hand, the melt in the middle of the molten pool is purer and approaches to eutectic composition than the superficial coating. As a result, the amorphous phase is concentrated in this area and reveals higher microhardness. On the contrary, the area near the substrate is diluted by the matrix under the high temperature. Therefore, this area shows lower microhardness than the middle of the coating and the value of microhardness decreased along the depth.
It is also seen from the picture that the microhardness varies with the X changing. When the X increases from 0 to 1, the microhardness of the coating reaches maximum. This is because of the amorphous phase in the coating increases with the X increasing. On the other hand, when the X increases further, the microhardness of the coating plays down instead of increasing. The reason for this is that the amorphous phase in the coating lessens instead of augment. As we can see in Fig. 3 and Table 1, the microstructure of the coatings changes with the X increasing. When X is equal to 0, as shown in Fig. 3(a), the microstructure is a lot of big grains embedded in the amorphous phase and the degree of crystallization is 47.53%. This kind of morphology exhibits a lower microhardness. When X is equal to 1, the major phase in the coating is amorphous phase and the crystallization phase is minority. The degree of crystallization in which is 2.07% and the grains in it is the smallest among the three layers. The grains strengthen the amorphous substrate and the coating exhibits the highest microhardness. To sum up, the microhardness of the coating reaches maximum when the coating contains an amorphous phase with a few crystalline phase on it. That is, the X is equal to 1. 4. Conclusion Fe38Ni30 XSi16B14V2MX (X = 0, 1, 2) Fe-based amorphous composite coatings are synthesized from low purity of raw materials by a small amount of elements addition using laser cladding. The multi-component addition is proved to have positive effect on the glass forming ability of the Fe-based amorphous alloy. The microstructure of the coatings change with the X increasing. X influences the coatings not only on the content of the amorphous phase, but also on the shape and distribution of the crystallization grains. The elements addition has its optimal quantity. When X is equal to 1, the microstructure of the coating has an amorphous phase with a few crystalline phases on it. As a result, the microhardness of the coating reaches maximum. The multi-component addition method is an effective factor to develop amorphous composite coatings with low purity raw materials by laser cladding. References
Fig. 5. Variation of the microhardness with depth.
[1] W. Klement, R.H. Willens, P. Duwez, Nature 187 (1960) 869. [2] A. Inoue, A. Takeuchi, Mater. Trans. JIM 43 (2002) 1892. [3] H.A. Davis, in: F.E. Luborsky (Ed.), Amorphous Metallic Alloys, Butterwoths, London, 1983, p. 14. [4] A. Inoue, T. Zhang, T. Masumoto, Mater. Trans. JIM 30 (1989) 965. [5] A. Inoue, Acta Mater. 48 (2000) 279. [6] W.H. Wang, C. Dong, C.H. Shek, Mater. Sci. Eng. R 44 (2004) 45. [7] T.D. Shen, R.B. Schwarz, Acta Mater. 49 (2001) 837. [8] D. Carvalho, S. Cardoso, R. Vilar, Scripta Mater. 37 (1997) 523. [9] H. Yoshioka, K. Asami, A. Kawashima, K. Hashimoto, Corros. Sci. 27 (1987) 981. [10] Y. Wang, G. Li, C. Wang, Y. Xia, B. Sandip, C. Dong, Surf. Coat. Technol. 176 (2004) 284. [11] X. Wu, Y. Hong, Surf. Coat. Technol. 141 (2001) 141. [12] T.M. Yue, Y.P. Su, H.O. Yang, Mater. Lett. 61 (2007) 209. [13] Y. Hu, M.X. Pan, L. Liu, Y.H. Zhao, D.Q. Zhao, W.H. Wang, Mater. Lett. 57 (2003) 2698. [14] X. Wu, B. Xu, Y. Hong, Mater. Lett. 56 (2002) 838.
Synthesis of Fe-based amorphous composite coatings with low purity materials by laser cladding Zhu Qingjun*, Qu Shiyao, Wang Xinhong, Zou Zengda School of Materials Science and Engineering, Shandong University, Jinan 250061, PR China Received 23 December 2006; received in revised form 1 February 2007; accepted 11 February 2007 Available online 23 February 2007
Abstract Amorphous composite coatings Fe38Ni30 XSi16B14V2MX (X = 0, 1, 2) (M contains Al, Ti, Mo, and C) were prepared with low purity of raw materials by laser cladding. X-ray diffraction and transmission electron microscopy results show that the coating have an amorphous structure with a few crystalline phase on it. The amorphous phase is the primary phase. The glass forming ability as well as the microhardness of the Fe-based alloy made from low purity raw materials can be much enhanced by adding small amount of multi-components. However, the elements addition has its optimal quantity. When X is equal to 1, the microstructure of the coating contains 97.93% amorphous phase and 2.07% crystalline phase on it. As a result, the microhardness of the coating reaches maximum. With further increasing of the additions, the amorphous phase in the coating lessens instead of augment and the crystalline phase begins to accumulate, which result in the decrease of the microhardness. # 2007 Elsevier B.V. All rights reserved. Keywords: Amorphous; Composite coating; Elements addition; Laser cladding
1. Introduction Since the metallic glasses had been developed by Duwez in 1960 [1], the materials have attracted increasing attention because of their excellent properties, such as higher hardness and wear resistance [2]. Most of all, the synthesis and properties studies of multi-component Fe-based bulk metallic glasses (BMGs) have attracted more and more attention for their potential industrial application. However, it is well know that the Fe-based amorphous alloys found before 1990 require high cooling rates above 105 K/s and the resulting sample thickness is limited to less than about 50 mm [3]. Further improvement of the Fe-based amorphous alloys was restricted by their poor amorphous forming ability and brittleness or toughness. Fortunately, the research on metallic glasses gained more momentum after Inoue succeeded in finding new multicomponent alloy systems in 1990s [4,5]. After that, a series of Fe-based amorphous alloys have been developed [6]. This gives a bright future for the utilization of Fe-based amorphous alloys. Nevertheless, in order to fabricate the Fe-based BMGs, a lot of
* Corresponding author. Tel.: +86 531 8839 6145; fax: +86 531 8261 6431. E-mail address: [email protected] (Q. Zhu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.055
exceptive methods have to be used in case of eliminating heterogeneous nucleation, such as fluxing and high vacuum in rapid quenching [5,7]. Laser cladding is an effective method to prepare amorphous composite coatings on metallic substrates. It is a significant way to improve the surface properties of materials by surface modification [8]. The first report on fabricating amorphous composite coatings using laser cladding was published in 1975 and carried out by Yoshioka et al.[9]. The results exhibited that the amorphous composite coatings could be prepared only when the components of the alloys were restricted in a narrow range. The requirements of laser cladding were more restrict than the traditional quenching methods, which indicated the difficulty of amorphization by laser cladding. Nevertheless, researchers have done different works on a series of BMGs using laser cladding. For example, Wang et al. has cladded a Zr–Al–Ni–Cu amorphous alloy on Ti substrate [10]. Their results show that the coating consists of a mixture of intermetallic compound, including a nanocrystalline phase and an amorphous phase. However, the conditions of synthesis amorphous composite coatings are severely rugged, such as protective container and high purity of powders [10–12]. This is because of oxygen and other impurities would counteract the formation of amorphous phase in the coatings dramatically.
Q. Zhu et al. / Applied Surface Science 253 (2007) 7060–7064
That is, the heterogeneous nucleation would be trigger and partial elements of the alloy would be oxygenated, which causes the glass forming ability (GFA) of the amorphous alloys depressed deeply. The high purity of raw materials and the strict processing cause a high cost for producing amorphous composite coatings, and restrict their applications [13]. To our knowledge, there is no report on fabricating amorphous composite coatings by laser cladding using raw materials in low purity and without protection while the powders were mixed. In this paper, powders with the nominal composition of Fe38Ni30Si16B14V2 (at.%) are selected as a specific Fe-based alloy and an exploratory study have been carried out to prepare the Fe-based amorphous composite coatings more economically and easily by laser cladding. The processing consisted of melting the base metal with pre-coated Fe-based alloy powders and then forming a protective layer with different properties from those of the substrate. The results show that a small amount of multi-components addition is effective in improving the GFA of Fe38Ni30Si16B14V2 alloy even though the raw materials are in low purity. 2. Experiments Rectangular specimens of AISI 1045, 100 mm long, 10 mm wide and 10 mm thick were used as the base metal. A powder mixture with compositions of Fe38Ni30 XSi16B14V2MX (at.%) was selected for laser cladding, where X was 0, 1 and 2, M contained Al, Ti, Mo, and C. The size of the particles was in the range from 100 to 200 meshes. It is worthwhile to point out that the materials used in this study were industrial purity and the purity of iron, nickel and silicon is 98 wt.%. The iron boron alloy contains 79.8 wt.% of Fe, 18.4 wt.% of B and other components, such as aluminum, silicon, carbon, sulfur, phosphor and their oxides in remainder. Similarly, the iron vanadium alloy contains 59.8 wt.% of Fe, 35 wt.% of V and other components either. The content of boron and vanadium in the alloy can be adjusted by adding different amount of the iron boron alloy and iron vanadium alloy, which are much cheaper than pure elements such as pure B [14]. The powders were evenly mixed in air by a self-made mixer. After that, the mixed powders were pre-coated onto the specimen surface homogeneously using waterglass. The thicknesses of the pre-coated coatings were about 1 mm. And then, the samples were dried by electric hair dryer. A 5 kW continuous CO2 laser (HL-T5000) was used to prepare the coating. The laser processing, which was carried out under a industrial condition, was divided into three sections. Firstly, the fusion parameter was 2000 W and the scanning speed was 300 mm/min while the laser beam diameter was 3 mm. Secondly, a velocity of 600 mm/min was used to re-melt the coating under the same fusion parameter and laser beam diameter. Finally, the parameters of laser power, laser beam diameter and scanning speed were 4800 W, 2 mm, 3500 mm/ min, respectively. Argon served as protective gas in order to prohibit the alloy powders from oxidation during laser processing. The gas feed rates were 15 L/min during all of the three sections.
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After being etched with aqua regia, the coatings were characterized by Hitachi scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS) (S-2500), the detecting error of which is less than 0.5%. Rigaku X-ray diffractometry (D/max-rB, 4 kW) was used to classify amorphous coatings with Cu Ka irradiation (l = 0.154060 nm) while the scanning speed was 48 per minute and the step size was 0.028. The microstructures of the coatings were identified with Hitachi transmission electron microscopy (TEM) (H-800). The microhardness was measured using a Vickers hardness tester (MH-6) with a load of 200 g, employing a loading time of 15 s. 3. Results and discussion 3.1. Synthesis of the amorphous coating Fig. 1(a) exhibits the picture of laser cladded Fe38Ni30 X Si16B14V2MX (X = 0, 1, 2) alloys coatings. All of the layers have smooth outer surface and good metallic luster, which illustrates the argon atmosphere can prevent the alloys from being oxidized effectively. The low-magnification optical morphology at the transition region between the layer and the substrate shows the coating has no pore and incomplete fusion. The bond zone exhibits a good metallurgical bonding between the substrate and the coating. At the same time, no
Fig. 1. The low-magnification optical micrograph of the Fe38Ni30 X Si16B14V2MX (X = 0, 1, 2) alloys coatings (a) and microstructure of interface in the laser-cladding coating (b).
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crack exists both in the coating and the bond zone. The coating reveals a featureless pattern contrast with an etched substrate. Fig. 1(b) shows SEM morphology of the bonding region between the coating and the substrate. In this area, a thin film of the substrate is melted under heat conduction in the course of laser processing, and mix with the amorphous alloy in liquid stage, which is the source of dilution. The composition and cooling rate is different from other zones in the coating. Therefore, the microstructure changed obviously at the interface. It can be seen that the coating is diluted obviously by the substrate. In particular, the substrate reveals an epitaxial growth and grows into the coating in the form of columnar crystallites because of the high temperature gradient and concentration gradient. Unmelted or semi-fused grains of the substrate benefit the epitaxial growth. However, the sharp change of the composition and cooling rate in the coating restricts the growth of the columnar crystallites. Therefore, the microstructure of the coating turns to no obviously characteristic in the coating zone. 3.2. Influence of the X on the fabrication of the coatings Fig. 2 shows the XRD patterns at a distance of 0.4 mm away from the top surface of the laser cladding Fe38Ni30 X Si16B14V2MX (X = 0, 1, 2) alloys with different elements additions. Without elements addition (X = 0 in Fig. 2), whole amorphous structure could not be fabricated, as illustrated in Fig. 2 that several peaks corresponding to crystalline phase imposed on the diffused diffraction maxima, which means that crystalline phases exist in the alloy. The crystalline phase is identified as Fe2B and g-Fe,Ni. Therefore, the GFA of the alloy is rather lower without elements addition and the coating only can be synthesized with partial amorphous phase. The crystallization cannot be avoided during the laser processing. Varies experiments with different parameters obtain the same result. Compared with other alloys, in which X is equal to 1 or 2, GFA of the alloy can be considerably enhanced by addition elements. For example, when X is equal to 1, the broad halo peak
is much wider than the previous and the diffraction peaks corresponding to crystalline phase are much lower. When X is equal to 1, the crystalline peaks is feeble and the coating reveals an apparent amorphous structural characteristic within the detect limitation of the XRD. In other words, the crystallization of Fe2B and g-Fe,Ni is depressed and the amorphous phase is principal in the coating. However, when X is equal to 2, the diffraction peaks corresponding to Fe2B can be seen again. This means the amorphous phase is no more the priority one in the coatings. Although the multi-component addition can improve the GFA of the Fe38Ni30 XSi16B14V2MX alloys, superabundant additions acts the villain of the piece. On the other hand, in the alloy in which X is equal to 2, the intensity of the diffraction peaks is lower than the alloy without additions, and the crystalline phase is less either, for example, the g-Fe,Ni phase cannot be detected. In a word, the system of crystal growth in these alloys is changed with the addition of multi-component. Further addition reveals the same trends. For X is equal to 2 or more, the crystalline peaks boom again and the broad halo peak becomes smaller. The results prove that the Fe-based amorphous composite coating can be fabricated by using low purity raw Fe element through addition of multi-components. The possible reason is that the multicomponents can alter the system of crystallization during the laser cladding processing and facilitate the forming of amorphous phase. According to the theory developed by Inoue [5], the GFA of the amorphous alloys is related to the atomic structure greatly. Structurally, the effective addition of Al, Ti, Mo and C causes the sequential change of atomic size in the previous alloy. At the same time, new atomic pairs with various negative heats of mixing have been generated. This means the atomic rearrangement is difficulty in the molten pool, leading to a decrease of atomic diffusivity and an increase of viscosity. The topological and chemical short-range orders are enhanced, leading to the formation of highly dense random packed structure with low atomic diffusivity in the supercooled liquid. However, with more elements addition (X > 2), the crystalline phases become the priority phase again. This means that excess elements addition increases crystallization instead of bring further improvement to GFA. Therefore, a proper addition can improve the GFA of the Fe38Ni30 XSi16B14V2MX alloy, and too much of additions (X > 2) also lead to the boom of crystalline phase. The degree of crystallization can be calculated by MIDI-JADE5, which is shown in Table 1. 3.3. Influence of the X on the microstructure of the coatings Fig. 3 is cross-section SEM image of the laser cladded coatings, in which the X is equal to 0, 1 and 2, respectively. Although the coatings have experienced the same processing Table 1 Degree of crystallization with different multi-component additions
Fig. 2. The XRD patterns of the laser cladded Fe38Ni30 1, 2) alloys coatings.
XSi16B14V2MX
(X = 0,
X=0 X=1 X=2
Crystallization phase (%)
Amorphous phase (%)
47.53 2.07 23.11
52.47 97.93 76.89
Q. Zhu et al. / Applied Surface Science 253 (2007) 7060–7064
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Fig. 3. SEM pattern of the laser cladded coatings: (a) X = 0; (b) X = 1; (c) X = 2.
and etching, the morphologies presents various characteristics for their different contains of X. For the coating in which X is equal to 0, as shown in Fig. 3(a), lots of crystalline grains embedded in the amorphous substrate. For the one in which X is equal to 1, as we can see in Fig. 3(b), only a little crystalline grains embedded in the amorphous substrate. Even more important, the crystalline grains are much smaller and regular than in the previous alloy and distributes more homogeneously. In other words, multistage crystallization is existed in the laser processing and the homogeneous nucleation sites generates in the amorphous phase. However, the addition of the solute element with low atomic diffusivity at the nanocrystal/ amorphous interface suppressed the growth reaction. For the layer in which X is equal to 2, the grains accumulate and grow up. The microstructure of which is shown in Fig. 3(c). However, the degree of crystallization is much lower than the alloy without element addition. This is in accordance with the calculated result, which is shown in Table 1.
In order to identify the microstructure of the coating, in which X is equal to 1, TEM was carried out. Fig. 4 is TEM image and selected-area diffraction pattern (SADP) at a distance of approximately 0.4 mm away from the coating surface. Fig. 4(a) shows the TEM pattern of the laser cladded coating. The microstructure in which is the specific characteristic of the amorphous phase. Fig. 4(b) shows SADP of the coating. The pattern indicates that amorphous phase exists in the coating. However, a few nanocrystalline is also can be seen in the TEM patterns, which shows crystallization cannot be suppressed completely. This is in accordance with the XRD pattern in Fig. 2. 3.4. Influence of the X on the microhardness of the coatings Fig. 5 shows the microhardness of the amorphous composite coating from top of the coating to the substrate. Each of the data points in Fig. 5 is the average of five measurements. For alloy in which the X is equal to 0, a gradient distribution of the
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Fig. 4. TEM pattern of the laser cladded coating: (a) morphology of the amorphous alloy; (b) the halo ring diffraction pattern.
microhardness can be seen ranging from 891 to 1208 in the coating. The hardest area is in the midst of the coating. It is also seen from Fig. 5 that the value of microhardness in the coating increases from 1177 at the surface to 1208 in the middle. The same trends can be seen in the other two alloys. The reason is that the powders are mixed in air and the superficial coating is exposed to the air, which usually causes the oxidation of the pre-coated powders on the substrate. Even more important, the oxidation during the laser processing is unavoidable. All of this deviate the amorphous alloy from eutectic composition, which enhances the critical cooling rate of the amorphous alloy and decreases the GFA. On the other hand, the melt in the middle of the molten pool is purer and approaches to eutectic composition than the superficial coating. As a result, the amorphous phase is concentrated in this area and reveals higher microhardness. On the contrary, the area near the substrate is diluted by the matrix under the high temperature. Therefore, this area shows lower microhardness than the middle of the coating and the value of microhardness decreased along the depth.
It is also seen from the picture that the microhardness varies with the X changing. When the X increases from 0 to 1, the microhardness of the coating reaches maximum. This is because of the amorphous phase in the coating increases with the X increasing. On the other hand, when the X increases further, the microhardness of the coating plays down instead of increasing. The reason for this is that the amorphous phase in the coating lessens instead of augment. As we can see in Fig. 3 and Table 1, the microstructure of the coatings changes with the X increasing. When X is equal to 0, as shown in Fig. 3(a), the microstructure is a lot of big grains embedded in the amorphous phase and the degree of crystallization is 47.53%. This kind of morphology exhibits a lower microhardness. When X is equal to 1, the major phase in the coating is amorphous phase and the crystallization phase is minority. The degree of crystallization in which is 2.07% and the grains in it is the smallest among the three layers. The grains strengthen the amorphous substrate and the coating exhibits the highest microhardness. To sum up, the microhardness of the coating reaches maximum when the coating contains an amorphous phase with a few crystalline phase on it. That is, the X is equal to 1. 4. Conclusion Fe38Ni30 XSi16B14V2MX (X = 0, 1, 2) Fe-based amorphous composite coatings are synthesized from low purity of raw materials by a small amount of elements addition using laser cladding. The multi-component addition is proved to have positive effect on the glass forming ability of the Fe-based amorphous alloy. The microstructure of the coatings change with the X increasing. X influences the coatings not only on the content of the amorphous phase, but also on the shape and distribution of the crystallization grains. The elements addition has its optimal quantity. When X is equal to 1, the microstructure of the coating has an amorphous phase with a few crystalline phases on it. As a result, the microhardness of the coating reaches maximum. The multi-component addition method is an effective factor to develop amorphous composite coatings with low purity raw materials by laser cladding. References
Fig. 5. Variation of the microhardness with depth.
[1] W. Klement, R.H. Willens, P. Duwez, Nature 187 (1960) 869. [2] A. Inoue, A. Takeuchi, Mater. Trans. JIM 43 (2002) 1892. [3] H.A. Davis, in: F.E. Luborsky (Ed.), Amorphous Metallic Alloys, Butterwoths, London, 1983, p. 14. [4] A. Inoue, T. Zhang, T. Masumoto, Mater. Trans. JIM 30 (1989) 965. [5] A. Inoue, Acta Mater. 48 (2000) 279. [6] W.H. Wang, C. Dong, C.H. Shek, Mater. Sci. Eng. R 44 (2004) 45. [7] T.D. Shen, R.B. Schwarz, Acta Mater. 49 (2001) 837. [8] D. Carvalho, S. Cardoso, R. Vilar, Scripta Mater. 37 (1997) 523. [9] H. Yoshioka, K. Asami, A. Kawashima, K. Hashimoto, Corros. Sci. 27 (1987) 981. [10] Y. Wang, G. Li, C. Wang, Y. Xia, B. Sandip, C. Dong, Surf. Coat. Technol. 176 (2004) 284. [11] X. Wu, Y. Hong, Surf. Coat. Technol. 141 (2001) 141. [12] T.M. Yue, Y.P. Su, H.O. Yang, Mater. Lett. 61 (2007) 209. [13] Y. Hu, M.X. Pan, L. Liu, Y.H. Zhao, D.Q. Zhao, W.H. Wang, Mater. Lett. 57 (2003) 2698. [14] X. Wu, B. Xu, Y. Hong, Mater. Lett. 56 (2002) 838.