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Feb 7, 2013 - Amorphous surrounded nano polycrystals (ASNP) are found in this study. An amorphous nanocrystal-reinforced coating was fabricated on an ...
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Physical Properties and Formation Mechanism of Copper/Glass Modified Laser Nanocrystals-Amorphous Reinforced Coatings Jianing Li,†,§,∥ Huijun Yu,‡ Chuanzhong Chen,*,† and Shuili Gong§,∥ †

Key Laboratory for Liquid−Solid Structural Evolution and Processing of Materials (Ministry of Education), School of Materials Science and Engineering, Shandong University, Jinan 250061, People's Republic of China ‡ School of Mechanical Engineering, Shandong University, Jinan 250061, People's Republic of China § Science and Technology on Power Beam Processes Laboratory, Beijing, People’s Republic of China ∥ Beijing Aeronautical Manufacturing Technology Research Institute, Beijing, People’s Republic of China

ABSTRACT: Amorphous surrounded nano polycrystals (ASNP) are found in this study. An amorphous nanocrystal-reinforced coating was fabricated on an important aeronautical material Ti-6Al-4V titanium alloy by laser cladding of the Al3Ti-TiB2-SiCCu-Y2O3 mixed powders. Transmission electron microscope and scanning electron microscope test results indicated that with Cu addition, the TiCu2Al nano polycrystals were produced in the matrix of laser clad coating, which grew along (200), (220), (222), and (400) planes; high resolution transmission electron microscope image indicated that with glass addition, the particles grew up, leading to the formation of a micronano structure. Moreover, Cu addition led the agglomeration of ASNP to decompose then distributed uniformly in the coating matrix. This research provided essential theoretical and experimental basis to promote the application of laser cladding technique in the modern aviation industry. als,35,36 while less work has been carried out on the formation mechanism and physical properties of nano polycrystals in laser clad coatings. Through experiments, it was found that laser cladding of the Al3Ti-TiB2-SiC-Cu-Y2O3 mixed powders on a Ti-6Al-4 V titanium alloy can form an amorphous-nanocrystals reinforced coating. The Al3Ti-TiB2-SiC-Cu-Y2O3 powders are interesting in the scheme of research concerned with laser cladding of hard, amorphous, and nanocrystal-reinforced coatings on titanium alloys. Because of the action of the sufficiently rapid heating and cooling rates of laser-cladding technique, many TiB2 nanocrystals were produced in such coating, and the TiCu2Al nano polycrystals were also obtained with the Cu addition. In this study, the physical properties and formation mechanism of the glass/copper-modified laser amorphous nanocrystal-reinforced coatings on the Ti-6Al-4 V alloys were investigated in detail.

1. INTRODUCTION Laser cladding is an useful surface modification technique, because of the following advantages: optimal bond properties, high process flexibility, rapid solidification, and no requirement for post process treatment.1−12 Laser cladding is also a promising approach to produce the amorphous nanocrystalreinforced coatings on the metals.13−23 Recently, amorphous materials have attracted an increasing amount of attention due to their unique physical, mechanical, and chemical properties.24−27 Moreover, in the past decade nanocomposite coatings have become very popular because of their high toughness and stiffness along with superior hardness, which made the nanocomposite coatings promising candidates in mechanical and tribological applications.28−31 Later, laser clad coatings with typical bulk metallics glasses and nanocrystals compositions were reported for the alloy system, such as Ni−Cr−Mo−Zr− P−B,32 Zr−Al−Ni−Cu33 and Fe−Cr−Mo−W−C−Mn−Si− B,34 and so forth. In the past few years, the efforts have always focused on microstructure performance and phase constitutions of the nanoscale ceramic-reinforced laser-treated coatings on met© 2013 American Chemical Society

Received: November 10, 2012 Revised: February 3, 2013 Published: February 7, 2013 4568

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2. EXPERIMENTAL SECTION The materials used in this experiment include the following: Ti6Al-4 V samples with the size of 10 mm ×10 mm ×10 mm were abraded with abrasive paper prior to the coating operation. The preplaced powders of Al3Ti (≥99.5% purity, 50−200 μm), TiB2 (≥99.5% purity, 100−200 μm), SiC (≥99.5% purity, 150−250 μm), Cu (≥99.5% purity, 150−250 μm), Y2O3 (≥99.5% purity, 100 nm∼2 μm), and glass (Na2O·CaO·6SiO2) powder (≥99.5% purity, 100 nm to 2 μm) were used for the laser cladding. Clad powders were preplaced on the surface of substrates using a water glass (Na2O·nSiO2), to form a layer of 0.8 mm thickness, and during the laser-cladding process a number of Si were released from the water glass. A cross-flow CO2 laser was employed to melt the surface of the samples. Argon gas at a pressure of 0.4 MPa was fed through a nozzle that was coaxial with the laser beam. Process parameters of laser cladding include the following: laser power P = 1.0 kW, scanning velocity V = 3−7.5 mm/s, and the laser beam diameter D = 4 mm. The diameter of the laser beam was obviously less than the width of the sample, so four-track lap-cladding layer was adopted in order to cover the whole cladding plane, and an overlap of 35% between successive tracks was selected. The parameters of all the samples were the same during the cladding process, and the materials in the lasercladding process are shown in Table 1. Figure 1. SEM micrographs of the coatings in samples 1 (a), 2 (b), 3 (c), and 4 (d).

Table 1. The Materials of Laser Cladding Process in Experiment sample number

powders composition (wt.%)

1 2 3 4 5

71Al3Ti-15TiB2-7SiC-6Cu-1Y2O3 69Al3Ti-15TiB2-7SiC-6Cu-1Y2O3-2Glass 77Al3Ti-15TiB2-7SiC-1Y2O3 75Al3Ti-15TiB2-7SiC-1Y2O3-2Glass 72Al3Ti-15TiB2-7SiC-3Cu-1Y2O3-2Glass

sample 4. According to the analysis previously, it was considered that the Cu addition led the nanoscale particles to distribute uniformly in laser clad coatings, and glass powder addition made the micro/nano particles produce in the laser clad coatings. As shown in Figure 2a, the TiB nanorods are produced in the coating of sample 1. The fact that TiB grows faster in the direction of [010] than the (100), (101), (102), and (001) planes, favoring to the formation of stick-shape precipitates.37,38 HRTEM image indicates that the TiB high-resolution lattice is produced in test location of thin film, and its fringe spacing of 0.186 nm is observed, which corresponds its (301) crystal plane (see Figure 2b). Figure 2c,d shows the TEM test location and its corresponding selected area (electron) diffraction (SAED) pattern, respectively. The amorphous and incomplete polycrystalline rings in the pattern indicated that many of the amorphous phases just began to crystallize when the laser molten pool had completed the solidification process, favoring the formation of the nanocrystals. The SAED pattern also indicated that the TiCu2Al polycrystals grew along (200), (220), (222), and (400) planes. A number of the small spots in the direction of white line in SAED pattern proved the existence of the nanorods. Moreover, the HRTEM image indicates that the TiCu2Al high-resolution lattice is produced, and the fringe spacing of 0.212 nm is observed (see Figure 2e). A number of the irregular amorphous lattice are also produced in the location of TiCu2Al polycrystals, which proves the production of amorphous phases. Moreover, the lattice distortions are also in existence in this image. The production of the nanocrystals led the free energy to enhance, which increased the density of point defect, leading to the formation of the hypersaturated state of point defect, causing the lattice distortions.

Metallographic samples were prepared using standard mechanical polishing procedures and then were etched in a solution of HF, HNO3, and H2O in a volume ratio of 1:1:5 to reveal the growth morphologies of the compounds in laser clad coatings. Microstructural morphologies of the coatings were analyzed by means of a QUANTA200 scanning electron microscope (SEM) and a JEM-2100 high-resolution transmission electron microscope (HRTEM). Phase constitutions were determined by X-ray diffraction (XRD) using a D/MAXRC equipment. Elements’ distributions of the coatings were measured using a E/DAXGenesis2000 energy dispersive spectrometer (EDS).

3. RESULTS AND ANALYSIS As shown in Figure 1a, the ultrafine nanoscale particles are dispersed uniformly in the coating matrix of sample 1. The fact that the atomic sizes of the Ti−Cu−Al alloy system have the great difference, which own the aggregation enthalpy of grain boundary, favoring the formation of ultrafine nanocrystals. With glass powder addition, the particles grew up in coating of sample 2, which were significantly larger than those in the coating of sample 1 (see Figure 1b). It is noted that the agglomeration of fine particles are present in some location of the coating of sample 3 (see Figure 1c). As shown in Figure 1d, many of the micro/nano particles gather together, leading to the formation of coarse block-shape precipitates in coating of 4569

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As shown in Figure 3a, the block-shape particles are produced in the coating of sample 2. The HRTEM image of such block-shape particle indicates that the particle mainly consists of the amorphous phases (see Figure 3b). The EDS test is performed in the center of the block-shape particles, indicating that the O, Na, Si, and Ca diffraction peaks are present in EDS spectrum. The glass consists of Na2O·CaO·6SiO2, so it is confirmed that the glass is included in the particles of sample 2. Moreover, it is interesting to note that the diffraction peaks of Al, Ti, and Cu are also present in the EDS spectrum, which proves the existence of TiCu2Al. Thus, combining the HRTEM and EDS test results, it can be confirmed that during the solidification process of molten pool, TiCu2Al precipitated first, which gave nucleation sites for the future grains. In the same process, the glass softened then precipitated around the TiCu2Al nucleus, leading to the formation of a coarse structure. Schematic diagram of such process is shown in Figure 3c. From this diagram, it is noted that with glass addition, the nanoscale particles have grown up after the laser-cladding process, that is, this kind of particle is ASNP. Figure 3d,e shows the TEM image of Y2O3 in coating of sample 2 and its corresponding SAED pattern, respectively. As shown in Figure 3d, the selected area electron diffraction index calibration of the Y2O3 [101] crystal zone axis can be located, which proves the existence of the undecomposed Y2O3 particles. In fact, the energy distribution of the laser beam is nonuniform, which shows a gauss distribution.39 Thus, many of the Y2O3 particles in the edge of facula were able to retain their original size during cladding process, which diffused into the every location of the molten pool due to their high-diffusion coefficient.40 Moreover, it is also noted that an amorphous ring is obviously present in such the SAED pattern. In fact, laser cladding is one of the surface amorphization technologies due to its rapid heating and cooling rates that inhibits the long-range diffusion and avoids crystallization; when the cooling rate reached the critical cooling rate of the amorphous phases, the amorphous phases were produced.41,42 On the other hand, a series of

Figure 2. SEM, TEM, and HRTEM images of the coating in sample 1: SEM image of TiB (a), HRTEM image of TiB (b); TEM image (c), and its corresponding SAED pattern (d); HRTEM images of TiCu2Al nano polycrystals (e).

Figure 3. SEM, EDS, TEM, and HRTEM images of the coating in sample 2: SEM micrograph of ASNP and its corresponding EDS pattern (a); HRTEM image of ASNP (b); schematic diagram of the formation and distribution of ASNP (c); TEM image of Y2O3 (d), and its corresponding SAED pattern (e). 4570

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Figure 4. SEM images of the middle-coating of samples 1 (a), 2 (b), 5 (c), and 4 (d); Ca diffraction peaks in different coatings of samples (e); schematic diagram of the formation and distribution of ASNP with Cu addition (f).

amorphous alloys with high-glass forming ability in Si- and Ybased alloy systems are obtained in such coating.43−47 As shown in Figure 4a−d, the EDS tests are performed in the locations of precipitates in samples 1, 2, 4, and 5, respectively. As shown in Figure 4e, without the glass addition the Ca diffraction peak in sample 1 is very weak due to the low water glass content. As mentioned previously, it is known that the glass is the main composition of ASNP, so the value of the Ca diffraction peak can be used to determine the ASNP content. After the glass powder addition, it was found that the Ca content increased with the decrease of the Cu content. Thus, it can be considered that the Cu addition led the agglomeration of ASNP to decompose and then distributed uniformly in the coating matrix. Reversely, with the decrease of the Cu content, ASNP gathered together, leading to the formation of the coarse bulk; the schematic diagram of this process is shown in Figure 4f. Figure 5a,b shows the test location of TiB2 and its corresponding HRTEM images in coating of sample 1, respectively. TiB2 crystal structure is the alternating series of the hexagonal symmetrical titanium and boride layers, and its cylinder index is {1010}; moreover, the (0001) plane of TiB2 is vertical to cylindrical surface, and its growth direction is along the (1010) lateral plane.48 It is known that TiB2 crystal does not include the crystal plane and the direction of obvious growth advantage, favoring the formation of the block-shape precipitates.49 Because of the sufficiently rapid cooling rate of laser molten pool, TiB2 did not have enough time to grow up, leading to the formation of nanocrystals. Figure 5c shows the HRTEM images of the Ti3SiC2 and TiB nanocrystals that appear at the amorphous region of the coating in sample 1, and the fringe spacing of 0.183 or 0.254 nm is observed, which corresponded its (107) or (201) crystal plane, respectively. In fact, −SiC2 system alloying compounds exhibited the high melting points, which were able to be produced when the temperature of laser molten pool reached a certain value.50

Figure 5. TEM image of TiB2 (a), and its corresponding HRTEM image (b) in coating of sample 1; HRTEM image of the coating in sample 1 (c); XRD diagrams of the coatings in samples 1 and 2 (d).

As shown in Figure 5d, the X-ray diffraction patterns indicate that the laser clad coatings in samples 1 and 2 mainly consisted of Ti−Al intermetallics, Ti−B compounds, Ti3SiC2, and TiCu2Al, and these compounds were produced through the in situ metallurgical reactions. It is noted that with glass powder addition, the broad diffraction peaks appeared at 2θ = 16−35°, 38−46°, and 69−80°, indicating that much of the amorphous phases were produced in such coating with addition of the glass powder. Thus, it is considered that added glass powder plays an important amorphization effect on laser clad coatings. 4571

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4. CONCLUSIONS In summary, laser cladding of the Al3Ti-TiB2−SiC−Cu-Y2O3 mixed powders on the Ti-6Al-4 V titanium alloy substrate can form an amorphous nanocrystal-reinforced coating. Such coating mainly consists of the amorphous phases and also the Ti−Al intermetallics, Ti−B compounds, Ti3SiC2, and TiCu2Al crystalline phases. The TiCu2Al nano polycrystals are produced in such coating matrix, which grew along (200), (220), (222), and (400) planes. During the solidification process of the laser molten pool, many of the amorphous phases just began to crystallize when the laser molten pool had completed the solidification process, favoring to the formation of nanocrystals. Moreover, Cu addition led the agglomeration of the particles to decompose and then distributed uniformly in the coating. With the glass addition, ASNP were produced in coating, leading to the formation of a micronano structure. Added glass powder also played an important amorphization effect on laser clad coatings.

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AUTHOR INFORMATION

Corresponding Author

*Address: Jinan 250061, Jingshi Road # 17923, Shandong. Email: [email protected]. Tel: +86 531 88395991. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Development Project of Science and T ech no lo gy of Shan do ng Pro vince (2010GGX10403), the National Natural Science Foundation of China (50874069), and China Postdoctoral Science Foundation funded project (2012M520135). The authors hereby express their heartfelt thanks.

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