CRYSTAL GROWTH & DESIGN
Analysis of the Growth Morphology of TiB and the Microstructure Refinement of the Coatings Fabricated on Ti-6Al-4V by Laser Boronizing
2008 VOL. 8, NO. 2 700–703
Y. S. Tian,* Q. Y. Zhang, D. Y. Wang, and C. Z. Chen Key Laboratory of Liquid Structure and Heredity of Materials, Shandong UniVersity, Jinan 250061, P.R. China ReceiVed NoVember 15, 2006; ReVised Manuscript ReceiVed October 17, 2007
ABSTRACT: Coatings mainly containing the stick TiB strengthening phase have been fabricated on a Ti-6Al-4V alloy by laser boronizing. Results show that the coating microstructure is refined by the addition of boron. The growth model of stick TiB is mainly attributed to the lattice characteristics of TiB, and the mechanism of the microstructure refinement is attributed to the preeutectic TiB crystal embryos acting as heterogeneous nuclei and the growth obstruction of the formed crystals caused by TiB during the solidification. Titanium alloys are now widely used in aeronautical and marine industries for their excellent properties such as high specific strength and corrosion resistance. But, their applications in tribological situations are limited due to their lower hardness and wear resistance. Surface modification technologies is a promising way to increase the surface hardness and the wear resistance of titanium alloys, and among them, laser surface treatment is a commonly used method due to its high energy density and ability to be easily conducted to where needed. Coatings containing various strengthening phases have been laser-fabricated on titanium alloys to improve the wear resistance. In a nitrogen environment and using SiC powder as an alloying addition, Selamat and Mridha et al. laser fabricated composite layer containing titanium nitrides and silicides on Ti-6Al-4V.1,2 Wang et al. produced compositionally graded coatings by laser alloying of Ti-6Al-4V with TiC powder.3 Titanium borides have excellent properties and wear resistance and composite coatings containing titanium borides have been observed on titanium alloys with different surface boronizing processing technologies.4,5 Zhu and Cheng et al.6,7,12 reported that a suitable amount of boron addition can significantly refine the structure of an as-cast titanium alloy and improve the mechanical properties. In this study, laser boronizing of titanium alloy was performed with different boron content. The TiB growth morphologies and the coating microstructures were investigated.
1. Experimental Procedures The samples of titanium alloy Ti-6Al-4V, 10 mm × 10 mm × 12 mm in size, were abraded with SiC grit paper prior to the coating operation. Powder mixtures of boron and titanium with different weight ratios, an average particle size of about 10 µm, blended with diluted polyvinyl alcohol solution, were precoated on the surface of the samples in a thickness of approximately 0.5 mm. A 1500 W continuous wave CO2 laser, with an output power of 1200 W, beam size of 3 mm, and scanning speed of 3.5 mm/s, was employed to melt the surface of the samples, and the tracks were 50% overlapped. Argon gas at a pressure of 0.3 MPa was fed through a nozzle which was coaxial with the laser beam to protect the melt pool from oxidation during laser processing. Metallographic samples were prepared using standard mechanical polishing procedures and then etched in a solution of HF, HNO3, and H2O in a volume ratio of 2:1:47 to reveal growth morphologies of the * Corresponding author. Tel.: +86-531-8395991. Fax: +86-531-8392313. E-mail address:
[email protected].
Figure 1. XRD spectrum of sample 2. Table 1. Weight Ratio of the Powders samples
1
2
3
B:Ti
B ) 100%
1:4
1:8
titanium borides, and the microstructures were characterized using JXA-8800R electron probe microanalysis (EMPA) and an H-800 transmission electron microscope (TEM). The phase compositions of the coatings were identified using a D/max-RC X-ray diffractometer (XRD) with Cu KR radiation operated at a voltage of 40 kV, a current of 40 mA, and a scanning rate of 4°/min. The used alloy powders, and their weight ratios are shown in Table 1.
2. Analysis of Growth Morphologies of TiB The XRD pattern of the laser boronized layer (see Figure 1) shows that the titanium borides in the coatings are mainly TiB, and this can be explained according to the binary Ti-B phase diagram (see Figure 2). It is seen that the boron concentration needed to form TiB is lower than that of TiB2, and the formation of TiB can be carried out in a quite broad concentration area. In addition, the formation temperature of pre-eutectic TiB is in a relatively large range of approximately 1540-2200 °C, and at 1540 °C, the eutectic reaction takes place to produce TiB + (β-Ti). During the present process, the laser irradiation time is short (about 1 s), thus the boron concentration in the melt is low and the formation of TiB is easier than that of TiB2. Figure 3 shows that the borides in the coating are sticklike, and the growth mechnism of the stick TiB crystals may be mainly attributed to its lattice structure. Reference 8 indicated that the crystal structure of TiB is orthorhombic (a ) 6.12 Å, b ) 3.06 Å, and c ) 4.56 Å). It consists of trigonal prisms
10.1021/cg0608099 CCC: $40.75 2008 American Chemical Society Published on Web 01/09/2008
TiB Coatings Fabricated on Ti-6Al-4V
Figure 2. Equilibrium binary Ti-B phase diagram.
Figure 3. EMPA micrographs of sample 3 (B:Ti ) 1:8).
stacked in columnar arrays sharing only two of their three rectangular faces with neighboring prisms. The B atom lies at the center of a trigonal prism of six Ti atoms, and thus, the B atoms form a zigzagging chain along the [0l0] direction, as depicted in Figure 4. This is due to the fact that the bond energy of B-B is stronger than that of B-Ti and Ti-Ti. According to the periodic bond chain theory,9 the faster-growth direction of TiB should be along the B-B bond zigzagging chain, and therefore, stick TiB crystals are formed with their stick axis parallel to the [0l0] direction.
3. Refinement Mechanism of the Coating Microstructures Figure 5 is the micrograghs of the laser-treated samples. It is seen that the microstructure of the sample laser boronized is finer than that of the laser remelted, which may be explained as follows. First, it is attributed to the inoculation effect of the
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Figure 4. Schematic showing a stacking growth of the TiB crystal.8
Figure 5. OM microstructure of the samples: (a) laser remelting; (b) laser-boronizing.
pre-eutectic TiB. References 10 and 11 reported that the crystallographic relationships between TiB and the titanium matrix are [010]TiB//[011j 0]Ti, (100)TiB//(2j 110)Ti, (001)TiB// (0002)Ti, (101j)TiB//(42j21)Ti, and [001]TiB//[011j0]Ti, (010)TiB// (2j110)Ti. In the present study, we also found that the crystallographic relationships between TiB and the titanium matrix are [110]TiB//[213j3]R-Ti, (001)TiB//(1j101)R-Ti, and (11j1)TiB// (1j012)R-Ti (see Figure 6). Therefore, it can be inferred that the pre-eutectic TiB crystal embryos may act as heterogeneous nuclei during the solidification and promote the microstructure refinement of the coatings. On the other hand, during the solidification, a solute-rich (boron) area can be built up in the liquid ahead of the solid–liquid interface due to constitutional supercooling. If the melt contains a higher concentration of boron, more boron solute would be ejected into the liquid ahead of the solid–liquid
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Figure 6. TEM micrographs and selected area electron diffraction pattern (SADP) of sample 2.
Figure 7. TEM micrographs and SADP of sample 2.
interface where the TiB nucleation would easily take place. Cheng7 thought it possible for even only 0.5% boron addition
to form sufficient constitutional supercooling for such nucleation to occur and this process would be responsible for boron-induced
TiB Coatings Fabricated on Ti-6Al-4V
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structure of the coatings can be further refined. Figure 8 illustrates the growth obstruction of the formed crystals caused by TiB. Therefore, the final microstructure of the coatings is significantly affected both by the boron concentration and the constitutional supercooling extent of the melt. Nevertheless, from Figure 9, it can be seen that the titanium borides in the coating alloyed with 100% boron are coarser than that alloyed with a powder mixture of boron and titanium. This means that although a high concentration boron can promote TiB nucleation during solidification, an overabundance of boron will induce the formation of coarser stick TiB. Figure 8. Schematic illustration of the microstructural refinement by borides.
4. Conclusions Laser boronizing of titanium alloy Ti-6Al-4V was carried out with powder mixtures of boron and titanium with different ratios. XRD pattern show that the strengthening phase in the coatings is mainly TiB. The formation of stick TiB is mainly attributed to the lattice characteristics of TiB. The microstructure refinement of the coatings by boron addition is attributed to the pre-eutectic TiB crystal embryos acting as heterogeneous nuclei and the growth obstruction of the formed crystals caused by TiB. During solidification, although high concentration boron can promote TiB nucleation, an overabundance of boron will induce the formation of coarser stick TiB.
References
Figure 9. Microstructures of the samples laser boronized with (a) 100% B and (b) B:Ti ) 1:4.
grain refinement. After the laser beam was removed, the melt pool solidified quickly because of the chill caused by the matrix; therefore, high constitutional supercooling would be formed in the liquid ahead of the solid–liquid interface, and thus, TiB nuclei form and grow up quickly and then block the growth of the previously formed crystals (see Figure 7). So, the micro-
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