2020 Volume 61 Issue 6 Pages 1090-1095
Titanium matrix composite materials with high strength and toughness are expected to be obtained from trimodal composites (metal matrix (titanium)/ceramic (TiC)/precipitated metal (titanium) in ceramic) by titanium precipitation in brittle ceramic particles. Titanium carbide (TiC) particles dispersed in titanium matrix composites are prepared by the arc-melting method, and the effect of nitrogen content on the microstructural changes in the TiC particles is revealed. Titanium precipitation in the TiC particles is observed when more than 2 at% nitrogen is added. The mechanism of titanium precipitation is discussed in terms of the crystallographic relationship between titanium and metastable Ti2C.
Fig. 4 Representative high resolution TEM micrograph of the interface between TiC and precipitated Ti for the 5N5TiC (a) and SAD pattern (b) with its key diagram (c).
Titanium and its alloys have been used for many practical applications because of their light weight, high strength and excellent corrosion resistance. Therefore, they have been used for aircraft, aero-engines and components in chemical processing equipment. Furthermore, they have been used in consumer products, such as components for bicycles, cameras and biomedical products.1,2) Among the many titanium-based alloys, Ti–6Al–4V possesses mechanical properties superior to all others; however, vanadium may be a harmful element and aluminum may have an effect on the human body.3) Additionally, titanium is used not only as a monolithic metal and in alloys, but also in composites with ceramic particles.
To date, many studies have reported on the relation between the microstructures and mechanical properties of titanium matrix composites reinforced with ceramic particles, such as TiC4) and TiB.5) We have studied the fabrication of TiC particles reinforced Ti matrix composites from Ti–C–N system powder mixtures and Ti–C–gaseous N by the arc-melting method.6)
Furthermore, in previous studies, microstructural changes of TiC particles in (α + β)- or β-titanium matrix composites prepared by the arc-melting method from Ti–Fe–C–N7) and Ti–Cr–C–N8) powder systems were studied. It was found that α-titanium precipitation in TiC particles was observed in the Ti–Fe–C–N system when the N content was more than 3 at% and the Fe content was less than 8 at%. In the Ti–Cr–C–N system, α-titanium precipitated in TiC particles when the N content was more than 2 at% and the Cr content was less than 11 at%.
So far, similar precipitation in TiC/Ti and TiC/Ti–Al–V composites has been reported.9–11) However, detailed analysis of the microstructure has not been performed. Therefore, we focus on the structural change of TiC particles by N addition and the mechanism of α-titanium precipitation in TiC particles in α-titanium matrix composites.
The trimodal composite, which is a metal matrix (α-titanium)/ceramic (TiC)/precipitated metal in ceramic (α-titanium), may be a desired composite material with excellent strength and ductility. It is essential to determine the detailed microstructure to develop a unique composite material having both strength and ductility that can be fabricated by the arc-melting method. In this study, by means of optical and transmission electron microscopy, the microstructural changes of a TiC particle and α-titanium matrix are observed when the N component is increased from 0 to 10 at%. From the crystallographic point of view, the mechanism of titanium precipitation in TiC particles is discussed.
Titanium powder (99.9%, 350 mesh), carbon powder (Graphite, 99.98%, 25 µm) and TiN powder (99.5%, 350 mesh) were used as starting materials. The volume percentage of TiC particles (5 vol%) in the titanium matrix composites were fixed, and the N content was varied from 0 to 10 at%. Powder mixtures of cylindrical compacts with a diameter and height of 10 mm × 10 mm were prepared by one-directional pressing with a pressure of 230 MPa. After that, using a non-consumable arc-melting facility, 30 g of button ingots with full density were fabricated from the compacts in an Ar gas atmosphere. X-ray diffraction (XRD) with the Cu Kα line and optical microscopy were performed on all the samples. Furthermore, transmission electron microscopy (TEM; JEM-2000FX and HF-2000) operating at 200 kV was carried out to investigate the microstructures of the TiC particles and titanium matrix.
XRD was performed to identify the products in the samples prepared by the arc-melting method. Figure 1 shows the XRD patterns of the samples of Ti–5 vol% TiC composites with a N content from 0 to 10 at%. Diffracted peaks of α-titanium (hereafter referred to as titanium) and TiC were observed in all the samples, which indicated that TiC/Ti composites including nitrogen could be synthesized by using the arc-melting method. The angles 2θ of the diffracted peak for the titanium matrix, especially the $(01\bar{1}2)$ and $(01\bar{1}3)$ planes related to c-axis, were shifted to lower angles as the nitrogen content was increased. Nitrogen can be dissolved in titanium to a maximum of 23 at%,12) and it was considered that nitrogen dissolved in titanium extended the c-axis of titanium during the arc-melting. Thus, it is presumed that the lattice constant of titanium, particularly the value of the c-axis, increases.
XRD patterns of the composites as a function of N content.
However, the diffracted peaks of TiC were shifted to higher angles compared with the stoichiometric composition of TiC. This suggested that a non-stoichiometric composition of TiC was formed by the arc-melting method. In general, TiC is a representative non-stoichiometric compound. TiC has a titanium-rich composition because TiC contains many atomic vacancies in its unit cell, and it has been experimentally confirmed that the lattice constant of TiC decreases for a titanium-rich composition.13) On the other hand, nitrogen is solid-solved not only in titanium but also in TiC, so TiC becomes just like Ti(C, N). Furthermore, because TiC forms a pseudo-binary solid solution with TiN, Ti(C, N) synthesized by the arc-melting method can be considered as a TiC–TiN pseudo-binary solid solution. As the lattice constant of TiN is smaller than that of TiC, the lattice constant of Ti(C, N) becomes smaller than that of TiC as the amount of nitrogen increases. Therefore, as the nitrogen content increased the angles 2θ of diffracted peaks for TiC were clearly sifted to higher angle side, suggesting decreasing the lattice constant of TiC.
3.2 Optical microscopyOptical micrographs of selected samples are shown in Fig. 2. The microstructure of the Ti–5 vol%TiC–0%N is shown in Fig. 2(a), in which TiC was a very small particle with a diameter of approximately 1 to 2 µm. When the amount of added nitrogen was 2%, as shown in Fig. 2(b), TiC lumps grew to a width of 2–5 µm and a length of approximately 20 µm. When the added nitrogen was 5%, lump-shaped TiC of approximately the same size as that in the 2% nitrogen added composites was observed, as shown in Fig. 2(c).
Optical micrographs of composites with different amounts of N.
Next, our attention turned to the characteristics of the microstructure of TiC particles in the samples. The upper right of Fig. 2(a), (b) and (c) show the enlarged micrographs of the square sections in each micrograph. For the microstructure on the surface of TiC particles, with less than 2% nitrogen, only a slight increase in the TiC particle size was observed, and no difference in the microstructure of TiC at 0% nitrogen was observed. However, when the nitrogen content was 2%, a linear black contrast was observed, as shown in Fig. 2(b). As the amount of nitrogen was further increased, the number of straight- and black-line contrasts increased, which showed a very specific microstructure, as shown in Fig. 2(c).
3.3 Transmission electron microscopyTEM observations and selected area diffraction (SAD) analysis were carried out to examine the detailed microstructures of the titanium precipitates in the TiC particles. TEM bright images and SAD patterns of the TiC particles in samples containing 0, 2 and 5 at% nitrogen are shown in Fig. 3(a)–(f).
TEM micrographs and SAD patterns in TiC particle for 0N5TiC (a), (d), for 2N5TiC (b), (e), and for 5N5TiC (c), (f), respectively. All incident beam directions are parallel to [011]TiC.
When the sample did not contain nitrogen, no significant structure existed in the TiC particle, as shown in Fig. 3(a); however, a diffused spot in 1/2[111]TiC was clearly observed (Fig. 3(d)). Analysis of SAD showed that these spots arose from Ti2C, which has the superlattice structure of TiC.14,15) Details will be described later.
For the sample with 2% nitrogen added, as shown in Fig. 3(b), a thin plate-like contrast was observed in TiC. Figure 3(e) shows the SAD pattern corresponding to Fig. 3(b). The intensity of the spot of Ti2C in the SAD pattern was weaker than that without nitrogen, and new spots other than TiC and Ti2C appeared. Detailed analysis of the spots revealed that these were titanium spots. Here, unless otherwise specified, the α-titanium had the hexagonal close packed (hcp) structure. Therefore, Ti2C and Ti coexisted in the TiC particles of the samples to which 2 at% of nitrogen were added.
When 5% nitrogen was added (Fig. 3(c)), plate-like titanium, the same as that shown in Fig. 3(b), precipitated in two directions. The spot of Ti2C disappeared, and only TiC and titanium spots were observed, as shown in Fig. 3(f). These phenomena implied that Ti2C disappeared owing to the addition of nitrogen and the transformation to titanium.
A representative high resolution TEM micrograph for the 5% nitrogen added sample of the interface between TiC and precipitated titanium is shown in Fig. 4(a). Additionally, the corresponding SAD and its key diagram are shown in Fig. 4(b) and (c). A clear product was not formed at the interface between TiC and the precipitated titanium, and a fringe of TiC and titanium was continuously observed. In addition, fringes of 0.24 nm and 0.47 nm could be clearly observed in the (111) plane of TiC and the (0001) plane of titanium. From the TEM observation results, the crystallographic relationship between the Ti2C, TiC and precipitated titanium could be described as follows;
| \begin{align*} &(111)_{\text{Ti2C}}\mathrel{/\!/} (111)_{\text{TiC}}\mathrel{/\!/} (0001)_{\text{Ti}},\\ & [110]_{\text{Ti2C}}\mathrel{/\!/} [110]_{\text{TiC}}\mathrel{/\!/} [11\bar{2}0]_{\text{Ti}}. \end{align*} |
Representative high resolution TEM micrograph of the interface between TiC and precipitated Ti for the 5N5TiC (a) and SAD pattern (b) with its key diagram (c).
First, the formation of matrix Ti and TiC from the melt will be examined here. During solidification, matrix β-titanium and TiC crystallize out from the melt through the eutectic reaction shown in the Ti–C binary phase diagram (Fig. 5).16) Thereafter, the temperature decreases, and matrix changes from β-titanium to α-titanium. At 773 K (500°C) or lower, α-titanium and TiC (Ti (C, N) when N is contained) coexist. It is found that the presence of metastable Ti2C is also suggested in the Ti–C phase diagram.
Ti–C binary phase diagram.
Although the presence of Ti2C could not be confirmed by XRD, the SAD pattern obtained from TiC particles in the matrix with no nitrogen revealed the presence of Ti2C. In addition, it was shown that the intensity of diffused Ti2C spots in SAD became weaker with the increase of the N content, and conversely, a diffraction spot of titanium newly appeared when N was added. This suggested that Ti2C was deeply involved in titanium precipitation in the TiC particles. Therefore, only based on Ti–C binary phase diagram and Ti–C–N ternary phase diagrams,17) it is difficult to understand titanium precipitation from Ti(C, N). Thus, next the coherency of the crystal structure of titanium and Ti2C will be focused to consider titanium precipitation from Ti(C, N).
In general, TiC crystallizes in the sodium chloride type compound (B1 structure, space group Fm3m) with a very wide range of compositions (TiCx, 0.48 < x < 1.0), and an off-stoichiometric composition is common for this compound.13) Additionally, in the composition range of (TiCx, 0.5 < x < 0.7), there are two kinds of compounds, cubic Ti2C (space group Fd3m; which will be referred to as Fd3m-Ti2C) and trigonal Ti2C (space group $R\bar{3}m$; which will be referred to as $R\bar{3}m$-Ti2C).18) These are long-range ordered structures of TiC, and many vacancies exist in their lattice. Neglecting the trigonal distortion, the two ordered defect structures have identical atomic distances and atomic pair correlations.19)
Figure 6 shows the schematic view of the unit cell of Fm3m-TiC and Fd3m-Ti2C as a representative of Ti2C. The lattice constant of cubic Fd3m-Ti2C has experimentally been determined to be 0.86 nm,14) which is twice the lattice constant of Fm3m-TiC. The lattice constant of one of eight of the unit cells of cubic Fd3m-Ti2C is 0.43 nm, which is comparable to Fm3m-TiC (0.4328 nm). The feature of the Fd3m-Ti2C structure is that there are some vacancies on the plane where C exists.
Unit cell of Fm3m-TiC and Fd3m-Ti2C crystal structure.
Schematic views of the structures of Fm3m-TiC, Fd3m-Ti2C and $R\bar{3}m$-Ti2C are shown in Fig. 7(a).20,21) In this figure, the schematics of the two Ti2C crystal structures show one of eight of each unit cell. Additionally, to avoid complication, titanium atoms are removed and a sublattice of only C atoms and vacancies are shown. In the figure, one black or white circle was a carbon atom, and the other was a vacancy. Figure 7(b) shows schematic SAD patterns considered to result from the Fm3m-TiC, Fd3m-Ti2C and $R\bar{3}m$-Ti2C structures when the electron beam was parallel to the [110] direction.22) Gray circles in the SAD patterns represent the spots from TiC, and the black circles represent superlattice spots from Ti2C. Furthermore, owing to the difference in the structural symmetry of Fd3m-Ti2C and $R\bar{3}m$-Ti2C, the appearance of superlattice spots should be different, for example, as shown in Fig. 7(b). Therefore, if the SAD pattern can be obtained from a single domain, it is possible to identify which structure it represents. However, if many domains contribute to the diffraction, the information from many domains will be obtained. In this case, when more than two diffraction patterns corresponding to the $R\bar{3}m$-Ti2C structure are mixed, it becomes equivalent to the diffraction pattern of Fd3m-Ti2C, so it becomes difficult to distinguish the two structures.22) To date, we have been unable to determine which Ti2C appears in the TiC. However, it was found that as the amount of added nitrogen increased, both types of Ti2C disappeared and titanium was newly precipitated. Therefore, here we consider the mechanism of titanium precipitation in TiC particles using the Fd3m-Ti2C structure.
Crystal structures of Fm3m-TiC, Fd3m-Ti2C and $R\bar{3}m$-Ti2C (a), and corresponding selected area diffraction patterns (b).
To consider titanium precipitation in the TiC particles from the crystallographic point of view of Fd3m-Ti2C and titanium (hcp, space group P63/mmc), their schematic views are shown in Fig. 8. Fd3m-Ti2C is a superlattice structure of TiC, and the lattice constant of Ti2C (0.86 nm) is approximately twice that of TiC (0.4328 nm). As described before, the TEM observation results revealed that the (111) plane of Ti2C and the (0001) plane of titanium exist in parallel. Therefore, in this figure, the (111) plane of Ti2C and the (0001) plane of titanium are drawn parallel to each other. As shown in Fig. 8, the spacing between three titanium planes in Ti2C (two-thirds of the length in the ⟨111⟩ direction of Ti2C) was 0.497 nm, and the spacing between (0001) titanium was 0.468 nm. This indicated that the difference between the plane spacings of both structures was small. The above values were for a 0% nitrogen content, and it is believed that the lattice constant of TiC becomes smaller with an increase of the nitrogen addition amount, and the lattice constant of Ti2C also becomes smaller at the same time. In contrast, the increase in the amount of nitrogen addition gradually lengthens the c-axis of titanium. Therefore, with the addition of nitrogen, the distance between three titanium atomic planes in Ti2C and the spacing of the (0001) plane of titanium become almost identical, so it is proposed that titanium precipitation from Ti2C occurs in TiC particles. It is also inferred that this precipitation is related to Ti2C having a unique structure containing many vacancies, and that this is recognized as a metastable phase in the Ti–C binary system. Furthermore, coherency between Ti2C and titanium may enhance the kinetics of titanium precipitation.
Crystal structures of Fd3m-Ti2C and Ti.
TiC particles dispersed in titanium matrix composites were prepared from titanium, carbon and TiN powders by the arc-melting method, and the microstructural change of the TiC particles with the amount of nitrogen content was investigated. The obtained results are summarized as follows:
This work was financially supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (C) JP16K06802. One of the authors (HT) would like to thank Mr. Yukio Tamura who was a student of Osaka Prefecture University.
