Reactions between Ti and Ti₃SiC₂ in temperature range of 1 273-1 573 K

GU Wan-li(谷万里)¹, ZHOU Yan-chun(周延春)

1. School of Mechanical Engineering, Shandong University of Technology, Zibo 255049, China;

2. High-Performance Ceramic Division, Shenyang National Laboratory for Materials Science,

Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

Received 6 March 2006; accepted 22 August 2006

Key words: Ti₃SiC₂; Ti; high-temperature reaction

1 Introduction

Recently, the layered ternary carbide Ti₃SiC₂ has attracted attention due to its unique properties such as high toughness, high fatigue-crack growth threshold and elastic modulus, plasticity at high temperature, excellent electrical and thermal conductivity, and easy machinability[1-6]. The unique combination of these properties makes Ti₃SiC₂ as a promising structural material for high temperature applications. In addition, the high modulus, metallic conductivity and self-lubricated properties also make Ti₃SiC₂ as a potential reinforcement for soft metals such as Al, Cu, Ti and Ni. The advantage of Ti₃SiC₂ reinforced soft metals over other ceramics is that high strength will be achieved without loss of conductivity. During preparing Ti₃SiC₂ reinforced metal matrix composites, reactions between the matrix and Ti₃SiC₂ is one of the key factors controlling the microstructure and properties of the composites. The stability of Ti₃SiC₂ in metal matrix is also important in selecting the preparation method and processing parameters. The knowledge of compounds formed and the diffusion path between the ceramic and metal interface is necessary to control the interface structures, which essentially control the mechanical properties.

In practice, when used as a high-temperature structural material, Ti₃SiC₂ often needs to be joined with structural metals to make a complex component. While as a reinforcement of soft metals, Ti₃SiC₂ particulates are incorporated uniformly into metal matrix to make metal matrix composites with controlled microstructure. In both of these applications, understanding the interfacial chemical reactions and interfacial microstructure/ chemistry is of vital importance in selecting processing parameters and optimizing properties.

However, very limited information of the interaction between Ti₃SiC₂ and metals is available. Our previous works reported the reaction between Ti₃SiC₂ and Al[7], Ti₃SiC₂ and Cu[8], respectively. Recently, we are intrigued in investigating the interfacial reactions in Ti-Ti₃SiC₂ systems. The interests stem from the need in developing high temperature self-lubricated materials such as Ti/Ti₃SiC₂ composite, as well as joining Ti₃SiC₂ with Ti-based alloys for high-temperature applications. This paper reports the reaction between Ti₃SiC₂ and Ti powder, which is useful in both selecting processing conditions of Ti/Ti₃SiC₂ composite and controlling the interfacial microstructure of Ti/Ti₃SiC₂ joint.

To investigate the possible reactions during processing of Ti/Ti₃SiC₂ composites, Ti and Ti₃SiC₂ powders were mixed and hot-pressed at 1 273-1 573 K. The average particle size of Ti powders and Ti₃SiC₂ powders was 40 mm (from General Research Institute For Nonferrous Metals, China) and 10 mm (prepared at author's lab), respectively. The Ti₃SiC₂ powder used in this work was fabricated by using the in-situ hot pressing solid-liquid reaction process, which was described in Ref.[9]. The as-prepared powders contained a small amount of TiC, which was described in the previous work[9].

According to the phase diagram of Ti-Si system [10], at low Si content (less than 1% Si, mole fraction) the solid solution of Ti(Si) forms; beyond the solubility there are five kinds of Ti-Si compounds, which are Ti₃Si, Ti₅Si₃, Ti₅Si₄, TiSi and TiSi₂. If Ti reacts with Si in Ti₃SiC₂ when Ti₃SiC₂ and Ti mix together at high temperatures, Ti₃SiC₂ will not be stable, and the reaction products depend on the content of Si, i.e. the content of Ti₃SiC₂. Supposing that Ti reacts with Si from the decomposed Ti₃SiC₂ and the Ti₃Si, Ti₅Si₃, Ti₅Si₄, TiSi and TiSi₂ can be formed, respectively, their mass ratio of Ti to Ti₃SiC₂ can be calculated as 3/4, 1.67/4, 1.25/4, 1/4 and 0.5/4, respectively. To confirm this hypothesis and
understand the high temperature reactions between Ti and Ti₃SiC₂, Ti/Ti₃SiC₂ composites with above mass ratios were prepared in the temperature range of 1 273-牋? 1 573 K under a pressure of 20 MPa.

The phase compositions of the composites prepared under different conditions were identified by X-ray diffraction(XRD), wherein the data were collected by a step-scanning diffractometer (Rigaku D/max-2500PC, Japan) using ?/span>Cu K_α radiation. To understand the mechanism of the reaction and the interfacial microstructure, bulk couple of Ti/Ti₃SiC₂ was hot pressed at 1 473 K with a pressure of 20 MPa for 60 min. The cross-section of the hot pressed Ti/Ti₃SiC₂ couple was observed in an S-360 scanning electron microscope (Cambridge Instrument Ltd., UK) equipped with an energy dispersive spectroscope(EDS) system. X-ray dot maps and concentration profiles were determined to identify the interfacial chemistry.

3 Results

To study the possible reactions between Ti₃SiC₂ and Ti, we should know the thermal stability of Ti₃SiC₂. According to previous works, the dissociation temperature of Ti₃SiC₂ was reported from 1 573 K to 牋2 073 K depending on the purity, atmosphere and sample form (bulk or powders)[11-14]. To determine the dissociation temperature of Ti₃SiC₂ powders prepared by the fluctuation method, the as-prepared Ti₃SiC₂ powders were annealed at different temperatures.

Fig.1 shows the X-ray diffraction patterns of the as-prepared Ti₃SiC₂ powders and those annealed at 1 573, ?/span>1 673 and 1 773 K for 30 min, respectively. In the 牋?/span>as-prepared Ti₃SiC₂ powders there is a small amount of TiC and SiC, which is the main impurity phase in Ti₃SiC₂[9]. After annealing at 1 573 K, the intensity of reflections from TiC_x (Here C replaced by C_x is because that the atomic ratio of Ti and C is 3 to 2 in Ti₃SiC₂, the subscript x in the new-formed TiC_x is less than 牋牋牋?/span>1) increase and a new phase TiSi₂ emerges. The increase in the intensity of TiC_x and the appearance of TiSi₂ reveal that Ti₃SiC₂ powder is decomposed obviously over the temperature of 1 573 K. At 1 673 K, the intensities of diffraction peaks of TiC_x are almost as high as that of Ti₃SiC₂, indicating that more Ti₃SiC₂ powders are decomposed. At 1 773 K, the reflections from Ti₃SiC₂ almost disappear but SiC and graphite emerge as decomposition products. The above results are similar to result of RACAULT et al[13] and different from reports of BARSOUM et al[11] and RADHAKRISHNAN et al[12] on the thermal stable of Ti₃SiC₂. The observed low decomposing temperature is attributed to the fact that the Ti₃SiC₂ powders studied in our present work have large surface area and higher activity than bulk material used in previous works[11,12]. Owing to the decomposition of Ti₃SiC₂ powder above 1 573 K, the temperature range for the reaction in Ti-Ti₃SiC₂ system is chosen below 牋牋?/span>1 573 K.

Fig.1 XRD patterns of original Ti₃SiC₂ powders and those annealed at 1 573, 1 673 and 1 773 K for 30 min, respectively

Fig.2 shows the X-ray diffraction patterns for samples of Ti/Ti₃SiC₂ composites with a mass ratio of 3/4, which were hot pressed at 1 173, 1 273, 1 373, 1 473 and 1 573 K under a pressure of 20 MPa for 30 min, respectively. For the sample hot pressed at 1 173 K, there are very weak diffraction peaks from TiC_x and the main phases are Ti and Ti₃SiC₂, which implies that the reaction between Ti and Ti₃SiC₂ almost can be neglected at the temperature of 1 173 K. For the sample hot pressed at 牋1 273 K, the intensities of reflections from TiC_x are higher than those at 1 173 K. In addition, very weak diffraction peaks from element carbon and titanium- silicon compound Ti₅Si₃ can be identified, which indicates that the starting reaction should be in the range of 1 173-1 273 K. For the sample prepared at 1 373 K, the diffraction peaks of TiC_x become even higher and the reflections from Ti and C disappear, indicating that the amount of TiC_x increases with the reaction temperature increasing. The reflections from Ti₃SiC₂ are weaker than those from TiC at 1 473 K and almost disappear at 1 573 K. All the above results demonstrate that Ti reacts with Ti₃SiC₂ at temperatures above 1 273 K and the reaction extent is temperature dependent.

To further investigate the effect of Ti₃SiC₂ content on the reaction products, Ti/Ti₃SiC₂ powders with a mass ratio of 1.67/4 were hot pressed under a pressure of 20 MPa at 1 273, 1 373, 1 473 and 1 573 K for 30 min, respectively, and the X-ray diffraction patterns of these composites are shown in Fig.3. The difference between Figs.3 and 2 is that new reflection from TiSi₂ is detected in Fig.3 besides TiC_x and Ti₅Si₃. Meanwhile, the reflec-

Fig.2 XRD patterns for samples of Ti/Ti₃SiC₂ composites with mass ratio of 3/4 hot pressed at 1 173, 1 273, 1 373, 1 473 and 1 573 K for 30 min, respectively tions from graphite emerge again, which is similar to the situation in Fig.2. The emerging of the graphite (002) peak implies that Ti₃SiC₂ is decomposed at 1 273 K and the element carbon is one of the products of the decomposition.

Fig.4 shows that the X-ray diffraction patterns for samples of Ti/Ti₃SiC₂ composites with a mass ratio of 0.5/4, which were hot pressed under a pressure of 20 MPa at 1 273, 1 373, 1 473 and 1 573 K for 30 min, respectively. No new phase can be detected besides TiSi₂ and Ti₅Si₃. It can be found that the diffraction peak (200) from TiC_x increases with the increasing temperature. In another hand, the diffraction peaks of graphite at 1 273 K and 1 373 K are higher than the same peak detected in

Fig.3 XRD patterns for samples of Ti/Ti₃SiC₂ composites with mass ratio of 1.67/4 hot pressed at 1 273, 1 373, 1 473 and 1 573 K for 30 min, respectively

Fig.4 XRD patterns for samples of Ti/Ti₃SiC₂ composites with mass ratio of 0.5/4 hot pressed at 1 273, 1 373, 1 473 and 1 573 K for 30 min, respectively

Fig.3. Combined with Fig.3, it is clear that the intensity of graphite's peak has a close relationship not only with the content of Ti₃SiC₂ but also with the reaction temperature. Briefly, when the content of Ti is relatively low the diffraction peak of graphite is relatively high, meanwhile, for a certain mass ratio of Ti to Ti₃SiC₂, the diffraction peak of graphite is high at a low temperature.

As mentioned above, there are five compounds in Ti-Si system. However, the XRD results shown in Figs.2-4 indicate that only Ti₅Si₃ and TiSi₂ can be detected during the reaction process. The important question is whether other three compounds would be detected if the mass ratio is changed. In fact, there are marked disagreements concerning Ti₃Si and Ti₅Si₄. HANSEN et al[15] has not shown either of these compounds in their paper about the Ti-Si phase diagram. According to Ref.[10], this judgement is based on: 1) the probable purity of the alloys; 2) the fact that the phases of composition 25% and 45% (mole fraction) were observed by different authors, and 3) the present thermodynamic analysis of the system[10]. Based on these facts, it is acceptable that no Ti₃Si is detected in the product of reaction. On the other hand, Ti₅Si₄ and TiSi will be formed above temperature of 1 843 and 1 753 K [10], respectively. It is impossible for these two compounds to be formed under 1 573 K. To conform this deduction, a sample of Ti/Ti₃SiC₂ composites with a mass ratio of 1/4 (with the mass ratio the compound of TiSi should be formed) is hot pressed at 1 573 K for 30 min and the XRD result is shown in Fig.5. It is clear that all the diffraction peaks of Ti-Si compound are from Ti₅Si₃ and TiSi₂ and no TiSi can be detected. This result implies that only Ti₅Si₃ and TiSi₂ can be formed under the condition of our investigation.

Based on the experimental results described above, the reaction between Ti₃SiC₂ and Ti can be described as the following equation:

Ti₃SiC₂+Ti→TiC_x+Ti₅Si₃+TiSi₂            (1)

The equation represents a typical reaction and maybe only one kind of Ti-Si compound can be detected for certain content as shown in Fig.2. The subscript x in TiC_x represents the atomic ratio of titanium and carbon.

4 Discussion

In the above section, we have known that during the high temperature processing of Ti-Ti₃SiC₂ composite, Ti reacts with Ti₃SiC₂ and the reaction depends on both Ti₃SiC₂ content and the temperature. To reveal the reaction process a bulk Ti₃SiC₂ was polished and buried in powder of Ti, then hot pressed at 1 573 K with a pressure of 20 MPa for 60 min. The back-scattered SEM image and the chemical composition profile across the interface of the bonded Ti/Ti₃SiC₂ are shown in Fig.6. The composition profiles(Fig.6(b)) detected along the

Fig.5 XRD patterns for samples of Ti/Ti₃SiC₂ composites with mass ratio of 1/4 hot pressed at 1 573 K for 30 min

Fig.6 Back-scattered SEM image and chemical composition profile across interface of bulk Ti₃SiC₂ buried in powder of pure Ti hot pressed at 1 573 K with pressure of 20 MPa for 60 min

line as shown in Fig.6(a), indicate the diffusion of Si occurred on the boundary at high temperature. However, the diffusion of other elements can be neglected. On the other hand, Ti reacts with the Si in Ti₃SiC₂ to form Ti-Si compound (dim part of interface marked 3 in Fig.6(a)), which results in the formation of TiC_x (dark part marked 4 in Fig.6(a)) near the interface. The EDS semi- quantitative analysis of Ti and Si at the five different points in Fig.6(a) is listed in Table 1, which implies that at layer A in Fig.6(a) Ti₅Si₃ forms and at layer B in Fig.6(a) TiC_x forms. Fig.6(b) also reveals that the composition of Si declines with a steep gradient on the boundary of Ti and the layer A, which implies that the diffusion of Si from Ti₃SiC₂ to Ti is limited by the forming of Ti-Si compound.

Previous study on the structure characteristics of Ti₃SiC₂ revealed that the structure of Ti₃SiC₂ can be described as a layer of Si intercalated into the {111} twin

Table 1 Mole fraction of elements in different regions in Fig.6(a) (%)

boundary of TiC, and the chemical bonding between Si and Ti is relatively weak compared to the strong Ti桟 bonding[16-18]. And the decomposition of this layered ternary compound is usually caused by the de-intercalation of Si from it[16-18]. In other words, if Si forms solid solution or a stable compound with other elements, Ti₃SiC₂ will decompose. In this work, the Si-Ti compound such as Ti₅Si₃ and TiSi₂ forms during the reaction process, which gives a direct evidence for the de-intercalation of Si from Ti₃SiC₂. In addition, the presence of elemental carbon in the Ti-Ti₃SiC₂ reaction product implies that the decomposition mode of Ti₃SiC₂ in Ti may not solely follow the one described above. The experimental results shown in Figs.2-4 reveal that Ti₃SiC₂ can be decomposed with other ways besides the de-intercalation of Si from Ti₃SiC₂ and the formation of TiC_x.

Analyzing the graphite's peak shown in Figs.3 and 4 we would like to hypothesize that the graphite from the decomposed Ti₃SiC₂ comes into existence at 1 273 K and then reacts with titanium to form TiC_x at higher temperature. If the titanium is insufficient or the temperature is low, the reaction will not be sufficient and the graphite will be left. To confirm this hypothesis the reaction time is extended to 90 min for sample Ti/Ti₃SiC₂ composites with a ratio of 0.5/4 at temperatures of 1 273 and 1 373 K, respectively. The XRD results of the samples prepared in 90 and 30 min are compared and shown in Fig.7. It is clear that the graphite's diffraction peaks disappear for the sample with a longer reaction time. This result implies that the element carbon is an interim phase during the reaction process; the carbon from the decomposed Ti₃SiC₂ will react with Ti furthermore if the reaction time is long enough. Combining with the above result, Ti₃SiC₂ may be decomposed in two different modes. The first mode is caused by the de-intercalation of Si from it and the TiC_x is formed by the remained titanium and carbon; the second mode is that the carbon is separated from the Ti₃SiC₂ and reacts with titanium furthermore. Thus, the reaction route for the high temperature reaction between Ti₃SiC₂ and Ti can be described by the following procedures.

Fig.7 XRD patterns for samples of Ti/Ti₃SiC₂ composites with mass ratio of 0.5/4 hot pressed at 1 273 and 1 373 K for 30 and 90 min, respectively

Ti₃SiC₂+Ti→TiC_x+Si+Ti→TiC_x+Ti_xSi_y             (2)

Ti₃SiC₂+Ti→Ti_xSi_y+C+Ti→Ti_xSi_y+TiC_x                     (3)

The molecular formula Ti_xSi_y represents all the two sorts of titanium silicon compound detected in the reaction process. The subscript x in TiC_x represents the atomic ratio of titanium and carbon.

In Fig.2, as calculated above, the mass ratio of Ti and Ti₃SiC₂ corresponds to the compound Ti₃Si. According to this mass ratio, the element Ti should be left after the reaction is finished, because the detected compound Ti₅Si₃ has a relatively low Ti content compared to Ti₃Si. However, the experiment result indicates that no element Ti remains. This result implies that the value of subscript x in TiC_x may be less than 0.67, because more Ti will be consumed for a small value of x. On the other hand, the XRD result in Fig.4 shows that besides TiSi₂, the compound should appear for this content, Ti₅Si₃ is detected as well. This result implies that the value of subscript x in TiC_x may be larger than 0.67. All these experiment results suggest that the subscript x in TiC_x will change in a large range. From Ti-C phase diagram[19] we know that the range of x will be changed from 0.5 to 1.

Considering the crystal structure of TiC_x, the change of x will bring the change of lattice spacing and the remove of the diffraction peak. As described above, the results in Figs.2-4 show that the diffraction peaks of TiC_x is low at 1 273 K and becomes higher and wider at 1 373 K and 1 473 K. This result implies the composite of TiC_x is uneven in the different region. The peak becomes very sharp at 1 573 K, which implies a higher temperature is in favor of the harmoniousness in composition.

Our work demonstrates that Ti reacts with Ti₃SiC₂ at high temperatures and the reaction depends on the temperature and Ti₃SiC₂ content. The result of this work is instructive for selecting the preparation method and for processing parameters of Ti-Ti₃SiC₂ composite. Due to the reaction between Ti matrix and the Ti₃SiC₂ reinforcement at temperatures above 1 273 K, it is desirable to make Ti₃SiC₂ reinforced Ti below 1 273 K.

5 Conclusions

The chemical reactions between Ti and Ti₃SiC₂ have been investigated during the preparation of Ti-Ti₃SiC₂ composites in the temperature range of 1 273-1 573 K. The results demonstrate that Ti₃SiC₂ reacts with Ti to form TiC_x, Ti₅Si₃ and TiSi₂. It is shown that the hot pressing temperature is the key factor that determines the extent of reactions. The relative proportion of Ti and Ti₃SiC₂ has a relationship with the product of reaction. The Si diffused from Ti₃SiC₂ to Ti forms stable compounds like Ti₅Si₃ is believed to be the main mechanisms for the reaction. Because the reactions between Ti and Ti₃SiC₂ occur at 1 273 K, which is lower than the melting point of Ti (1 943 K), it is difficult to synthesize composite with titanium and Ti₃SiC₂.

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(Edited by YUAN Sai-qian)

Foundation item: Project(59925208) supported by the National Outstanding Young Scientist Foundation; Project(59772021) supported by the National Natural Science Foundation of China

Corresponding author: GU Wan-li; Tel: +86-533-2786499; E-mail: wlgu001@sina.com.cn