International Journal of Modern Physics B Vol. 23, Nos. 6 & 7 (2009) 1444–1448 World Scientific Publishing Company
EFFECT OF SINTERING TEPMERATURE ON MICROSTRUCTURE OF Ti6Al4V MATRIX COMPOSITES LUJUN HUANG, FUYAO YANG, YONGLIANG GUO, JIE ZHANG and LIN GENG* School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001 *
[email protected] (TiBw+TiCp)/Ti6Al4V composites were fabricated by reactive hot-pressing at the temperature range of 800~1200°C using the starting materials of TiB2, C and Ti6Al4V powders. The XRD results suggested that the reaction between C and Ti happened 900°C and above, while the reaction between TiB2 and Ti happen at 1100°C and above. SEM results also suggested that the reaction between C and Ti was prior to that between TiB2 and Ti. With the increase of the sintering temperature, the size of TiC particle and TiB whisker reinforcements increased gradually. The TiC particle was formed at the boundaries of original Ti6Al4V particles, while the TiB whiskers grew toward the inside of Ti6Al4V particles, which resulted in a strong bonding between neighboring Ti6Al4V particles. The results of hardness and relative density tests showed that the (TiBw+TiCp)/Ti6Al4V composites sintered at 1100°C had the highest hardness and relative density compared with that sintered at other temperatures. Keywords: Titanium alloy; metal matrix composites; sintering; microstructure.
1. Introduction Titanium matrix composites (TMCs) offer a combination of good mechanical properties and high temperature durability that render them attractive materials for commercial automotive, aerospace and military applications.1 The fabrication methods of TMCs include conventional ex situ methods and novel in situ methods.2 When compared to conventional methods, in situ methods afford the prepared composites many advantages, such as strong interface bonding which is beneficial to the mechanical performance.2-5 According to the current literatures, extensive studies have pointed out that TiB whisker and TiC particle are the best reinforcements for TMCs. The TMCs reinforced with TiB whiskers and TiC particles have higher mechanical properties than the TMCs reinforced by TiB whiskier or TiC particle, respectively.1,6-8 The Ti-6Al-4V alloy, used as matrix in the present study, has been commonly chosen as the matrix of TMCs for its good mechanical properties and its wide applicability in the industries.9 TMCs are usually fabricated by powder metallurgy techniques, and microstructure of the TMCs is affected greatly by the fabrication processes. Therefore, it is of much interest to understand the effect of fabrication parameters on microstructure and properties of TMCs. In this paper, the effect of sintering temperature on microstructural characteristic of a (TiBw+TiCp)/Ti-6Al-4V composite will be discussed.
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2. Materials and Experimental Procedure To prepare the in situ composites, fine TiB2 powder, C powder and the Ti-6Al-4V powder were used as the starting materials. The Ti-6Al-4V powders with a narrow distribution of particle size ranging from 10 to 40µm are shown in Fig. 1(a). The prism TiB2 powders with the particle size ranging from 1 to 8µm are shown in Fig. 1(b). The flake C powders with average size of 0.5µm are shown in Fig. 1(c). Using the above materials, the (TiBw+TiCp)/Ti-6Al-4V composite was fabricated by a hot reaction pressing method. Ti-6Al-4V powders, TiB2 powders and C powders were mechanically blended for 8 hours using a planetary blender. The blended powder mixtures were hot-pressed in vacuum at 800-1200°C for 60 min with a pressure of 20MPa. Phase identification of the as-extruded composites was conducted using a Philipx’pert X-ray diffraction meter and CuKα radiation. Microstructural examination was performed by using a Hitachi S-3000N scanning electron microscopy (SEM). The relative densities were determined by Archimedes method using water immersion.
(a)
(b)
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Fig. 1. SEM micrographs of the starting materials: (a) Ti–6Al–4V powder, (b) TiB2 powder, and (c) graphite powder.
3. Results 3.1. Thermodynamic analyses The (TiBw+TiCp)/Ti-6Al-4V composite was in situ synthesized through the chemical reactions between the compacted reactant powders of TiB2 and Ti, C and Ti, respectively. The reaction can be described as follows:
＝ 2TiB, Ti ＋ C ＝ TiC,
Ti + TiB2
∆G = −184765 + 12.55T ∆G = −186600 + 13.22T
(1) (2)
According to the equations (1) and (2), The values of the Gibbs free energy are negative, indicating that chemical reactions among TiB2, C and Ti will take place at higher temperatures and TiB and TiC phases will be formed during hot pressing. In general, TiB phase is thermodynamically more stable than TiB2 phase with excess amount of titanium.4 Therefore, TiB phase and TiC phase can be easily formed during the fabrication process. Similar TiBw/Ti composites have been obtained from TiB2 and Ti system by Ma.4 According to reactions (1) and (3), a 10vol.%(TiBw+TiCp)/Ti-6Al-4V
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composite with a TiB/TiC volume ratio of 1:1 was fabricated by hot reaction pressing at various temperatures. 3.2. XRD analyses Figure 2 shows the XRD patterns of the (TiBw+TiCp)/Ti-6Al-4V composites sintered at various temperatures. As seen from Fig. 2, TiC and TiB phases were formed during hot pressing above 800°C and 900°C, respectively. Only a small amount of TiC was formed and C still existed in the composite sintered at 800°C. It means that the reaction between Ti and C is completed at 900°C. In another hand, TiB was formed in the composite sintered at 900°C and TiB2 was not used up until 1100°C. It means that the reaction between Ti and TiB2 is completed at 1100°C. Therefore, the (TiBw+TiCp)/Ti6Al4V composites need to be sintered above 1100°C.
2θ, Fig. 2. X-ray diffraction patterns of the (TiBw+TiCp)/Ti6Al4V composites by hot-pressing at various temperatures.
3.2. Microstructures Figure 3 shows the SEM microstructures of the composites fabricated at various sintering temperatures. As seen from Fig. 3(a), only fine particles can be observed, and the average size of this particle is about 500nm. Figure 3 also shows that the size of the particle increases with increasing the sintering temperature. Combining with the XRD analyses, this particle is determined to be TiC reinforcement. The “fiber-like” phase can be observed in the composite sintered at 900°C, and this “fiber-like” phase is usually called whisker phase. Combining with the XRD analyses, this whisker phase is determined to be TiB reinforcement. It can be seen form Fig. 3 that the size of TiC particle and TiB whisker increase with increasing the sintering temperature and keep stable above 1100°C. The TiB whiskers can be clearly observed in the composite sintered at 1100°C. These TiB whiskers are dispersed in the Ti-6Al-4V matrix in 3D random array. It can be observed that the reinforcements, especially the TiC paticles, were not dispersed in a homogeneous manner in the composites. To investigate this phenomenon,
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another sample was slightly etched and observed. It can be found from Fig.4 that the TiB whiskers grow toward the inside of Ti matrix like dowels pins, resulting in a strong bonding between neighboring Ti-6Al-4V particles. However, the TiC particles were formed only on the boundaries of Ti-6Al-4V particles. (a)
(c)
(b)
(d)
(e)
Fig. 3, SEM micrographs of the (TiBw+TiCp)/Ti6Al4V composites fabricated by hot-pressing at various temperatures: (a) 800°C, (b) 900°C, (c)1000°C, (d) 1100°C, (e) 1200°C.
Fig. 4. SEM micrograph of the (TiBw+TiCp)/Ti6Al4V composite fabricated by hot-pressing at 1200°C.
Figure 5 shows that the effect of sintering time on hardness of the (TiBw+TiCp)/Ti6Al4V composites. As seen from Fig. 5, due to the increasing amount of the ceramic phases, especially the TiB whisker, the hardness of the composites increases gradually with the sintering temperature. In addition, the grain refinement caused by reinforcement addition can increase the hardness of the composites. However, the hardness is stable when the sintering temperature is above 1100°C. Figure 6 shows that the relative density of the composites changes with the increasing sintering temperature. As seen from Fig. 6, the changing tendency of relative density is similar to that of hardness. The main reason is that the reactions between TiB2
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and Ti and between C and Ti have accomplished at 1100°C. Combining with the above analyses, the optimal sintering temperature for fabricating the (TiBw+TiCp)/Ti6Al4V composites is 1100°C.
Fig. 5. Dependence of Vickers Hardness of the composites on the sintering temperature.
Fig. 6. Dependence of relative density of the composites on the sintering temperature.
4. Conclusion (1) The reaction between C and Ti is prior to that of TiB2 and Ti. With the increase of the sintering temperature, the size of TiC particle and TiB whisker reinforcements is gradually increased. (2) TiC particle is formed at the boundaries of original Ti6Al4V particles by the reaction between C and Ti. While the TiB whisker grows toward the inside of Ti matrix, which results in a strong bonding between neighboring Ti6Al4V particles. (3) For the (TiBw+TiCp)/Ti6Al4V composites, the optimal sintering temperature is 1100°C. Acknowledgment This work is financially supported by National Natural Science Foundation of China under grant No. 50771039. References 1. 2. 3. 4. 5. 6. 7. 8. 9.
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