Raman spectroscopy sheds new light on TiC formation during the controlled milling of titanium and carbon
A magneto ball mill was used to mill titanium and carbon elemental powder mixtures with compositions of Ti50C50 and Ti60C40 under a helium atmosphere. Previous studies on the milling of titanium and carbon powder mixtures have reported a sudden increase in the temperature of the milling vial, which occurs after a specific milling interval, referred to as tig [Z.G. Liu, J.T. Guo, L.L. Ye, G.S. Li, Z.Q. Hu, Appl. Phys. Lett. 65 (1994) 2666–2668; G.B. Schaffer, J.S. Forrester, J. Mater. Sci. 32 (1997) 3157–3162; N.Q. Wu, S. Lin, J.M. Wu, Z.Z. Li, Mater. Sci. Technol. 14 (1998) 287–291; Z. Xinkun, Z. Kunyu, C. Baochang, L. Qiushi, Z. Xiuqin, C. Tieli, S. Yunsheng, Mater. Sci. Eng. C 16 (2001) 103–105; C. Deidda, S. Doppiu, M. Monagheddu, G. Cocco, J. Metastable Nanocryst. Mater. 15/16 (2003) 215–220]. This sudden temperature increase has been found to correspond to the formation of TiC via a rapid, highly exothermic reaction. In these cases, XRD analysis did not detect TiC in powder sampled before tig. These results, combined with those from studies suggested that the milling period prior to tig represents an incubation period during which the powders become mechanically activated and that no reaction between the starting powders occurs during this time [L. Takacs, J. Solid State Chem. 125 (1996) 75–84; B.K. Yen, T. Aizawa, J. Kihara, J. Am. Ceram. Soc. 81 (1998) 1953–1956; M. Puttaswamy, Y. Chen, B. Jar, J.S. Williams, Mater. Sci. Forum 312–314 (1999) 79–84; G.B. Schaffer, P.G. McCormick, Metall. Transact. A 23A (1992) 1285–1290; M. Mingliang, L. Xinkuan, X. Shenqui, C. Donglang, Z. Jingen, J. Mater. Process. Technol. 116 (2001) 124–127].
In the current investigation a sudden increase in the temperature of the milling vial was also observed after a specific milling duration of tig. XRD analysis of powder sampled before tig did not detect TiC, whilst for powder sampled after tig, XRD analysis indicated that the powder had almost completely transformed into TiC. However, Raman spectroscopy showed the formation of non-stoichiometric TiC in Ti50C50 and Ti60C40 powders sampled before tig. It is believed that the current, and many previous XRD analyses did not detect the formation of TiC prior to tig either because the TiC grain size was too small and/or the phase represented too small a volume fraction of the powder samples to be detected by the XRD method employed. These Raman spectroscopy results suggest that a significant component of the heat generated at tig may be due to a combination of growth of TiC accompanied by consumption of unreacted Ti and carbon, and/or recrystallisation of the TiC formed prior to tig, rather than the direct formation of TiC.