Year

2005

Degree Name

Doctor of Philosophy (PhD)

Department

School of Mechanical, Materials and Mechatronic Engineering - Faculty of Engineering

Abstract

The aim of this work is to improve the understanding of the reaction to form titanium carbide (TiC) during the high-energy milling of titanium and carbon powder mixtures because a more thorough understanding of this reaction is required if high-energy milling is to be developed as a commercially viable process for the production of TiC and/or TiC-Ti powders. Titanium and activated carbon powder mixtures with compositions of Ti100-xCx (x = 50, 40, 30) were milled under a helium atmosphere using a magneto ball mill. For Ti50C50 and Ti60C40 powder mixtures milled in the higher milling energy impact mode, the combined results of external mill temperature monitoring and X-ray diffractometry indicated that, after a specific milling period of tig, TiC was formed rapidly via a highly exothermic mechanically-induced reaction. However, contrary to the current understanding of mechanically-induced self-propagating reactions, Raman spectroscopy clearly showed the formation of non-stoichiometric TiC in Ti50C50 and Ti60C40 powders prior to tig. This result has not been reported in previous studies that used only XRD analysis to characterise the as-milled powders. It is now thought that a significant component of the heat generated at tig may be due to a combination of rapid grain growth and/or recrystallisation of the pre-existing TiC, rather than the direct formation of TiC. When milling Ti70C30 in impact mode, the reaction to form TiC proceeded gradually as milling progressed. It was found that, when milling powder mixtures with compositions of Ti100-xCx (x = 50, 40, 30) in the lower milling energy shearing mode, TiC was formed gradually as milling progressed. It was also found that, for the starting powder compositions investigated in this study, increasing the carbon content of the titanium and activated carbon starting powder mixture results in a slowing of the milling process. This slowing of the milling process is thought to be due to the volume of powder in the milling vial increasing as the carbon content is increased. This would have a similar effect to decreasing the ball to powder weight ratio, which is known to slow the milling process. This study also revealed that moisture adsorbed by the activated carbon starting powder could prevent, or at least delay, the reaction to form TiC during controlled ball milling. It is thought that the moisture adsorbed by the activated carbon may result in the formation of an oxide layer on the newly created titanium surfaces produced during milling, which prevents the reaction to form TiC. The above results demonstrate that controlled ball milling of titanium and carbon powder mixtures using a magneto ball mill can be used to produce nanocrystalline TiC and TiC-Ti powders. Such powders could be used for the synthesis of high-performance TiC reinforced titanium matrix materials.

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