Grain Refinement of Cast Titanium Alloys

β-grain size is an influential microstructural parameter on the properties of titanium components. A reduction of β-grain size is generally associated with improvements to ductility, strength, corrosion and fatigue resistance of many α, α/β and β titanium alloys. During production of wrought titanium components, the β-grain size is carefully controlled during thermomechanical processing but there is currently no control of the β-grain size during solidification of cast components. As such, this inability to control the β-grain structure during solidification may limit the applications for solidification based technologies including casting, welding and direct metal deposition. Due to the limited knowledge of grain refinement practices and the lack of commercial grain refiners for the titanium system, this thesis investigates the mechanisms of β-grain refinement during solidification of cast titanium alloys. In this thesis, generalized theories for grain refinement that have been developed from research into other metallic systems are applied to the titanium system. Similar to the findings from aluminium and magnesium research, it is shown that grain refinement of cast titanium alloys requires the addition of growth restricting solutes which provide constitutional undercooling as well as the presence of potent nucleant particles. It is demonstrated that commercially pure titanium contains a natural distribution of nuclei particles which may originate from the mould wall and when powerful growth restricting solutes are introduced, significant prior-β grain refinement is achievable. All solutes investigated do not interact or poison the naturally occurring nucleants enabling the grain size of the titanium alloys to be predicted by an empirically determined relationship based on the growth restriction factor. A full list of growth restriction factors for various elements in titanium is determined and it is proven that growth restriction theory is valid in the titanium system. A further reduction in β-grain size is achievable by introducing additional nucleant particles to titanium castings in conjunction with growth-restricting solutes. Using a novel technique, titanium powder was introduced to the melt stream prior to solidification and was mixed throughout the liquid. The powder particles partially melted and the oxide surface layer dissolved allowing intimate substrate-liquid contact, enabling the titanium substrates to act as sites for heterogeneous nucleation. Using this technique, it was possible to grain refine commercially pure titanium without foreign elemental addition and when growth restricting solutes were present it was possible to obtain approximately an order of magnitude grain size reduction. The results and concepts developed from this work may aid the future development of a commercial grain refiner for titanium. If a grain refiner is developed, its application will not just be limited to the titanium casting industry but may also benefit other solidification based technologies such as welding, direct metal deposition and wrought billet production