Titanium in the Gas Turbine Engine

Abstract

The development of the gas turbine engine over the past 60 years has been mirrored by the success of the titanium industry, with a clear symbiant relationship existing between the two industries. Immediately apparent in the early days of the evolution of the gas turbine was the need for a material which could provide the strength required for component operation, whilst at the same time providing a low enough density to allow for successful flight applications. Whilst aluminium based alloys offer an excellent strength to weight ratio, their operation is limited to temperatures below approximately 1300C, reducing possible applications within the gas turbine to a minimum. 300 series stainless steels offer a similar strength to most conventional titanium alloys, but come with a significant density penalty of over 50% and, whilst offering reductions in cost, do not provide significant benefits in terms of operating temperatures. Titanium however, has long been viewed as having a desirable balance of properties for applications towards the front end of the gas turbine engine (i.e. fan discs/blades, compressor discs/blades, along with other smaller components). Titanium has a density of 4.5g/cm3 (which, apart from a limited number of alloys such as Ti811, does not vary significantly in alloys considered for aerospace applications) which is higher than aluminium, but lower than nickel and steel alloys. Titanium is allotropic with a HCP lattice ( phase) stable to 8820C, transforming to a BCC (β phase) lattice above this temperature. Alloying elements act to stabilize either of these phases (Al, Sn for example stabilize the alpha phase, whereas Mo, V, Cr stabilize the beta phase) meaning that the transformation temperature can be altered, and subsequently the proportions of each phase existing at room temperature can be varied. The morphology of these phases may however vary, dependent on the process history, with alpha phase material being classed as primary alpha (persisting during heat treatment in the  phase field) or secondary alpha (structures arising from the → phase transformation). This allows for the development of a range of bimodal microstructures which provide titanium alloys with inherent strength and also allows for further refinement of properties through various heat treatment and processing regimes. For example designers requiring creep strength and good elevated temperature properties may choose to opt for alloys with more alpha stabilizers (alpha or near alpha alloys), whereas metastable beta alloys, which are heavily beta stabilized offer improved forgeability. Alphabeta alloys contain a more balanced mix of stabilizers and are widely used due their balance of properties. Ti6-4 (Ti-6Al-4V) for example has been a stalwart of the titanium industry since the 1950s due to its good weldability, relatively high strength and good fatigue properties.