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ABSTRACTβ-21s titanium is a recently developed aerospace grade titanium alloy and is attractive for jet engine nacelles, exhausts and nozzle assemblies due to its high strength and superior resistance to oxidation and embrittlement from aircraft fluids such as Skydrol. Its metastable β composition upon solution treatment permits deep hardenability and its ageing response is studied in depth at high temperatures. However, its response at low-medium temperatures is insufficiently characterised in literature. Throughout this thesis, the decomposition mechanics, hardening response and microstructural changes from isothermal ageing is studied at two different temperatures, 350°C and 500°C. Microhardness tests performed on the sheet thickness face were performed to characterise the age hardening response, while microstructural observations were captured using optical microscopy and Scanning Electron Microscopy (SEM). It was found that ageing at the two different temperatures resulted in substantially different properties which stem from the decomposition process of the metastable β phase. At 500°C ageing, heterogeneous nucleation of α on lattice defects The mechanism by which the stable α state is reached from ageing this alloy is of great practical importance as mechanical and microstructural properties are heavily effected; particularly the embrittlement caused by 350°C which poses catastrophic risks for aerospace applications. ACKNOWLEDGEMENTS