Characterising the microstructure and properties of thermally aged aerospace β-21s titanium alloy, Ti - 15Mo - 3Nb - 3Al - 0.2Si

Abstract

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

β21S titanium alloy heat treatment development and improvement

“…The peak hardness condition of an alloy is essential to be aware of when testing or heat-treating a material. Work from past UQ students Hagan and Campbell [20,21], were incorporated into the broader study for Dr.…”

“…Due to this beta phase breakdown inFigure 12(c), the average interparticle spacing has clearly decreased with the high temperature aging application. This has therefore decreased the alloy's ability to lock dislocation motion, and in turn decreases the hardness in the material [20]. This is supported further by the thick, dark alpha lines scattered throughout the microstructure, and the minimal beta phase represented as white spaces.…”

“…The clear variation between this sample's microstructure andFigure 12 (a) and (b) is the increased coarsening of the alpha precipitates; whilst alsoshowing a lack of, and therefore decomposed beta grain boundaries. The larger alpha precipitates in the alloy early in the heat treatment process begins to consume the smaller precipitates to reduce the interfacial energy of precipitates and achieve a more stable state [20]. Due to this beta phase breakdown inFigure 12(c), the average interparticle spacing has clearly decreased with the high temperature aging application.…”