Microstructure control in Ti-6246
Creators: name: Ackerman, Abigail
Publisher: name: Imperial College London
Year: 2018

description: Titanium alloys are widely used in gas turbine engines due to their high specific fatigue strength and ability to retain that strength up to temperatures in the range of 600°C. The alloy Ti-6Al-2Sn-4Zr-6Mo is used in the intermediate pressure compressor of the gas turbine engine, with a yield strength of around 1.1GPa. It has a microstructure of primary α laths within a β matrix. Between the primary α are much finer secondary α laths, creating a basketweave microstructure. These laths are multioriented, which gives the alloy its supreme strength and fatigue properties, which make it a prime candidate for an gas turbine engine component material. Within this thesis, microstructural control in Ti-6246 is examined in detail. The effect of chemistry, temperature and processing is investigated using a wide range of advanced microscopy techniques, including high resolution scanning transmission electron microscopy (HR-TEM), focussed ion beam (FIB) and nano beam electron diffraction (NBED). In addition to these techniques, mathematical approaches to the scientific questions are applied and compared. The kinetics of primary α are first examined using electro thermal mechanical testing (ETMT) to calculate the velocity of primary α growth at a variety of temperatures. This is then compared to an isothermal model, based upon the concentration of molybdenum in the matrix, and is found to be in good agreement with experimental results, with a peak in growth 40K below the β transus. Furthermore, the analytical model is used to compare the growth velocity of other titanium alloy systems, such as Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo, to examine the effects of oxygen and vanadium on the kinetics of lamellar α growth. The concentration of molybdenum at the primary α interface is also studied, and an increased concentration of molybdenum is found at the interface using scanning transmission electron microscopy-electron dispersive x-ray (STEM-EDX) and atom probe tomography (APT). This is also described mathematically with a model based on Fick's second law and matches the experimental results well. The α/β interface is characterised via high resolution scanning transmission electron microscopy (HR-STEM), both in a clean primary α interface (no secondary α) and a sample that has been cold rolled to 10% of its original height. Semi-coherent interface steps, or disconnections, can be seen around 4.5 atoms in length. This step feature has been mathematically predicted by both the PTMC and topological models, which are in good agreement. Additionally, a centre of symmetry analysis has been completed, highlighting a 1nm thick region approaching the interface with contraction along the [10-10]α and elongation along the [-12-10]β as the α lattice gradually approaches the β lattice. In the deformed sample, larger, incoherent interface stepsare seen, connected by {110}β slip bands. Finally, the introduction of dislocations via both hot and cold rolling is achieved, stimulating nucleation from defects within the β matrix. This results in the formation of multi-variant fine scale secondary α phase precipitation. This has been observed in situ by HR-TEM and nano beam electron diffraction (NBED), used to map the evolution of strain over time. Growth of fine scale secondary α occurred along {110} slip bands, with compression along the growth axis and elongation perpendicular to this. The fine scale α has been indexed using transmission Kikuchi diffraction (TKD). A processing route has been developed using lab based methods, through this proved problematic due to increased oxygen content and cooling rate fluctuation. Samples of as received (AR), straight through lab processing (STR) and straight through route with the addition of a warm roll (STR + Edit) to encourage nucleation from dislocations have been processed. High cycle fatigue (HCF), low cycle fatigue (LCF), fatigue crack growth (FCG) rate and threshold have been measured and compared. The STR has the highest HCF strength, whereas STR + Edit exhibits lower than expected HCF and LCF, though the fatigue crack growth rate is improved.