Ordering and the deformation mechanisms of Ti-Al alloys
Creators: name: Radecka, Anna
Publisher: name: Imperial College London
Year: 2015

description: The fatigue behaviour of near-alpha and alpha/beta titanium alloys depends strongly on the microstructure, the environment and the load cycling conditions. It is known that Ti-Al alloys can suffer from a chemical decomposition on ageing above 500C or air cooling which reduces fatigue life. The factors promoting the ordering, as well as its effect on fatigue crack nucleation and propagation are still not sufficiently understood. This project aims to provide an improved understanding of how ordering affects the extent of planar slip occurring during fatigue in the a phase of aluminium-alloyed titanium alloys. For this purpose, samples of Ti-7Al, Ti-6Al-4V and IMI 834 were aged to promote ordering and the formation of nanometre-scale Ti3Al, at a range of temperatures between 450-700C for times up to 84 days. Ti-7Al wt.% (12 at.%) was selected as a model of the a phase in Ti-6Al-4V which has also been studied in the literature. A reliable method for producing the alloy with the desired microstructure has been established. The effects of post-rolling heat treatment on ordering of Al were analysed using light microscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). The evolution of intergranular lattice strain was characterised using in situ time-of-flight (TOF) neutron diffraction during room temperature tensile loading. On ageing, at 550C and 625C, dislocations were observed to travel in pairs, and in planar arrays, which has been attributed to the presence of ordering. A slight change in c/a was observed, from 1.6949 to 1.6945, and a slight increase in the macroscopic modulus. However, no changes were observed in the residual lattice strains, which are the grain orientation average elastic strains produced by localised plasticity. Therefore it is inferred that the changes in deformation mechanisms caused by ordering that result in an enhanced vulnerability to dwell fatigue affect primarily the extent of slip localisation. At long ageing times, a chemical decomposition results in the formation of Ti3Al precipitates in the a phase of alpha/beta titanium alloys. At reduced times or elevated temperatures, diffuse electron or neutron diffraction peaks can be observed, indicative of short range ordering (SRO). Here, correlative TEM and atom probe tomography (APT) results are presented showing that the reaction proceeds through the formation of ordered Al-rich clusters in Ti-6Al-4V. In addition, the V-containing a phase of Ti-6Al-4V formed ordered clusters much faster than the binary alloy Ti-7Al. This implies that the ternary addition of b stabilisers exacerbates the problem of Ti3Al precipitate formation in commercial dual phase titanium alloys. Precipitate evolution was also studied in the a phase of IMI 834 using TEM after ageing at 550, 625 and 920C for times up to 28 days. The TEM investigation was complemented by APT to give compositional information. It is found that precipitation occurs much faster, and is more prolific in samples that were heat treated at higher temperatures. Additionally, a change in precipitate morphology was observed. Particles were spherical after ageing at 550C while after ageing at 625C become ellipsoids with the major axis lying close to the [0001] direction. In summary, it was found that precipitation of the ordered alpha2 phase appears to occur much faster, and in greater proportions, in multi-component Ti alloys than in the binary alloy. Additionally, the appearance of superlattice spots was much clearer, and occurred after shorter ageing times, in Ti-6Al-4V and IMI 834 compared to Ti-7Al, despite all the alloys having a similar Al content in the a phase. This indicates that the addition of the ternary elements to binary Ti-Al system accelerates the formation of the alpha2 phase.