Aspects of microstructure and texture evolution in the near-u, titanium alloy, Timetal834 were investigated. The electron back scatter diffraction (EBSD) technique was used extensively to acquire microtexture data. To aid analysis of this data, a computer program was written in Visual Basic .NET. The two main functions ofthe program were: (1) to separate the two forms of u phase, primary-u (up) and secondary-u (us) that are present in bimodal microstructures at room temperature, and (2) to reconstruct the parent P phase, which transforms to Us on cooling. The P reconstruction was validated using a metastable titanium alloy, which retains enough P phase at room temperature to allow orientations to be measured directly using EBSD. Parent P orientation maps were successfully reconstructed for several alloys, cooled from above and below the P transus, at a range of cooling rates. The limitations of the method are discussed. Timetal 834 and other titanium alloy forgings can contain regions of closely aligned up grains called macro zones that can have a deleterious effect on fatigue performance. Understanding the origin of macro zones and how they change during processing is an important step in optimising process routes for these alloys. A detailed characterisation was carried out on a slice from a Timetal 834 billet using optical metallography, EBSD and neutron texture analysis. Results showed there was a variation in texture and macro zones through the billet, related to the ingot to billet forging process. The effect of cooling rate on the p~us transformation in a bimodal microstructure was investigated using a Jominy' end quench test. At the highest cooling rates, a martensitic microstructure was observed. At lower cooling rates, a diffusional transformation to WidmansHitten Us, allotriomorphic u at PIP grain boundaries and growth of the existing up grains was observed. Variant selection in Widmanstatten Us was observed at PIP boundaries with specific misorientations and at uplP boundaries with a specific orientation relation. Forging to produce a bimodal microstructure was simulated using laboratory plane strain compression tests at 101OoC. The majority of the strain was accommodated in the P phase. The P phase undergoes dynamic recovery resulting in a subgrain structure. At strain rates above 0.18s-1 , selective recrystallisation in the P phase occurred in the vicinity of PIP grain boundaries and up grains. The up grains deform by slip and additionally by deformation twinning at strain rates above 1.8s-1 Some localised dynamic recrystallisation was observed but the majority of the up phase remained unrecrystallised. Macrozones remained at a strain of 0.63 but were less apparent at a strain of 1.23.

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Titanium alloys are often used in aerospace applications in which they are subjected to cyclic loading. It has been found that the addition of a hold at peak load greatly decreases the fatigue life of these alloys. This is referred to as "cold-dwell fatigue". In order to improve component life prediction, it is desirable to understand the mechanisms leading to initiation of fatigue cracks. Due to the high anisotropy of a-titanium, it is expected that deformation on a microstructural scale will he significantly heterogeneous. To understand the process of crack nucleation, the mecha- nisms of this deformation must first he understood. This thesis will present a series of experiments which will attempt to give some insight into this process. Two main lines of inquiry have heen undertaken. The first method uses electron hackscat- tered diffraction (EBSD) to obtain the crystal orientations within a selected region. Optical mi- croscopy and digital image correlation methods were then used to track the accumulation of strain in the region during deformation. Results were obtained for monotonic tension, cyclic fatigue, and cold-dwell fatigue. Behaviour in local strain peaks differed hetween loading modes, hut strain concentrations in the range of 2-4 times mean strain were typical. The second method uses the cross-correlation based EBSD technique developed by Wilkinson et al [1] to derive the dislocation densities present in a crystal. By tracking the changes in dislocation density after different deformation regimes, some insight can be gained into the local deformation processes that produced the dislocation densities. Strong links were found between the local crystallography and inhomogeneities in strain and dislocation density accumulation. In particular, interactions between grains of differing orientations was found to have a strong impact on deformation behaviour. Additionally, a link was found between high strain accumulation and crack initiation in both fatigue and dwell fatigue.

This work characterises microstructures and textures of welds in Ti-6A1-4V produced using a range of fusion welding methods, and uses the results to discuss solid state transformations and microstructure formation in titanium alloys in general. Welded samples are particularly useful for this, because of the wide range of cooling rates exhibited by welds produced by different techniques.

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No description available.

Two gamma-TiAl alloy groups were the subject of this investigation. From the first group, Ti-40Al-5Mn and Ti-40Al-10Mn alloy ingots, produced by cold-hearth plasma arc melting were characterised using light and electron microscopy and XRD. Their dendritic microstructures comprised of alpha2/gamma lamellar areas and single-phase gamma, beta and Laves (Mn2Ti) grains or precipitates. The lamellar alpha2/gamma Volume fraction was less in the Ti-40Al-10Mn alloy, due to the influence of its higher Mn content. The alloys were heat treated at 1200°C for 4 hours, resulting in nearly lamellar and duplex microstructures in the Ti-40Al-5Mn and Ti-40Al-10Mn alloys, respectively, hi both cases, the lamellar colonies contained poorly formed, discontinuous alpha2 lamellae. Raising the heat treatment temperature to 1300°C provided both alloys with fully lamellar microstructures with well formed, linear and continuous alpha2 lamellae. From the second alloy group, the Ti-47Al-4(Nb, Mn, Cr, Si, B) alloy known as gamma-TAB, was produced as hot isostatically pressed billets. Its microstructure comprised of some "feathery" lamellar colonies and single phase gamma-matrix. Unique phases based on Ti-B and Ti-Si were identified as being TiB and Ti5Si3, respectively. The boride stringers were profusely spread in the microstructure, whereas the Ti5Si3 was very elusive and found only once in TEM analysis. Heat treatment at 1400°C for 6 hours resulted in a fully lamellar Ti-47Al-4(Nb, Mn, Cr, Si, B) alloy with wide interlamellar spacing and greater gamma lamellae volume fraction; however the lamellar morphology was generally linear and continuous. In order to assess microstructural and lamellar morphological stability of the fully lamellar alloys, isothermal ageing at 800°C for durations of 500, 1000, 1500 and 2000 hours was conducted. Major microstructural changes took place in the Ti-Al-Mn alloys, whereby Ostwald ripening of alpha2 agglomerates and beta grains in the early stages, gave way to Laves phase lamellae and grains in the latter stages of ageing. This was more extensive in the Ti-40Al-10Mn alloy due to phase equilibrium requirements, whereby more of the alpha2/gamma lamellar content was eliminated. However, the specific resilience of alpha2 lamellae was inferior in the Ti-40Al-5Mn, as greater serration, fragmentation and termination migration within lamellar regions was witnessed. Termination migration within lamellar regions was considered as being independent to termination migration of the colony boundaries, which was distinguished as recession or contraction of the lamellar region, with an equivalent extension of the gamma-matrix. Boundary splitting was not precisely seen, but must have been the precursor to termination migration. In the Ti-47Al-4(Nb, Mn, Cr, Si, B) alloy, no Ostwald ripening of alpha2 agglomerates or formation of beta or Laves phases was observed. The gamma-matrix did progressively extend to eliminate alpha2/gamma lamellar grains to some degree, and gamma lamellae coarsened profoundly. Fragmentation and termination migration of alpha2 lamellae was more prominent after 1500 hours ageing. Mean interlamellar spacing (lambda) and coarsening ratio (r) increased the most in the Ti-40Al-5Mn alloy, due to the extent of alpha2 lamellar fragmentation and termination migration, followed by the Ti-47Al-4(Nb, Mn, Cr, Si, B) and Ti-40A1-10Mn alloys, respectively. Modes of morphological evolution that had not taken place during thermal ageing in all alloys studied, were direct cylinderisation, edge spheroidisation and discontinuous coarsening. Overall, the Ti-47Al-4(Nb, Mn, Cr, Si, B) alloy provided the best model for development of gamma-TiAl alloys with enhanced morphological stability, if issues such as the low alpha2 phase volume fraction and coarse gamma lamellae could be resolved. The role of silicon in gamma-TiAl alloys also needs to be examined, and whether it has any bearing on lamellar morphological stability.

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No description available.

Titanium alloys are strong candidates for the aerospace industry and biomaterial applications because of their low density, high strength-to-weight ratio and very high strength even at temperatures up to 600°C. Like many other engineering alloys, titanium alloys are prone to strong preferred crystallographic orientation development during thermomechanical processing. Part of the titanium processing route is to heat treat the material above the β transus for the purpose of homogenization and associated phase transformation. This heat treatment dramatically affected the microstructure and texture evolution. Theoretically, such heat treatment should result in a nearly random texture if all variants during α→β→α phase transformation are active. In reality, significant textures are observed after such a heat treatment process. The present project aims at developing a detailed understanding of the root cause for this relatively strong texture by means of EBSD and in-situ neutron diffraction studies. The effect of β grain growth on variant selection during β to α phase transformation has been investigated by using two variants of Ti-6Al-4V with and without 0.4 wt% yttrium addition. The aim of adding yttrium was to control β grain growth above the β transus by pinning grain boundaries with yttria. Both materials were first thermomechanically processed to generate a similar starting microstructures and crystallographic textures. Subsequently, both materials were solution heat treated above the β transus followed by slow cooling to promote growth of the α lath structure from grain boundary α. Detailed EBSD and in situ neutron diffraction analysis were carried out to study microstructure and texture evolution. The variant selection calculation suggests that more variant selection occurred in convectional material with a large grain size compared to material with yttrium addition. In situ measurements showed that β texture strengthened significantly above the β transus with increasing β grain size. There was no significant variant selection during α→β transformation; variant selection noticeably increased during β→α transformation with increasing β grain size. Additional interrupted cooling experiments followed by EBSD analysis showed early nucleation of α variants with a 'butterfly morphology' from β grain boundaries that have a pair of β grain with a common <110> pole. These observations suggest reduced nucleation energies for α formation in such circumstances allowing extensive growth of these α variants into unoccupied β grains making it a dominant variant. The influence of rolling temperatures (i.e. at 800 ºC and 950 ºC) to produce different starting texture, on texture evolution and variant selection during α→β→α transformation was also investigated. Laboratory X-ray, EBSD and in-situ neutron diffraction texture analyses were carried out. Even though the transformation texture is stronger at 800 ºC, the degree of variant selection is stronger in materials rolled at950 ºC compared to material rolled at 800 ºC. Here, the enhanced variant selectionfor the material rolled at 950 ºC was related to the different β texture. It is suggested that the combination of a particular β texture components promote variant nucleation that can increase the likelihood of having β grain pairs with a common <110> pole.