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Title: Alloy and process development in cast TiAl-based materials
Author: Huang, Aijun.
Awarding Body: University of Birmingham
Current Institution: University of Birmingham
Date of Award: 2005
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A new quenching and ageing treatment to refine microstructures of a number of cast .TiAI-based alloys has been assessed. This has been done by using Jominy end quenching, to better understand the continuous cooling transformation behaviour of the different alloys, followed by an investigation of the response of the transformed samples to ageing in optimally quenched samples. Ti-46AI-8Ta, Ti-46Al-8Nb, Ti-46AI-5Nb-W, Ti-47AI-2Nb-lMn-lW-O.2Si and Ti-48AI-2Nb-2Cr, have been investigated and the importance of boron content, grain size and oxygen content have been investigated for Ti-46AI-8Nb. It has been found that heavy elements such as Nb and Ta broaden the range of cooling rates over which a fully massively structure can be obtained. The addition of boron leads to the formation of fully lamellar structures over a wide range of cooling rates by suppressing the massive transformation, the feathery and the Widmanstatten transformation. The prior a. grain size also suppresses the massive transformation independent of whether the grain size was achieved by heat treatment or by addition of boron. It has been found that the responses of both high and low oxygen-containing samples are similar at high cooling rates, but are very different at intermediate and low cooling rates. The amount of massive gamma which appears to be nucleated away from the original alpha boundaries is also strongly influenced by cooling rate and oxygen content. It is proposed that the low diffusivity of such heavy elements retards diffusion-controlled phase transformations (feathery and lamellar) so that the partitionless massive transformation can take place at lower cooling rates. In fine-grained samples the dominance of the lamellar microstructure is interpreted in terms of the role of grain boundaries in nucleating lamellae at temperatures above the massive start temperature. The complex role of oxygen is interpreted in terms of the ·stabilisation of a. by oxygen and by the extent of the segregation of oxygen during quenching. The observations made using the Jominy end quenching technique led to the selection of salt bath quenching as the optimum technique to develop crack-free fully massively transformed samples and these have been used to investigate the influence of different ageing schedules (carried out using the HIP (hot isostatic pressing) cycle used for • structural castings) on the microstructure and properties of quenched and aged samples. It has been found that the formation of subgrains during the massive transformation, precipitation of a on the sub grain boundaries and precipitation of a on the four {Ill} planes of the gamma within these sub grains during ageing lead to \:onsiderable refinement of microstructures. The influence of the ageing treatment has been studied by ageing at different temperatures within the two phase field and by two step ageing at low and high temperatures. It has been found that the finest microstructures are obtained by low temperature ageing followed by short, higher temperature ageing. The influence of the ageing sequence on the microstructures is explained in terms of the factors influencing the nucleation and growth, of a precipitates at different ageing temperatures. Room temperature tensile tests have been carried out on salt bath quenched samples aged using either one step or two step ageing. It has been found that one-step ageing brings a more significant improvement in room temperature tensile properties than two-step ageing, where the best properties are found in samples aged at low temperatures, followed by a short excursion into the single a phase field. Two separate appendixes, studying mechanism of Widrnanstiitten transformation and embrittlement of TiAl-based alloys after exposure at service temperatures are given at the end of the thesis.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available