Thermo-Mechanical Processing of Third Generation γ – Titanium Aluminides
The γ – TiAl based intermetallics are characterized by unique combination of the properties which make these materials amenable for the application in gas turbines. However, due to problems related to limited plasticity and strong solidification texture, the application of these materials is limited to low pressure turbine regions. In order to utilize the full potential of these materials, new chemistries have been developed, which are currently known as the third generation γ titanium aluminides. Further it is desirable to optimize the processing conditions of these newly developed materials, and provide a fundamental understanding of processing-microstructure relationship. The present thesis deals with examining the thermo-mechanical processing response of third generation titanium aluminides with nominal compositions Ti-45Al-5Nb-0.2B-0.2C and Ti-45Al-10Nb-0.2B-0.2C. The study involves through-processing microstructural modification and characterization. The entire work consists of four parts. In the first part, the solidification microstructure and texture of the cast material and the modification brought due to hot isostatic pressing (HIPping) has been investigated. A clear comparison has been brought out in terms of niobium content of the alloys. Further, these alloys undergo complex phase transformations during heating and cooling. In the second part of the study, the microstructural changes during phase transformation have been investigated for both the alloys along with the oxidation behavior. The difference in terms of Nb content leads to differences in the transformation temperature as well as in oxidation response. In the third part of the study, thermo-mechanical processing (TMP) of the cast plus HIPped alloys has been investigated by plotting strain rate sensitivity maps. The results indicate that processing is easier in the domain of α phase, where in higher deviation from the orientation relationship between the two constituent phases have been observed indicating the occurrence of dynamic recrystallization. In the last part, an alternative route for processing has been followed by including a pre-deformation heat treatment cycle to spheroidize the lamellae by the thermal cycling. Such a microstructural feature is expected to provide a more amicable and broader processing domain. The materials with prior spheroidised microstructure were subjected to thermo-mechanical processing and a validation of the hypothesis has been carried out by plotting the strain rate sensitivity maps. The predictions of the maps have been validated by examining the microstructures of the as-deformed materials. Different possible pathways have been proposed for tailoring suitable microstructures that would enable to design the appropriate processing schedule for the third generation TiAl base intermetallics. The thesis has been divided in to eight chapters. The first three chapters (chapters 1-3) are dedicated to the introduction, a detailed review of the relevant literature and the experimental methodologies followed in the present investigation. The investigations and the outcomes pertaining to the present thesis, as described above, are detailed in the next four chapters (chapters 4-7). The overall outcome of the thesis is presented in chapter 8, along with the recommendation for future investigations.