Thermo Mechanical Processing of Third Generation γ Titanium Aluminides

Abstract

The and#947; 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 and#947; 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 and#61543; 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 and#945; phase, where in higher deviation from the orientation relationship between the two constituent phases have been observed indicating the occurrence of dynamic recrys