Effect of thermo-mechanical treatment on the evolution of microstructure and mechanical properties in a β and a β rich α+β titanium alloy

Abstract

Near β and metastable β-titanium alloys are difficult to process due to narrow processing window. Lower processing temperature causes difficulty in recrystallization of these alloys especially in the two phase conditions, which plays an important role in achieving microstructural control for the desired mechanical properties. Furthermore, most of the studies in this field are either in single phase β or about the globularization behavior of mostly α-laths in the (α+β) regions. Hence, there is dearth of studies on the microstructural state and role of the β-phase during deformation in these conditions. Therefore, aim of this thesis was to address the aforementioned gaps in the literature pertaining to recrystallization behavior of α and β phases during hot deformation of β-titanium alloys in (α+β) phase field.

To achieve the above mentioned goals in the present work, a metastable β titanium alloy, Ti10V-2Fe-3Al, and a near β titanium alloy, SP-700, subjected to uniaxial compression tests in (α+β) regions as a function of temperature and strain rates with lamellar starting microstructure to systematically study the development of microstructures and textures in the dynamic conditions. Subsequently, the alloys were hot rolled up to 90 pct. in the (α+β) region followed by static annealing as a function of temperature and time to elucidate the microstructural and textural evolution at length. Additionally, fatigue crack growth (FCG) of Ti-10-2-3 and, superplastic behaviour of SP-700 alloys were also evaluated as characteristic properties of these alloys. EBSD analysis was extensively employed to substantiate the aforementioned microstructural developments.

While kinking and bending is the dominant mechanism in α phase; CDRX grains are formed in the β-phase of both the alloys at lower temperature and at lower strain rate. The best processing conditions were identified for DRX of α and β phases through the strain rate sensitivity maps for both the alloys. Signatures of the deformed texture are witnessed in α and β phases of both the alloys in all the annealed conditions. Grain boundary sliding was found to be one of the dominant deformation mechanisms during superplastic testing.