ABSTRACT

In the present work, metastable β Ti alloy Ti - 5Al - 5Mo - 5V - 3Cr (Ti 5553) and its derivative alloy Ti - 5Al - 3.5Mo - 7.2V - 3Cr (Ti 5373) with same Mo equivalent value of 8.15 were subjected to three different heat treatments to obtain varying volume fraction of α in the β matrix. In the first part of the work, workability limit diagrams of the heat treated specimens were plotted using the data from upsetting experiments on cylindrical specimens at room temperature. In the second part, hot deformation behaviour of these alloys with different initial microstructures were investigated by hot compression testing. The activation energies for deformation for all conditions were determined, and the material constants for hot deformation were found out. A model for the prediction of flow stress at strain of 0.5 in terms of Zener - Hollomon parameter for all cases has been established. The flow curves and processing maps for these alloys with different initial microstructure conditions were generated from the compression test data. The optimum hot workability domains and regions of instability were determined and microstructural characterization of hot compressed samples at different temperature and strain rate was carried out to validate these observations. The Cockroft and Latham criteria, Brozzo et al. criteria and the Oh et al. criteria can be used for both the base and deriviative alloy system as valid workability or ductile fracture criteria. However, the Freudenthal criteria cannot be used as a ductile fracture criterion for these alloys. From the flow curves generated from high temperature testing, the activation energies for deformation were found to be 230 kJ/mol, 206 kJ/mol and 220 kJ/mole for Ti 5553 with initial α + β microstructure, Ti 5333 with largely β microstructure and Ti 5373 with initial α + β microstructure respectively. From the processing maps generated, wider optimum processing range was identified for Ti 5553 with largely β initial microstructure (temperature range of 750oC to 950oC and strain rates of 0.001 to 0.1 s-1) compared to Ti 5553 with α+ β initial microstructure (temperature range of 800oC to 950oC and strain rates of 0.001 to 0.1/s).