DEFORMATION STUDIES OF y+a2 TITANIUM ALUMINIDES PROCESSED THROUGH REACTION SYNTHESIS

Abstract

Gamma Ti aluminde has been an important aerospace material due to its high temperature properties and lower density as compared to other high temperature materials. However, it has poor ductility at room temperature, which limits it processing and application. The alloy has been extensively studied and several methods have been applied to bring this class of alloy to industrial application. Alloy design and process design both were equally attempted and limited success have been reported. In the area of alloy design, Ti48Al with small addition of Cr, Nb and B has been noted to be promising and in the area of process design, reaction synthesis (RS) alongwith isothermal working is found to be a potential option. Considering all these, a base alloy (Ti48Al2Cr2Nb, at%) has been selected and studied in the present work. The alloy has been modified with small addition of boron and Ni-P coated boron (0.1 at%) to obtain the benefit of microstructural refinement. RS was selected to obtain uniform and homogenous alloy as compared to ingot metallurgy route. Effect of Ti particle size is included in the work by selecting two different sizes of particles. This study is attempted to understand the effect of particle size on reaction synthesis, homogenization and subsequent processing, through microstructure development. Calorimetry study has been carried out using non-isothermal differential scanning calorimetry (DSC) of various combinations of powder mixtures to examine the reaction of titanium aluminide formation. Reaction temperatures were noted to optimize the RS process parameters and phases formed due to transformation were analyzed. Deformation studies of the developed alloys were carried out through isothermal and near isothermal working under vacuum and ambient atmosphere. Hot isothermal compression tests at different strain rates and at different temperatures were conducted to evaluate the flow stress and to study the deformation behaviour of the alloys. Processing maps and constitutive equations were developed using hot isothermal compression test data. Finally, heat treatment response of all the deformed samples were studied to obtain lamellar microstructures through a designed heat treatment cycle based on the understanding of phase transformations. Characterization of the alloys was carried out through X-ray diffraction, optical microscopy, Scanning electron microscopy with EDAX and elemental mapping, Transmission electron microscopy, microhardness, hardness etc.