EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

It is known that fine-grained and equiaxed microstructure of two-phase titanium alloys enables superplastic behavior, which is the ability of a material to exhibit large plastic deformation at high temperature and a specific strain rate without necking. One of the main challenges of superplastic forming is to improve its technological and financial efficiency by lowering the forming temperature and reducing the forming time. One way of improvement is to optimize the microstructure (as grain refinement and/or spatial distribution of the β phase). In fact, different strain and accommodation mechanisms may be involved during superplastic forming depending on alpha grain size and its fraction, the temperature, the strain rate in the case of Ti-6Al-4V alloy. This study focuses on the mechanical behavior and the microstructural evolution for a wide range of temperatures (750°C-920°C) and strain rates (10-2s-1-10-4s-1) of a Ti-6Al-4V alloy with two different alpha grain size (i.e. 0.5 and 3µm). The microstructural evolutions were tracked using the synchrotron radiation facilities during thermomechanical loading. In situ experiments were accompanied by SEM observations and EBSD analyzes for thermomechanically loaded samples at different strain values. The phase amounts, the d-spacing and the FWHM variations were determined by Rietveld refinement during different thermomechanical loading. The complementarity of characterization tools clearly highlighted changes in the microstructure depending on the conditions (temperature, strain rate and initial alpha grain size) with a decrease in the number density of grain size (coarsening) and a change in the morphology and the texture. In addition, the combined analysis of d-spacing and FWHM has shown the deviations from linearity, which amplitude depends on the conditions. These variations will be analyzed in regard of internal stress relaxation or changes of chemical composition.