Mechanically driven phase transformation in titanium and Ti alloys at high pressure torsion

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Severe plastic deformation (SPD) is increasingly intensively used for processing of nanostructured titanium and Ti-based alloys, at least on the laboratory scale. However, the SPD process does not only refine the microstructure, but it also initiates concomitant phase transformations. Depending on the experimental technique and on the pressure environment used, the phase transition of α-Ti to the high-pressure ω-Ti occurs between 2 and 12 GPa. Furthermore, external shear stresses can provide an additional driving force for martensitic phase transformation. In this respect, the application of SPD by high pressure torsion (HPT) is advantageous for studies of the α-Ti to ω-Ti transition, because this technique applies high pressure and shear strain simultaneously, and under well-controlled conditions. During the first funding period it was revealed that the HPT-induced phase transitions in Ti alloys depends on the amount alloying elements, especially of the elements stabilizing high-temperature β-Ti.The aim of this project is to investigate mechanically driven phase transformations in the Ti–Fe and Ti–Co alloys that were induced by HPT at elevated and cryogenic temperatures, in order to be able to describe the interplay between the diffusion processes and the martensitic (diffusionless) mechanism during the α → ω and β → ω phase transformations. Based on the results of the first funding period, we want to describe the effect of the microstructural features, in particular the effect of microstructure defects, on the stability of metastable phases, and to explain the relationship between the nanoscale structure achieved (local phase composition, nature of the grain and phase boundaries) and the resulting mechanical properties. Finally, a way for describing the stability of the metastable phases by using the CalPhaD method should be found. The microstructure and phase composition of the samples and the thermal stability of individual phases will be determined by using XRD (including in situ XRD at high temperatures), DSC, SEM, conventional and analytic TEM (including ACOM TEM), and Atom Probe Tomography. The influence of the initial state of the material (alloying, phase composition and microstructural features) and processing parameters (pressure, temperature, strain and strain rate) on the phase transformations will be evaluated. The experimental work will be complemented by the calculations of the high pressure phase diagrams. The proposed investigations will contribute to the basic knowledge regarding the mechanically driven phase transitions in Ti-based alloys. The obtained results will be important not only for fundamental materials science, but they will also allow to elaborate the principles of the thermo-mechanical treatments of ultrafine grained materials providing the high level of strength and ductility.