Kristína Václavová

Evolution of Microstructure and Microhardness in Ti-15Mo β-Ti Alloy Prepared by High Pressure Torsion

The main aim of this study is to analyze the effect of the severe plastic deformation (SPD) on the mechanical properties and defect structure of metastable beta Ti alloys. Experiments were performed on two different β-Ti alloys: Ti-15Mo and Ti-6.8Mo-4.5Fe-1.5Al which were subjected to severe plastic deformation (SPD) by high pressure torsion (HPT). The increase of hardness with increasing equivalent strain was determined by microhardness mapping. Dislocation density was studied by advanced techniques of positron annihilation spectroscopy (PAS). Microhardness and dislocation density increases with increasing equivalent strain inserted by severe plastic deformation.

Ultra-fine grained microstructure of metastable beta Ti-15Mo alloy and its effects on the phase transformations

Processing of metastable titanium alloys by severe plastic deformation provides an opportunity to achieve exceptional grain refinement, to enhance the strength and to affect phase transformations occurring during thermal treatment. The main aim of this study is to investigate the microstructure of ultra-fine grained (UFG) material and effect of microstructural changes on phase transformations in metastable β-Ti alloy Ti-15Mo. Metastable β-Ti alloys are currently the most studied Ti-based materials with prospective use in medicine. Ti-15Mo alloy after solution treatment contains metastable β-phase. Metastable ω-phase and stable α-phase particles are formed upon annealing,. Solution treated Ti-15Mo alloy was deformed by high pressure torsion (HPT) at room temperature. Severely deformed structure after HPT with grain size of ~200 nm was studied by transmission electron microscopy. In-situ electrical resistance measurements showed significant changes in undergoing phase transformations when compared to coarse-grained (CG) material. Scanning electron microscopy revealed heterogeneous precipitation of α-particles at grain boundaries (GB). Due to the high density of GBs in UFG structure, these precipitates are very fine and equiaxed. The study demonstrates that SPD is capable of enhancing mechanical properties due to grain refinement and via affecting precipitation processes in metastable β-Ti alloys.

Microstructure evolution in ultrafine-grained Ti and Ti-6Al-7Nb alloy processed by severe plastic deformation

Unique in-situ electric resistivity measurement was utilized to identify microstructural changes in ultra-fine grained commercially pure titanium and Ti-6Al-7Nb alloy. Both materials were prepared by equal channel angular pressing. Changes in resistivity evolution during in-situ heating were compared to scanning electron microscopy observations. Both materials are stable up to 440°C. Further heating at rate 5°C/min causes recovery and recrystallization of UFG structure. At 650°C the microstructure is fully recrystallized. High resolution in-situ electric resistivity measurement is capable of detecting recovery and recrystallization in UFG CP Ti and Ti-6Al-7Nb alloy.