Abstract
It is well known that severe plastic deformation not only leads to strong grain refinement and material strengthening but also can drive phase transformations. A study of the fundamentals of α → ω phase transformations induced by high-pressure torsion (HPT) in Ti–Nb-based alloys is presented in the current work. Before HPT, a Ti–3wt.%Nb alloy was annealed at two different temperatures in order to obtain the α-phase state with different amounts of niobium. X-ray diffraction analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were applied for the characterisation of phase transitions and evolution of the microstructure. A small amount of the β-phase was found in the initial states, which completely transformed into the ω-phase during the HPT process. During HPT, strong grain refinement in the α-phase took place, as did partial transformation of the α- into the ω-phase. Therefore, two kinds of ω-phase, each with different chemical composition, were obtained after HPT. The first one was formed from the β-phase, enriched in Nb, and the second one from the α-phase. It was also found that the transformation of the α-phase into the ω-phase depended on the Nb concentration in the α-Ti phase. The less Nb there was in the α-phase, the more of the α-phase was transformed into the ω-phase.
Highlights
Transmission electron microscopy (TEM) studies were carried out using a TECNAI G2 FEG super TWIN (200 kV) (FEI, Hillsborough, OR, USA) with an energy-dispersive X-ray (EDS) spectrometer produced by EDAX (AMETEK, Inc., Berwyn, PA, USA)
The study of selected area, electron diffraction patterns (SAED), obtained by transmission electron microscopy (TEM), made it possible to identify precipitates of the second phase as a β-phase surrounded by the α-matrix (Figure 1c)
Measurement of the chemical composition by means of EDS during the TEM study showed an increase of niobium content in the α-matrix from 2.2 to 3.0 wt.%, with an increase of the annealing temperature from 450 to 600 ◦ C
Summary
It is well known that mechanical properties, especially strength, can be significantly improved by applying severe plastic deformation (SPD) as a result of the increased density of crystal lattice defects and strong grain refinement reaching ultrafine size or even the nanometer scale [6,7]. In this context, it became possible to strengthen ternary and quaternary Ti-based alloys with niobium as a β-stabilizer, using SPD techniques such as equal channel angular pressing (ECAP) or high-pressure torsion (HPT) [8,9,10,11]. SPD is quite effective for stimulating allotropic transformations, for example, in titanium, zirconium or hafnium alloys which possess a high-temperature body-centred cubic Im3m β-phase, a low-temperature hexagonal close-packed crystal structure P63/mmc α-phase and a high-pressure hexagonal P6/mmm ω-phase [1,18,19]
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