Abstract
The pressure influence on the α → ω transformation in Ti–Co alloys has been studied during high pressure torsion (HPT). The α → ω allotropic transformation takes place at high pressures in titanium, zirconium and hafnium as well as in their alloys. The transition pressure, the ability of high pressure ω-phase to retain after pressure release, and the pressure interval where α and ω phases coexist depend on the conditions of high-pressure treatment. During HPT in Bridgeman anvils, the high pressure is combined with shear strain. The presence of shear strain as well as Co addition to Ti decreases the onset of the α → ω transition from 10.5 GPa (under quasi-hydrostatic conditions) to about 3.5 GPa. The portion of ω-phase after HPT at 7 GPa increases in the following sequence: pure Ti → Ti–2 wt % Co → Ti–4 wt % Co → Ti–4 wt % Fe.
Highlights
Titanium and titanium alloys are attractive materials for various applications due to their high strength-to-density ratio, excellent corrosion resistance, and good biocompatibility [1]
Our results demonstrate that the addition of cobalt decreases the starting pressure of α/β → ω transformation in comparison with pure titanium
The presence of shear strain during high pressure torsion (HPT) decreases the onset of the α → ω transition in pure Ti from 10.5 GPa to about 3.5 GPa
Summary
Titanium and titanium alloys are attractive materials for various applications due to their high strength-to-density ratio, excellent corrosion resistance, and good biocompatibility [1]. The structure and properties of titanium alloys can be tailored by using the combination of alloying, as well as thermal and mechanical treatments [1]. They are especially effective because titanium has different allotropic modifications. The crystal structure of ωTi is hexagonal (space group P6/mmm), and the Ti atoms occupy the Wyckoff positions 1a and 2d. Metastable ω-phase can be achieved without high pressure in certain Ti-based alloys as well. It depends on the alloying content, quenching rates or isothermal annealing conditions [4,5]
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