Characteristics of the allotropic phase transformation in titanium processed by high-pressure torsion using different rotation speeds

Abstract

An investigation was initiated to examine the effect of rotation speed on the allotropic α to ω-phase transformation in titanium. A grade 2 commercial purity titanium was processed by high-pressure torsion (HPT) at room temperature up to a maximum of 10 turns using a pressure of 5.0 GPa and different rotation speeds from 0.5 to 2 rpm. It is shown that the allotropic phase transformation occurs during HPT at different rotation speeds but the volume fraction of the ω-phase decreases when the rotation speed is increased. The results indicate that the hardness and strength are decreased by processing the specimens at higher rotation speeds. Additional annealing for 15 min at 423 and 473 K after HPT processing is also effective in changing the volume fractions of the ω-phase and producing an ω to α reverse phase transformation. The results are interpreted in terms of the temperature rise during HPT processing.

An evaluation of the hexagonal close-packed to face-centered cubic phase transformation in a Ti-6Al-4V alloy during high-pressure torsion

“…Nevertheless, this phase was observed in commercially pure Ti as an additional plastic deformation mode comparable to twinning [16,17] after the same experimental procedure. An important parameter governing the α to ω transformation is the role of impurities in Ti, in particular the oxygen content [38] and it seems that this parameter is very important in suppressing the α to ω transformation in the Ti-6Al-4V alloy which contains 6% Al and 4%…”

“…High-pressure torsion (HPT) is a well-established SPD procedure [11][12][13] and in pure Ti it was established that there may be an α (hcp) to ω (hcp) phase transformation during HPT processing [14][15][16][17]. Similarly, HPT-induced β to ω [18] and α" to ω [19] phase transformations were reported in a Ti-Nb alloy.…”

Development of an ω-Phase in Grade 2 Titanium Processed by HPT at High Hydrostatic Pressure

“…The large hydrostatic stress developed during this processing operation prevents cracking and permits processing at room temperature. Thus grain sizes in the range of a few hundreds of nanometers have been reported in CP-Ti processed by HPT [15][16][17][18][19][20][21] . Also, titanium exhibits a phase transformation at high pressures into an hexagonal ω-phase 22,23 .…”

“…It was reported that the ω-phase is denser despite the less-packed structure and this was attributed to the smaller atomic size 22 . Recent research shows that the ω-phase is formed in CP-Ti processed by HPT 16,17,20,21,24 . The high pressures developed in this process lead to a phase transformation and the imposed torsional strain stabilizes the hexagonal phase upon pressure release.…”

“…The high pressures developed in this process lead to a phase transformation and the imposed torsional strain stabilizes the hexagonal phase upon pressure release. It is known that the α to ω phase transition is favored with decreasing oxygen content in the alloy 16 , with increasing pressure 17 , with increasing the numbers of turns in HPT 16,17,20,21 and with decreasing the rotation rate 21 . However, although the phase transformation has been reported at pressures as low as 5 GPa in Ti-0.05% O 16 and in grade 2 Ti 21 , the ω-phase was not detected in CP-Ti of similar grade even after processing by HPT using a pressure of 6 GPa for up to 10 turns 19 .…”