Pressure-induced alpha-omega transformation in titanium: Features of the kinetics data

Abstract

The increase in the electrical resistance of the specimen, associated with the α→ω transformation in Ti, has been used to obtain the fraction ζ of the ω phase as a function of time under isobaric–isothermal conditions in the pressure range 4–9 GPa and at 300±3 K. In the entire pressure range ζ–t data fit an equation of the form: ζ=1−exp−(t/τ)n, where τ and n are constants at a given pressure. The results indicate that τ decreases rapidly and n slightly with increase in pressure. An analysis of τ–p data shows that (i) the activation free energy for the total process is very high at low pressures and decreases rapidly with increasing pressure, and (ii) the activation volume in the 6–9 GPa range is −4.3 cm3 mol−1. The activation enthalpy has been obtained by measuring τ at different temperatures, and the average value in the pressure range 5–8 GPa is 12±0.5 kCal mol−1.

Evidence of α → ω phase transition in titanium after high pressure torsion

“…x transformation occurs via a nucleation and growth mechanism, when stress-assisted nucleation controls the overall transformation rate. It is interesting to compare our results with previous investigations of [13,15] where the fraction of x-phase was measured during isochronal pressure exposure in the pressure range between 4 and 10 GPa. These authors had determined two parameters: ance of the first nuclei of the omega phase under certain pressure, and (ii) the time necessary to transform more than 50 % of the a-phase to x-phase.…”

“…Indeed, we observe *25 % of x-phase in samples deformed for c = 785 (N = 5, which corresponds to 5 min) under a pressure of 4 GPa, whereas without shear deformation it would take 27 h to obtain the first nuclei! It should be noted that the results presented in [13,15] were obtained in in-situ measurements, whereas ours are estimated by the amount of phase remaining after unloading which had inevitably led to an underestimation of the x-phase fraction. In Table 1 the portions of HPT-induced omega phase are presented together with corresponding applied pressures, shear strains and time of deformation (duration of pressure exposure).…”

“…In Table 1 the portions of HPT-induced omega phase are presented together with corresponding applied pressures, shear strains and time of deformation (duration of pressure exposure). For comparison reasons the incubation time necessary to obtain the first nuclei of the x-phase under the applied pressure without any deformation [13] is also shown. Obviously, the reason for such an accelerated formation of the x-phase is related to a considerable change in the driving force provided by the external shear stress.…”

The kinetics of the ω to α phase transformation in Zr, Ti: Analysis of data from shock-recovered samples and atomistic simulations

“…Here we consider the pressure driven martensitic a (hexagonal close-packed) to x (simple hexagonal) transformation in zirconium and titanium, which is thought to be a combination of isothermal-athermal processes [6][7][8], with significant implications in engineering and medicine [9]. Analysis of quasi-static pressure data (5-9 GPa) based on resistivity measurements suggest that the kinetics of the a !…”

Annealing of defects in Fe after MeV heavy-ion irradiation

“…Recent investigation of Ti [2,14] under high energy ion irradiation showed phase transformation from α phase to metastable ω phase. It is known that Ti undergoes this kind of transformation when subjected to high static pressure of 8 Gpa [15,16] for about 24 hours.…”