The α → ω Transformation in Titanium-Cobalt Alloys under High-Pressure Torsion

Abstract

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.

Diffusive and Displacive Phase Transformations Under High Pressure Torsion

“…HPT leads to the transitions of α-and β-phases to the ω-phase which remain in the samples after pressure release [123][124][125][126]. The alloying of titanium with β-stabilizers (like Fe, Co or Nb) strongly modifies the α-β-ω-transformations [126][127][128][129][130]. For example, the β-to-ωtransformation in Ti-alloys is martensitic [126][127][128].…”

“…The alloying of titanium with β-stabilizers (like Fe, Co or Nb) strongly modifies the α-β-ω-transformations [126][127][128][129][130]. For example, the β-to-ωtransformation in Ti-alloys is martensitic [126][127][128]. There are special orientation relationships between β and ω lattices which ensure the diffusionless martensitic transformations where long- range mass transfer is not needed and the atoms conserve more or less their neighbors before and after transition [131].…”

Deformation-Induced Phase Transformations

“…The present Special Issue includes two review papers [1,2] and seven scientific papers [3][4][5][6][7][8][9] covering compositional, microstructural, and crystallographic aspects of deformation-induced phase transformations in titanium alloys [2,5,6], steels [3,4,[7][8][9], and shape memory alloys [1].…”

“…Deformation-induced martensitic transformation plays a remarkable role in the so-called Transformation-Induced Plasticity (TRIP) effect [6,7] in both advanced steels and metastable Ti alloys [2,5,6]. However, to date, the clear understanding of various factors influencing the occurrence of this event remains elusive as a result of the complexity of their different interactions.…”

“…However, to date, the clear understanding of various factors influencing the occurrence of this event remains elusive as a result of the complexity of their different interactions. Both material-dependent parameters, such as a stacking fault energy of austenite in steels and β phase stability in Ti alloys [2,5], as well as external/loading conditions (temperature, uniaxial tensile, plane strain compression, cyclic loading, high pressure torsion) [2,[4][5][6][7] are examined in this Special Issue.…”