Review - Phase Transitions in Ti Alloys Driven by the High Pressure Torsion

Abstract

The high pressure torsion (HPT) of various binary Ti alloys with ¢-stabilizers (Fe, Co, Ni, Mo, Nb) was studied. Before HPT, the samples were annealed and contained (i) pure ¢-phase, (ii) ¡+¢ mixture with different portion of phases, (iii) ¡A or ¡AA martensites, (iv) the mixture of ¡-Ti and respective intermetallic phase. The microstructure of Ti alloys before and after HPT was studied by scanning and transmission electron microscopy (also high resolution one), X-rays diffraction (including the high-temperature in situ one), differential scanning calorimetry, atomic probe tomography, synchrotron irradiation. During HPT the (usual) strong grain refinement took place. Also, it was observed that HPT can lead to various phase transitions in the Ti alloys. In particular, the metastable high-pressure ½-phase and ¡A martensite can form. The composition of phases (similar to their grain size) reached the steady-state value after about 1.5 plunger revolutions. In some cases, the equifinality of the HPT-driven phase transitions was observed. Equifinality means that the composition and portion of phases after HPT do not depend on the composition and portion of phases before HPT.The HPT-driven phase transitions can be martensitic (i.e. without or almost without mass transfer) or diffusional (i.e. with mass transfer). In case of martensitic ¢-to-½ or ¡-to-½ phase transitions, the certain orientation relations between ¢ and ½ or ¡ and ½ phases were observed. The thermal stability of the ½-phase obtained by HPT has been studied by the in-situ X-rays diffraction at high temperatures. The ½-phase in the HPT-treated Ti alloys with ¢-stabilizers can remain in the samples up to 500600°C. It is much higher than in pure titanium (³180°C). Thus, the HPT-driven phase transitions open the new way for tailoring of grain size and phase composition of Ti-based alloys. In turn, it gives the new instrument in hands of engineers to improvement of the technologically important properties of Ti-based alloys. It is especially important for the medical application like the teeth or bone prosthesis.

Severe Plastic Deformation of Light Metals (Magnesium, Aluminum and Titanium) and Alloys by High-Pressure Torsion: Review of Fundamentals and Mechanical/Functional Properties

“…13,14) Such materials are sometimes referred to as superfunctional materials due to their superior properties. 15,16) For the most recent progress in the field of SPD, the readers are referred to a 2023 special issue of Materials Transactions, 17) which covered various aspects like processing methods, [18][19][20][21][22][23][24][25][26] structural/microstructural features, [27][28][29][30][31][32][33] mechanical/functional properties, nanostructured metallic and non-metallic materials [56][57][58][59][60][61][62][63][64][65][66][67][68] and heterostructured materials. [69][70][71][72] Among various SPD methods, ECAP and ARB have the potential for commercialization.…”