Formation and Thermal Stability of the ω-Phase in Ti–Nb and Ti–Mo Alloys Subjected to HPT

Abstract

This paper discusses the features of ω-phase formation and its thermal stability depending on the phase composition, alloying element and the grain size of the initial microstructure of Ti–Nb and Ti–Mo alloys subjected to high-pressure torsion (HPT) deformation. In the case of two-phase Ti–3wt.% Nb and Ti–20wt.% Nb alloys with different volume fractions of α- and β-phases, a complete β→ω phase transformation and partial α→ω transformation were found. The dependence of the α→ω transformation on the concentration of the alloying element was determined: the greater content of Nb in the α-phase, the lower the amount of ω-phase that was formed from it. In the case of single-phase Ti–Mo alloys, it was found that the amount of ω-phase formed from the coarse-grained β-phase of the Ti–18wt.% Mo alloy was less than the amount of the ω-phase formed from the fine α′-martensite of the Ti–2wt.% Mo alloy. This was despite the fact that the ω-phase is easier to form from the β-phase than from the α- or α′-phase. It is possible that the grain size of the microstructure also affected the phase transformation, namely, the fine martensitic plates more easily gain deformation and overcome the critical shear stresses necessary for the phase transformation. It was also found that the thermal stability of the ω-phase in the Ti–Nb and Ti–Mo alloys increased with the increasing concentration of Nb or Mo.

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

“…It is lower than in the TiFe alloys, comparable with TiCo alloys and in any case higher than in pure titanium. 87,166) Two binary titanium alloys with 2 and 18 wt.% Mo were annealed in such a way that they contained only ¡A-martensite (Ti2 wt.% Mo alloy) or only ¢-phase (Ti18 wt.% Mo alloy). 166) It was found that after HPT the amount of ½-phase formed from the coarse-grained ¢-phase was less than the amount of the ½-phase formed from the fine-grained ¡Amartensite.…”

Experimental Investigation of the Impact of Niobium Additions on the Structural Characteristics and Properties of Ti–5Cr–xNb Alloys for Biomedical Applications

“…Various binary Ti alloys, such as Ti-Cr [5], Ti-Nb [6], Ti-Mo [6], and Ti-Sn [7], have been systematically evaluated for dental and orthopedic applications. However, these binary alloys still require further refinement of their mechanical properties, particularly in terms of reducing the elastic modulus.…”