**1. Introduction**

The interest in metastable β-Ti alloys has gradually increased due to their high specific strength, which make them ideal for applications in the aerospace industry [1]. Manufacturing of medical implants and devices is another high-added-value field and constitutes a prospective application of metastable β titanium alloys [2]. These alloys offer higher strength levels than commonly used α + β alloys due to controlled precipitation of tiny particles of α phase [3]. The strength of titanium and Ti-based alloys can be further improved by achieving an ultra-fine grained (UFG) structure via severe plastic deformation methods (SPD) [4–7]. Manufacturing of UFG metastable β-Ti alloys is of significant interest as demonstrated by recent reports [8–11]. This study focuses on the effect of microstructure refinement by SPD on the precipitation of α phase upon subsequent thermal treatment.

The α phase forms in metastable β Ti alloys by a standard mechanism of nucleation and growth. The kinetics of α phase formation in a solution-treated material is generally given by chemical composition of the alloy and the temperature of ageing. However, α phase precipitation also strongly depends on the microstructure since α phase particles nucleate at preferential sites such as grain and subgrain boundaries and dislocations [12,13].

In some β-Ti alloy, nanosized ω phase particles can also act as preferential nucleation sites for precipitation of α phase, although the exact mechanism of ω formation has not been fully resolved yet [14–18]. It is therefore of significant interest to investigate α phase precipitation in the presence of high concentration of lattice defects (UFG material) in an alloy prone to the ω phase formation. Binary metastable Ti15Mo alloy used in this study consists, similarly to other β-Ti alloys with similar degree of β stabilization, of a mixture of β and ωath (athermal ω) phases in the solution-treated (ST) condition [19,20]. This ωath phase forms during quenching of the alloy from the temperatures above β transus by a displacive diffusionless mechanism [21]. However, it was also reported that ω phase can form as a result of high deformation, and is referred to as deformation induced ω [8,22]. The β → ωath transformation is reversible up to a temperature of 110 ◦C [23]. Upon ageing at higher temperatures, the ωath particles become stabilized by diffusion i.e., in Ti15Mo alloy by expelling Mo. This phase is referred to as ωiso (isothermal ω) [24]. The size of particles of ωiso phase is typically in the range of few nanometers up to 100 nm [19,24].

Solution-treated Ti15Mo alloy was processed by high-pressure torsion (HPT) [5] in order to achieve ultra-fine grained (UFG) microstructure with a high density of lattice defects. The equivalent von Mises strain achieved by HPT is heterogeneous and can be calculated according to Equation (1) [25]:

$$
\varepsilon\_{\rm rM} = \frac{2\pi Nr}{\sqrt{3}\hbar} \tag{1}
$$

where *r* represents the distance from the center of the sample, *h* is the thickness of the specimen, and *N* is the number of revolutions. The equivalent inserted von Mises strain after a single HPT rotation (*N* = 1, *r* = 10 mm and thickness *h* of 1 mm) ranges from 0 (the exact center) up to 35 = 3500% (periphery). Such extreme strain results in a dislocation density exceeding <sup>ρ</sup> = 5 <sup>×</sup> 10<sup>14</sup> m−<sup>2</sup> and grain size in the range of hundreds of nanometers [11,26].

The objective of this study was to investigate the effect of SPD on the mechanisms and kinetics of α phase precipitation and to compare it with the precipitation in the non-deformed solution-treated material. Both laboratory X-ray diffraction (XRD) and high-energy XRD using synchrotron radiation (HEXRD) were employed for this experimental study.
