This is the final version of the article. It first appeared from Elsevier via https://doi.org/10.1016/j.actamat.2016.09.03

The decomposition of the β phase during rapid cooling of the near β titanium alloy Ti–5Al–5Mo–5V–3Cr has been studied using in situ X-ray synchrotron diffraction combined with ex situ conventional laboratory X-ray diffraction and transmission electron microscopy (TEM). Evidence is found supporting the suggestion by De Fontaine et al. (Acta Mater. 1971;19) that embryonic ω structures form by the correlation of linear (1 1 1)β defects at high temperatures. Further cooling causes increased correlation of these defects and the formation of athermal ω structures within the β matrix at temperatures ∼500 °C. Post-quench aging at 570 °C resulted in the nucleation of α laths after ∼90 s at temperature, with the laths all initially belonging to a single variant type. Aging for 30 min produced an even distribution of α precipitates with a lath morphology ∼1.5 μm × 0.2 μm in size composed of both the expected Burgers variants. Mechanical property data suggests that the ω structures alone have no real effect; however, hardness increases were observed as the α phase developed. The utilization of thermal regimes similar to those presented in this paper could offer a method to engineer the α phase in near β titanium alloys and hence control mechanical properties. Keywords Titanium alloys Phase transformations TEM Synchrotron radiation 1 Introduction Ti–5Al–5Mo–5V–3Cr (Ti-5-5-5-3) is a relatively new near β alloy based on the Russian alloy VT22. This high-strength alloy is of particular interest to the aerospace industry for applications such as bogie beams in aircraft landing gear. Economic manufacturing of these large components is achieved by hot near net shape forging [1] , which yields the desired mechanical properties. However, many near β alloys are extremely sensitive to forming variables, even during isothermal processing. For example, Ti–10V–2Fe–3Al (Ti-10-2-3), which is used in landing gear forgings for the Boeing 777, has been shown to be particularly sensitive to small fluctuations in both temperature and strain rate [2] . Ti-5-5-5-3 meanwhile, has exhibited less sensitivity to forming variables and offers a larger processing window due to the less steep β transus approach curve; hence it is becoming the alloy of choice for airframe forgings [3,4] . One of the major reasons near β alloys are selected for such applications is their ability to achieve strengths well over 1 GPa [5] . One explanation as to the source of this high strength is the development of fine-scale α precipitates (∼20 nm × 200 nm in Ti-10-2-3 [6–8] ). These precipitates create a high number of α/β interfaces that act as dislocation barriers, increasing the alloy strength [6] . Producing an evenly distributed fine-scale α structure requires multiple site nucleation and limited growth. Sympathetic nucleation from primary α or nucleation of α from martensitic (α″) plates would require the larger structural units already to be present in the microstructure. Therefore to achieve the desired fine precipitates, nucleation would need to occur from dislocations, grain boundaries, vacancies and/or another phase, such as the ω phase. Ti-5-5-5-3 has been shown to vary from VT22 in that when quenched from ∼50 °C above its β transus a 100% retained β structure is not achieved. Several researchers have reported the formation of athermal ω when Ti-5-5-5-3 is rapidly cooled from the β phase field [3,9,10] . The formation mechanism of the compositionally invariant athermal ω, by the collapse of β {1 1 1} planes, has been well documented [6,11,12] . The resulting precipitates are usually found to be small (∼2–10 nm) ellipsoidal structures aligned along 〈1 1 1〉β [12] . It has been suggested that at high temperatures, well above the supposed ω start temperature (300–500 °C in near β alloys [6] ), embryonic ω or “ω-like fluctuations” are present [11,13,14] . These structures can be thought of as a body-centred cubic (bcc) lattice defect produced by atomic jumps in the 〈1 1 1〉 direction causing local plane collapse. The highly correlated displacements give significant concentrations, even at high temperatures, where the structure is maintained for longer than a typical atomic vibration [12] . At higher temperatures the embryos persist as uncorrelated line defects aligned with the 〈1 1 1〉 direction, meaning that each diffracts differently until cooled closer to the ω transition temperature, when the correlation increases and discrete ω particles are formed [12] . The development of the athermal ω phase has been studied using high-resolution transmission electron microscopy (HRTEM) [15] . It has been suggested that as alloy solute concentration increases, the athermal ω structure becomes incommensurate, leading to the commonly observed diffuse streaking in electron diffraction patterns [15] . Lattice imaging has shown a deviation of the apparent ω c -axis away from the 〈1 1 1〉β, rotating by up to 10° towards the [1 1 0]. This has been linked to the nature of the stepped interface between the ω and β formed by the coalescence of multiple ω embryos. The local selection of the collapsing plane for a given ω structures is random and for each individual case will be aligned with the 〈1 1 1〉β. When two structures are joined using a single vector, a deviation from the 〈1 1 1〉β can arise [15] . Due to the small size of athermal ω, many researchers have subsequently aged samples at low temperatures (∼300–400 °C), often below...