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A metastable β titanium alloy, Ti–2Al-9.2Mo–2Fe (wt.%), with athermal ω phase after quenching has been subjected to high strain rate (∼3000 s−1) compression at ambient temperature by Spilt Hopkinson Pressure Bar arrangement. Reverse β-to-ω transformation was investigated by transmission electron microscopy during deformation. Furthermore, the novel distribution of athermal ω phase in {332} twinning and stress-induced martensitic was observed. We reveal that such novel distribution is attributed to the influence of ω phase on {332} twinning and stress-induced martensite transformation (SIMT) based on the viewpoint of the instability of the lattice structure of metastable β phase. Furthermore, a new process of twinning and SIMT in current alloy with ω phase was proposed.

The mechanical behavior of a metastable β titanium alloy, Ti-2Al-9.2Mo-2Fe (wt%), was studied through high compression strain rate (~3000 s−1) at ambient temperature by Spilt Hopkinson Pressure Bar (SHPB) tests after solution treatment at 850 ℃ for 0.5 h and 950 ℃ for 1 h. The mechanism of deformation and microstructure evolution was investigated by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Multiple deformation mechanisms, including primary and secondary {332} twinning, stress-induced martensite, and stress-induced ω-phase and dislocation slips, were identified in specimens subjected to solution treatment at 850 ℃ for 0.5 h. In contrast, only primary and secondary {332} twinning and dislocation slips were detected in specimens subjected to solution treatment at 950 ℃ for 1 h. The athermal ω precipitation produced after solution treatment and elevated temperature generated during dynamic loading played a crucial role in the different deformation mechanisms observed in the two different specimens. Moreover, the deformation mechanisms active during plastic deformation in the different strain regimes affected the development of different textures in the Ti-2Al-9.2Mo-2Fe alloys from each regime.

Hot deformation behaviors of Ti–2Al–9.2Mo–2Fe alloy with boron were investigated in a hot compression test at temperatures ranging from 850 to 1000 °C and strain rates ranging from 0.01 to 10.00 s−1. With strain rate decreasing and deformation temperature increasing, dynamic recrystallization (DRX) was promoted in both alloys. The nucleation mechanism of DRX involved grain boundary bulging and subgrain rotation. In case of deformation at low temperatures and high strain rates, the boron-free alloy only showed dynamic recovery (DRV) phenomenon. However, due to the particle-stimulated nucleation mechanism of recrystallization by TiB particles, the boron-containing alloy shows DRX at that condition. These DRX grain nuclei are formed by lattice rotation and subgrain growth from deformation zone in the initial grains, which are close to those of TiB particle. Moreover, the flow stress of the boron-containing alloy is lower than that of the boron-free alloy due to the grain refinement during hot deformation process.

The microstructure, room temperature and 650 °C tensile properties of an (α + β) titanium alloy were investigated after aging over a temperature range of 600–750 °C following solution treatment. The results exhibit that both aging temperature and aging time influence the precipitation behavior of α2 phase, and the size of α2 phase affects the tensile properties. The growth speed of α2 phase gets quicker with the aging temperature increasing under the same aging time. For a given aging temperature, the size of α2 phase gets larger when prolonging the aging time. There are significant improvements in yield and ultimate tensile strength at room temperature and 650 °C after aging at 600 °C for 2 h, but these values decrease with the size of α2 phase increasing. It is found that α2 phase, less than 7 nm, does not impair the room-temperature ductility. However, when the size of α2 phase grows up from 7 to 15 nm, ductility decreases drastically. During 650 °C tensile process, with the particle size increasing from 3 to 15 nm, the interaction between moving dislocations and coherent particles changes from cutting mechanism to bypass mechanism. With the α2 particle size increasing from 3 to 15 nm, the interaction between moving dislocations and coherent particles changes from cutting mechanism to bypass mechanism at 650 °C, and 6 nm is in the range of critical radius, which corresponds to the maximum strengthening effect of α2 phase at 650 °C

In order to study the aging hardening characteristics and isothermal aging process of Ti-6Cr-5V-5Mo-4Al-1Nb alloy the time-temperature-transformation (TTT) curve of the alloy was measured by aging hardness method, and the microstructure changes during isothermal process were analyzed by XRD and SEM. It was found that the alloy had a high aging response speed at other aging temperatures except for the slow aging at 300°C and 400°C. When the aging time is prolonged at medium and low temperatures, the alloy can also produce the same aging strengthening effect as that at medium temperature aging, indicating that the temperature window of the alloy aging is wide, the aging strengthening effect is good, and it has the operability of heat treatment strengthening, so it is an ideal structural material. The isothermal aging transformation curve of the alloy is C type, and it can be obtained from the figure that the nose temperature is about 545°C

In order to develop a new titanium alloy with a good combination of strength–ductility–toughness, a near-beta titanium alloy was designed based on the already widely used Ti-1023 alloy. To avoid beta fleck occurring in the microstructure, the new Ti–Al–Fe–V (Cr, Zr) alloy has been made through decreasing the content of Fe, based on molybdenum equivalency and Bo–Md molecular orbital method (a method for new alloy designing based on the molecular orbital calculating). After primary design computation, Ti–Al–Fe–V (Cr, Zr) alloy was optimized as Ti–3Al–4.5Cr–1Fe–4V–1Zr finally. The microstructure and tensile properties of this alloy subjected to several commonly used heat treatments were investigated. The results show that the tensile strength of the alloy after solution treated below the β-transus temperature comes between 850 and 1100 MPa, with elongation in the range of 12.5 %–17.0 %. In solution-treated and solution-aged samples, a low-temperature aging at 500 °C results in the precipitation of finer α phase. With the increase in aging temperature, the secondary α phase becomes coarser and decreases in amount. Thus, it will lead to the increase in tensile ductility, but reduction in strength. Eventually, after modulated aging treatment, the alloy can obtain high-strength level with acceptable ductility. The tensile strength of the alloy can achieve 1273 MPa, with an elongation of 11.0 %. At the same time, the fracture toughness (K IC) of the alloy achieves 83.8 MPa·m1/2. It is obvious that the newly designed alloy has achieved a good blend of strength–ductility–toughness.

The effect of Al addition on ω precipitation and age-hardening behavior of Ti-9.2Mo-2Fe and Ti-2Al-9.2Mo-2Fe alloy during aging treatment was investigated. The results showed that athermal and isothermal ω phase formation in Ti-2Al-9.2Mo-2Fe alloy was suppressed to a certain extent due to Al addition. In addition, a small amount of athermal ω phase was observed in the β matrix with a size of about ~5 nm during water quenching from above the β transus temperature for both alloys. Isothermal ω formation was also found during aging at temperatures ranging from 573 K to 773 K (300 °C to 500 °C) in both alloys, although it had a limited time of stability at 773 K (500 °C). The hardening due to isothermal ω precipitation exhibited no over-aging as long as ω phase existed in both alloys, and ω phase played a more important role in hardening than α phase. And the ω phase in 50 to 100 nm size exhibited the best hardening effect in Ti-9.2Mo-2Fe alloy. Similarly, α phase with 100 to 200 nm in length showed better hardening effects in Ti-2Al-9.2Mo-2Fe alloy. Both the alloys showed stronger age hardening at an intermediate temperature of 673 K (400 °C) and in the first aging stage at a higher temperature of 773 K (500 °C) due to the sufficiently fine size (50 nm), while they exhibited weaker age hardening at a lower temperature of 573 K (300 °C) and long period aging at a higher temperature of 773 K (500 °C) due to incomplete ω formation and/or coarsening of α phase. No over or peak aging stage was found at 573 K and 673 K (300 °C and 400 °C) during the aging for 72 hours, while the peak hardness values of both alloys aged at 773 K (500 °C) were obtained in the first stage of aging. The hardness of the alloys was very sensitive to size and volume fraction of ω phase, which depends on aging temperature, time, and composition of the involved alloys.

The reverse transformation temperature, recovery strain and pull-out force of TiNiFe alloy pipe couplings under different diameter expansion ratios (7%, 9%, 11%, 13%, 15%) were studied. The results show that the martensite reorientation occurs and the elastic strain energy released decreases with the increase of the expansion ratio, which leads to the increase of martensite reverse transformation temperature as’ and Af’. The recovery strain increases and the recovery rate decreases with the increase of expansion ratio. The expansion ratio of pipe couplings has a great influence on the pull-out force, which is related to the fit clearance and the diameter of the connected rod. When the fit clearance is the same, the larger the expansion ratio is, the greater the pull-out force is, and when the diameter of the connected rod is the same, the pull-out force decreases with the increase of the expansion ratio.

Compressive properties, microstructure features and deformation modes were investigated in binary Ti-(2,4,8) wt% V alloys during quasi-static (1 × 10−3 s−1) and dynamic (3 × 103 s−1) compressions. The compressive behavior shows a strong dependence on the loading strain rate and vanadium content contained in pure Ti, such that the flow stress increases with the increase in strain rate and vanadium content ranging from 2 wt% to 8 wt%. The microstructure features are clearly different from each other for alloys with different vanadium contents or under quasi-static and dynamic loading conditions. An examination of deformation microstructures by optical microscopy and electron backscattered diffraction indicates that twinning behavior occurs during quasi-static and dynamic compressions and the twinning density increases with strain rate increasing but decreases with vanadium addition. The existence of $$\left\{ {10\bar{1}2} \right\}$$ , $$\left\{ {11\bar{2}1} \right\}$$ and $$\left\{ { 1 1 {\bar{{2}}{2}}} \right\}$$ type twinning was further identified. With the help of the calculated Schmid factor map, the values of critical resolved shear stress of twinning types mentioned above have been obtained and verified to be rarely affected by the loading strain rate but sensitive to the vanadium content. In vanadium-rich alloys (Ti-8V), twins are rarely observed but dislocation slip mechanism is active by transmission electron microscopy investigations. With vanadium content increasing, both the critical resolved shear stress of twinning types and the content of β phase with abundant slip systems increase, reflecting a suppression of twinning but an active dislocation slip mechanism.