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The metastable ω and α″ in titanium alloys have a significant impact on the subsequent α precipitates and the final mechanical properties. Herein, the microstructure transformation of coexisting ω/α″ during aging and their effects on tensile properties have been systematically investigated in Ti-3Al-5Mo-4.5 V (wt%) α/β titanium alloy. The results demonstrate that the secondary phase within the β-transformed (β trans) grain evolves from embryonic ω + α″ to isothermal ω + α″, and ultimately transforms to secondary α (α s) with the aging temperature ascending from 300 °C to 550 °C. As embryonic ω transforms to isothermal ω, the yield strength increases from 1130.0 ± 14 MPa to 1259.5 ± 5.5 MPa with weak elongation due to the excellent strengthen effect of undeformed isothermal ω. After aging at 550 °C, coexisting micron-scale α p and nanoscale α s microstructures lead to enhanced elongation (increase by 93.7 %), sacrificing only 21.7 % yield strength. Both < a > and < c + a > dislocations are simultaneously presented in micron-scale α p grain, while nanoscale α s plates act as effective obstacles for dislocation motion resulting in accumulation of dense dislocations near the β/α s interfaces. This work provides new insights on strengthening strategy and developing new Ti-alloys. [Display omitted] • The α p , athermal ω and α″ precipitate in Ti-3Al-5Mo-4.5 V α/β titanium alloy after quenching. • The phase in β matrix changes from embryonic ω + α″ to isothermal ω + α″ and finally to α s as aging temperature ascends. • The coexisting α p , isothermal ω and α″ microstructures exhibit excellent strength but weak elongation. • The coexisting micro-scale α p and nanoscale α s microstructures display higher strength and ductility. [ABSTRACT FROM AUTHOR]

The stress-induced α″ martensite transformation always sacrifices the yield strength in titanium alloys with specific β stability. Here, a detailed analysis for the microstructure transformation of the stress-induced α″ martensite during isothermal aging is carried out using the traditional experimental characterizations based on the transmission electron microscope (TEM) in an α + β Ti-alloy, namely Ti-3Al-5Mo-4.5 V (wt%) alloy. The results show that the stress-induced α″ plates transform to the isothermal-α″ due to the local solute redistribution (Al, Mo, V) during the one-step isothermal aging at 400 °C. Subsequently, the ladder-like α s precipitates due to the decomposition of stress-induced α″ or isothermal-α″ after aging at 550 °C, eliminating the influence of stress-induced α″ martensite on the yield strength. Besides, the ultrafine α s is obtained by the refinement of isothermal ω during two-step aging at 550 °C. And the ultimate tensile strength increases slightly due to the precipitate of ladder-like and ultrafine α s after two-step aging at 550 °C. The present study provides a novel way to obtain the α lamellae and improve the mechanical properties of titanium alloys. • The stress-induced α″ precipitates during deformation and transforms to isothermal-α″ during one-step aging at 400 °C. • The isothermal-α″ exhibits the concentration fluctuation (Al, Mo, V) with the surrounded β matrix. • The decomposition of isothermal-α″/stress-induced α″ results in the ladder-like α s precipitate during aging at 550 °C. [ABSTRACT FROM AUTHOR]

It is generally accepted that the addition of Fe deteriorates the creep behavior of titanium alloys at elevated temperature. However, the effect of Fe at ambient temperature is not well documented. In this paper, the creep performance of an α/β titanium alloy with different Fe contents (0.05 and 0.20 wt%) was systematically studied at temperatures from the ambient to 500 °C. Creep resistance of samples with 0.20 wt% Fe was found to decrease at temperatures above 300 °C, but to increase at ambient temperature and 200 °C. The resulting stress exponent was in the range of 4.7–14.4, corresponding with dislocation creep. The fitted creep activation energy fell into two categories, i.e., 1) comparable with the self-diffusion activation energy of α-Ti at elevated temperature, and 2) remarkably higher than the former at 200–400 °C. The underlying mechanisms controlling creep processes at different temperatures were discussed. [ABSTRACT FROM AUTHOR]

Mo element significantly affects the microtexture evolution of α in α + β titanium alloys. In this study, the evolution of microtexture during thermomechanical processing in Ti-6Al-2Sn-4Zr-2Mo (Ti6242) and Ti-6Al-2Sn-4Zr-6Mo (Ti6246) alloy was investigated. The β annealing treatments and α + β solution-treatments were conducted to display the microstructure characteristics before thermal deformation. Then, the thermal compression in the (α + β) phase region was applied to trace the evolution of lamellae orientation in conjunction with the electron backscatter diffraction. The results show a strong prior colony microtexture before thermal deformation. {10−12} and {10−11} twins are activated in Ti6246 during thermal compression, which strongly weakens the microtexture of the prior α colony. Besides, the lower volume fraction of the primary α phase, the smaller α colony, the thinner lamellae and the lower aspect ratio of α grains contribute to weak prior colony microtexture in Ti6246 alloy. This study shows the effect of Mo content on the microtexture evolution during thermomechanical processing. [Display omitted] • Mo element significantly affects the α microtexture of α + β titanium alloys. • Strong prior α colony microtexture exists before thermal deformation. • {10–12} and {10–11} twins operate in Ti6246 alloy during thermal deformation. • The retained β phase inside α lamellae will preferentially break lamellae at elevated α + β temperature. • The smaller α colony will hinder the formation of a large-size microtexture. [ABSTRACT FROM AUTHOR]

The partitioning of alloying elements during the α+β solution treatment, accompanying the α↔β phase transformation, determines the composition and stability of the microstructure, and thus significantly influences the ultimate mechanical behavior of α+β titanium alloys. In the current study, the influence of two main factors, i.e., the solution temperature and the isothermal solution time, on element partitioning in Ti 6Al 4V alloy was investigated. The distribution of alloy elements in both phases was quantitatively analyzed by the electron probe micro-analyzer (EPMA). The results indicated that, with the increase of the solution temperature, an evident change in composition occurred in the β transformed structure (β t), while the composition in the primary α phase (α p) remained almost constant, because of the higher atomic mobilities in the β phase than in the α. With longer solution time, there appeared a steeper composition gradient and a smaller transition range of element concentration around the α p /β t boundary. Phase field simulation revealed the evolution of microstructure and composition toward equilibrium at a constant solution temperature and verified the above experimental results from the perspective of elemental diffusion and kinetics. Image 1 • Element partitioning accompanies structural change during α+β solution treatment. • The element content of β t varies more evidently as solution temperature changes. • The composition gradient near α p /β t interface turns steeper at long solution time. [ABSTRACT FROM AUTHOR]

The influence of microstructures ( β grain size, α / β colony size and width of α lath) and stress ratios ( R = 0.1, 0.7) on fatigue crack growth (FCG) behavior of beta-annealed Ti624 x ( x = 2, 4 and 6) alloys was investigated. The FCG rate curves, crack paths, and the plastic zone ahead of the crack tip were compared. The reducing FCG resistance from Ti6242 to Ti6246 was found to stem from decreasing α / β colony size. Increasing stress ratio also reduced crack deflection and crack closure, and thus decreased the FCG resistance. The relevant mechanisms were discussed in terms of K max and crack closure effect. [ABSTRACT FROM AUTHOR]

To obtain a synergistic combination of high strength and high ductility in titanium alloys, design and creation of heterogenous precipitate microstructure have attracted increasing attention. Herein, using Ti-3Al-5Mo-4.5V (wt.%, an α / β titanium alloy) as a model alloy, we demonstrated that a highly heterogenous α -phase precipitate microstructures with well-controlled length scale of spatial heterogeneity (i.e., micron-sized primary α and nano-scale secondary α precipitates), can be synthesized through activating the ω-assisted α nucleation transformation pathway that operates in metastable β titanium alloy alone. A detailed analysis of transformation pathway for ω phase and the underlying ω-assisted α nucleation mechanisms is carried out using the integrated advanced characterizations, theoretical calculations and simulations based on DFT, and phase-field modeling. Experimental results show unambiguously that the embryonic ω particles (also known as athermal ω) with partially collapsed structure are incapable of refining secondary α (α s). In contrast, the isothermal ω particles with a complete structural collapse play an important role in assisting α s nucleation, resulting in the formation of the ultrafine α s lamellae at nanoscales after two-step aging heat treatments. The contributions from the isothermal ω particles are then quantified using first-principles calculations and phase-field simulations. It is found that, even though the solute partitioning between a growing isothermal ω particle reduces the α s nucleation driving force in the surrounding β matrix, the elastic interaction between the ω particles and the α s nucleus provides more driving force for α s nucleation at specific locations of the ω/β interface. Our work extends the microstructure design strategy based on ω -assisted α nucleation mechanism from metastable β to α / β Ti-alloys, thereby significantly widening the application space of the strategy. [Display omitted] [ABSTRACT FROM AUTHOR]

A metastable titanium alloy Ti–3Al–6Fe–6V–2Zr (wt. %, Ti3662) was fabricated by laser powder bed fusion (LPBF), and high resolution digital image correlation (HRDIC) coupled with multiscale characterizations were employed, in order to uncover the effect of microstructural characteristics on the mechanical behavior of LPBF Ti3662 alloy. Results show that the LPBF Ti3662 alloy exhibits near equiaxial microstructure with fine β grain size and weak texture, due to the strong growth restriction effect from Fe addition. Mechanical tests and HRDIC results demonstrate that microstructural heterogeneity with a chessboard-like pattern has a pronounced influence on the microstrain evolution and mechanical properties of LPBF Ti3662. Strain localization is observed at the interfacial zones between the fine and coarse grains, and the irregular prior β grain morphology with rugged grain boundaries is significant in mitigating the adverse effect of grain boundary α phase. This is attributed to the successful suppression of strain localization and cracking at precipitate-free zones near the irregular grain boundaries. This study demonstrates that the irregular grain morphology is beneficial for the mechanical properties of metastable titanium alloy, whereas the heterostructure formed during LPBF causes strain localization. [ABSTRACT FROM AUTHOR]