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A sustained load holding period imposed during fatigue loading is detrimental to material performances, causing a sharp decline in the fatigue life of near-α titanium alloys. Therefore, the deformation discrepancies of dwell fatigue (DF) and low cycle fatigue (LCF) were studied for Ti834 titanium alloy with bimodal structures in this work. The fractographies after dwell fatigue and low cycle fatigue testing were characterized using scanning electron microcopy (SEM), and the crack propagation paths at the subsurface were investigated using an optical microscope (OM). In order to reveal the mechanism of fatigue damage, detailed dislocation structures were observed using transmission electron microcopy (TEM). The crack propagation paths in microscales and the dislocation distributions were observed in the LCF and DF. The reasons for the discrepancies are also discussed in this work, which effectively enhances the understanding of the dwell failure procedures. The results show that the near basal cracks are formed under dwell fatigue, and the deformation is highly localized at the boundary of αp grains under dwell fatigue. In contrast, during low cycle fatigue, the sample tends to deform homogenously. An intergranular fracture along the primary αp grains is formed due to the localized deformation during dwell fatigue. However, a transgranular fracture is formed in the primary αp grains under low cycle fatigue. [ABSTRACT FROM AUTHOR]

Isothermal compressions of as-cast near-α Ti90 titanium alloy were carried out on a Gleeble-3800 simulator in the temperature range of 860–1040 °C and strain rates of 0.001–10 s−1. The deformation behavior of the alloy was characterized based on the analyses of flow curves, the constructions of Arrhenius constitutive equations and the processing map. The microstructure evolution of the alloy was analyzed using the optical microscopic (OM), transmission electron microscope (TEM), and electron backscatter diffraction (EBSD) techniques. The results show that the kinking and dynamic globularization of α lamellae is the dominant mechanism of flow softening in the α + β two-phase region, while the dynamic recovery (DRV) of β phase is the main softening mechanism in the β single-phase region. The dynamic globularization of α lamellae is mainly caused by the wedging of β phase into α laths and the shearing of α laths due to imposed shear strain. The activation of prismatic and pyramidal slip is found to be easier than that of basic slip during the deformation in the α + β two-phase region. In addition, the Schmid factor of equiaxial α is different from that of lamellar α, which also varies with the angle between its geometric orientation and compression direction (CD). Based on the processing map, the low η region within the temperature range of 860–918 °C with a strain rate range of 0.318–10 s−1 should be avoided to prevent the occurrence of deformation instability. [ABSTRACT FROM AUTHOR]

Laser shock peening (LSP) is a novel surface treatment technique for strengthening metal materials. The microstructural response of lamellar Ti834 alloy was investigated after treated by LSP. Micro-hardness of LSPed specimens on the surface and along the depth direction was measured. Microstructural features of plastic deformation layer induced by LSP were identified by transmission electron microscopy (TEM). Experimental results showed that the surface micro-hardness of Ti834 alloy increases gradually with increasing impact times and the peak of the micro-hardness appear in the subsurface layer of the specimen. The depth of the strengthening layer was 200 μm and 500 μm for the one and two impacted specimens, respectively. The compressive residual stress along the depth of the specimen increases with increasing of impact times, and the maximum compressive residual stress at the depth of 100 μm from the surface of the specimen after two times impact. The microstructural evolution of Ti834 alloy with lamellar structure includes the formation of sub-grain and dislocation cell, twin–matrix (T-M) lamellae divided by the parallel MTs, MT-MT intersecting at an angle with the α-plates, the crossed arrangement of dislocation arrays (DAs-DAs) and the segmentation of DWs/DTs in the transverse direction. • The micro-hardness peaks appear in the subsurface layer of the LSPed specimen. • The maximum compressive residual stress at the depth of 100 μm from the surface of the specimen. • The depth of the strengthen layer was 200 μm and 500 μm for the one and two impacts specimen, respectively. • The microstructural evolution of the alloy includes the formation of sub-grain, dislocation cell and mechanical twin et al. [ABSTRACT FROM AUTHOR]

• Nb, V and Fe significantly tailor the microstructure and mechanical properties. • Phase stability and energy barrier of phase transformation were responsible for microstructure formation. • Solid-solution and phase-interface strengthening were responsible for various mechanical properties. [Display omitted] Binary Ti-4(Nb, V, Fe) alloys were prepared to investigate the effects of β-stabilizers (Nb, V and Fe) on microstructure formation and mechanical properties of titanium alloys by using experimental and theoretical methods. Microstructure observations revealed that addition of Nb, V or Fe significantly tailored the microstructure under different heat-treated conditions, i.e., furnace-cooled (FC), air-cooled (FC) and water-quenched (WQ). The FC Ti–4(Nb, V, Fe) alloys exhibited the Widmanstätten microstructure and the AC samples showed the spheroidized microstructure. The WQ Ti–4Fe exhibited the microstructure composed of equiaxed β grains and nanoscale ω particles while the WQ Ti–4Nb and Ti–4V showed the martensite microstructure. Differentials of microstructure can be attributed to the effects of Nb, V and Fe elements on the phase stability and energy barrier of phase transformation according to the empirical electron theory (EET) of solids and molecules. Results of tensile tests showed that Nb, V and Fe significantly influenced the tensile properties under the different heat treatments. Both the WQ Ti–4Nb and Ti–4V showed the larger tensile strength than that of the FC and AC sample due to the stronger solid-solution and phase-interface strengthening in martensite structure. The WQ Ti–4Fe sample exhibited the largest tensile strength (1104 MPa) compared to that of the FC (754 MPa) and AC (981 MPa) samples due to the strongest solid-solution and phase-interface strengthening from the ω phase. By executing the methods of the empirical electron theory (EET) of solids and molecules, effects of β-stabilizers (Nb, V and Fe) on microstructure formation and mechanical properties of titanium alloys were revealed on the view of valence electron. [ABSTRACT FROM AUTHOR]

Charpy impact properties and crack resistance of commercial pure (CP) Ti and Ti–6Al alloy with different oxygen contents were systematically investigated by using the instrumented Charpy impact testing machine. The scanning electron microscope (SEM) equipped with electron backscatter diffraction (EBSD) detector and transmission electron microscope were employed to detect the deformation mechanisms. The experimental results showed that decreasing the oxygen content significantly improved the impact toughness of CP Ti and Ti–6Al alloy. Furthermore, decreasing the oxygen content simultaneously strengthened the resistance to crack initiation and propagation of CP Ti while only improved the resistance to crack propagation of Ti–6Al alloy. EBSD analysis demonstrated the significant effects of oxygen content on dislocation activities and deformation twinning behaviors in CP Ti and Ti–6Al. Decreasing the oxygen content significantly increased the dislocation activities and deformation twinning behaviors in CP Ti and Ti–6Al alloy which were mainly responsible for the improved impact properties and the strong crack resistance of CP Ti and Ti–6Al alloy. A mathematical model was introduced for the first time to uncover the interactions between oxygen atoms and dislocation as well as the deformation twinning on the view of valence electron by employing the Yu Rui-huang electron theory. By conducting the systematically experimental and theoretical investigations, we are aiming to provide deep insights into the relationships between interstitial oxygen and impact toughness as well as the crack resistance of titanium alloys and promote the production of titanium alloys with excellent impact properties. • Decreasing oxygen content significantly improved the impact properties of CP Ti and Ti–6Al. • Various oxygen contents extremely influenced dislocation activities and deformation twinning behaviors in CP Ti and Ti–6Al. • The increased dislocation activities and deformation twinning behaviors were responsible for the improved impact properties. [ABSTRACT FROM AUTHOR]