The roles of microstructure in plastic deformation and crack growth mechanisms of a titanium alloy with a trimodal microstructure have been systematically investigated. The results show that thick intragranular α lath and a small number of equiaxed α phases avoid the nucleation of cracks at the grain boundary, resulting in branching and fluctuation of cracks. Based on electron back-scattered diffraction, the strain partition and plastic deformation ahead of the crack tip were observed and analyzed in detail. Due to the toughening effect of the softer equiaxed α phase at the grain boundary, crack arresting and blunting are prevalent, improving the crack growth resistance and generating a relatively superior fracture toughness performance. These results indicate that a small amount of large globular α phases is beneficial to increase the crack propagation resistance and, thus, a good combination of mechanical property is obtained in the trimodal microstructure.

The gradient nanostructured (GNS) layer forms beneath the surface of Zr-4 samples by the surface mechanical grinding treatment (SMGT) process, which increases the fatigue strength apparently due to the synergistic effect of the gradient nanostructured layer and compressive residual stress. The SMGTed Zr-4 samples are subjected to annealing to remove residual stress (A-SMGT) and the individual effect of the GNS layer and compressive residual stress can be clarified. The results show that the gradient nanostructure in the surface is stable after annealing at 400 °C for 2 h but residual stress is apparently removed. Both SMGTed and A-SMGTed Zr-4 samples exhibit higher fatigue strength than that of coarse-grained (CG) Zr-4 alloy. The fatigue fracture of Zr-4 alloy indicates that the hard GNS surface layer hinders fatigue cracks from approaching the surface and leads to a lower fatigue striation space than that of CG Zr-4 samples. The offset fatigue strength of 10⁶ cycles is taken for SMRT-ed, A-SMRT-ed, and CG Zr-4 samples and the results indicate clearly that the GNS surface layer is a key factor for the improvement of fatigue strength of the Zr-4 alloy with surface mechanical grinding treatment.

With the increase of time, the shrinkage of materials at fixed temperature could enhance the failure of fasteners. We report a potential way to alter the volume/length of alloy automatically through isothermal aging due to pseudospinodal decomposition mechanism. The volume of Ti-10V-2Fe-3Al alloy first shrunk and then expanded during isothermal aging at 550 °C. It can fit tightly and make up for volume loss. Transmission electron microscopy observation exhibits no obvious coarsening of intragranular α phase with the increasing time. However, composition evolution with time shows a gradual change through energy dispersive spectrometer analysis. The result shows that β stabilizers, V and Fe, are prone to diffuse to the β matrix, while α stabilizers, Al, prefer to segregate to the α phase. First principle calculations suggest that the structure transition for β to α cause the first decrease of volume, and the diffusion of V, Fe and Al is the origin of the later abnormal increase of volume.

A novel high-strength metastable β-Ti alloy Ti-5Al-4Zr-8Mo-7V has been successfully developed by improved Bo-Md map combined with critical Mo equivalent criteria. Near β forging combined with different heat treatments was introduced to tailor the Bi-modal structure. The alloy exhibits good strength-ductility combination with UTS~1390 MPa and El~10% after solution treated (ST) at 800°C followed by 570°C aging. After β-transus forging combined with subsequent heat treatment, an excellent combination of strength UTS~1460MPa and El~10% is achieved which is attributed to the Tri-modal structure consisting of primary α (αp), sub-micro α rods (αr) and nano-scale α platelets (αs). Plastic strain is effectively partitioned in both αp, αr and β matrix during deformation, which results in a more homogeneously strain distribution and improved ductility. Nano-scale distribution of αs effectively block the dislocation motion in β matrix and strengthen the alloy.