No abstract

Thermohydrogen treatment (THT) is an effective way to refine microstructure and improve the mechanical properties of the titanium alloys. In the current work, as-cast Ti-6.5Al-2Zr-Mo-V alloy was hydrogenated with different hydrogen contents and processed solution aging. Accordingly, the microstructure evolution and phase transformation were analyzed. Results show that during solution aging, eutectoid decomposition occurs and the product is a mixture of coarse primary a, fine eutectoid product and undecomposed b H . The size of primary a is closely dependent on the hydrogen content, and large primary a can be obtained at medium hydrogen content. Further, the influence of hydrogen content on the growth of primary a phase was revealed. The primary a is much fine, and the eutectoid product is relatively homogeneous with 0.984 wt% H. After THT, the ultimate strength is beyond 1,100 MPa that has increased by 23.15% compared with that in as-cast state.

Substructures and microsegregation ofγ/γlaths are analyzed with HRTEM and HAADF-STEM. Results show that the substructures are generated during evolution of shear transformation on the(111-)plane ofγlath. At the beginning, shear transformation evolves in a singleγlath, and a superstructure intrinsic stacking fault (SISF) forms in theγlath. After the formation of the SISF, the shear transformation may evolve in two different ways. If the shear transformation evolves into neighboringγlaths, the SISF also penetrates into neighboringγlaths and a ribbon of SISFs forms. If shear transformation continues to evolve in the original lath, complex substructures begin to form in the original. If shear transformation in the original lath is homogeneous and complete, secondary twin forms which may further grow into twin intersection. Incomplete shear transformation could not form secondary twins but generates a high concentration of planar faults on the(111-)plane. These planar faults may further penetrate theγ/γlath interface, grow into adjacent laths, and form a ribbon of planar faults.