Obtaining bimodal microstructure in laser melting deposited Ti-5Al-5Mo-5V-1Cr-1Fe near β titanium alloy

Bimodal microstructures, consist of equiaxed primary α (α_p) and lamellar secondary α (α_s), usually lead to good comprehensive properties for wrought titanium alloys. However, for laser melting deposited titanium alloys, only lamellar microstructures are usually obtained, which result in relative low ductility. In this paper, to improve the ductility of laser melting deposited Ti-5Al-5Mo-5V-1Cr-1Fe titanium alloy, we try to introduce equiaxed α and obtain bimodal microstructures by heat treatments. Firstly, two kinds of heat treatment are applied to obtain equiaxed α i.e., subtransus anneal treatment and subtransus multi-anneal treatment. The subtransus anneal treatment is found to be able to promote α phase globularization, and the underlying mechanism is proposed by diffusion theory. However, it only leads to the elongated α phase with aspect ratio about 3.5. Then, inspired by the globularization mechanism, a novel subtransus multi-anneal treatment is designed, which can lead to near equiaxed α with the aspect ratio about 1.7. Afterwards, the subtransus multi-anneal and aging treatment are applied to obtain bimodal microstructure with near equiaxed α_p and lamellar α_s, which increases the elongation of the alloy to 11.5%, compared to 6.7% for the stress-relief anneal treated samples.