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

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Vol. 609; pp. 177 - 184
• Main Authors: Liu, C.M., Tian, X.J., Wang, H.M., Liu, D.
• Format: Journal Article
• Language: English
• Published: Kidlington Elsevier B.V 15-07-2014; Elsevier
• Subjects: Annealing; Applied sciences; Aspect ratio; Bimodal microstructure; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Deposition; Diffusion in solids; Ductility; Elongation; Exact sciences and technology; Heat treatment; Laser beam melting; Laser melting deposition; Materials science; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Microstructure; Other mechanical properties; Physics; Production techniques; Solid solution, precipitation, and dispersion hardening; aging; Subtransus multi-anneal treatment; Titanium alloy; Titanium base alloys; Transport properties of condensed matter (nonelectronic); Treatment of materials and its effects on microstructure and properties; Titanium alloy; Laser melting deposition; Bimodal microstructure; Subtransus multi-anneal treatment; Annealing; Heat treatment; Ductility; Ageing; Laser deposition; Equiaxed structure; Aspect ratio; Stress relaxation; Dual phase structure; Lamellar structure; Alpha phase; Diffusion; Laser fusion; Elongation (mechanics)
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/j.msea.2014.05.010

Bimodal microstructures, consist of equiaxed primary α (αp) and lamellar secondary α (αs), 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 αp and lamellar αs, which increases the elongation of the alloy to 11.5%, compared to 6.7% for the stress-relief anneal treated samples.