Development of an additively manufactured metastable beta titanium alloy with a fully equiaxed grain structure and ultrahigh yield strength

Abstract

Metastable β titanium alloys usually suffer from relatively low yield strengths, which restricts their applications as a structural material. Additive manufacturing (AM), due to its extremely high cooling rates, can generate a refined microstructure that is beneficial to yield strength. However, the intrinsic steep thermal gradients within melt pools often lead to development of columnar grains that can result in mechanical anisotropy. To address these issues, we propose to use potent β-stabilizing elements with large growth restriction factors as the main solute elements in Ti, specifically Fe and Co. d-electron theory is also used to design the detailed compositions of the new titanium alloys for AM. A novel metastable titanium alloy, Ti-xFe-xCo-1Mo (1.5< x <3.5 at%), is thus developed by laser powder bed fusion (L-PBF). With process optimization, the L-PBF-processed alloy was found to contain fully equiaxed β grains embedded with α laths and ω precipitates and the grain boundaries decorated by Ti 2 Co precipitates. The matrix consists of a certain number of micro-sized β flecks and a high density of Fe and Mo atomic clusters. Upon solution treatment (ST), the microstructure turned into equiaxed β grains embedded with ultrafine ω precipitates and Mo atomic clusters. While the L-PBF-processed alloy shows poor tensile properties probably due to the presence of isothermal ω precipitates, the L-PBF-ST-processed alloy demonstrates an unprecedented yield strength of 1.2 GPa and a decent elongation of 10~12%. The alloy deformed by dislocation slipping and failed in a ductile fracture mode.