Strong and Ductile Martensite Induced by Periodic Solute Segregation in an Additively Manufactured Titanium-Copper Alloy

Abstract

The fusion based metal additive manufacturing process, in particular laser powder bed fusion (L-PBF), revolutionizes the titanium industry due to its lower cost, shorter lead time and higher level of design flexibility. However, the most common product in the as-fabricated Ti alloys is martensite due to the high cooling rate, which is typically strong but intrinsically brittle. Herein we propose a new alloy design strategy to solve this dilemma by stimulating a high density of inter-variants transformation twins in an L-PBF Ti-5Cu alloy. The as-fabricated Ti-5Cu alloy comprises full martensite with a tensile strength of 1130 MPa and a fracture strain as high as 15%. Such excellent synergy of strength and ductility stems from the periodic segregation of solute Cu atoms on the inter-variant twin boundaries, which stabilizes thermodynamically the twin boundaries. Meanwhile, the fast diffusion of Cu atoms in Ti lattice facilitates such segregation is accomplished in a transient temperature field of the L-PBF process, without the need for any post-fabrication heat treatment. This manufacture-friendly, high-strength and high-ductility, martensitic alloy is expected to be a promising candidate for aerospace and biomedical applications.

Keywords: titanium alloys, Additive manufacturing, microstructure, mechanical property