Enhanced Strength-Ductility Synergy and Mechanisms of Heterostructured Ti6al4v-Cu Alloys Produced by Laser Powder Bed Fusion

Abstract

Limited slip systems of α structure always play a major role in hindering strength-ductility synergy improvement of titanium (Ti) alloys. To overcome the strength-ductility of Ti alloys, heterostructure is introduced into Ti6Al4V-xCu (x=0, 1, 3, 5 wt.%) alloys produced by laser powder bed fusion (LPBF), the formation mechanism and deformation behavior of heterostructures are investigated. The results show that the single α' in the LPBF-produced Ti6Al4V-xCu alloys can be decomposed into dual α and α" after heat treatment at 600-900 °C: α' → α + Ti₂Cu → α + β → α + α". The multistage transformation of α' to α and α" is driven by thermal activation (temperature > 800 °C), Cu addition and rapid cooling of water quenching. The heterostructure with α and α" in the LPBF-produced Ti6Al4V-5Cu alloy after heat treatment at 800 °C results in high tensile strength (~1.3 GPa) and large elongation (~15%). The enhanced strength-ductility synergy is attributed to the decomposition of brittle α' and Ti₂Cu, as well the soft-hard heterostructure of α" and α. Moreover, the deformation twins (DTs) in α" and the heterogeneous interfaces of α and α" can also improve the strength and ductility of the LPBF-produced Ti6Al4V-xCu alloys. These findings elucidate the influence of heterostructure (α and α") on strength and ductility, which is helpful for the designing of Ti alloys with excellent mechanical properties.

Keywords: Titanium alloy, Laser powder bed fusion, Deformation twins, Heterostructure, Mechanical behavior