Formation of a transition V-rich structure during the ?? to ? plus ? phase transformation process in additively manufactured Ti-6Al-4 V

Ti-6Al-4V parts fabricated by laser powder-bed fusion (L-PBF) additive manufacturing often suffer from poor ductility and low toughness due to the predominance of the acicular alpha' martensitic phase in their microstructures. This challenge can be overcome by manipulation of the L-PBF thermal history to introduce an alpha' -> alpha+ beta decomposition, resulting in a fine lamellar alpha+ beta structure with a combination of excellent strength and ductility. Understanding the details of the alpha' -> alpha+ beta phase transformation process is critical for fabricating titanium alloys with excellent mechanical properties. Through a systematic electron microscopy characterisation of the microstructural evolution of a Ti-6Al-4V alloy fabricated by LPBF, here we reveal that the alpha' -> alpha+ beta phase transformation occurs in two steps: alpha' -> alpha+ alpha HME (high in Mo eq ) and alpha HME -> beta, in which alpha HME is a newly discovered non-equilibrium structure with a hexagonal close-packed structure and with a composition close to that of the beta phase. (c) 2022 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

By studying the microstructural evolution of Ti-6Al-4V alloy fabricated by laser powder-bed fusion (L-PBF), it has been discovered that manipulating the thermal history can induce an alpha' -> alpha+beta phase transformation, resulting in a fine lamellar alpha+beta structure with excellent strength and ductility.