Massive transformation in Ti-6Al-4V additively manufactured by selective electron beam melting

The occurrence of the 13 (bcc) to alpha(m) (hcp) massive transformation in Ti-6Al-4V (wt.%) during additive manufacturing (AM) by selective electron beam melting (SEBM) has been identified. A variety of patch shaped massive grains including both grain boundary (GB)-crossing and non-GB-crossing instances were characterised and their formation temperature and growth rate were estimated. In contrast to conventional Ti-6Al-4V, the massive grains in SEBM-fabricated Ti-6Al-4V consist of ultrafine lamellar alpha and beta phases (100 nm wide beta strips) due to in-situ decomposition of the massive grains during AM. The resulting ultrafine alpha-beta lamellae obey the typical Burgers orientation relationship established for the lamellar alpha-beta structure in conventional Ti-6Al-4V. These findings show that the beta (bcc) to alpha(m) (hcp) massive transformation significantly contributes to development of the Ti-6Al-4V microstructure during AM by SEBM, providing an alternative microstructural design strategy for AM of Ti-6Al-4V and other titanium alloys.