Metastable phase and microstructural degradation of a TiAl alloy produced via selective electron beam melting

Abstract Selective electron beam melting (SEBM) is an attractive rapid prototyping technology to fabricate TiAl alloys with a complex structure. A near fully dense Ti–47Al–2Cr–2Nb alloy was fabricated by SEBM by using a low-energy density of 20.04 J/mm3. The phase transformation, microstructural degradation and microhardness of the SEBM-produced TiAl alloy were investigated. The results showed that the microstructure was inhomogeneous along the building direction. Multiple phase transformations were observed during the in-situ thermal cycle. Disordered α phase was observed in the last solidifying tracks in the top surface due to the rapid cooling rate. A metastable Ti2Al phase was observed due to the relatively high residual stress that was caused by a quick cooling rate when fabrication occurred under a low energy density of 20.04 J/mm3. The decomposition of α2 lamellae, continuous coarsening and discontinuous coarsening of lamellar structure were the three main routes for microstructural degradation for SEBM-produced TiAl alloys. The microhardness exhibited an increasing tendency from a height of 18 mm to the top surface due to the increasing content of the lamellar colony. Finally, the degradation mechanism of the lamellar colony was discussed in detail.