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Alternative-band microstructure and LPSO phase in TiAl alloy produced by direct laser deposition
X Zhang，C Li，M Zheng，H Zhong，J Gu
Abstract
Additive manufacturing is becoming an increasingly important process for TiAl alloys, but the microstructure always appears complicated. In the present study, Ti-47Al-2Cr-2Nb alloy was produced by direct laser deposition (DLD), and alternatingly arranged dendritic bands and equiaxed-colony bands were observed under optical microscope. Transmission electron microscopy analysis revealed that α2 phase and γ phase are respectively distributed in dendritic zones and interdendritic zones for the top dendritic band, but α2/γ lamellar structures and a long-period stacking ordered (LPSO) phase with 18R structure were observed in the middle dendritic bands. The equiaxed colonies are fully filled with α2/γ laths in middle equiaxed colony bands. This specific microstructure feature is considered to result from the cyclic thermal history caused by the back-and-forth laser scanning strategy. The LPSO phase is transformed from the interdendritic γ phase under the ageing effect of DLD.
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