Hot isostatic pressure treatment and high-temperature oxidation behavior of a LaB6/TiAl-based composite fabricated by laser powder bed fusion

Hot isostatic pressure (HIP) treatment was applied to densify and tailor the microstructure of the laser powder bed fused high-Nb TiAl composite reinforced by LaB₆ in this study. The results showed that the HIP treatment significantly decreased the crack density by 86.67 %. Besides, the HIP treatment further resulted in a microstructure evolution from a single α₂-phase with coarsen columnar grain to a heterogeneous microstructure of α₂+ γ lamellar colony and equiaxed γ grain. Meanwhile, due to HIP-induced continuous dynamic recrystallization and pining effect of in-situ La₂O₃ nanoparticles, the average grain size was remarkably refined from 8.607 μm to 4.722 μm. Subsequently, the high-temperature oxidation behavior of the HIP-ed TiAl composite was evaluated at 900°C. The oxidation progression was found to be mainly controlled by ionic diffusion. After oxidation for 100 h, the oxide scale exhibited a four-layer structure including a TiO₂ layer, an (Al, Cr)₂O₃ layer, a TiN/Ti₂AlN layer, and an AlNb₂ layer. Notably, the AlNb₂ layer was just formed at the interface of the oxide layer and matrix, thus effectively reducing oxygen penetration into the substrate. By comparing with other works, the HIP-ed sample in this work exhibited a much lower oxide layer thickness in the intact region free of any defects. On one hand, it can be attributed to a superior heterogeneous microstructure induced by HIP. On the other hand, the in-situ La₂O₃ nanoparticles with an excellent thermal stability also play a positive role in increasing diffusion barrier of oxygen. In general, this work presents a new LPBF-fabricated TiAl-based composite system and helps to give more insight into its high-temperature oxidation behavior.