Direct ink writing of near-fully dense TiAl alloys: synergistic toughening by in-situ MAX phase and hierarchical twin networks

Beam-based additive manufacturing of γ-TiAl alloys suffers from extreme thermal gradients, causing severe cracking and microstructural anisotropy. To circumvent this incompatibility, we report the first successful fabrication of structural intermetallics via direct ink writing (DIW). By coupling a stress-free DIW paradigm with hot isostatic pressing (HIP), we produced near-fully dense Ti-48Al-2Cr-2Nb alloys. This approach addresses the long-standing ‘densification-contamination’ paradox inherent to binder-based manufacturing. We established a unique ‘Residue-to-Reinforcement’ mechanism to thermodynamically control the phase evolution of amorphous carbon. Rather than inducing embrittlement, this residual carbon is in-situ converted into beneficial, semi-coherent Ti2AlC MAX phase precipitates. These precipitates act as physical barriers that deflect microcracks and resist deformation. Consequently, a hierarchically heterogeneous architecture comprising a fully lamellar matrix, nanoscale annealing twins, and Ti2AlC particles is formed. Therefore, the HIP-induced densification substantially eliminates critical volumetric defects, achieving an exceptional balance between ambient strength and high-temperature damage tolerance. Compared to the as-sintered baseline, the HIP-induced densification enhances the room-temperature ultimate tensile strength by 105.5% to 541.8 ± 22.3 MPa, alongside a compressive strength of 2453.0 ± 61.3 MPa. It outperforms conventional binder-based powder metallurgy TiAl variants. Mechanistically, this superior performance originates from a multi-scale deformation accommodation network involving geometric locking, continuous twin transmission, and dynamic recovery.