Effect of TiN addition on microstructural and mechanical behavior of Ti-22Al-25Nb-5Ta matrix composites: Experiments and first-principles calculations

Ti₂AlNb-based alloys are promising lightweight candidates for aerospace propulsion systems. In this work, TiN ceramic particles were introduced into the recently developed Ti-22Al-25Nb-5Ta alloy, which exhibits superior resistance to reheat cracking, to further optimize its microstructure and mechanical properties. TiN promotes the transformation of β/B2 phase to α₂/O phases and provides heterogeneous nucleation sites, leading to significant grain refinement (~31.7% at 2 wt% TiN addition). TiN particles are distributed both within grains and along grain boundaries, forming clean semi-coherent interfaces with the B2 matrix. The orientation relationship is identified as [100]_B2 // [110]_TiN, (011)_B2 3° from (11̅1)_TiN, and (01̅1)_B2 19° from (11̅1̅)_TiN. Such TiN/B2 interface bonding, accommodated by local lattice distortion and misfit dislocations, enables efficient load transfer. Consequently, the composite reinforced with 2 wt% TiN exhibits a significant enhancement in both hardness (~32.4%) and compressive yield strength (~45.3%) compared with the unreinforced alloy, while maintaining a relatively low density of ~5.860 g/cm³. First-principles calculations indicate that N preferentially occupies Ti-rich octahedral interstitial sites in the B2 phase irrespective of Ta substitution for Nb, and simultaneously enhances the thermodynamic driving force for the B2 → O transformation. Furthermore, the N-terminated TiN-N/B2-I configuration is identified as the most stable TiN(11̅1)/B2(011) interface, exhibiting the highest adhesion work and the smallest equilibrium interfacial spacing.