Ti2AlNb microlattices via 3D ink-extrusion printing and sintering of precursor powders

Abstract

Microlattices are 3D-extruded with inks containing a blend of precursor Ti + Nb + TiAl₃ powders, and their struts are then densified through a series of heat treatments to eliminate organic binder, sinter porosity, and achieve compositional Ti₂AlNb homogeneity. The phase evolution of an as-printed filament (representative of a microlattice strut) is examined using in-situ X-ray diffraction, revealing a series of steps: (i) TiAl₃ decomposition, starting at 710 °C and ending at 780 °C, to form TiAl; (ii) Nb and Al interdiffusion, initiating at 820 °C, accompanied by the formation of Nb₂Al and Nb₃Al phases; (iii) the α→β Ti phase transformation and (iv) Ti₃Al formation, starting at 870 °C. Fully-homogenized Ti₂AlNb microstructures with low residual porosity, comprising a B2 matrix and two types of α₂ and O (orthorhombic) secondary phases, are achieved after sintering at 1300 °C for 5 h. Under compression at 1000 °C, microlattices with struts ∼400 µm in diameter show a good combination of yield strength (138 MPa) and ductility (48 %, with no catastrophic failure). Because of their low density (∼3 g/cm³) and high strength at high temperatures, Ti₂AlNb microlattices exhibit a specific strength higher than existing Ni- and Co-based superalloy microlattices above 900 °C. Finally, a complex Ti₂AlNb prototype heat exchanger is created via layer-by-layer ink-extrusion and sintering.