Optimizing high Nb–TiAl alloy: Microstructural insights and mechanical performance under varied compression temperature

To reveal compressive properties and deformation mechanism of high Nb–TiAl alloys at different temperature, uniaxial compression tests were performed on Ti44Al7Nb alloy at 25 °C, 700 °C, 800 °C, 850 °C and 900 °C with strain rate of 1 × 10⁻³s⁻¹. Microstructure and fracture morphology were characterized. Results reveal a fully lamellar microstructure in Ti44Al7Nb with α₂, γ and a bit of B2 phase at room temperature. Compressive deformation properties increase gradually with the increase of temperature. The compressive yield strength gradually decreases from 700 MPa (25 °C) to 530 MPa (900 °C). Brittle-to-ductile transition temperature ranges from 800 °C to 850 °C. Cracks propagate in a trans -granular mode and obvious macro-cracks are exhibited on the profile surface of Ti44Al7Nb alloy after compression test at 25 °C and 700 °C. When crack propagation direction is perpendicular to the arrangement direction of lamellar phases or there is a large angle between them, the lamellar phases will hinder crack propagation effectively. For compression test at 850 °C and 900 °C, specimen will slip along the central plane with a 45° dip angle to its axial plane, as compressive stress increases. Microstructure displays that significant plastic deformation occurs in lamellar phases. And cracks propagate along the deformation direction of lamellar phases. TEM results indicate that as the compression temperature increases, the deformation mechanism of Ti44Al7Nb alloy progressively shifts from dislocation slip and dislocation interaction to twinning deformation, stacking faults. At 900 °C, the number of dislocations and stacking faults in lamellar phase is reduced obviously due to the dynamic softening effect.