Effect of Al Addition on Constituent Phases and Material Properties of Ti-18Nb-<i>x</i>Al Alloys

The Ti-18mass%Nb alloy with a quenched α” martensitic structure exhibited a high damping capacity. However, there are issues such as lower strength than annealed α+β structure and decreasing damping capacity due to heating until 400 K. Therefore, in this study, to address these issues, we investigated the effect of Al addition on the constituent phases and material properties of Ti-18Nb-xAl alloys. The crystal structure was determined by examining the lattice constant and unit volume using X-ray diffraction, and optical microscopy was also performed. The material properties were investigated by Vickers hardness, Young’s modulus, internal friction, tensile tests, and DSC measurements. Vickers hardness and tensile strength increased with increasing Al content. This is thought to be due to the combined effects of the refinement of the microstructure and solid-solution strengthening due to Al addition. The Young’s modulus increased slightly from 0Al to 1Al, but increased significantly to 4Al. Internal friction was highest for 0Al and decreased for 4Al, whereas 7Al showed a higher value than 1Al. In the DSC heating curves, there was a decrease in the exothermic peak starting temperature and an increase in the phase-transformation heat with the addition of Al, except for 1Al. It was suggested that these changes in Ti-18Nb-xAl alloys were influenced by the structure of the quenched α” phase, texture, and pseudoelasticity or phase transformation by deformation.