To broaden the scope of non-aerospace applications for titanium-based alloys, both hexagonal C40 binary TaSi2 and ternary Al alloyed TaSi2 nanocrystalline coatings were exploited to enhance the cavitation erosion resistance of Ti-6Al-4V alloy in acidic environments. To begin with, the roles of Al addition in influencing the structural stability and mechanical properties of hexagonal C40 Ta(Si1-xAlx)2 compounds were modelled using first-principles calculations. The calculated key parameters, such as Pugh's index (B/G ratio), Poisson's ratio, and Cauchy pressures, indicated that there was a threshold value for Al addition, below which the increase of Al content would render the Ta(Si1-xAlx)2 compounds more ductile, but above which no obvious change would occur. Subsequently, the TaSi2 and Ta(Si0.875Al0.125)2 coatings were prepared and their microstructure and phase composition were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Both the two coatings exhibited a uniform thickness of 15 µm and a densely packed structure mainly composed of spherically shaped nanocrystallites with an average diameter of about 5 nm. Nanoindentation measurements revealed that Al alloying reduced the hardness (H) and elastic modulus (E) values of the TaSi2 coating. Ultrasonic cavitation erosion tests were carried out by immersing coated and uncoated samples in a 0.5 M HCl solution. The cavitation-erosion analysis of the tested samples was investigated by various electrochemical techniques, mass loss weight and SEM observation. The results suggested that both coated samples provided a better protection for Ti-6Al-4V against the cavitation-erosion damage in acidic environments, but the addition of Al further improved the cavitation-erosion resistance of the TaSi2 coating.