Abstract Although niobium and its alloys are considered promising materials to use in the making of ultrasonic horn (sonotrodes) for the cavitation treatment of molten aluminum alloys, data on the performance of such sonotrode is lacking in the literature. In this study, a sonotrode tip made of Nb and another made of Nb–4%Mo alloy were used to introduce high-intensive ultrasonic vibrations in an aluminum melt under conditions close to those encountered in aluminum casting. The purpose was to investigate the characteristics of tip erosion and the effect of such erosion on said tips to produce acoustic cavitation in an aluminum melt. The results showed that the Nb–Mo tip has superior resistance to erosion than the Nb one, with the main difference being that the working surface of the Nb–Mo tip eroded uniformly in contrast to the non-uniform island-like erosion observed in the Nb tip. As a result, the Nb–Mo tip showed a longer life span and good property to produce cavitation in an aluminum melt even after 40h of ultrasonic irradiation. In contrast, the Nb tip lost its cavitation-producing property after 16h of ultrasonic irradiation. Mechanisms of cavitation erosion included fatigue damage and micro deformation of the tip surface, followed by the dissolution of Nb in the Al melt, either directly or by the formation of Al–Nb intermetallic compounds. It is assumed that the lower resistance of the Nb tip to cavitation damage is due to the Nb tip having a much lower value of tensile stress and hardness compared to that of the Nb–Mo alloy tip.