This research aimed to investigate the impact of friction stir processing (FSP) as a modifying technique on the microstructure and mechanical properties of Cu-10Al, Cu-10Al-5Ni, and Cu-10Al-5Ni-5Fe nickel-aluminum bronze (NAB) alloys under two different conditions of ambient and underwater submerged conditions con-cerning the as-cast state of these materials. Adding nickel as an alloying element to the Cu-10Al composition resulted in the formation of NiAl intermetallic compound and a 3 % increase in the volume percentage of sec-ondary phases. Furthermore, incorporating iron into the Cu-10Al-5Ni composition prevented the formation of detrimental gamma 2 phase and instead created kappa intermetallic compounds within the structure, leading to an overall increase of approximately 14 % in the volume percentage of secondary phases compared to the Cu-10Al composition. The initial coarse, rough, and heterogeneous structure of the cast samples was transformed into a fine, equiaxed, and homogeneous structure through the friction stirring modification conducted in both air and underwater environments for all alloy compositions, primarily due to action of dynamic recrystallization (DRX). Notably, a finer structure was achieved when the FSP was performed underwater in the submerged state. Regarding developments in mechanical properties, FSP treatment performed in air demonstrated an average enhancement of 15 % in strength and hardness across all three compounds. In contrast, the flexibility of Cu-10Al, Cu-10Al-5Ni, and Cu-10Al-5Ni-5Fe alloys in terms of elongation to failure was reduced by 70 %, 80 %, and 20 %, respectively, in comparison to the cast samples. Conversely, the underwater submerged FSP led to a notable increase of 55 % in both strength and hardness for all three compounds. However, the ductility of the alloys was reduced by 80 % compared to the cast states.
