Positron lifetime and Doppler broadening with a coincidence technique experiments have been performed in NiAl, Ni48Al45Mn7, Ni48Al45Fe7 and Ni50Al20Fe30 polycrystalline alloys. It has been found that there are structural vacancies and large open-volume defects in binary NiAl alloy. The addition of Fe to NiAl results in the decrease of the open volume of the defects and the concentration of structural vacancies. The effects of Mir and Fe on the distribution of electron momentum in the alloy have also been discussed.