Microdefects and Electron Densities in Near-Equiatomic NiTi Alloys Studied by Positron Annihilation Spectroscopy

Microdefects and free electron densities in near-equiatomic NiTi alloys of five different chemical compositions have been studied by positron lifetime measurements. The results indicate that some 3d electrons are localized to form covalent bonds when Ni and Ti atoms aggregate to form NiTi alloys, leading to the decrease of the free electrons participating in metallic bonds. The densities of free electrons in the bulk and defects of near-equiatomic NiTi alloys change with the increase of Ni content and reach the maximum value with 51.0% Ni (atom fraction). It is also found that the Ni51Ti49 alloys have the fewest defects, which contributes to the Martensite transformation. Consequently, the Ni51Ti49 alloys have the best shape memory effect.