Microdefects and electron densities in NiTi shape memory alloys studied by positron annihilation

The microdefects and free electron densities in B2, R and B19′ phases of Ni 50.78Ti 49.22 alloy were studied by positron lifetime measurements. Comparing the lifetime parameters of the Ni 50.78Ti 49.22 alloy measured at 295 K and 225 K, it is found that the free electron density of the R phase is lower than that of the B2 phase; the open volume of the defects of the R phase is larger, while the concentration of these defects is lower than that of the B2 phase. The Ni 50.78Ti 49.22 alloy exhibits B19′ phase at 115 K. In comparison with the R phase, the free electron density of the B19′ phase increases, the open volume of the defects of the B19′ phase reduces, and the concentration of these defects increases. The microdefects and the free electron density play an important role during the multi-step transformations ( B2→ R→ B19′ phase transformations) in Ni 50.78Ti 49.22 alloy with the decrease of temperature.