A neutron diffraction study of the martensitic transformations in aged Ni-rich NiTi alloys
Journal De Physique Iv 112: 643-646
The transformation of a Ni-rich NiTi alloy of nominal composition 50.7 atomic percent nickel which was solution annealed, water quenched and aged at 500degreesC for 20 hours was investigated. The material shows a two-step transition on cooling with differential scanning calorimetry peaks at 18degreesC and 7degreesC. On heating, a single peak occurs at 43degreesC. The aging leads to the formation of Ni4Ti3 precipitates in the microstructure, and in the presence of the precipitates the martensitic transformation occurs from the cubic B2 phase via the R-phase to B19' martensite. However, the first peak on cooling is not due to the formation of R-phase alone. Below the peak temperature, R-phase is present along with some B19' and with residual B2 and the precipitates. The second peak is related to the transformation of R-phase and B2 to B19'. This result confirms our earlier conclusions which were obtained for the same alloy aged at 400degreesC for 20 hours.