J. Phys. IV France 112 (2003) 643
A neutron diffraction study of the martensitic transformations in aged Ni-rich NiTi alloysH. Sitepu1, W.W. Schmahl1, G. Eggeler2, J. Khalil Allafi2 and D.M. Többens3
1 Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
2 Institut für Werkstoffe, Ruhr-Universität Bochum, 44780 Bochum, Germany
3 Hahn-Meitner-Institut (HMI), 14109 Berlin, Germany
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 500 °C for 20 hours was investigated. The material shows a two-step transition on cooling with differential scanning calorimetry peaks at 180 °C and 7 °C. On heating, a single peak occurs at 43 °C. The aging leads to the formation of Ni 4Ti 3 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 400 °C for 20 hours.
© EDP Sciences 2003