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Sitepu, H.; Schmahl, Wolfgang W.; Eggeler, Gunther; Khalil-Allafi, Jafar; Többens, D. M.; Pietikainen, J.; Soderberg, Eds. O. (2003): A neutron diffraction study of the martensitic transformations in aged Ni-rich NiTi alloys. In: Journal De Physique. IV : JP, Vol. 112 II
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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 18°C and 7°C. On heating, a single peak occurs at 43°C. 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 400°C for 20 hours.