Sitepu, H.; Schmahl, Wolfgang W.; Reinecke, T.; Khalil-Allafi, Jafar; Eggeler, Gunther; Pietikainen, J.; Soderberg, Eds. O.
Phase fractions of B2, B19′, R-phase and Ni4Ti3 in NiTi alloys during two-step phase transformations.
In: Journal De Physique. IV : JP, Vol. 112 II
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We investigated the phase composition of (i) solution annealed and subsequently aged Ti-50.7 at.-\%Ni alloy and (ii) solution annealed Ti-50.1 at.-\%Ni alloy cycled 30 times between 100°C and -196°C by x-ray diffraction through the martensitic phase transformation. Both samples show \"two-step\" transformation behaviour with two exothermic differential scanning calorimetry (DSC) peaks on cooling and one endothermic DSC peak on heating. For the aged Ni-rich material the x-ray data, which reflect the conditions at the sample surface only, indicate that in the presence of the Ni4Ti3 precipitates the B2 phase transforms to the R-phase, which then transforms to B19′ phase in the second step on cooling. The transition to B19′ starts well above the second peak. In the cycled, precipitate-free material, the R-phase coexists with B2, and/or B19′ throughout its limited temperature range of stability.