N. Kizakibaru, Kumamoto University, Kumamoto, Japan; T. Hara, National Institute for Materials Science, Tsukuba, Japan; M. Nishida, Kyushu University, Kasuga, Japan
It has been reported that the multistage martensitic transformation (MMT) appears in the aged Ni-rich Ti-Ni alloys. There are two reports on the transformation sequence of the MMT. Dlouhy et al. (1) have reported from their in situ TEM observation that the first peak in the differential scanning calorimetry (DSC) curve corresponds to the formation of R-phase at grain boundary regions containing precipitates and the second peak corresponds to the formation of B19′ martensite in the same region, the third peak corresponds to the B2 to B19′ transformation in precipitate-free grain interior regions. Fan et al. (2) have proposed the following transformation process of short-time aged Ti–50.6 at.% Ni polycrystalline alloy during cooling. The first peak is due to the B2 to R-phase transformation around grain boundary, where there is a high density of Ti3Ni4 particles. The second peak is attributable to the B2 to B19′ transformation of the grain interior, where there are no Ti3Ni4 particles essentially. The third peak is due to the R-phase to B19′ transformation around the grain boundary. In the present study, we demonstrate that the transformation sequence is changed by Ni contents, aging temperatures and periods. In other words, the sequence depends on the size and the density of Ti3Ni4 particles in the parent phase. 1. A. Dlouhy, J. Khalil-Allafi and G. Eggeler, Philos. Mag. A 83 (2003), p. 339.
2. G. Fan, W. Chen, S. Yang, J. Zhu, X. Ren and K. Otsuka, Acta Mater. 52 (2004), p. 4351.
Summary: We demonstrate that the transformation sequence of the multistage martensitic transformation in aged Ni-rich Ti-Ni alloys is changed by Ni contents, aging temperatures and periods. In other words, the sequence depends on the size and the density of Ti3Ni4 particles in the parent phase.
