M. Nishida, Kyushu University, Kasuga, Japan; T. Matsui, Kobelco Company, Kobe, Japan; T. Fujimoto - Matsui, M. Matsuda, S. Tsurekawa, Kumamoto University, Kumamoto, Japan
Ti-Ni shape alloys have anomalous ductility in the wide temperature range, irrespective of B19’ martensitic or B2 parent phases. The fracture mode of the parent phase in the Ti-Ni alloys is exceptionally transganular type in comparison to the other B2 type compounds such as Ni-Al. In order to clarify the anomalous ductility of the Ti-Ni alloy, we have to characterize not only the deformation substructure but also the grain boundary structure. The present authors have recently developed the recrystallization structure with a high fraction of low energy coincidence site lattice boundaries, by which we can complete the HREM imaging of some specific <110>B2 symmetric tilt boundaries such as {112}B2 ∑3, {114}B2 ∑9 and {115}B2 ∑27 boundaries. In the present paper, we report on the grain boundary control procedure of the Ti-Ni alloy. Subsequently, the atomic structure of the <110>B2 symmetric tilt boundaries is discussed on the basis of HREM observations. The obtained results are summarized as follows. The proportion of CSL boundary below ∑29 with low energy was determined to be about 88.7% in the specimen annealed after the tensile deformation about 20 to 40% at the martensitic state. The atomic structure of <110>B2 symmetric tilt boundaries is described by the periodical combination of structure units. The mechanism of the successful grain boundary control mentioned above is also discussed on the basis of lattice correspondence between the B19’ martensitic and the B2 parent phases.
Summary: We report on the grain boundary control procedure of the Ti-Ni alloy. Subsequently, the atomic structure of the <110>B2 symmetric tilt boundaries is discussed on the basis of HREM observations.
