Mechanism of B2-B19-B19′ transformation in shape memory Ti-Ni-Cu alloys

Abstract

Martensitic transformations which occur in Ti-45.5Ni-5.0Cu and Ti-44.0Ni-5.0Cu-1.0Al (at%) alloys were investigated by means of electrical resistivity measurements, X-ray diffraction analysis and transmission electron microscope observation. It was found that both the two alloys transform in two steps on cooling, i.e., from the high temperature cubic phase (B2-type structure) to the orthorhombic martensite (B19-type structure), and then to the monoclinic martensite (B19′-type structure). The two-step transformations in these alloys proceed as follows. On cooling, plates of orthorhombic martensite nucleate at certain sites in the cubic matrix. While the plates of orthorhombic martensite are in growth, plates of the monoclinic martensite nucleate in the orthorhombic martensite and grow consuming it. The transformation from the orthorhombic martensite to the monoclinic martensite occurs by shearing (001)o plane to [100]o direction. A plate of the monoclinic martensite which have grown in this way consists of thin plates of (001)m compound twin as a result of alternative shearing to [100]o and [1̄00]o direction. It is difficult to differentiate the successive two transformations of these alloys described above by electrical resistivity measurement or X-ray diffraction because the two transformations overlap and the amount of the orthorhombic martensite in the specimen through the two-step transformation is very small.