CRYSTAL-STRUCTURE OF 4H MARTENSITE OBSERVED IN TI-NI-CU SHAPE MEMORY ALLOYS

The crystal structure of a martensite with a 4H-like structure, which was reported so far to exist not only in binary Ti-Ni but also in ternary Ti-Ni-Cu alloys in addition to the major martensite with a B19' structure, has been examined by electron diffraction. As a result, the "4H" martensite was found to possess a monoclinic structure with a = 0.491, b = 0.720, c = 0.931 nm and gamma = 93.3-degrees, in which complicated atomic shuffles were expected to occur three-dimensionally. The crystal structure is thus not a simple 4H one as the so-called long period stacking order structure: The reciprocal lattice in the a*-b* plane was three times as dense as the corresponding one of, e.g. the gamma-2' martensite in Au-Cd or Ag-Cd alloys. Lattice correspondence between the "4H" martensite and the B2 matrix was expected to be as follows: [100]M --> [1BAR11]B2, [010]M --> [11BAR2]B2 and [001]M --> [220]B2. The volume change associated with the B2 --> "4H" martensitic which was estimated from this electron diffraction study, was larger than that of the B2 --> B19' transformation in absolute value. This may suggest that the thermal hysteresis of the former transformation is larger than that of the latter.